Muscarinic receptors mediate negative and positive inotropic effects in mammalian ventricular myocardium: differentiation by agonists

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Abstract
1 The concentration-dependence of the negative and positive inotropic effect of choline esters and of oxotremorine was studied in isometrically contracting papillary muscles of the guinea-pig. The preparations were obtained from reserpine-pretreated animals and were electrically driven at a frequency of 0.2 Hz. 2 In the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methyl xanthine (IBMX, 100 μmol I−1), choline esters and oxotremorine produced concentration-dependent negative inotropic effects. Oxotremorine exhibited the highest negative inotropic potency (with a half-maximal effective concentration, EC50, of 20 nmol l−1) followed by carbachol (139 nmol l−1), methacholine (490 nmol l−1), acetylcholine in the presence of lO μmol l−1 physostigmine (1.36 μmol l−1 and bethanechol (10 μmol l−1)). Atropine was a competitive antagonist of the negative inotropic effects. Carbachol and oxotremorine decreased Vmax, overshoot and duration of slow Ca2+-dependent action potentials which had been elicited in the presence of 100 μmol l−1 IBMX. 3 Choline esters produced a concentration-dependent positive inotropic effect. With an EC50 of 32 μmol l−1, carbachol was the most potent compound, followed by methacholine (35 μmol l−1), acetylcholine in the presence of 10 μmol l−1 physostigmine (46 μmol l−1) and bethanechol (142 μmol l−1). Compared to carbachol and methacholine which increased force by 100% of control, the increase induced by acetylcholine and bethanechol was only 64 and 58%, respectively. Atropine shifted the concentration-effect curves of all choline esters to higher concentrations. Choline esters caused intracellular Na+ activity to increase in the quiescent papillary muscle. This effect was reversed by atropine. 4 Oxotremorine produced a small concentration-dependent positive inotropic effect (about 30% of the maximal effect of carbachol) which was resistant to atropine. Oxotremorine was a potent inhibitor of the positive inotropic effect of choline esters, and did not cause an increase in intracellular Na+ activity in the quiescent papillary muscle. 5 The results show that muscarinic receptors of the ventricular myocardium mediate two inotropic effects, which are opposite in direction and differ in their concentration-dependence by a factor of 100. Although agonists differentiate between both inotropic effects, it is unknown whether the receptors involved represent receptor states or separate receptor subpopulations. The negative inotropic effect of choline esters and of oxotremorine can be best explained by adenylate cyclase inhibition. While stimulation of phosphoinositide hydrolysis might have been responsible for the positive inotropic effect of choline esters via modulation of cation-fluxes across the cell membrane, such a mechanism was not involved in the positive inotropic effect of oxotremorine.