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 > M3) and HHSiD (p-fluorohexahydro-siladifenidol hydrochloride; M3 > or = 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. (ABSTRACT TRUNCATED AT 250 WORDS)