Carbachol promotes Na1 entry and augments Na/Ca exchange current in guinea pig ventricular myocytes

Saeki, Tomoaki, Jian-Bing Shen, and Achilles J. Pappano. Carbachol promotes Na1 entry and augments Na/Ca exchange current in guinea pig ventricular myocytes. Am. J. Physiol. 273 (Heart Circ. Physiol. 42): H1984–H1993, 1997.— The effect of carbachol (CCh) on the Na/Ca exchange current (INa/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 245 mV and tail current amplitude on repolarization; CCh had no effect on the L-type Ca21 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(baminoethyl ether)-N,N,N8,N8-tetraacetic acid and are characteristic of INa/Ca. Depolarization from 280 to 230 mV elicited the rapid Na1 current followed by a slowly decaying inward INa/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 INa/Ca also suppressed the CCh effect. Sarcoplasmic reticulum (SR) Ca21 release participated in generating INa/Ca because 10 mM caffeine or 1 μM ryanodine blocked INa/Ca and the effect of CCh. Rapid superfusion of 10 mM caffeine induced inward INa/Ca at 275 mV; a caffeineinduced charge transfer gives an SR Ca21 content of 67 μM. CCh increased caffeine-induced current; SR Ca21 content rose to 98 μM. CCh also augmented the amplitude of steady-state intracellular Ca21 transients and contractions during a train of voltage-clamp pulses (275 to 30 mV for 200 ms) at 1 Hz. CCh elevated intracellular Na1 (M. Korth and V. Kühlkamp. Pflügers Arch. 403: 266–272, 1985) by inducing a background Na1 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 INa/Ca at the sarcolemma through a mechanism that requires the SR and that this action accounts for the increased contractions.