Cholinergic receptor mechanisms in amphibian vasoconstrictor responses

The four choline esters, namely acetylcholine, carbachol, methacholine and bethanechol, produced constriction of frog's perfused systemic blood vessels, acetylcholine and carbachol being more potent than methacholine and bethanechol. Pentolinium and dihydroergotamine, in submaximal receptor-blocking doses, also produced a uniform partial inhibition of all choline esters and other agonists including the direct-acting barium chloride. On the other hand atropine markedly inhibited the responses of the four choline esters without affecting the adrenaline and barium responses. Compound AHR-602, a specific ganglionic muscarinic excitatory receptor stimulant, did not produce any effect in these experiments. Eserine in higher doses produced slight vasoconstriction and markedly potentiated acetylcholine responses, but it inhibited partially those of the other three choline esters without significantly affecting adrenaline and barium responses. The results provide strong evidence in favor of involvement of the postganglionic muscarinic receptors only in vasoconstriction after choline esters. There seems to be a considerable nonspecific inhibitory activity in ganglionic and alpha-adrenergic blockers in their submaximal receptor-blocking doses. It may be that in frog the adrenergic and nicotinic receptors are not as well differentiated as muscarinic receptors are. The ganglionic muscarinic receptors also seem to be absent in the frog.