Interaction between Cyclic Adenosine Monophosphate and Cyclic Guanosine Monophosphate in Guinea Pig Ventricular Myocardium

The purpose of this study was to examine the mechanisms underlying adrenergiccholinergic antagonism in ventricular myocardium. Myocardial contractility, cyclic adenosine monophosphate (AMP) levels, and cyclic guanosine monophosphate (GMP) levels were measured in isolated guinea pig ventricles after treatment with various inotropic agents given alone and simultaneously with acetylcholine. Acetylcholine alone markedly elevated cyclic GMP levels but did not substantially change myocardial contractility. However, the same concentration of acetylcholine significantly attenuated the inotropic effect of isoproterenol and histamine, two drugs that may act by increasing myocardial levels of cyclic AMP. The decrease in the inotropic response to isoproterenol did not appear to be due to a decrease in the generation of cyclic AMP, because cyclic AMP levels were similar in hearts receiving isoproterenol alone and those receiving isoproterenol with acetylcholine. Dibutyryl cyclic GMP also antagonized the inotropic action of isoproterenol. Acetylcholine did not alter the inotropic effects of ouabain, an agent that increases myocardial contractility without changing cyclic AMP levels. These results suggest that cyclic GMP mediates the antiadrenergic effects of acetylcholine by specifically antagonizing the inotropic actions of cyclic AMP. • Histological studies done since the early 1960s have documented the presence of cholinergic innervation in the ventricular myocardium (1, 2). In addition, a number of physiological experiments have demonstrated that stimulation of vagal nerves or administration of cholinergic drugs can depress, although often minimally, the contractile state of the ventricle (3-7). The ventricular depressant effect of the cholinergic system appears to be enhanced in the presence of adrenergic stimulation (8-13). This complex interaction between adrenergic and cholinergic effects on the heart, in which the cholinergic depressant action is magnified during sympathetic stimulation, has been termed accentuated antagonism by Levy (14). Several studies, all done with broken cell preparations or tissue slices, have also shown that cholinergic drugs can attenuate catecholamineinduced increases in cyclic adenosine monophosphate (AMP) levels in cardiac tissues (15, 16). This From the Departments of Medicine and Pharmacology and The Krannert Institute of Cardiology, Indiana University School of Medicine, Indianapolis, Indiana 46202. This investigation was supported by U.S. Public Health Service Grants HL 06308, HL 14159, HL 05363, and HL 05749 from the National Heart and Lung Institute, by the American Heart Association, Indiana Affiliate, and by the Herman C. Krannert Fund. Dr. Watanabe is the recipient of a Pharmaceutical Manufacturers Association Foundation Faculty Development Award in Clinical Pharmacology. Received February 19, 1975. Accepted for publication June 6, 1975. attenuation of the amount of cyclic AMP generated has logically been proposed as the mechanism, at the subcellular level, for the antagonism between the adrenergic and the cholinergic systems. With the recent demonstration that the level of cyclic guanosine monophosphate (GMP) in the myocardium is increased after cholinergic stimulation (16, 17), it became of interest to investigate whether the interaction of the adrenergic and cholinergic systems at the subcellular level of the heart is occurring between cyclic AMP and cyclic GMP. The series of experiments reported in this paper was performed with the objective of attempting to correlate the contractile state of intact ventricular muscle with induced changes in cyclic nucleotide (cyclic AMP and cyclic GMP) levels. Positive inotropic drugs thought to act by increasing cyclic AMP levels as well as ouabain, which increases contractility without changing cyclic AMP levels, were administered alone or simultaneously with acetylcholine. The effects of acetylcholine on the inotropic actions of these various interventions and on cyclic nucleotide levels were then studied.