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On the dog isolated right ventricular muscle, experiments were carried out in order to elucidate the characteristics of the cholinergic antagonism against the positive inotropic action (PIA) induced via different subcellular mechanisms. The relationship between cyclic AMP and cyclic GMP levels, and contractile force during cholinergic antagonism was assessed. Carbachol (10 microM) by itself decreased only slightly the tension developed, but inhibited prominently the PIAs of isoprenaline, histamine, glucagon, theophylline and papaverine. The action of dibutyryl cyclic AMP was inhibited less than PIAs of other agents mentioned above. In contrast, carbachol did not affect the PIAs of calcium and g-strophanthin. The antagonism by carbachol of PIAs of isoprenaline, histamine, glucagon, theophylline and papaverine was accompanied by a reduction of the intracellular cyclic AMP level elevated previously by these agents, and by an elevation of the intracellular cyclic GMP level. A good correlation was found between changes in the tension developed, and cyclic AMP and cyclic GMP levels during the cholinergic antagonism of PIAs induced by these agents in the dog ventricular myocardium.