Low efficiency of Ca2+ entry through the Na(+)-Ca2+ exchanger as trigger for Ca2+ release from the sarcoplasmic reticulum. A comparison between L-type Ca2+ current and reverse-mode Na(+)-Ca2+ exchange.

It has been proposed that Ca2+ entry through the Na(+)-Ca2+ exchanger can contribute significantly to the trigger for Ca2+ release from the sarcoplasmic reticulum (SR). We have compared the characteristics of Ca2+ release triggered by reverse-mode Na(+)-Ca2+ exchange and by L-type Ca2+ current (ICaL) during depolarizing steps in single guinea pig ventricular myocytes (whole-cell voltage clamp, fluo 3 and fura-red as [Ca2+]i indicators, 36 +/- 1 degrees C, K(+)-based pipette solution with 20 mmol/L [Na+]). Conditioning pulses to +60 mV ensured comparable Ca2+ loading of the SR. In the presence of ICaL, [Ca2+]i transients typically have an early and rapid rising phase reflecting Ca2+ release, which has a bell-shaped voltage dependence with a peak at +10 mV. With Ca2+ entry through Na(+)-Ca2+ exchange only (20 mumol/L nisoldipine), Ca2+ release flux from the SR is decreased and directly related to the amplitude of the depolarizing step. Ca2+ release is preceded by a significant delay (81 +/- 21 ms at +20 mV, 24 +/- 4 ms at +70 mV) related to Ca2+ entry through the exchanger. Triggered release interrupts Ca2+ entry, as evidenced by reversal of the exchanger current. At potentials positive to +40 mV, Ca2+ influx through Na(+)-Ca2+ exchange, calculated from the outward exchange current, reaches magnitudes comparable to ICaL, but Ca2+ release due to reverse-mode Na(+)-Ca2+ exchange still has a significant delay. We calculated trigger efficiency as the ratio between the maximal rate of Ca2+ release and the Ca2+ influx preceding this release; efficiency of reverse-mode Na(+)-Ca2+ exchange is approximately four times less than that of ICaL. With both ICaL and reverse-mode Na(+)-Ca2+ exchange present, Ca2+ release is triggered by ICaL, and a contribution of reverse-mode Na(+)-Ca2+ exchange to the trigger could not be detected at potentials below +60 mV. These characteristics of reverse-mode Na(+)-Ca2+ exchange predict that its role as a trigger for Ca2+ release during the action potential is likely to be negligible.