Demonstration of functional M 3-muscarinic receptors in ventricular cardiomyocytes of adult rats

Abstract

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 [ 3H]-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 μmol 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 μmol l -1) or the M 2-receptor antagonists AF-DX 116 and himbacine (1 μmol 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.