Carbachol promotes Na+ entry and augments Na/Ca exchange current in guinea pig ventricular myocytes

Abstract

The effect of carbachol (CCh) on the Na/Ca exchange current ( I Na/Ca) was studied in voltage-clamped ventricular myocytes isolated from guinea pig hearts and superfused with Tyrode solution at 35°C. CCh (100 μM) increased outward current during depolarizations (10–200 ms) from −45 mV and tail current amplitude on repolarization; CCh had no effect on the L-type Ca2+current. Amplitudes of the outward and tail currents declined with increasing duration of the depolarizing clamp pulse. Ouabain produced similar current changes that are suppressed by intrapipette ethylene glycol-bis(β-aminoethyl ether)- N, N, N′, N′-tetraacetic acid and are characteristic of I Na/Ca. Depolarization from −80 to −30 mV elicited the rapid Na+ current followed by a slowly decaying inward I Na/Ca (J. C. Gilbert, T. Shirayama, and A. J. Pappano. Circ. Res. 69: 1632–1639, 1991.) that was reversibly increased by CCh. Atropine (1–3 μM) prevented the CCh effect. All procedures that suppressed I Na/Caalso suppressed the CCh effect. Sarcoplasmic reticulum (SR) Ca2+ release participated in generating I Na/Cabecause 10 mM caffeine or 1 μM ryanodine blocked I Na/Ca and the effect of CCh. Rapid superfusion of 10 mM caffeine induced inward I Na/Ca at −75 mV; a caffeine-induced charge transfer gives an SR Ca2+ content of 67 μM. CCh increased caffeine-induced current; SR Ca2+ content rose to 98 μM. CCh also augmented the amplitude of steady-state intracellular Ca2+ transients and contractions during a train of voltage-clamp pulses (−75 to 30 mV for 200 ms) at 1 Hz. CCh elevated intracellular Na+ (M. Korth and V. Kühlkamp. Pflügers Arch.403: 266–272, 1985) by inducing a background Na+ current [K. Matsumoto and A. J. Pappano. J. Physiol. (Lond.)415: 487–502, 1989]. Together with these data, the present results are consistent with the hypothesis that CCh, via muscarinic receptors, eventually promotes I Na/Ca at the sarcolemma through a mechanism that requires the SR and that this action accounts for the increased contractions.