Carbachol exhibited positive inotropic effect on rat ventricular myocytes via M₂ muscarinic receptors.

The present study was aimed to investigate the positive inotropic mechanism of carbachol (CCh) on rat ventricular myocytes. The effects of CCh on L-type calcium current (ICa,L) and Na+/Ca2+ exchange current (INa/Ca) were investigated in isolated rat ventricular myocytes. After loading myocytes with Fura-2/AM, electrically triggered Ca2+ transient and cell shortening in single myocyte were measured simultaneously using ion imaging system with charge-coupled device (CCD) camera. CCh (100 μmol/L) increased INa/Ca in forward mode from (1.18±0.57) pA/pF in the conrol group to (1.65±0.52) pA/pF (P0.01) and that in reverse mode from (1.11±0.49) pA/pF in the control group to (1.53±0.52) pA/pF (P0.01), respectively. CCh had no effect on ICa,L. The stimulatory effect of CCh on INa/Ca was blocked by application of atropine, a non-selective M muscarinic receptor antagonist, and methoctramine, a selective M_2 muscarinic receptor antagonist. CCh (100 μmol/L) increased cell shortening from (3.00±0.67) μm in the control group to (3.55±1.21) μm. Ca2+ transient was also increased from 203.8±50.0 in the control group to 234.8±64.3 in 100 μmol/L CCh group. KB- R7943, a selective inhibitor of reverse mode Na+/Ca2+ exchange, did not change the baseline level of cell shortening and Ca2+ transient, while completely abolished CCh-induced increments of both Ca2+ transient and cell shortening. CCh increased cell shortening and Ca2+ transient in the presence of nicardipine, indicating that the positive inotropic effect of CCh was through activation of Na+/Ca2+ exchange. Calcium sensitivity was not changed by CCh. Both atropine and methoctramine abolished the positive inotropic effects of CCh, demonstrating that CCh induced positive inotropism via the M_2 muscarinic receptor. The results suggest that CCh increases cell contraction and Ca2+ transient in rat ventricular myocytes. This positive inotropic effect of CCh is through activation of reverse mode Na+/Ca2+ exchange, and M_2 receptors are involved in mediating CCh-induced contraction.