Forward-Mode Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> Exchange during Depolarization in the Rat Ventricular Myocytes with High EGTA

Forward-Mode Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> Exchange during Depolarization in the Rat Ventricular Myocytes with High EGTA

<P> During depolarization, extrusion of Ca<SUP>2&#8290;</SUP> from sarcoplasmic reticulum through forward-mode Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> exchange was studied in the rat ventricular myocytes patch-clamped in whole-cell configuration. In order to confine the Ca<SUP>2&#8290;</SUP> responses in a micro-domain by limiting the Ca<SUP>2&#8290;</SUP> diffusion time, rat ventricular myocytes were dialyzed with high (14 mM) EGTA. K<SUP>&#8290; </SUP>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<SUP>&#8291;</SUP> 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&#8273;1% of which was suppressed by 0.1 mM CdCl<SUB>2</SUB>. 10 mM caffeine increased the remaining inward current after CdCl<SUB>2</SUB> in a cAMP-dependent manner. This caffeine-induced inward current was blocked by 1μM ryanodine, 10μM thapsigargin, 5 mM NiCl<SUB>2</SUB>, or by Na<SUP>&#8290;</SUP> and Ca<SUP>2&#8290;</SUP> omission, but not by 0.1μM isoproterenol. The I∼V relationship of the caffeine-induced current elicited inward current from &#8291;45 mV to &#8290;3 mV with the peak at &#8291;25 mV. Taken together, it is concluded that, during activation of the rat ventricular myocyte, forward-mode Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> exchange extrudes a fraction of Ca<SUP>2&#8290;</SUP> released from sarcoplasmic reticulum mainly by voltage-sensitive release mechanism in a micro-domain in the t-tubule, which is functionally separable from global Ca<SUP>2&#8290;</SUP><SUB>i</SUB> by EGTA.