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