Enhancement of muscle blood cell flux and pO2 by cromakalim (BRL 34915) and other compounds enhancing membrane K+ conductance, but not by Ca2+ antagonists or hydralazine, in an animal model of occlusive arterial disease
Vasodilator drugs relax vascular smooth muscle, either directly or indirectly through adrenergic neurones. Different vasodilators may act principally on arteries, principally on veins, or on both arteries and veins. Dilatation of arterioles reduces peripheral vascular resistance and hence lowers arterial blood pressure. Dilatation of veins increases peripheral venous pooling of blood, reducing venous return to the heart and tending to lower cardiac output and arterial blood pressure. Reflex sympathetic tachycardia tends to restore cardiac output and blood pressure. There is sound rationale and proof of efficacy for using vasodilators in three disorders-hypertension, angina, and heart failure. In hypertension blood pressure is lowered by reducing peripheral vascular resistance. In angina the reductions in venous return (reducing preload) and arterial blood pressure (reducing afterload) reduce the workload of the left ventricular myocardium and hence reduce its requirement for oxygen. In heart failure reduction of the workload of the ventricles improves their performance. Of the newer vasodilators, prazosin, calcium antagonists, and captopril are covered in separate articles in this series. Minoxidil, reviewed here, is a recently introduced arterial vasodilator for treating severe hypertension. Although the efficacy of vasodilators in peripheral and cerebral vascular disease has not been proved, new drugs continue to be marketed and these are considered later in this article.