Phenoxybenzamine (PBZ), a haloalkylamine that blocks a1 and a2 receptors, is covalently conjugated with a receptors. Consequently, receptor blockade is irreversible and restoration of cellular responsiveness to a receptor agonists probably requires the synthesis of new receptors. Table 10-3 summarizes salient properties of PBZ, the major physiological effects of which result from blockade of a receptors in smooth muscle.
PBZ causes a progressive decrease in peripheral resistance, an increase in cardiac output (due, in part, to reflex sympathetic nerve stimulation), tachycardia accentuated by enhanced release of NE (due to a2 blockade), and decreased clearance of NE (due to inhibition of NET and ENT by the drug). PBZ impairs pressor responses to exogenously administered catecholamines and causes "epinephrine reversal" (b2 predominance in the vasculature). PBZ has little effect on supine blood pressure in normotensive subjects but causes a marked fall in blood pressure on standing (antagonism of compensatory vasoconstriction). The drug impairs the ability to respond to hypovolemia and anesthetic-induced vasodilation. At higher doses, PBZ also irreversibly inhibits responses to 5-HT, histamine, and ACh.
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