Forward-mode Na+ - Ca2+ exchange during depolarization in the rat ventricular myocytes with high EGTA

ABSTRACT

During depolarization, extrusion of Ca2+ from sarcoplasmic reticulum through forward-mode Na+ - Ca2+ exchange was studied in the rat ventricular myocytes patch-clamped in whole-cell configuration. In order to confine the Ca2+ responses in a micro-domain by limiting the Ca2+ diffusion time, rat ventricular myocytes were dialyzed with high (14 mM) EGTA. K+ current was suppressed by substituting KCl with 105 mM CsCl and 20 mM TEA in the pipette filling solution and by omitting KCl in the external Tyrode solution. Cl- current was suppressed by adding 0.1 mM DIDS in the external Tyrode solution. During stimulation roughly mimicking action potential, the initial outward current was converted into inward current, 47+/-1% of which was suppressed by 0.1 mM CdCl2. 10 mM caffeine increased the remaining inward current after CdCl2 in a cAMP-dependent manner. This caffeine-induced inward current was blocked by 1 muM ryanodine, 10 muM thapsigargin, 5 mM NiCl2, or by Na+ and Ca2+ omission, but not by 0.1 muM isoproterenol. The IapprxV relationship of the caffeine-induced current elicited inward current from -45 mV to +3 mV with the peak at -25 mV. Taken together, it is concluded that, during activation of the rat ventricular myocyte, forward-mode Na+ - Ca2+ exchange extrudes a fraction of Ca2+ released from sarcoplasmic reticulum mainly by voltage-sensitive release mechanism in a micro-domain in the t-tubule, which is functionally separable from global Cai2+ by EGTA.