Plasma cyclic GMP: response to cholinergic agents.

S.c. injections of cholinergic agents, carbachol, methacholine and bethanechol, into fasted rats caused rapid increases in the plasma concentration of cyclic GMP, with a sharp peak at 5--10 min after the injection. Acetylcholine gave rise to a rapid accumulation of cyclic GMP in plasma only when administered together with physostigmine which produced only a slight, if any, potentiation of the action of the cholinesterase-resistant choline esters. Cyclic AMP also increased after these drugs, but only subsequently to the rise of cyclic GMP; the primary action of the cholinergic drugs appeared to be the increase in cyclic GMP. Atropine was effective not only in abolishing the increase in plasma cyclic GMP induced by cholinergic drugs but also in lowering the baseline level of cyclic GMP. It was concluded that the plasma concentration of cyclic GMP could serve as a good parameter of cholinergic activity in rats.