Muscarinic Receptor Heterogeneity in Neonatal Rat Ventricular Myocytes in Culture

Carbachol increased ventricular automaticity in a concentration-dependent fashion from a control rate of 72 +/- 5 (mean +/- SEM) to 86 +/- 4 beats per minute at 10(-4) M carbachol. Pirenzepine, an M1-selective antagonist, and AFDX 116, an M2-selective antagonist, both at 10(-7) M, did not block the carbachol-induced positive chronotropic response. In contrast, 10(-7) M HHSiD, an M3-selective antagonist, completely blocked the positive chronotropic effect of carbachol. Carbachol stimulated the accumulation of IP1 in a concentration-dependent manner at concentrations > or = 3 x 10(-6) M. AFDX 116 had no effect on carbachol-induced IP1 accumulation. HHSiD significantly inhibited IP1 accumulation at concentrations > or = 3 x 10(-8) M, while pirenzepine inhibited IP1 accumulation only at concentrations > or = 10(-5) M. McN A343 and methacholine, two muscarinic receptor agonists with minimal M2 activities, and carbachol did not alter basal cAMP concentration, but all three agonists significantly attenuated the increase in cAMP accumulation in response to isoproterenol. Carbachol inhibited isoproterenol-mediated cAMP accumulation at concentrations > or = 10(-7) M. AFDX 116, HHSiD, and pirenzepine blocked the carbachol-induced inhibition of isoproterenol-stimulated cAMP accumulation. At equimolar concentrations, the inhibitory effects of HHSiD and AFDX-116 were similar, while that of pirenzepine was much less. Pretreatment with pertussis toxin for 24 h did not prevent the carbachol-mediated positive chronotropic response or accumulation of IP1 but completely abolished the inhibition of isoproterenol-stimulated cAMP accumulation. These results indicate that (a) neonatal ventricular myocytes in culture have a heterogeneous population of muscarinic (M2 and M3) receptors, (b) the M3 receptor is coupled to pertussis toxin-sensitive and pertussis toxin-insensitive G proteins, (c) M3 receptor stimulation activates phosphoinositide hydrolysis and increases automaticity via a pertussis toxin-insensitive G protein-dependent pathway, and (d) both M2 and M3 receptors couple to pertussis toxin-sensitive G protein(s) to mediate the inhibition of intracellular cAMP accumulation in response to isoproterenol stimulation.