Carbachol promotes Na+ entry and augments Na/Ca exchange current in guinea pig ventricular myocytes.
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 degrees C. CCh (100 microM) 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(beta-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 microM) prevented the CCh effect. All procedures that suppressed I(Na/Ca) also suppressed the CCh effect. Sarcoplasmic reticulum (SR) Ca2+ release participated in generating I(Na/Ca) because 10 mM caffeine or 1 microM 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 microM. CCh increased caffeine-induced current; SR Ca2+ content rose to 98 microM. 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.
Saeki
T
T
Department of Pharmacology, University of Connecticut Health Center, Farmington 06030, USA.
Shen
J B
JB
Pappano
A J
AJ
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D005260:Female; D006168:Guinea Pigs; D006352:Heart Ventricles; D008297:Male; D018721:Muscarinic Agonists / Q000494:pharmacology*; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D011930:Reaction Time; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000502:physiology*
D018721:Muscarinic Agonists; D018727:Muscarinic Antagonists; D019831:Sodium-Calcium Exchanger; D002110:Caffeine; D001285:Atropine; D002217:Carbachol; D012964:Sodium; D002118:Calcium