Acetylcholine modulation of phosphorylase and contractility in rat hearts exposed to anoxia or isoproterenol

The effects of acetylcholine (ACh) on glycogen phosphorylase activated by either a cyclic AMP-dependent (isoproterenol) or a cyclic AMP-independent (anoxia) mechanism were examined. Cyclic adenosine 3' : 5'-monophosphate and cyclic guanosine 3' : 5'-monophosphate content, protein kinase and glycogen phosphorylase activities, and the contractile force of isolated perfused rat hearts exposed to either isoproterenol or anoxia were determined. Isoproterenol (1.6 × l0^-7M) produced an increase in cyclic AMP, activated protein kinase and glycogen phosphorylase, and increased intraventricular pressure developed by the myocardium. Acetyleholine did not alter basal phosphorylase activity or contractility. Acetylcholine, infused concurrently with isoproterenol, produced an increase in cyclic GMP and a decrease in cyclic AMP as well as in protein kinase and phosphorylase activity. The effects of ACh on cyclic GMP and phosphorylase were observed at 10^-5 M. Exposure of isolated perfused hearts to anoxia decreased the intraventricular pressure developed. This negative inotropic effect was accompanied by an activation of glycogen phosphorylase that was independent of alterations in cyclic AMP or cyclic GMP. A high concentration of acetylcholine (10^-4 M) further diminished the contractile activity of the heart and abolished the activation of phosphorylase. These effects also occurred in the absence of alterations in cyclic AMP but were coincident with an elevation of cyclic GMP. A lower concentration of ACh (10^-5 M) infused during anoxia, however, elevated cyclic GMP without concurrent effects on cyclic AMP, protein kinase, phosphorylase or contractility. Thus, phosphorylase activated by a cyclic AMP-independent mechanism was not affected by doses of acetvlcholine that were capable of suppressing phosphorylase activated by a cyclic AMP-dependent mechanism. These data support the concept that a reduction in cyclic AMP may be involved in mediating the effects of ACh on catecholamine-stimulated phosphorylase in myocardial tissue. The data do not support a role for cyclic AMP in the inhibition of anoxia-stimulated phosphorylase activation and, thus, some other modulating factor may be operative under these conditions. The role of cyclic GMP in this response remains in question.