In the human isolated myocardium, acetylcholine (10−9 to 10−3 M) elicited a biphasic inotropic effect (a decrease in the lower and an increase in the higher concentration range) in atrial and a positive inotropic effect in ventricular trabeculae. However, under conditions of raised contractility achieved by exposure to noradrenaline (10−5 M), only negative inotropic effects were observed in both atria and ventricles. Atropine (10−6 M), but not propranolol (10−6 M), antagonized both positive and negative inotropic effects of acetylcholine, thus showing that the responses were mediated by muscarinic acetylcholine receptors. The use of subtype selective muscarinic receptor antagonists (10−7 to 10−5 M), pirenzepine (M1 > M3 > M2), AF-DX 116 (11-({2-[(diethylamino)-methyl]-1-piperidyl}acetyl)-5,11-dihydro-6H-pyridol[2,3-b][1,4]benzodiazepine-6-one base; M2 > M1 > M and HHSiD (p-fluorohexahydro-siladifenidol hydrochloride; M3 ≥ M1 ⪢ M2) revealed that the negative inotropic effect of acetylcholine in atrial as well as the positive inotropic effect in ventricular trabeculae were best antagonized by AF-DX 116 and not by pirenzepine, suggesting the involvement of the muscarinic M2 receptor subtype, possibly linked to different second messenger systems. On the other hand, the positive inotropic effect of acetylcholine (10−6 to 10−3 M) in the atrial tissue, observed only in preparation with depressed contractility, was not effectively antagonized by either AF-DX 116 or HHSiD, but was significantly reduced by pirenzepine. Furthermore, the selective muscarinic M1 receptor agonist McN-A-343 (4-(m-chlorophenylcarbamoyloxy)-2-butynyltrimethyl ammonium chloride; 10−9 to 10−3 M), which failed to significantly change the baseline contractility in either atrial or ventricular trabeculae, produced a positive inotropic effect in atrial preparations when contractility had been depressed by prior treatment with acetylcholine (10−9 to 10−7 M). This effect of McN-A-343 was effectively antagonized by pirenzepine (10−5 M). These data show that, besides the muscarinic M2 receptor mediating both negative (atria) and positive (ventricle) inotropic effects, muscarinic M1 receptors, capable of reversing depressed atrial contractility, are present in the human heart.

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European Journal of Pharmacology
Department of Pharmacology

Du, X., Schoemaker, R., Bos, E., & Saxena, P. R. (1995). Characterization of the positive and negative inotropic effects of acetylcholine in the human myocardium. European Journal of Pharmacology, 284(1-2), 119–127. doi:10.1016/0014-2999(95)00384-W