Vascular Smooth Muscle Activation

The actions of many drugs discussed in this chapter affect vascular smooth muscle cell (VSMC) contraction, and a basic understanding of contractile regulation is essential to understanding their mechanism of action. Contraction of vascular smooth muscle involves a sliding filament mechanism similar to that observed in other smooth muscle or in skeletal muscle. This topic has been reviewed in depth previously,¹ is covered in detail in Chapter 3, and is therefore only briefly discussed here. The classical paradigm, depicted in Figure 6-1, is that increases in intracellular calcium lead to formation of a calcium-calmodulin complex. Calcium–CaM then binds and activates myosin light chain kinase (MLCK), which then phosphorylates myosin light chain (LC-20). Phosphorylation of LC-20 increases myosin adenosine triphosphatase (ATPase) activity, which leads to cross-bridge cycling and contraction. Myosin light chain phosphatase negatively regulates this process by dephosphorylating LC-20. Myosin light chain phosphatase is in turn inhibited by the small G-protein Rho and Rho kinase, which phosphorylates a subunit of myosin light chain phosphatase known as the myosin-binding subunit (MBS), leading to inhibition of phosphatase activity and favoring contraction. Myosin phosphatase is also inhibited by a 17-kDa protein known as CPI-17 (protein kinase C [PKC]–potentiated inhibitory protein of 17 kDa) that in turn is activated by PKC. Thus, activation of PKC can indirectly reduce myosin phosphatase activity, increase myosin phosphorylation, and promote vasoconstriction.

Figure 6-1 Vascular smooth muscle contractile regulation.

ATP, adenosine triphosphatase; CaM, calmodulin; cAMP, cyclic adenosine monophosphate; CCB, calcium channel blocker; cGMP, cyclic guanosine monophosphate; CPI-17, protein kinase C-potentiated inhibitory protein of 17 kDa; GMP, Guanosine monophosphate; GTP, guanosine triphosphate; MLCK, myosin light chain kinase; NO, nitric oxide; PDE, phosphodiesterase; pGCase, particulate guanylyl cyclase; PGI², prostacyclin; PKC, protein kinase C; PKG-1, type 1 protein kinase G; sGC, soluble guanylyl cyclase.

An important counterregulatory pathway in this scheme is the nitric oxide (NO) pathway. Nitric oxide acts on soluble guanylyl cyclase (sGC), which catalyzes the conversion of guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP). In turn, cGMP acts as the only substrate for type 1 protein kinase G (PKG), which phosphorylates MBS, increasing its phosphatase activity and promoting vasodilation. Protein kinase G also phosphorylates and inhibits Rho, further reducing the propensity for vasoconstriction and promoting vasodilation. These pathways are targets of myriad vasoactive drugs that will be considered in greater depth in this chapter and are depicted in Figure 6-1.

Traditionally, precapillary arterioles with diameters of approximately 25 μm were thought to regulate blood flow. While this is true in some organs such as the kidney, in many organs, vascular resistances are distributed over a wider range of vessel sizes. In the coronary circulation, fully half of this resistance lies in vessels between 100 and 300 μm in diameter, and the remainder exists in vessels smaller than 100 μm and in venules. Interestingly, many vasoactive agents variably affect these different-sized vessels. As an example, organic nitrates act predominantly on the larger arteries and veins and have minimal effect on arterioles. In contrast, adenosine potently dilates resistance vessels and has less effect on larger vessels. Vasopressin is a potent constrictor of resistance arterioles and causes endothelium-dependent vasodilation of conductance arteries. These differential effects cause various drugs and hormones to selectively affect factors such as venous capacitance, large (conductance) vessel diameter, and blood flow in the intact circulation.

It is now apparent that many pharmacological agents not only modulate vascular tone but also vascular growth, remodeling, inflammation, thrombosis, and vascular repair. As examples, many components of the contractile pathway discussed earlier exist in endothelial cells (ECs), including actin, myosin light chain, MLCK, Rho, and Rho kinase. These regulate endothelial shape, migration, cell-cell contact, and permeability. Myosin light chain kinase activation controls EC calcium entry, NO production, and release of endothelium-derived hyperpolarizing factor (EDHF).² The Rho/Rho kinase pathway works in concert with other GTPases to modulate endothelial production of NO and reactive oxygen species (ROS) and gene expression.³ These aspects of vascular control have been the subject of substantial recent research, and new drugs have been developed to affect these targets. In addition, these pathways seem to be affected in an off-target fashion by several existing pharmacological agents.

Pharmacokinetics and Pharmacodynamics

Before discussing specific pharmacological agents, the basic concepts of pharmacology should be reviewed. Drug absorption, distribution, metabolism, and clearance are the principal concepts of pharmacokinetics, and these concepts are detailed elsewhere in pharmacology texts. Certain disease states, such as renal insufficiency, liver insufficiency, or heart failure, may have important effects on drug pharmacokinetics within individuals, and relevant situations will be discussed with specific drugs. Pharmacodynamics is the study of drug mechanism of action and physiological effects. Because some agents produce persistent effects beyond their clearance from the circulation, a drug with a short half-life can have a longer dosing interval than that predicted by its clearance. For example, aspirin irreversibly inactivates the cyclooxygenase (COX) enzyme and achieves long-lasting platelet inhibition despite rapid clearance from the circulation. Diuretic agents may have a persistent antihypertensive effect after drug cessation, at least until a new level of sodium balance is achieved. Other effects may become evident only after drug withdrawal. For example, β-blockers increase receptor sensitivity as well as circulating catecholamines, and sudden drug withdrawal of their β-blockade can result in rebound hypertension. Therefore, consideration of both pharmacokinetic properties and pharmacodynamic effects is essential for understanding drug action.

The nature of a drug response helps classify the drug as a full or partial agonist, antagonist, or an inverse agonist (Fig. 6-2A) and may provide insight into the mechanism of drug action. For receptor conformation–specific drugs, pure antagonists stabilize the active and inactive conformations equally and have no net effect on basal activity. Inverse agonists preferentially stabilize the receptor’s inactive form, and agonists stabilize the active conformation.

Figure 6-2 Comparison of receptor agonist activity.

A, Conformation-specific receptor agonists (see text). B, Drug potency is compared by the concentration necessary to achieve 50% maximal drug response (EC₅₀). C, Drug efficacy is compared by the observed maximal response achieved.

The potency of a drug refers to the molar concentration necessary to achieve a desired response (e.g., 50% maximal stimulation or inhibition; Fig. 6-2B), whereas efficacy reflects the drug’s maximal response relative to other agents (Fig. 6-2C). Clinical differences in drug potency may be overcome by increasing the dosage, whereas differences in drug efficacy cannot.

Receptor antagonists can be assessed by the response to a known stimulus in the presence of increasing antagonist concentration (Fig. 6-3). Antagonists that reversibly bind to the receptor can be overcome with increasing concentration of agonist (Fig. 6-3A). Antagonists that irreversibly bind their target impair the maximal response with increasing concentration (Fig. 6-3B). A number of drugs act in an allosteric manner by binding to a site on the receptor that is distinct from the native ligand, inducing a conformational change. Allosteric modulators can either increase or decrease agonist response by binding to a site distinct from the agonist binding site. An allosteric antagonist dose-response curve appears similar to that of a noncompetitive antagonist. Allosteric potentiators shift the agonist curve to the left (see Fig. 6-3A), while competitive antagonists shift the curve to the right.

Figure 6-3 Receptor antagonism.

A, With increasing concentration of a competitive antagonist, the agonist dose response curve is shifted to the right. Allosteric potentiators produce a left shift of this curve. B, Noncompetitive antagonists and allosteric antagonists shift the agonist response curve to the right and impair maximal response in a nonlinear manner.

Nitrovasodilators

The nitrovasodilators produce their biological effects either by releasing NO or closely related molecules that are converted to NO in cells. The most commonly employed nitrovasodilators are the organic nitrates and sodium nitroprusside. It is useful to begin a discussion of these agents by comparing sodium nitroprusside and nitroglycerin, which are illustrated in Figure 6-4. As apparent, the oxidation state of the nitrogen that is ultimately released as NO differs in these molecules, and this basic structural property provides insight into their pharmacological profiles. Sodium nitroprusside requires a one-electron reduction to release NO, and this is readily accomplished nonenzymatically by a variety of reductants in the circulation, the interstitial space, and the cell. Thus, when infused intravenously, nitroprusside begins to release NO throughout the circulation and potently dilates all vessels. Moreover, given the short half-life of NO, the vasodilation caused by nitroprusside is short-lived once its infusion is discontinued.

Figure 6-4 Nitrovasodilator agents.

Structure of nitric oxide (NO) donor agents. The nitroprusside cyanide ligands (C≡N) are highlighted in red.

As is apparent from its structure, nitroprusside possesses five cyanide groups in each molecule (highlighted in red in Fig. 6-4), and prior studies have shown that each of these is reduced prior to the release of NO. The cyanide radicals react with hemoglobin (Hb) to form methemoglobin and are converted to thiocyanate in the liver. When these metabolic pathways are depleted, cyanide toxicity occurs, characterized by central nervous system (CNS) dysfunction, metabolic acidosis with a base deficit, and elevated plasma lactic acid concentrations.⁵ Fortunately, cyanide toxicity is infrequent during brief administration of sodium nitroprusside but occurs more commonly when infusion rates exceed 2 μg/kg/min and when the drug is infused for prolonged periods. In addition, the risk of cyanide toxicity is increased in patients with renal or hepatic failure, so sodium nitroprusside should be avoided in patients with these conditions. Owing to its capacity to rapidly release NO, sodium nitroprusside produces potent systemic vasodilation and is effective as an antihypertensive. It is still used for treatment of severe hypertension and, in some cases, for afterload reduction in patients with severe heart failure; however, newer agents with less potential toxicity are now more commonly used.

In contrast to sodium nitroprusside, nitroglycerin and other organic nitrates require a 3-electron reduction to yield NO. In the last several years, it has become clear that this is in large part accomplished by the action of the mitochondrial enzyme aldehyde reductase-2 (ADH2).⁶ Mice lacking this enzyme are resistant to the actions of nitroglycerin. Notably, about 40% of East Asians have a dominant negative mutation of ADH2 that causes intolerance to ethanol and markedly impaired vasodilator responses to nitroglycerin.⁷

As mentioned earlier, organic nitrates preferentially dilate larger arteries and veins while having less effect on arterioles, particularly at lower doses.⁸ This response profile is likely beneficial in alleviating angina because potent arteriolar dilators are prone to cause coronary steal and paradoxically worsen myocardial ischemia. Moreover, venous dilatation reduces left ventricular (LV) filling, alleviates pulmonary congestion, and can improve subendocardial perfusion in ischemic regions of the myocardium.

Traditionally, organic nitrates have been employed to either alleviate or prevent the chest pain associated with myocardial ischemia. For acute angina, nitroglycerin is administered either as a sublingual tablet or an oral spray. For prevention of angina, long-acting nitroglycerin preparations or related organic nitrates (e.g., isosorbide mononitrate, isosorbide dinitrate, pentaerythritol tetranitrate, transdermal nitrates) are commonly employed. Nitroglycerin is often administered intravenously for treatment of acute coronary syndromes (ACS).

Experimental studies have shown that NO inhibits platelet adhesion, expression of adhesion molecules, and vascular smooth muscle proliferation and migration. Thus, one might expect that NO donors such as nitroglycerin would reduce atherosclerosis progression and potentially reduce major cardiovascular events in patients with coronary artery disease (CAD). Despite extensive use for alleviation of myocardial ischemia for almost a century and a half, no clinical trials have shown that these drugs reduce ischemic cardiovascular events. The GISSI-3 and ISIS-4 trials examined the effect of nitrates following myocardial infarction (MI) but failed to show a significant improvement in outcome.^9,10 These trials only observed patients for 5 weeks to 6 months following MI, and therefore did not determine whether long-term nitrates might have a beneficial effect on outcome in patients with ischemic heart disease. Given the many putative beneficial effects of NO on vascular function, longer-term treatment might impart a beneficial effect on atherosclerosis, inflammation, vascular remodeling, or plaque stability. Indeed, a recent analysis of the GRACE registry, which includes patients admitted for ACS, showed that chronic nitrate users were much more likely to present with non–ST-segment elevation MI NSTEMI) than non-nitrate users.¹¹ These data must be interpreted with caution because the use of nitrates was not randomized, and conclusions were derived from a retrospective analysis.

In addition to their use as antianginal agents, the long-acting nitrates are now often employed for treatment of congestive heart failure (CHF), commonly in combination with hydralazine. Unlike the case for treatment of CAD, prospective randomized trials have shown that long-acting nitrates improve survival, reduce hospitalizations, and enhance quality of life in patients with CHF, particularly among African Americans.¹² Precise mechanisms underlying the beneficial effects are unclear; however, long- acting nitrates appear to synergize with hydralazine as afterload- and preload-reducing agents. These agents might also improve renal hemodynamics and promote diuresis and, via release of NO, have beneficial effects on vascular and cardiac remodeling.

A major limitation to prolonged use of organic nitrates is development of tolerance. Within about 12 hours of administration, the hemodynamic effects of organic nitrates begin to abate, in part due to extravascular adaptations such as volume redistribution and neurohormonal activation. After several days of continuous nitrate therapy, the direct vascular actions of nitrates are lost, even when vessels are removed from the animal or human. The mechanisms of nitrate tolerance, and in particular this latter form of true vascular tolerance, remain uncertain but have been attributed to formation of ROS, nitrosation and oxidation of guanylyl cyclase, and changes in activity of ADH2.¹³ A number of strategies have been proposed to prevent nitrate tolerance, but the only approach accepted clinically is to allow a drug “holiday”; that is, to withdraw the nitrate for about 12 hours daily. The commonly employed isosorbide mononitrate preparations accomplish this by increasing blood levels of the drug for about 12 hours during waking hours, after which blood levels fall to near-undetectable levels. Experimental studies have shown that hydralazine prevents nitrate tolerance by reducing oxidative stress,¹³ which might explain the benefit of hydralazine when added to long-acting nitrates in the treatment of heart failure. The long-acting nitrate pentaerythritol tetranitrate seems not to cause tolerance in experimental animals, but this has not been proven in clinical studies.

Intravenous nitroglycerin has occasionally been used for treatment of hypertensive emergencies. This condition is often associated with a contracted blood volume. Owing to nitroglycerin’s propensity to produce venular dilation rather than arteriolar dilation, it has the potential to reduce cardiac output in this setting and may produce untoward effects in patients with compromised coronary, renal, or cerebral perfusion.¹⁴ Low-dose nitroglycerin might be useful in combination with other agents in treating a hypertensive emergency, particularly in patients with acute pulmonary edema, but other agents are available and likely more effective.

In addition to its reaction with sGC, NO can react with other heme proteins and radicals. Higher oxides of NO can also react with thiols, leading to formation of nitrosothiols.¹⁵ An important example of these reactions is the reversible NO reaction with cytochrome C, which modulates mitochondrial respiration and superoxide production.¹⁶ It is uncertain as to how important these reactions are in the overall response to nitrovasodilators.

Related to the chemistry mentioned earlier are reactions of inorganic nitrate () and nitrite (). Although these are oxidation products of endogenously produced NO, they are also derived from dietary sources such as green leafy vegetables. Nitrate is rapidly converted to nitrite by bacteria in the oral cavity and gastrointestinal tract. Nitrite, in turn, can be reduced by various heme proteins, including deoxyhemoglobin, to NO. Studies have shown that the reaction of nitrite with deoxyhemoglobin promotes NO formation and vasodilation in regions of the circulation where oxygen tension is low, thereby improving oxygenation of hypoxic tissues.¹⁷ Thus, once considered an inactive metabolite of NO, nitrite likely has physiological significance and might have therapeutic utility.¹⁸

Molsidomine (see Fig. 6-4) has also been used as an NO donor for treatment of angina, but it is not commonly employed clinically. The liver metabolizes molsidomine to release SIN-1, which in turn decomposes to NO and superoxide in equimolar amounts. These species can react rapidly with one another to yield the strong oxidant peroxynitrite. Because of this chemistry, SIN-1 oxidizes lipoproteins, damages DNA, and depletes antioxidants. This capacity to generate peroxynitrite has dampened enthusiasm for clinical use of molsidomine and related drugs, but SIN-1 is commonly used to produce peroxynitrite in experimental settings.

There are other agents used experimentally as NO donors. Two classes that deserve mention are the S-nitrosothiols (SNOs) and the NONOates. S-nitrosothiols can be formed either by reactions of thiols with higher oxides of NO or by the reaction of NO with thiyl radicals. There is substantial evidence that SNOs are formed in vivo, where they serve as reservoirs for NO, and that the attachment of NO to thiols in proteins affects protein function. As an example, S-nitrosylation of Hb has been implicated in oxygen affinity and delivery. S-nitrosothiols are simple to synthesize and, depending on the thiol backbone, have different stabilities such that they can release NO in times ranging from seconds to minutes. S-nitrosothiols can also undergo heterolytic scission, yielding the nitrosonium cation (NO⁺), which acts as a nitrosating agent to form various nitroso compounds. The NONOates are commercially available nucleophilic/NO complexes often used experimentally as NO donors. These release only NO and are very useful because their varying structures permit controlled NO delivery over widely varying times, ranging from seconds to hours. Neither SNOs nor NONOates are clinically used at present.

Unique Modulators of Soluble Guanylyl Cyclase

Soluble guanylyl cyclase contains a heme group that is responsible for binding and activation by NO. Agents such as 1 H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1 (ODQ), ferricyanide, or methylene blue can inactivate sGC by oxidizing the heme group. Owing to this enzymology, ODQ and methylene blue have been used as pharmacological probes to prove that a biological response is dependent on guanylyl cyclase. In a similar vein, oxidation of the heme group by superoxide or hydrogen peroxide might alter NO-dependent enzyme activity and therefore impair endothelium-dependent vasodilation under conditions where endogenous production of ROS is increased. Thus, in addition to oxidative inactivation of NO, superoxide and related oxidants can impair NO function by inactivating sGC.

Compounds have been developed that activate sGC in an NO-independent fashion.¹⁹ Some of these, such as the pyrazolopyridine BAY 41-2272 and YC-1, interact with the heme group independent of NO, or can markedly enhance NO-stimulated enzyme activity. Others, such as BAY 58-2667 and HMR-1766, activate sGC in a heme-independent fashion and can stimulate cGMP formation even when the heme group is oxidized. Because these agents do not depend on endogenous production of NO, they have potential advantages over PDE inhibitors (see later discussion) in diseases where NO production is impaired. They also potentially bypass the problem of tolerance observed with various NO donors. These agents produce vasodilation, lower blood pressure, inhibit platelet aggregation, and have been shown to have therapeutic benefit in experimental models of systemic hypertension, pulmonary hypertension (PH), and heart failure.¹⁹ Like NO, they inhibit neointima formation following balloon injury in rats and therefore might be effective in treatment or prevention of restenosis and atherosclerosis. They also hold promise for treatment of erectile dysfunction (ED), liver fibrosis, and renal disease. Currently, clinical trials are underway to examine the efficacy of some of these agents in the treatment of heart failure and PH.

Natriuretic Peptides

Natriuretic peptides, including atrial (ANP), brain (BNP), and C-type (CNP) natriuretic peptides, are 17-amino-acid ring structures with an internal disulfide bond and are secreted as prohormones. Atrial natriuretic peptide and BNP are predominantly produced by atrial and ventricular myocytes; CNP is produced by vascular endothelial cells, the brain, and other peripheral tissues.²⁰ Urodilatin, a related peptide processed from the ANP prohormone, is released from distal tubular cells of the kidney.²¹ The A and B natriuretic peptide receptors are homodimers that are widely distributed, particularly in the cardiovascular system and kidney.²¹ The cytoplasmic tails of these contain a guanylyl cyclase domain that is activated by binding with natriuretic peptides.²⁰ There also exists a C-type natriuretic receptor that has a short cytoplasmic tail without a guanylyl cyclase domain and seems predominantly involved in clearing natriuretic peptides from the circulation.

As mentioned, ANP and BNP are produced predominantly in atrial myocytes. In the setting of a variety of conditions (e.g., heart failure, cardiac inflammation, fibrosis, hypoxia), BNP is expressed in large amounts by ventricular myocytes, leading to an elevation of circulating BNP. Thus, BNP and pro-BNP are commonly used as biomarkers for detection of various cardiac pathologies, and in particular for diagnosis and management of volume overload states.²²

Activation of the A- and B-type natriuretic receptors leads to vasodilation and a variable diuretic and natriuretic response, depending on volume status. For this reason, a synthetic form of BNP known as nesiritide has been marketed and employed for treatment of decompensated heart failure. Like the nitrovasodilators, nesiritide infusion lowers pulmonary capillary wedge pressure (PCWP), right atrial pressure, and systemic vascular resistance, and improves symptoms of dyspnea.²³ This agent also lowers circulating catecholamines, aldosterone, and angiotensin-(Ang) II levels, and aids diuresis. One study suggested that nesiritide was more effective that intravenous nitroglycerin treatment of patients with severe heart failure.²³ An early meta-analyses suggested that nesiritide therapy was associated with an increase in mortality within 30 days of treatment, for uncertain reasons²⁴; however, more recent meta-analysis of six randomized clinical trials showed no change in outcome at 10, 30, or 180 days following administration of this agent.²⁵ A randomized trial of more than 7000 subjects has shown that treatment with nesiritide acutely improves patients with class IV heart failure, without worsening long-term outcome.²⁶ This positive study is tempered by a very recent large study of 7143 patients with acute heart failure that showed no benefit of nesiritide in reducing symptoms or improving outcome at 30 days.²⁷

Phosphodiesterase Inhibitors

As reflected in Figure 6-1, cGMP is rapidly inactivated to GMP by cellular PDEs. There are 11 PDE isoenzymes with varying specificities for the different cyclic nucleotides. Phosphodiesterases 5, 6, and 9 are highly selective for cGMP, while PDEs 3 and 10 are preferentially activated by cyclic adenosine monophosphate (cAMP). Phosphodiesterases 1, 2, and 11 have dual substrate specificity.²⁸ In the cardiovascular system, the predominant PDEs are PDE1, 2, and 5. The PDEs are subject to substantial posttranslational regulation. As examples, PDE1 is calcium/CaM-dependent, cGMP stimulates PDE2 inactivation of cAMP, and binding of cAMP to PDE3 is inhibited by cGMP.

Several naturally occurring PDE inhibitors, such as caffeine, theophylline, and theobromines, are present in coffee, chocolates, and tea, and have been used since antiquity as stimulants.²⁹ These are among the most widely distributed drugs in the world. Like cAMP and cGMP, the PDE inhibitors commonly contain a purine structure with linked pyrimidine and imidazole rings. These agents occupy the cAMP or cGMP PDE binding sites and inhibit respective PDE isoenzymes with varying degrees of selectivity.²⁹ The immediate cardiovascular effects of nonselective PDE inhibition include vasodilation due to accumulation of cGMP and cAMP, increases in cardiac contractility due to accumulation of cAMP, and improvement in diastolic relaxation (lusitrophy) mediated by cAMP and cGMP.

In the past 30 years, a variety of PDE5 inhibitors, including sildenafil, tadalafil, and vardenafil, have been developed and are now used clinically (Table 6-1). Experimental studies have shown that the vasodilator effect of PDE5 inhibitors is almost exclusively dependent on endogenous NO release, and is prevented by inhibition of NOS and in conditions in which endogenous NO production is impaired.²⁸ These agents also affect cardiac function. The PDE5 inhibitors acutely reduce cardiac contractility and precondition cardiac myocytes to reduce necrosis and apoptosis caused by experimental ischemia.^30,31 Chronic PDE5 inhibition with sildenafil prevents experimental cardiac hypertrophy caused by transaortic constriction.³²

The PDE5 inhibitors were developed as antihypertensive agents, but because of their potent effect on the corpus cavernosa, they were initially approved and have become widely employed for treatment of erectile dysfunction. These agents are also potent dilators of the pulmonary circulation. Sildenafil and tadalafil been approved by the U.S. Food and Drug Administration (FDA) for treatment of pulmonary arterial hypertension (PAH). This disorder, defined by the hemodynamic parameters of a mean pulmonary artery pressure (PAP) above 25 mmHg and a PCWP 15 mmHg or lower, occurs as a primary condition and in the setting of a variety of diseases that affect the pulmonary circulation.³³ (Also see Chapters 56 and 57.) A single dose of sildenafil was found to reduce PAP in patients with both primary and secondary PH and to augment the effect of inhaled NO in these subjects.³⁴ Clinical studies have shown that chronic administration of PDE5 inhibitors reduces PAP and right ventricular (RV) mass, and improves exercise tolerance and functional status in patients with PAH.³⁵ The recent SUPER-2 clinical trial showed that sildenafil improved 3-year survival in patients with PAH compared to historical controls.³⁶ For these reasons, PDE5 inhibitors are now considered a mainstay of therapy for PAH. They have also been used with some success in neonates with persistent pulmonary hypertension of the newborn (PPHN).³⁷

Phosphodiesterase type 5 inhibition has beneficial effects on hemodynamics and cardiac function in heart failure. In various experimental models of heart failure, PDE inhibitors prevent and reverse cardiac hypertrophy, reduce remodeling, and decrease myocardial fibrosis.^38,39 In a recent placebo-controlled clinical trial of patients with severe heart failure, sildenafil treatment for 1 year improved ejection fraction, improved parameters of diastolic function, and reduced left atrial size while improving functional capacity and clinical status.⁴⁰ This study was not designed to determine whether sildenafil improves survival; larger studies are needed with longer-term follow-up to discern whether PDE5 inhibition provides survival benefit. Nevertheless, these orally available agents, which avoid the problem of tolerance encountered with the nitrovasodilators, have substantial promise in treating ventricular dysfunction.

Cyclooxygenase Inhibitors

Cyclooxygenase inhibitors have been used since antiquity to alleviate pain and fever. Salicylic acid was purified from willow bark in the 18th and 19th centuries and was further modified to acetylsalicylic acid (ASA) in 1897. A large number of nonsteroidal antiinflammatory drugs (NSAIDs) have been developed to specifically inhibit COX enzymes, and together with ASA are the most commonly used drugs in the world. Drugs that specifically inhibit COX-2 were subsequently developed. These were intended to reduce gastrointestinal side effects and block inflammation caused by COX-2, although as mentioned later, they have unexpected and untoward effects that have reduced their popularity.

Aspirin has been studied extensively since the 1950s as a means of reducing cardiovascular events.⁴⁷ Numerous large clinical trials performed in the 1980s supported the concept that aspirin decreases the occurrence of MI and stroke. A recent large meta-analysis showed that aspirin was effective in both primary and secondary prevention of total coronary events, ischemic stroke, and serious vascular events, with the greatest benefit observed in the case of secondary prevention.⁴⁸ Another recent meta-analysis of nine trials that included 90,000 patients showed that aspirin is effective for primary prevention of nonfatal MI and total cardiovascular events, but not for stroke, cardiovascular mortality, or all- cause mortality.⁴⁹ Of interest, several recent meta-analyses have suggested that aspirin might not be useful for primary prevention of events in the diabetic population.^50,51

The beneficial effects of aspirin are generally considered a consequence of its antiplatelet effects and reduction of thrombosis. However, aspirin reduces levels of C-reactive protein (CRP) in patients with recent unstable coronary syndromes,⁵² and in experimental models of atherosclerosis, reduces atheroma burden, decreases inflammation, and improves endothelial function,^53,54 suggesting that it might also have direct vascular effects.

Although aspirin has proven effective in reducing cardiovascular events, there are no clinical trials showing that other COX inhibitors convey similar cardiovascular benefit, and paradoxically, there is substantial evidence that these agents are harmful. The most striking example is that of the COX-2 inhibitor rofecoxib, which was withdrawn from the market because of increased thrombotic events⁵⁵; however, other COX inhibitors might also increase cardiovascular risk, depending upon the relative COX-2–to–COX-1 selectivity.^56,57 The precise mechanisms underlying this increased risk remain undefined, and it is unclear why aspirin, which inhibits the same enzyme, albeit via different mechanisms, is beneficial. These differences might relate to inhibition of vascular PGI₂ and perhaps renal COX, which in turn could promote sodium retention and blood pressure elevation and worsen cardiovascular outcome. As previously mentioned, the downstream products and their receptors are myriad, so the in vivo actions of these agents are complex and difficult to predict. Nevertheless, NSAIDs other than aspirin should be used sparingly in patients with known cardiovascular diseases and currently have no role in preventing cardiovascular events.

Prostacyclin Analogs as Therapeutic Agents

Given its potent vasodilator effects, there is enormous interest in therapeutic use of PGI₂ and its analogs. Several preparations have been developed. The most commonly employed are epoprostenol, a freeze-dried synthetic preparation of PGI₂, and the PGI₂ analogs iloprost, treprostinil, and beraprost. These agents have become a mainstay of treatment for PAH. Epoprostenol was initially approved for treatment of PAH following a 12-week trial in 81 patients prospectively randomized to either epoprostenol or conventional therapy.⁵⁸ Among those treated with epoprostenol, there was improvement in exercise capacity and a decline in PAP. This was in contrast to those receiving conventional therapy, in whom walk time decreased and PAP increased. Patients treated with epoprostenol had greater symptomatic improvement, and most strikingly in this small study, eight patients died, all in the conventional therapy group. A second study showed that epoprostenol improved exercise duration and lowered PAP in patients with PH associated with scleroderma.⁵⁹ Interestingly, these subjects showed a trend toward improvement of digital ulcers, suggesting that systemic vasodilation caused by this drug might also be beneficial. Subsequent long-term follow-up in large registries have confirmed a beneficial effect of continuous intravenous epoprostenol in PAH.

A downside of epoprostenol therapy is that it requires chronic central line placement, which is accompanied by risk of infection that might be related in part to prostanoid-mediated immunosuppression. The drug also often requires up-titration to overcome tachyphylaxis and is expensive.⁶⁰ Owing to its short half-life, there is rebound PH that develops shortly after discontinuing the drug, which can have serious consequences. Common side effects include headaches, occasional cases of thyrotoxicosis, nausea, jaw pain, thrombocytopenia (in up to 34% of patients), flushing, skin rash, anorexia, arthralgias, and myalgias.

For the reasons mentioned, PGI₂ analogs (i.e., iloprost, treprostinil, beraprost) have been developed. These have longer half-lives and can be given intravenously, subcutaneously, via nebulizer, and in some cases orally. Numerous studies have shown that these improve exercise tolerance and quality of life, either alone or in combination with endothelin blockade and PDE5 inhibitors in patients with PAH. The subcutaneous and, intravenous forms of administration are frequently complicated by local pain, induration, and inflammation at injection sites. Inhaled forms avoid these complications but require frequent administration. Despite their limitations, these agents improve hemodynamics, increase exercise tolerance, and enhance quality of life.

Although these agents are potent vasodilators and have the potential to reduce pulmonary vascular resistance (PVR), it is actually unclear as to how they impart therapeutic benefit. Hemodynamic studies have shown that the decrease in pulmonary pressure following inhalation therapy is brief and unlikely to account for sustained benefit. Moreover, the pulmonary vasculature in these patients is often extensively occluded, questioning the potential benefit of vasodilation. These agents enhance RV performance, and they might decrease fibrosis and thrombosis within the pulmonary vasculature.

There is also lack of consensus on how to use these agents, which agents to use, and what dosing regimen is optimal.⁴⁸ As discussed elsewhere in this chapter, these agents are often used in conjunction with PDE5 inhibitors and endothelin-1 (ET-1) receptor antagonists, again without uniformity across various centers.

Vascular Parasympathetic System

Postganglionic parasympathetic fibers release acetylcholine, which stimulates muscarinic and nicotinic receptors. Most blood vessels lack parasympathetic innervation, although some notable exceptions exist (e.g., coronary arteries), and the physiological role of endogenous acetylcholine in vasodilation is uncertain.⁶² The vasculature does contain muscarinic receptors and responds to exogenously administered acetylcholine or mimetics (e.g., methacholine). Exogenous acetylcholine dilates blood vessels by its actions on the vascular endothelium, but it produces vasoconstriction if the endothelial layer is injured or removed. This discovery demonstrated the importance of the endothelium as an active participant in vascular reactivity and eventually led to the discovery of endothelium-derived relaxing factors (e.g., NO, PGI₂.⁶³ Patients with cardiovascular disease exhibit an impaired vasodilatory response to acetylcholine (endothelial dysfunction) but often have a normal response to direct vasodilators such as nitroprusside. Impaired vascular reactivity in both the coronary and forearm vasculature predicts future cardiovascular events,^64,65 and the endothelium-dependent response may be improved with drug therapy, exercise, or risk factor modification (e.g., smoking cessation).^66–68

Acetylcholine receptors (AchRs) are classified by their ability to respond to either muscarine (M₁-M₅) or nicotine (nAchR). Muscarinic receptors are classic G protein–coupled receptors (GPCRs), coupled to G_i, which inhibits cAMP production. Nicotinic AchRs are ligand-gated voltage channels. Vascular M₁, M₂, and M₃ receptors have been described and produce vasodilation via endothelial, or vasoconstriction via VSMC, receptors⁶⁹ (Table 6-2). Acetylcholine is a nonselective agonist; there are no clinically available subtype-selective agents, although a number of investigational drugs exist. Methacholine is frequently used in clinical research because of its longer half-life and stability. Atropine is a nonselective muscarinic antagonist used mainly to increase heart rate by its effects on cardiac M₂ and M₃ receptors. Muscarinic receptors are also located on postsynaptic sympathetic nerve terminals and inhibit NE release. Peripheral neuronal nicotinic AchRs (N_N) transmit sympathetic impulses in autonomic ganglia and adrenal medulla to stimulate NE and epinephrine release. Trimethaphan inhibits N_N and was one of the earliest antihypertensive agents available, although it is no longer used, owing to resulting severe autonomic impairment and intolerable side effects.

Adrenergic Receptors and Agonist Selectivity

Sympathetic postganglionic neurons richly innervate the vasculature and release NE, whereas the adrenal medulla secretes epinephrine in addition to NE. These catecholamines activate adrenergic receptors, which are classic seven-transmembrane receptors coupled to G proteins. They are further classified as either α (α₁ and α₂) or β receptors (β₁, β₂, and β₃). α-Receptor subtypes have also been identified (α_1A, α_1B, α_1D, α_2A, α_2B, and α_2C), although no subtype-specific antagonists are available. Their physiological effects have been determined in part by the study of receptor knockout models.^70,71 In general, α₁ is coupled to G_q (stimulation of phospholipase C/D/A₂), α₂ to G_i (inhibition of adenylate cyclase), and β-receptors to G_s (stimulation of adenylate cyclase).

Distribution of tissue adrenergic receptors is a major determinant of the agonist response because they are relatively nonselective for epinephrine and NE (see Table 6-2). Vascular smooth muscle cells (venous, arterial, and arteriolar) are richly innervated by sympathetic nerve terminals and possess adrenergic receptors (α₁, α₂, and β₂). These receptors can have opposing actions within the vasculature, as demonstrated by α-mediated vasoconstriction and β₂-mediated vasodilation, and the vascular response is determined by the relative activation of α₁, α₂, and β₂ receptors. Vascular α₁ receptors produce vasoconstriction, whereas presynaptic α₂ receptors suppress NE release. Cardiovascular β₁ receptors are expressed primarily within the cardiac conduction system and cardiomyocytes, rather than in the vascular bed. However, vascular β₁ receptors mediate vasodilation within coronary arteries and stimulate renin secretion in the renal juxtaglomerular apparatus.⁷² The β₃ receptor is primarily expressed on adipocytes, where it stimulates lipolysis; β₃ receptors may counteract adrenergic stimulation via β₁ receptors in cardiac myocytes and contribute to control of vasodilation by vascular ECs.

Pharmacological Interruption of Catecholamine Metabolism

Catecholamine metabolism is an important target of therapeutic drugs and other chemical agents. Catecholamines are produced locally within the sympathetic neurons by metabolism of tyrosine (Fig. 6-7) to dopamine. Dopamine is concentrated into vesicles via vesicular monoamine transporters. Once in the vesicles, dopamine is converted into NE. Norepinephrine is then secreted and activates adrenergic receptors, provides positive or negative feedback, or is taken back up into the cell via NE transporter (NET). Norepinephrine transporters and similar transporters also transport other neurotransmitters such as epinephrine, dopamine, and serotonin. Norepinephrine is metabolized via monoamine oxidases (MAO-A and MAO-B) after reuptake into the cell, or by catechol-O-methyltransferase (COMT) after diffusion into the circulation.

Figure 6-7 Norepinephrine (NE) release and reuptake.

NE is released from the sympathetic nerve terminal and can signal via vascular α or β receptors. NE also provides positive and/or negative feedback. NE is rapidly taken back up into the nerve terminal via NE transporters (NETs) and can be recycled into granules or metabolized via monoamine oxidase (MAO). Metabolism and/or receptor signaling can be interrupted at multiple steps in the pathway. COMT, catechol-O-methyl transferase; DA, dopamine; DβH, dopamine β-hydroxylase; DD, dopamine decarboxylase; E, epinephrine; TH, tyrosine hydroxylase.

Pharmacological agents that affect this pathway are used clinically for treatment of hypertension, depression, and movement disorders. Reserpine blocks vesicular dopamine/NE transport and depletes NE from the nerve terminals. Guanethidine is an antihypertensive agent that is taken up into vesicles, displaces NE, and reduces NE release during long-term therapy. Many herbal, over-the-counter, or illicit medications act by stimulating NE release (e.g., amphetamine, pseudoephedrine), activating adrenergic receptors (phenylephrine), or acting via mixed mechanisms (ephedrine). Cocaine and tricyclic antidepressants block NE reuptake into the cell and may transiently increase NE and produce hypertension. Antidepressant medications such as selective serotonin reuptake inhibitors (SSRIs) act similarly, and may also nonselectively block NET. Sibutramine is a nonselective serotonin reuptake/NET inhibitor previously used for appetite suppressant effects, but it has been withdrawn from the market because of increased risk of cardiovascular events. Monoamine oxidase inhibitors (MAOIs) are occasionally used to treat depression and can cause marked hypertension during ingestion of tyramine-containing foods, which stimulates NE release. COMT inhibitors and dopa are used to treat movement disorders and can cause orthostatic hypotension and blood pressure dysregulation.

Many weight-loss supplements, decongestant preparations, and herbal supplements act as α₁-agonists (direct sympathomimetics) or stimulate release of catecholamines (indirect sympathomimetics).^73–75 Whereas epinephrine and NE are rapidly metabolized via COMT, many synthetic sympathomimetic drugs are resistant to this effect, and are therefore effective when ingested by mouth. Ephedra (or ma huang) is a sympathomimetic herbal extract used for asthma treatment, weight loss, and enhanced athletic performance. It can cause severe hypertension, cardiovascular events, and even death in young, apparently healthy individuals. Caffeine coadministration likely exacerbates ephedra-related complications.⁷⁶ Performance athletes or enthusiastic weight lifters may also take sympathomimetic supplements, which comprise many of the medications banned by the World Anti-Doping Agency.⁷⁴

Adrenergic Agonists and Antagonists

Vascular α- and β-receptor agonists and antagonists are listed in Table 6-2, and their vascular actions can generally be inferred from the respective receptor actions. Physiological effects of endogenous and synthetic catecholamines are complex because they activate multiple receptors, exhibit dose-dependent responses, and activate compensatory reflexes.

Epinephrine is primarily secreted by the adrenal medulla, where it constitutes roughly 80% of total catecholamine content. Depending on the dose and route of administration, epinephrine may produce divergent vascular responses (Table 6-3). Acute intravenous administration produces marked vasoconstriction, tachycardia, and elevated blood pressure. Continuous infusion or subcutaneous administration of epinephrine increases heart rate and cardiac contractility, systolic blood pressure, and mean arterial blood pressure. Diastolic blood pressure is affected to a lesser extent, resulting in a marked increase in pulse pressure. At lower doses, epinephrine reduces vascular resistance secondary to β₂-receptor stimulation and vasodilation, which may reduce blood pressure. Epinephrine is commonly used to treat anaphylactic reactions, bronchoconstriction, and refractory bradycardia and hypotension. Epinephrine is less often used than NE for treatment of septic shock because of tachycardia and concerns for worsened splanchnic ischemia compared to other agents.

Norepinephrine produces vasoconstriction with lesser direct cardiac effects and β₂ activity than epinephrine, which increases both blood pressure and peripheral vascular resistance. Heart rate decreases due to the baroreflex response. Norepinephrine is useful for treating hypotension refractory to fluid resuscitation (e.g., septic shock). Although there is debate regarding the optimal vasopressor in septic shock, NE has proven as effective as comparable agents, possibly with fewer complications.^77–81 Norepinephrine appears to produce less splanchnic vasoconstriction and intestinal ischemia than epinephrine or phenylephrine.

Isoproterenol is a nonselective β₁/β₂ agonist that is commonly used to increase heart rate for treatment of sinus bradycardia or torsades de pointes. Although its predominant effect is to increase heart rate, vasodilation is also produced by vascular β₂ receptors. Dobutamine is more β₁ selective and is used for its relative selective effects on cardiac contractility.

α₁-Antagonists

Most clinically available α-antagonists are α₁-selective and produce vascular relaxation, vasodilation, and reduction in blood pressure (Table 6-4). These agents are most commonly used for treatment of urinary retention in prostatic hypertrophy because of their inhibitory actions on the prostatic urethra smooth muscle. They are therefore useful for hypertension treatment in patients with concomitant chronic urinary retention. Side effects are nasal congestion, fatigue, and those in common with other vasodilators (peripheral edema, reflex tachycardia, and postural hypotension). The major dose-limiting side effects are postural hypotension and fluid retention. α-Blockers have also been linked to the rare occurrence of “intraoperative floppy iris syndrome,” which may result in permanent vision loss after eye surgery. α-Blockers are not generally recommended as hypertension monotherapy, owing to side effects and increased occurrence of cardiovascular events, compared to the thiazide diuretic chlorthalidone in the Antihypertensive and Lipid-Lowering Treatement to Prevent Heart Attack Trial (ALLHAT) trial.⁸²

Nonselective α-antagonists (phenoxybenzamine and phentolamine) are used primarily for preoperative treatment of pheochromocytoma. Phenoxybenzamine is administered orally, produces irreversible inhibition, and has a long half-life, whereas phentolamine is given intravenously, acts competitively, and is rapidly cleared. When used for treatment of pheochromocytoma, α-blockade should be achieved before starting β-blockers because of the risk of unopposed α-receptor activation during β-blocker monotherapy. Some β-blockers also have α-blocking effects (e.g., carvedilol, labetalol), but they should not be used for sole therapy of pheochromocytoma or cocaine overdose because of the relatively low-potency α effects.

α₁-Agonists

Activation of the α₁ receptor stimulates vascular smooth muscle contraction and vasoconstriction. These agents are most commonly used in over-the-counter sinus preparations to treat nasal congestion. Phenylephrine is commonly used for treatment of hypotension in intensive care settings because of its relatively selective vascular effect without increasing heart rate.

α₂-Agonists

Activation of the α₂ receptor within the CNS provides negative feedback inhibition of sympathetic activity and NE release. Clonidine and other α₂-agonists (see Table 6-4) suppress sympathetic and increase parasympathetic activity by actions within the CNS. Evidence for the central effect is obtained from in vivo studies demonstrating no effect of clonidine after spinal cord transection. Clonidine can also produce vasoconstriction via activation of peripheral α_2B receptors, although this usually only occurs after intravenous administration or accidental overdose.⁸³ However, this effect may be evident after oral clonidine administration in some patients with autonomic dysfunction.⁸⁴ Methyldopa is metabolized similarly to NE and acts as a false transmitter and α₂-agonist. Methyldopa is commonly used in pregnancy for its history of safety, and also remains an effective alternative in resistant hypertension. Other α₂-agonists, such as tizanidine and dexmedetomidine, are used for their sedative effects but may affect blood pressure regulation as a side effect. Etomidate is a sedative with pressor effects that appear to be mediated via α_2B.

All α₂-agonists can produce sedation, fatigue, dry mouth, bradycardia, and orthostatic hypotension. Transdermal clonidine frequently produces localized skin irritation due to the adhesive, rather than a drug reaction. Methyldopa carries additional risks of hepatic dysfunction, hemolytic anemia, lupus-like syndrome, and thrombocytopenia. Long-term use of clonidine results in receptor hypersensitivity and rebound hypertension due to exaggerated sympathetic discharge after abrupt drug withdrawal. This syndrome is accompanied by sympathetic hyperactivity and can be minimized by gradual taper or treated with combined α- and β-blockade. Methyldopa is less likely to produce rebound, owing to the longer half-life of active metabolites, but caution should still be used when stopping this drug.

α₂-Antagonists

Antagonists of α₂-receptors are infrequently used in clinical practice but have a few specific clinical applications. Yohimbine is an α₂-antagonist that increases sympathetic activity in patients with orthostatic hypotension and may also be useful for treatment of ED. Subtype-specific antagonists are not available. Although these drugs are not available commercially, herbal supplements with α₂-antagonist activity are commonly available.

β-Adrenergic Antagonists

Historically, β-adrenergic antagonists (β-blockers) have been classified by receptor subtype specificity and intrinsic sympathomimetic activity (ISA). In addition, some β-blockers also inhibit α₁ receptors, producing a vasodilatory effect. Intrinsic sympathomimetic activity reflects the drug’s ability to activate receptors when administered in the absence of any endogenous sympathetic activity (e.g., sympathetic denervation). This effect likely reflects the relative stabilization of the inactive/active receptor conformations as discussed in the pharmacodynamics section. Drugs with ISA tend to produce less bradycardia and may directly reduce vascular resistance, although evidence that this translates into hard clinical outcomes remains debatable. Analyses suggest that β-blockers with ISA do not reduce cardiovascular mortality and may actually worsen outcomes.⁸⁵

Drugs with β-blocking ability are summarized in Table 6-5. Propranolol was the first clinically available β-blocker, and is nonselective. Second-generation agents offer increased β₁ selectivity. Recently, vasodilatory β-blockers entered the market and produce additional blood pressure lowering effects via α₁ blockade and possibly via β₃ activation.⁸⁶ β-Blockers are commonly used to treat hypertension, acute MI, heart failure, angina, and supraventricular arrhythmias. In acute MI, atenolol reduces in-hospital mortality by 15%, although benefit with prolonged therapy is less well established. Perioperative β-adrenergic blockade also reduces in-hospital mortality in patients with high cardiovascular risk.⁸⁵

In the past, β-blockers were withheld in patients with systolic heart failure, owing to concerns of worsening contractile function and intolerance. However, the observation that the sympathetic nervous system is activated in severe heart failure and predicted mortality supported the concept of sympathetic blockade in CHF.⁸⁷ Randomized clinical trials have definitively demonstrated that metoprolol, bisoprolol, and carvedilol improve systolic function and reduce mortality in CHF. These agents should be introduced gradually and titrated upwards as tolerated in patients with severe CHF.

All β-blockers can produce side effects related to their mechanism of action (bradycardia, heart block, hypotension). β-Blockers can also worsen hyperglycemia (especially when combined with thiazide diuretics) or blunt the compensatory response to hypoglycemia. They should not be used as primary treatment for pheochromocytoma, cocaine intoxication, clonidine-withdrawal hypertension, or other hyperadrenergic crises, owing to the possibility of unopposed α-receptor activation. Sotalol is a unique β-blocker with antiarrhythmic effects due to potassium channel blocking activity, which requires close monitoring for QT prolongation and proarrhythmia.

Dopamine and Dopaminergic Agonists

Dopamine is endogenously produced in both peripheral and central neuronal cells and in the adrenal gland via the action of dopa decarboxylase on dopa (see Fig. 6-7). Dopamine acts on one of 5 G protein–linked receptors, termed D1 through D5, which are further classified into two major groups termed D1 and D2. The D1 class of dopamine receptors, D1 and D5, are G_αs-linked receptors that activate adenylyl cyclase; the D2 class receptors are linked to G_αi/o and inhibit adenylyl cyclase. Perturbations of dopamine signaling in the CNS have been linked with a variety of disorders, including Parkinson’s disease, Huntington’s disease, Tourette’s syndrome, schizophrenia, and major depression. In addition to CNS receptors, dopamine receptors are widely present in peripheral tissues including the kidney, gastrointestinal tract, heart, adrenal glands, and vasculature. Dopamine receptor signaling has recently been reviewed in depth.⁸⁸

The predominant clinical use of dopamine has been for circulatory support in critically ill patients in settings such as shock or the postoperative period. The clinical response to dopamine is complex and depends on the dose. At low doses (1-4 μg/kg/min), often referred to as “renal doses,” dopamine acts on D1-like receptors and β-adrenergic receptors to promote renal arterial vasodilation and improve renal blood flow. As the dose is increased, dopamine begins to exert greater effects at β- and α-adrenergic receptors, and the α-adrenergic effects begin to predominate at doses exceeding 10 μg/kg/min. There is also substantial variability in these responses, such that the precise effect of dopamine in an individual patient is difficult to predict. The potential increase renal blood flow, due to D1-like receptor activation, has not proven to have significant clinical benefit. Recent clinical trials have shown no benefit of dopamine over NE infusion in patients with septic shock, with substantially more cardiac arrhythmias and sinus tachycardia caused by dopamine.^89,90

Owing to the mixed effects of dopamine on multiple receptors, agonists have been developed that have greater specificity for D1-like receptors, and therefore would serve as potent vasodilators with limited off-target effects. Fenoldopam is such an agent that has been approved by the FDA for treatment of severe hypertension. This agent is a potent vasodilator with rapid onset of action that produces dose-dependent reductions in blood pressure when administered intravenously to patients with hypertension. It is devoid of the α- and β-adrenergic effects of dopamine, so less prone to cause off-target effects. Early studies showed that it preferentially increased renal plasma flow, in keeping with preferential dilation of the renal vasculature, and dramatically enhanced renal sodium excretion.

Despite these potentially beneficial effects of fenoldopam, its clinical use in severe hypertension remains limited, largely because several other drugs are quite effective. In prior clinical trials, fenoldopam showed no benefit over sodium nitroprusside in lowering blood pressure,⁹¹ and it is considerably more expensive.

Based on its ability to enhance renal perfusion and sodium excretion, fenoldopam has been used as a renal protectant in critically ill patients. A recent meta-analysis of 16 randomized trials involving 1290 patients indicated that fenoldopam reduced the need for renal replacement therapy, in-hospital mortality, and length of stay in the intensive care unit in postoperative or critically ill patients.⁹² Similar results were obtained from a meta-analysis of patients undergoing cardiovascular surgery.⁹³ Such analyses can be flawed by publication bias, and prospective trials are needed to establish a benefit of fenoldopam in this setting.

There was initial enthusiasm for use of fenoldopam to prevent contrast-induced nephropathy. However, a rigorous randomized prospective trial showed no benefit of this agent in preventing changes in renal function in patients undergoing angiography procedures,⁹⁴ and its use in this setting is no longer recommended. Dopexamine, which is a combined D1-like and β₂-adrenergic agonist, has been studied in a variety of settings involving critically ill patients, but it has not proven beneficial in randomized prospective trials.^95,96

Regulation of the Renin-Angiotensin-Aldosterone System

The renin-angiotensin-aldosterone system (RAAS) is highly coordinated to maintain blood volume and blood pressure, and is of major importance during sodium and/or fluid depletion (Fig. 6-8). The RAAS is stimulated under pathological conditions that cause reduced renal perfusion, such as heart failure, aortic coarctation, or renal artery stenosis. In addition, this system is inappropriately activated in obesity and diabetes.

Figure 6-8 The renin-angiotensin-aldosterone system (RAAS).

ACE, angiotensin-I converting enzyme; Ang, angiotensin; AT₁, angiotensin-II type 1 receptor; ENaC, epithelial sodium channel; MR, mineralocorticoid receptor.

Renin secretion by renal juxtaglomerular cells, the rate-limiting step in the RAAS cascade, is stimulated by reduced sodium chloride delivery to the macula densa, reduced renal perfusion pressure, and sympathetic stimulation.¹⁰¹ Upon release into the circulation, renin cleaves circulating angiotensinogen to angiotensin (Ang-I). Although Ang-I is inactive, it is rapidly converted into Ang-II by angiotensin-converting enzyme (ACE), which is abundantly expressed within the pulmonary vasculature and to a lesser extent in the peripheral circulation. In addition to the endothelial membrane-bound form, ACE also circulates in a soluble form. Angiotensin-II is a potent vasoconstrictor, acting directly on the Ang-II type 1 receptors (AT₁) on VSMCs. Within the kidney, Ang-II acts upon the renal afferent and efferent arteriole, to a greater extent on the efferent arteriole. During periods of volume depletion, this efferent selectivity serves to preserve glomerular filtration by increasing intraglomerular pressure. Angiotensin-II also stimulates aldosterone secretion from the adrenal gland. Aldosterone reinforces the vasoconstrictor effect of Angiotensin-II by increasing renal sodium reabsorption and expanding intravascular volume via the mineralocorticoid receptor (MR) in principal cells of the kidney and activation of the epithelial sodium channel (ENaC). Angiotensin-converting enzyme is the principal metabolizing enzyme for a number of other vasoactive peptides, notably bradykinin, which may confer some of the beneficial antihypertensive and antithrombotic effects observed during ACE inhibition.

Receptors and Novel Mediators in RAAS Signaling

The AT₁ and AT₂ receptors are the principal Ang-II receptors in humans and are widely expressed, including in areas important for blood pressure regulation (vascular smooth muscle, kidney, adrenal cortex, brain). AT₁ is a classic seven-transmembrane domain GPCR that signals via G_αq and phospholipase C, as well as other G protein–independent pathways.^102,103 Upon Ang-II binding, angiotensin receptor–associated protein (ATRAP) facilitates AT₁ internalization and desensitization.¹⁰⁴ AT₁ mediates the classic Ang-II effects including vasoconstriction, adrenal aldosterone secretion, and renal proximal tubule sodium reabsorption. In addition, Ang-II participates in a negative feedback loop in the kidney to inhibit renin secretion via AT₁. In mice, two AT₁ receptors have been identified, AT_1a and AT_1b, with AT_1a responsible for most of the pressor and mitogenic effects, although a single AT₁ receptor is present in humans.

In general, the actions of the AT₂ receptor tend to oppose those of the AT₁ receptor, although some effects are inconsistent with this generalization.^105,106 AT₂ stimulation produces vasodilation, in part via an increase in bradykinin and receptor heterodimerization with the bradykinin receptor. AT₂ has an antinatriuretic effect within renal tubules. However, AT₂ and AT₁ similarly suppress renin secretion. Although investigational agonists and antagonists for the AT₂ receptor are available, these agents are not available clinically. Therefore, the clinical implication of the AT₂ receptor remains unproven. Angiotensin-II decreases during ACE inhibition but increases during AT₁ antagonism. However, the AT₂ receptor remains available for Ang-II activation during chronic AT₁ antagonism and may promote beneficial effects. This rationale has led some to argue the benefit of AT₁ antagonism over ACE inhibition.

Aldosterone and other corticosteroids activate the MR within principal cells in the cortical collecting duct. Angiotensin-II, aldosterone, and MR activation induce multiple proteins that coordinate to increase renal sodium and water reabsorption.^107–109 The MR is a classic nuclear receptor localized to the cytosol in its inactive form, which dimerizes and translocates to the nucleus and activates nuclear transcription when activated. Although aldosterone appears to be the critical physiological stimulus, cortisol, corticosterone, and other steroids have a similar affinity for the MR. However, within epithelial target tissues, 11-β-hydroxysteroid dehydrogenase type 2 (11βHSD2) inactivates these hormones and prevents inappropriate MR activation. Either inhibition of this enzyme by licorice or genetic deficiency produces unregulated MR activation and hypertension with metabolic alkalosis and hypokalemia. The MR is also expressed within vascular smooth muscle and ECs, where it may contribute to vascular injury via activation of NADPH oxidase, generation of ROS, and inflammation.

Greater complexity of the RAAS has emerged with the discovery of novel angiotensin peptides and receptors. Angiotensin-(1-7) is formed by cleavage of Ang-I by neprilysin or prolyl-endopeptidase or from cleavage of Ang-II by ACE2.¹¹⁰ Angiotensin-(1-7) acts via the Mas receptor, a G protein–coupled cell-surface receptor generally opposing AT₁ effects.¹¹¹ Angiotensin-(1-7) and ACE2 confer protection against Ang-II-mediated cardiovascular injury, and ACE2-deficient mice have accentuated Ang-II-induced injury.¹¹² Angiotensin-II is also metabolized in vivo by aminopeptidase A to Ang-III and Ang-IV, which may have important physiological effects within the CNS.¹⁰³

Additional interest has focused on the (pro)-renin receptor (PRR), which binds either renin or prorenin.¹¹³ The PRR can exist as a full-length transmembrane protein, a soluble circulating form, or a truncated protein (transmembrane/cytoplasmic portion). The full-length transmembrane PRR can bind and activate prorenin by inducing a conformational change that exposes the catalytic site. In addition, (pro)-renin activates PRR and cellular signaling events (e.g., mitogen-activated protein kinase [MAPK] pathways) independent of renin activity.¹¹⁴ Prorenin circulates in marked excess of active renin, and the prorenin/renin ratio is further increased in diabetes, raising the possibility that (pro)renin-PRR signaling or PRR-induced activation of prorenin and local angiotensin production could contribute to cardiovascular injury.

Drugs That Inhibit the Renin-Angiotensin-Aldosterone System

The first ACE inhibitor was serendipitously discovered as a bradykinin-potentiating factor isolated from venom of the pit viper Bothrops jararaca. Subsequent studies demonstrated its activity against ACE, suggesting that this enzyme played a key role in regulating both the RAAS and the kallikrein-kinin systems. Isolation of the responsible peptide sequences led to development of captopril, one of the earliest examples of structure-based drug design.¹¹⁵ Captopril’s success in treatment of cardiovascular disease was critical to the development of other drugs that block the RAAS (Table 6-7). Drugs are now clinically available to block the RAAS cascade at nearly every level (see Fig. 6-8).

Direct renin inhibitors are the most recent class of RAAS blocking agents. Although renin is the rate-limiting enzyme in the RAAS pathway and a logical drug target, development of clinical renin inhibitors was hindered by poor potency, stability, and oral bioavailability.¹¹⁶ Development of aliskiren overcame these issues, and other agents are in clinical studies. Aliskiren selectively inhibits renin activity and dose-dependently reduces Ang-I and Ang-II production and blood pressure. Renin secretion markedly increases during aliskiren therapy, and attention to the assay method is needed if plasma renin concentration is measured.¹¹⁷ Plasma renin activity (assessed by in vitro Ang-I generation) remains inhibited, and thus compensatory renin secretion does not appear to overcome the effect of aliskiren or increase blood pressure.¹¹⁸ Aliskiren is well tolerated and has a low rate of side effects, which are principally gastrointestinal. Aliskiren effectively reduces blood pressure when used in alone or in combination with diuretic therapy, ACE inhibitors, or ARBs.^{116,119–121} Addition of aliskiren to maximal-dose losartan reduced proteinuria compared to placebo in a population with diabetic proteinuira.¹²² Aliskiren provided similar LV mass reduction compared to losartan in a group of overweight subjects with hypertension but provided no additional benefit in combination.¹²³ Further studies are needed to investigate hard cardiovascular endpoints.

Angiotensin-converting enzyme inhibitors are used to treat hypertension, diabetic nephropathy, CHF, and prior MI or stroke. Their antihypertensive effect is generally less effective in African Americans because of a higher prevalence of low-renin hypertension, but concurrent thiazide diuretic administration improves responsiveness. Many orally administered ACE inhibitors are given as a prodrug, which are rapidly metabolized into active metabolite via enteric metabolism (e.g., enalapril to enalaprilat). Enalaprilat is the active metabolite of enalapril and is available for intravenous administration. Most ACE inhibitors are renally excreted and require careful monitoring in patients with renal insufficiency.

Angiotensin-II can also be generated by enzymes other than ACE (e.g., chymase, cathepsin G), providing a rationale for combination therapy with ARBs and ACE inhibitors. Angiotensin-II type 1 receptor antagonists (ARBs) also provide an alternative treatment option for patients who are intolerant of ACE inhibitors. Early studies were done with saralasin, an intravenous peptide Ang-II analog, which demonstrated effectiveness of ARBs and led to the development of orally available agents.¹²⁴ Since then, multiple agents have been developed and approved for hypertension treatment and prevention of cardiovascular complications (see Table 6-7). Angiotensin receptor blockers are remarkably well tolerated and may even reduce the incidence of some complaints such as headache. All of the ARBs are reliably absorbed, highly protein bound, and selective for the AT₁ receptor. Telmisartan and the losartan metabolite EXP3174 can also activate peroxime proliferator-activated receptor gamma (PPARγ), which may explain improvement in insulin sensitivity.^125,126 Losartan has a short half-life, but has an active metabolite (EXP3174) with a long half-life. Elimination is primarily hepatic for most ARBs, but dose adjustment is usually needed only in severe liver impairment.

RAAS blockade with any of these drugs (ACE inhibitors, ARBs, or renin inhibitors) is contraindicated during pregnancy because of the risk of congenital renal and other malformations and should be used with extreme caution in women of childbearing potential.¹²⁷ These agents also carry a risk of hyperkalemia and worsening renal insufficiency.¹²⁸ RAAS blockade should not be used in the setting of bilateral renal artery stenosis because of the risk of worsening renal failure. Angiotensin-converting enzyme inhibitors rarely cause potentially fatal angioedema, which is more frequent in African Americans. Angiotensin-converting enzyme inhibitors commonly cause a cough, which may be bothersome enough to require cessation, but is a separate pathogenesis than angioedema.¹²⁹

Spironolactone and eplerenone are MR antagonists whose main effect is mediated by antagonizing MR activity in the distal kidney. These agents also produce systemic vascular effects such as reducing inflammation, improving vascular endothelial function, and promoting fibrinolysis. Mineralocorticoid receptor antagonists reduce mortality in patients with chronic heart failure and after acute MI, and are very effective in the treatment of drug-resistant hypertension.^130–133 Spironolactone possesses progesterone-like activity and can cause gynecomastia and/or breast tenderness in 5% to 10% of patients, which resolves upon cessation. Eplerenone does not cause gynecomastia because it is MR specific and therefore provides an alternative for those who are spironolactone intolerant. As opposed to other diuretics, MR antagonists do not require filtration into the urinary space to achieve their effect, but renal insufficiency carries an increased risk of hyperkalemia, and they should be used with caution if at all in this setting. Eplerenone carries additional risk of drug interactions due to moderate CYP3A4 inhibition.