Demonstration of functional M3-muscarinic receptors in ventricular cardiomyocytes of adult rats.

1 Muscarinic receptors (M-receptors) in the mammalian heart are predominantly of the M(2)-subtype. The aim of this study was to find out whether there might exist an additional myocardial non-M(2)-receptor. 2 For this purpose, we assessed, in adult rat isolated ventricular cardiomyocytes, carbachol-induced [(3)H]-inositol phosphate (IP) formation, and its inhibition by M-receptor antagonists. 3 Carbachol (10(-7)-10(-3) mol l(-1)) increased IP-formation (maximal increase: 14+/-3% above basal, n=6). This increase was significantly enhanced by pretreatment with pertussis toxin (PTX, 250 ng ml(-1) for 20 h): maximal increase was 31+/-5%, pEC(50)-value was 5.08+/-0.33 (n=6). 4 In PTX-pretreated cardiomyocytes 100 micromol l(-1) carbachol-induced IP-formation was inhibited by atropine (pK(i)-value: 8.89+/-0.10) and by the M(3)-receptor antagonist darifenacin (pK(i)-value: 8.67+/-0.23) but was not significantly affected by the M(1)-receptor antagonist pirenzepine (1 micromol l(-1)) or the M(2)-receptor antagonists AF-DX 116 and himbacine (1 micromol l(-1)). 5 In conclusion, in adult rat cardiomyocytes there exists an additional, non-M(2)-receptor, that is coupled to activation of the phospholipase C/IP(3)-pathway; this receptor is very likely of the M(3)-subtype.