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

Abstract

 In this study, the effects of three different voltage protocols on the Na⁺-Ca²⁺ exchange current (I _Na-Ca) of rabbit right ventricular myocytes were studied. Whole-cell patch-clamp recordings were made using a Cs⁺-based internal dialysis solution and external solutions designed to block major interfering currents. I _Na-Ca was measured at 35–37°C as (5 mM) Ni-sensitive current elicited by: a 2 s descending ramp (DR: +80 to –120 mV); a 2 s ascending ramp (AR: –120 to +80 mV) and 500 ms voltage steps (VS) between –120 and +80 mV. DR and AR were applied from –40 mV and elicited I _Na-Ca with reversal 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=10; 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 waveforms suggested a possible mechanism for these differences. Increases in subsarcolemmal Ca caused by Ca entry (coupled to Na extrusion) during the initial positive potential phase of the DR might have induced I _Na-Ca reversal at less negative potentials than observed with AR, during the initial phase of which subsarcolemmal Ca would not have accumulated. These data suggest that I _Na-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 Ca buffering in the bulk cytosol and can occur on a pulse-to-pulse basis.