Are the Pre- and Postsynaptic α-Adrenoceptors in Human Vascular Smooth Muscle Atypical?

Abstract

Summary: To sec whether human vascular post- and presynaptie α-receptors are similar to “classic” α-and α₂,-receptors, respectively, the effects of varions (v-ieeeptor agonists and antagonists on human isolated digital arleries and metacarpal (or metatarsal) veins have been studied. The relative poteney of phenylephrine eompared with noradrenaline on the postsynaptie receptors was signifieantly greater in arteries than in veins. The rank order ol“ potency of varions α-agonists on postsynaptie receptors was the same in arteries and veins exeept that in veins adrenaline was more potent than clonidine, whereas the reverse was the case in arteries. Phentolamine showed no seleetivity in either antagonizing postsynaptic responses or enhaneing stimulation-indueed trans-mitter release hy antagonism at presynaptie α-receptors. Yohimbine potently. but not seleetively, antagonized postsynaptic responses to noradrenaline and phenylephrine in both arteries and veins. and enhanced stimulation-indueed transmuter release to a greater extent in arteries than in veins. Prazosin re-dueed the maximum response to noradrenaline (but not phenylephrine) to a signifieantly greater extent in veins than in arteries. and in veins alone shifted the phenylephrine eurve to a greater degree than it did the noradrenaline eurve. Prazosin did not signifieantly inerease stimulation-indueed transmitter release in either arteries or veins. Phenoxybenzamine produeed large inereases in stimulation-indueed tritium outflow. which were signifieantly greater in arteries than in veins (even in the presence of cocaine). Clonidine signifieantly redueed stimulation-indueed tritium outflow to the same extent in arteries and veins, It is coneluded that the postsynaptie and possibly the presynaptie α-adrenoceptors of these tissues are not a homogeneous population.