Effects of α-adrenoceptor agonists and antagonists on pre- and postsynaptically located α-adrenoceptors

Abstract

The effects of α-adrenoceptor agonists and antagonists have been examined at pre- and post-synaptically located α-adrenoceptos in the pithed rat. The presynaptic receptors were those located at the cardiac sympathetic nerve terminals and the postsynaptic receptors were those present in vascular smooth muscle. Clonidine was approximately equipotent at pre- and post-synaptic α-adrenoceptors, whilst LSD and BAY-1470 were more active at the pre- than at post-synaptic sites. Oxymetazoline, naphazoline, methoxamine and phenylephrine were all much more active at the postsynaptic α-adrenoceptors. Phentolamine was the most potent antagonist at both pre- and post-synaptic α-adrenoreceptors. Piperoxan, yohimbine and tolazoline were about 3-7× less potent than phentolamine at both sites. Thymoxamine was about10× less potent than phentolamine at postsynaptic α-adrenoceptors but about 1000× less active at the presynaptic receptors. The differential actions of both agonists and antagonists at pre- and post-synaptic α-adrenoceptors suggest that the receptors may be of different types. © 1976.