Pharmacologic and physiologic considerations of adrenoceptor blockade.

The beta-adrenergic receptor blocking drugs have been in use for the treatment of hypertension for almost two decades. Although the mechanism of their antihypertensive action still is not precisely known, they have become an established major class of therapy for the disease. Most agents produce an immediate reduction in heart rate and cardiac output, later followed by a reduction in pressure. The exceptions include: those agents that possess intrinsic sympathomimetic activity and produce little reduction in heart rate and output; and labetalol, an agent that reduces pressure immediately (associated with the cardiac effects) because it possesses alpha- as well as beta-adrenoceptor blocking effects. Just because a beta-blocking drug reduces cardiac output significantly, it does not follow that renal blood flow will decrease; this depends upon the number and affinity of receptors in the renal circulation. Most beta blockers (including labetalol) reduce renal vascular resistance in patients with uncomplicated hypertension. Other actions of this class of adrenoceptor blocking agents are discussed. As we learn more of the physiologic effects of adrenoceptor blocking agents, there is no doubt that we shall gain more insight into the underlying mechanisms of hypertensive diseases as well as their pharmacologic properties.