Vascular Smooth Muscle Cell Contraction and Relaxation

Smooth muscle contraction is ultimately regulated by intracellular Ca⁺⁺ concentrations. Excitation-contraction coupling occurs by several mechanisms. Depolarization of vascular smooth muscle cell membranes allows Ca⁺⁺ entry through voltage-gated channels. When these channels open, Ca⁺⁺ flows into the cell down its concentration gradient (Fig. 24-1). Activation of receptors for certain vasoconstrictor substances can also open Ca⁺⁺ channels. In addition to elevating intracellular Ca⁺⁺ by opening channels, receptor activation can also increase intracellular Ca⁺⁺ by activating phospholipase C, which hydrolyzes phosphatidylinositol 4,5-bisphosphate to diacylglycerol and inositol 1,4,5-trisphosphate, both of which contribute to contraction (see Chapters 1 and Chapters 9). Inositol trisphosphate releases Ca⁺⁺ from intracellular stores, whereas diacylglycerol activates protein kinase C, an enzyme that phosphorylates several substrates involved in the contractile response. When Ca⁺⁺ enters the smooth muscle cell, it combines with calmodulin, and the Ca⁺⁺-calmodulin complex activates myosin light-chain kinase, which in turn phosphorylates the myosin light chain, promoting the interaction of myosin and actin and cross-bridge formation, leading to contraction. Because Ca⁺⁺-channel antagonists block or limit the entry of Ca⁺⁺ through voltage-gated channels, these drugs dilate blood vessels that have some endogenous degree of vasoconstrictor tone, or limit vasoconstriction caused by endogenous or exogenous vasoactive stimulants (see Chapter 20).

FIGURE 24–1 Mechanisms of contraction of vascular smooth muscle cells and its inhibition by Ca⁺⁺-channel blockers. Increases in intracellular Ca⁺⁺ can occur by Ca⁺⁺ entry through channels opened by a change in membrane potential, by receptor activation, or by Ca⁺⁺ release from sarcoplasmic reticulum (SR), an event triggered by inositol 1,4,5 trisphosphate (IP₃). IP₃ is formed by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) by phospholipase C (PLC). Ca⁺⁺ interacts with calmodulin (CM), which activates myosin light-chain kinase (MLCK). The latter phosphorylates myosin, which interacts with actin, resulting in contraction. Ca⁺⁺-channel blockers act by limiting Ca⁺⁺ entry through membrane channels.

Increases in cyclic adenosine monophosphate (cAMP) also lead to smooth muscle relaxation. Increased cAMP activates cAMP-dependent protein kinase A, which phosphorylates several proteins, leading to decreased intracellular Ca⁺⁺ as a consequence of reduced influx, enhanced uptake into the sarcoplasmic reticulum, and/or enhanced extrusion through the cell membrane (Fig. 24-2, A). Myosin light-chain kinase may also be phosphorylated, leading to enzyme inactivation and inhibition of contraction. Because adrenergic β receptor agonists such as isoproterenol activate adenylyl cyclase and increase cAMP, these agents lead to relaxation of vascular smooth muscle. Similarly, drugs that inhibit phosphodiesterases (PDEs), which metabolize cAMP and cyclic guanosine monophosphate (cGMP), promote smooth muscle relaxation. Drugs such as papaverine may act by this mechanism.

FIGURE 24–2 Mechanisms for relaxation of vascular smooth muscle cells. A, β Adrenergic receptor agonists cause relaxation by stimulating formation of cAMP, which activates protein kinase (PK) and decreases intracellular Ca⁺⁺. This occurs by activating Ca⁺⁺ pumps in the sarcoplasmic reticulum membrane or cell membrane to either sequester Ca⁺⁺ or pump it from the cell. B, Nitrovasodilators activate soluble guanylyl cyclase by releasing NO or related compounds, leading to an increase in cGMP and activation of cGMP-dependent protein kinase. NO may influence contractility by limiting Ca⁺⁺ entry through channels or by directly decreasing the sensitivity of contractile proteins to Ca⁺⁺. Type 5 PDE inhibitors, such as sildenafil, inhibit cGMP breakdown in cells where they are expressed (such as penile corpus cavernosum), and potentiate its vasodilatory actions. C, K⁺-channel activators, such as minoxidil, increase K⁺ conductance, which hyperpolarizes the cell, causing relaxation.-hyperpolarization, -depolarization.

Nitrovasodilators

Nitrovasodilators are organic nitrates that provide a source of nitric oxide (NO), which activates a soluble guanylyl cyclase in vascular smooth muscle, causing an increase in intracellular cGMP, which activates a cGMP-dependent protein kinase (see Fig. 24-2, B). This kinase leads to the phosphorylation of proteins, which results in smooth muscle relaxation. Although the cellular mechanisms involved are not entirely clear, they may include decreased entry of Ca⁺⁺ through membrane channels, inhibition of phosphatidylinositol hydrolysis, stimulation of Ca⁺⁺ pumps to extrude or sequester Ca⁺⁺, and decreased sensitivity of contractile proteins to Ca⁺⁺.

NO is one of the most important vasodilator factors formed in and released from the endothelial cells of blood vessels. Endothelial cells line all vessels of the body and release factors that affect both the contractile state and growth of smooth muscle cells. NO, a short-lived radical, is formed from L-arginine by a class of enzymes known as NO synthases. Two isoforms of this enzyme are particularly important with respect to vascular biology. The “constitutive” form is present in endothelium under normal physiological conditions, and its activity is dependent upon the concentration of Ca⁺⁺-calmodulin. There is also an “inducible” form of NO synthase expressed in smooth muscle in response to trauma or pathological stimuli, such as invading bacteria. The activity of this isoform does not depend on intracellular Ca⁺⁺-calmodulin concentrations and is not easily regulated. In severe septicemia, NO generated by this enzyme can cause harmful hypotension due to vasodilation. In all cases NO-induced vasodilation is associated with elevated levels of cGMP.

NO may be the final common mediator for several vascular smooth muscle relaxants. In addition to nitrovasodilators, which may form NO or a related molecule, some endogenous agents that cause vasodilation do so in whole or in part by releasing NO from endothelial cells. Included among these are bradykinin, histamine, adenosine triphosphate, adenosine diphosphate, substance P, and acetylcholine (Fig. 24-3). Because the endothelium is an important structure for communicating between the blood and the vascular media, it has the potential to be an important target for vasodilator therapy.

FIGURE 24–3 Endothelium-dependent relaxation produced by vasodilators. These substances act on endothelial cells at their respective receptors to release NO. The latter diffuses into the vascular smooth muscle cell, increases soluble guanylyl cyclase activity and cGMP concentration, and promotes relaxation. L-Arginine is converted to NO by NO synthases.

K⁺ Channel Activators

Agents such as minoxidil cause vasodilation by activating K⁺ channels in vascular smooth muscle. The increased K⁺ conductance results in hyperpolarization of the cell membrane and relaxation (see Fig. 24-2, C). The hyperpolarizing effect also counteracts stimulants that act by depolarization, promoting Ca⁺⁺ entry.

Phosphodiesterase Type 5 Inhibitors

Sildenafil, tadalafil, and vardenafil are selective inhibitors of cGMP-specific PDE type 5, which is found in high concentrations in the penile corpus cavernosum and is responsible for the degradation of cGMP. NO release during sexual stimulation activates guanylyl cyclase, resulting in increased levels of cGMP, which causes smooth muscle relaxation in the corpus cavernosum, allowing the inflow of blood. The PDE5 inhibitors prevent the catabolism of cGMP, thereby prolonging its actions (see Fig. 24-2, B). These drugs have no direct relaxant effect on isolated human corpus cavernosum and, at recommended doses, have no effect in the absence of sexual stimulation. Rather, they cause specific vasodilation in the presence of appropriate sexual stimulation and have become increasingly popular for treating erectile dysfunction in men.

Other Agents

Although hydralazine was one of the first antihypertensive drugs, its mechanism of action has not been definitively elucidated. It is a vasodilator with effects on arterioles and not venules. Hydralazine has been shown to decrease intracellular Ca⁺⁺ concentrations, and recent evidence suggests that this action may be due to its ability to inhibit the inositol 1,4,5-trisphosphate-induced release of Ca⁺⁺ from the sarcoplasmic reticulum in vascular smooth muscle cells (see Fig. 24-1). Additional sites of action have also been proposed (see Table 24-1).

Nitrovasodilators and Angina Pectoris

The goal of therapy in coronary artery disease is to reduce pain and increase the patient’s exercise tolerance. This can be accomplished by administration of organic nitrates, the prototype of which is nitroglycerin. Organic nitrates are the mainstay of antianginal therapy, used effectively for this purpose for approximately 100 years.

The pharmacological properties of the organic nitrates that make them useful depend on the underlying cause of the angina. If pain is associated with atherosclerosis, the chief benefit arises from actions on the peripheral circulation and not on coronary vessels. Nitrates produce vasodilation of the venous vasculature. Dilation of venous capacitance vessels diminishes venous return to the heart, reducing ventricular volume and pressure. This decreases ventricular wall tension, a major contributor to the O₂ demands of the heart (Fig. 24-4). Thus, by decreasing preload on the heart, O₂ needs diminish, and demand is consistent with supply.

FIGURE 24–4 Mechanism of vasodilator action in the therapy of CHF. The four main determinants of cardiac function act to determine the myocardial O₂ demand. Nitrates decrease preload and afterload but do not affect contractile force, thereby decreasing O₂ demand. Heart rate may increase slightly as a result of the baroreceptor reflex.

Other consequences of nitroglycerin administration also contribute to its beneficial effect in angina. For example, nitrates cause relaxation of resistance vessels of the arterial circulation. This decreases afterload placed on the heart, or the impedance against which the heart must pump. Reducing afterload decreases O₂ demands of the heart, just as reducing preload does. The nitrate effect on resistance vessels generally requires somewhat higher concentrations than those needed for venodilation.

Another beneficial feature of organic nitrate actions in angina pectoris is redistribution of blood flow to the subendocardial regions of the heart, which are especially vulnerable to ischemia. Perfusion of the subendocardial region occurs most prominently during early diastole. Later in diastole, as the ventricle fills, subendocardial arteries are constricted because of pressure in the ventricles, with the subsequent decrease in perfusion of these arteries. By decreasing preload, nitrates reduce ventricular filling pressure and increase the time available for endocardial perfusion.

In management of angina pectoris caused by coronary artery spasm, the organic nitrates, in addition to effects described previously, are useful because they can dilate constricted coronary vessels. Nitrates are available in many dosage forms, including sublingual, transdermal, and longer acting oral preparations. The choice of nitrate preparation depends on the necessity for a rapid onset or a longer duration of action.

Other Drugs for Angina Pectoris

Additional drugs used for the treatment of angina pectoris are adrenergic β receptor antagonists and Ca⁺⁺ channel-blocking drugs (see Chapters 11 and 20). The beneficial effect of β receptor blockers in angina is their ability to decrease O₂ demands of the heart. These drugs decrease heart rate and ventricular contractile force (see Chapter 11). Heart rate and contractile force, together with ventricular wall tension, are major determinants of myocardial O₂ demand (see Fig. 24-4). In addition, chronic therapy with β-blockers reduces blood pressure. Thus these drugs also decrease afterload. Unlike organic nitrates, β-blockers are not used to terminate an acute attack of angina pectoris but rather to increase exercise tolerance of the patient and to reduce the frequency of anginal attacks.

Vasodilators and Congestive Heart Failure

Recent studies indicate that vasodilator therapy is extremely effective in treatment of CHF. Drugs used more frequently for treating CHF are those that increase the force of cardiac contraction (see Chapter 23) or minimize Na⁺ and H₂O retention (see Chapter 21). Cardiac glycosides affect only two of several determinants of cardiac function (e.g., contractility and rate).

Among the major mechanisms by which vasodilators increase cardiac performance are afterload reduction, preload reduction, and the resulting increased left ventricular diastolic compliance. Afterload reduction, by use of other vasodilators and high concentrations of nitrates, is accomplished by dilating arterioles and thereby decreasing systemic vascular resistance. This increases cardiac output and tissue perfusion. Venodilators, including low doses of nitrates, predominantly decrease preload, reducing systemic and pulmonary venous pressures. Ventricular volume is also affected by the decreasing preload. The venodilators do not increase the force of contraction, and the heart rate is generally unchanged, so that the work of the heart remains the same. The overall effect, therefore, is a reduction in myocardial O₂ consumption and demand on the heart. The vasodilator drugs may also improve left ventricular diastolic performance by shifting the diastolic pressure-volume curve to the left (i.e., to pump the same volume at a lower pressure). This shift also moves the ventricular function curve to the left, demonstrating an improvement in left ventricular performance.

Patients who are refractory to cardiac glycosides frequently do well if treated with vasodilators. Among the direct-acting vasodilators used to treat CHF are the nitrates, hydralazine, minoxidil, and sodium nitroprusside. Angiotensin-converting enzyme (ACE) inhibitors such as captopril enalapril, and lisinopril (see Chapter 20 and 23) are also of proven effectiveness, as is the adrenergic α₁ receptor antagonist prazosin. Long-term treatment with hydralazine alone is only minimally effective in treatment of CHF, but the combination of hydralazine with a nitrate, isosorbide dinitrate, effectively decreases mortality. Minoxidil is also generally not very effective when used alone.

Vasodilators and Peripheral Vascular Disease

Peripheral vascular diseases are either vasospastic or occlusive. In Raynaud’s disease, a vasospastic disorder, blood flow to the extremities is reduced as a result of a reversible vasoconstriction. Therefore vasodilators may be helpful to these patients by dilating the blood vessels of the skin. Direct-acting vasodilators, adrenergic α₁ receptor blockers, Ca⁺⁺ channel-blocking drugs, prostaglandins, adrenergic β receptor agonists, and ACE inhibitors are used to treat Raynaud’s phenomenon. Nitroglycerin ointment may be helpful as an adjunctive agent in Raynaud’s disease. Other nonspecific vasodilators, such as cyclandelate, papaverine, ethaverine, and nicotinyl tartrate, have been used but are of questionable efficacy.

Vasodilators are of limited usefulness in occlusive disease when organic obstruction is significant, and they do not generally improve flow to either skeletal muscle or skin. In some instances vasodilator therapy may actually be harmful when blood is shunted away from diseased areas.

Vasodilators and Hemostasis

Vasodilators may be used as aids during surgical procedures. They can be used to provide a more satisfactory surgical field, to minimize large blood volume losses, and to improve cardiac performance by reducing preload or afterload.

Phosphodiesterase Type 5 Inhibitors and Impotence

A new and evolving use of direct vasodilators is in treatment of erectile dysfunction in men. The use of PDE5 inhibitors, including sildenafil, tadalafil and vardenafil, has dramatically altered therapeutic treatment for this disorder. Parasympathetic nerves innervating blood vessels of the penile corpus cavernosum release NO when activated during sexual activity. Sildenafil and related drugs potentiate the action of NO on cGMP by blocking cGMP hydrolysis, resulting in prolonged dilation of cavernosal blood vessels, thereby enhancing erection.

Nitrovasodilators

As with all vasodilators, orthostatic hypotension and tachycardia are adverse effects of nitrate therapy. Vascular headache is quite common but rapidly disappears on continued use. Tolerance to the vascular effects of nitrates does occur; however, this is not of great clinical significance, except possibly in treatment of chronic CHF. Cross-tolerance exists between nitroglycerin and other nitrate esters, but this and other nitrate tolerance can be reduced by only a short period of nitrate abstention. Orthostatic hypotension can be minimized by careful adjustment of dose and by having the patient avoid the upright position when taking rapid-acting preparations. Physical dependence has been observed in munitions workers exposed continuously to very high concentrations of nitrates. In these individuals withdrawal from the industrial environment may result in angina. This phenomenon is not observed in patients normally taking therapeutic doses of nitrates but can occur in individuals who have been taking large doses for a long time.

Nitrates can be reduced to nitrites, which in turn can oxidize the ferrous iron of hemoglobin, converting it to methemoglobin. The latter reduces O₂ delivery to tissues. Methemoglobinemia is not a problem with normal nitrate therapy but may be observed in accidental poisoning or overdose.

Phosphodiesterase Type 5 Inhibitors

Type 5 PDE inhibitors can cause abnormalities in color vision, although this is most common with sildenafil.

Inhibition of PDE5 in other tissues, such as esophageal smooth muscle, can result in a reduced tone of the esophageal sphincter and increased gastroesophageal reflux, as well as dyspepsia. In addition, as a consequence of PDE type 5 inhibition in the brain, these compounds have been reported to result in emotional, neurological, and psychological side effects. They also have the potential to cause hypotension.