Comparison of Na+-Ca2+ exchange current elicited from isolated rabbit ventricular myocytes by voltage ramp and step protocols

In this study, the effects of three different voltage protocols on the Na+- Ca2+ exchange current (INa-Ca) of rabbit right ventricular myocytes were st udied. Whole-cell parch-clamp recordings were made using a Cs+-based intern al dialysis solution and external solutions designed to block major interfe ring currents. INa-Ca was measured at 35-37 degrees C as (5 mM) Ni-sensitiv e current elicited by: a 2 s descending ramp (DR: +80 to -120 mV), a 2 s as cending ramp (AR: -120 to +80 mV) and 500 ms voltage steps (VS) between -12 0 and +80 mV. DR and AR were applied from -40 mV and elicited INa-Ca with r eversal potentials (E-rev) of -17.6+2.5 mV (mean+/-SEM; n=16) and -46.2+/-4 .1 mV (n=10: P=0.0001) respectively This difference was maintained when the holding potential was -80 mV (-44.0+/-2.1 mV, n=24 and -86.3+/-4.8 mV. n=1 0; P=0.0001), when the internal Ca chelator (EGTA) was replaced with BAPTA (-19.5+/-1.8 mV and -46.3+/-1.6 mV, n=6; P=0.0003) and when DR and AR were applied alternately to the same cell. Experiments using modified ramp wavef orms suggested a possible mechanism for these differences. Increases in sub sarcolemmal Ca caused by Ca entry (coupled to Na extrusion) during the init ial positive potential phase of the DR might have induced INa-Ca reversal a t less negative potentials than observed with ARI during the initial phase of which subsarcolemmal Ca would not have accumulated. These data suggest t hat INa-Ca during voltage-clamp experiments can be significantly influenced by the type of voltage protocol chosen, as the protocol appears to induce subsarcolemmal changes in Ca and Na concentration that are independent of C a buffering in the bulk cytosol and can occur on a pulse-to-pulse basis.