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

The effects of alpha adrenoceptor agonists and antagonists have been examined at pre- and post-synaptically located alpha-adrenoceptors 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 alpha-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 alpha-adrenoceptors. Phentolamine was the most potent antagonist at both pre- and post-synaptic alpha-adrenoceptors. Piperoxan, yohimbine and tolazoline were about 3-7X less potent than phentolamine at both sites. Thymoxamine was about 10X less potent than phentolamine at postsynaptic alpha-adrenoceptors but about 1000X less active at the presynaptic receptors. The differential actions of both agonsists and antagonists at pre- and post-synaptic alpha-adrenoceptors suggest that the receptors may be of different types.