Mechanism of Action

Available evidence suggests that a single unifying mechanism does not exist but rather that various vasodilators may act at different places in the series of processes that couple excitation of vascular smooth muscle cells with contraction. For example, the vasodilators known as calcium channel antagonists block or limit the entry of calcium through voltage-dependent channels in the membrane of vascular smooth muscle cells. In this way, the calcium channel blockers limit the amount of free intra-cellular calcium available to interact with smooth muscle contractile proteins (see Chapter 14).

Other vasodilators, such as diazoxide and minoxidil, cause dilation of blood vessels by activating potassium channels in vascular smooth muscle. An increase in potassium conductance results in hyperpolarization of the cell membrane, which will cause relaxation of vascular smooth muscle.

Another group of drugs, the so-called nitrovasodila-tors, of which nitroprusside is an example, activate soluble guanylate cyclase in vascular smooth muscle, which brings about an increase in the intracellular levels of cyclic guanosine monophosphate (cGMP). Increases in cGMP are associated with vascular smooth muscle relaxation. The action of the nitrovasodilators appears to be quite similar to that of the endogenous vasodilator released by a variety of stimuli from endothelial cells of blood vessels. This substance, originally named en-dothelial-derived relaxing factor, or EDRF, is nitric oxide or a closely related nitrosothiol compound. The knowledge that the nitrovasodilators generate nitric oxide in vivo suggests that this substance may be the final common mediator of a number of vascular smooth muscle relaxants.

This chapter describes four vasodilators in detail. Two of these agents, hydralazine and minoxidil, are effective orally and are used for the chronic treatment of primary hypertension. The other two drugs, diazoxide and sodium nitroprusside, are effective only when administered intravenously. They are generally used in the treatment of hypertensive emergencies or during surgery.

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