Characterization of the positive and negative inotropic effects of acetylcholine in the human myocardium

Abstract

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 (M 1 > M 3 > M 2), AF-DX 116 (11-({2-[(diethylamino)-methyl]-1-piperidyl}acetyl)-5,11-dihydro-6 H-pyridol[2,3- b][1,4]benzodiazepine-6-one base; M 2 > M 1 > M and HHSiD ( p-fluorohexahydro-siladifenidol hydrochloride; M 3 ≥ M 1 ⪢ M 2) 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 M 2 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 M 1 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 M 2 receptor mediating both negative (atria) and positive (ventricle) inotropic effects, muscarinic M 1 receptors, capable of reversing depressed atrial contractility, are present in the human heart.