G. G. Nahas

Acetylcholine and the cyclic AMP system in smooth muscle

Abstract The effect of acetylcholine on the cyclic AMP system and its interaction with the epinephrine effect on this system was studied in vitro in rat uterus. In the presence of acetylcholine (1–100 μMolar), epinephrine-induced cyclic AMP formation in intact tissue is decreased; this inhibitory effect of acetylcholine is more pronounced at higher concentrations of epinephrine and is reversed by atropine. In broken cell preparations, acetylcholine does not alter the activity of adenyl cyclase or phosphodiesterase, nor the stimulatory effect of epinephrine. It appears that acetylcholine has an effect on the cyclic AMP system of rat uterine smooth muscle only when the structural and functional relationships in the cell are preserved. The possible mechanisms are discussed.

Cyclic AMP and Vascular Smooth Muscle Function

Mediation of the catecholamine relaxing effect in uterine smooth muscle by cyclic AMP was suggested in our previous reports (1, 2). Further experimental work confirmed this and indicated that cyclic AMP has an even broader role in the regulation of smooth muscle function (2): compounds inhibiting cyclic AMP phosphodiesterase and thereby increasing intracellular cyclic AMP, consequently relax the smooth muscle (3); they also potentiate the relaxing effect of compounds which stimulate adenyl cyclase (2). Other compounds, e.g. halothane, acetylcholine, nitroprusside, may also exert part of their functional effects on smooth muscle through the cyclic AMP system. We have shown previously (4) that the cyclic AMP system is present in arterial smooth muscle and is a part of the regulatory mechanism of arterial tone and contractility. The present data add further information about the role of the cyclic AMP system in the control of vascular smooth muscle function.