Demonstration of functional M3‐muscarinic receptors in ventricular cardiomyocytes of adult rats

Muscarinic receptors (M‐receptors) in the mammalian heart are predominantly of the M2‐subtype. The aim of this study was to find out whether there might exist an additional myocardial non‐M2‐receptor. 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. 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%, pEC50‐value was 5.08±0.33 (n=6). In PTX‐pretreated cardiomyocytes 100 μmol l−1 carbachol‐induced IP‐formation was inhibited by atropine (pKi‐value: 8.89±0.10) and by the M3‐receptor antagonist darifenacin (pKi‐value: 8.67±0.23) but was not significantly affected by the M1‐receptor antagonist pirenzepine (1 μmol l−1) or the M2‐receptor antagonists AF‐DX 116 and himbacine (1 μmol l−1). In conclusion, in adult rat cardiomyocytes there exists an additional, non‐M2‐receptor, that is coupled to activation of the phospholipase C/IP3‐pathway; this receptor is very likely of the M3‐subtype. British Journal of Pharmacology (2003) 138, 156–160. doi:10.1038/sj.bjp.0704997