A postsynaptic location of alpha2 adrenoceptors in vascular smooth muscle: in vivo studies in the conscious rabbit

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Abstract
The rise in blood pressure after intravenous administration of a range of α-adrenoceptor agonists and the effect of α adrenoceptor antagonists was studied in groups of conscious rabbits. Phentolamine was equally effective at blocking the pressor response to the α1 adrenoceptor agonist phenylephrine and the mixed α1/α2 adrenoceptor agonist noradrenaline. Phenoxybenzamine which acts preferentially on α1 adrenoceptors and the more specific α1 adrenoceptor antagonist prazosin were more effective against phenylephrine induced rises in pressure, while yohimbine and α yohimbine which preferentially act on α2 adrenoceptors were more effective against noradrenaline. The immediate pressor response to intravenous injections of the α2 adrenoceptor agonists clonidine and guanabenz was blocked more effectively by phentolamine than prazosin when doses which gave a similar degree of α1 adrenoceptor blockade were compared. These studies are consistent with a postsynaptic location of both α1 and α2 adrenoceptors both of which mediate vasoconstrictor responses in vascular smooth muscle.