Voltage dependence of cardiac excitation-contraction coupling -: Unitary Ca2+ current amplitude and open channel probability

Excitation-contraction coupling in cardiac myocytes occurs by Ca2+ -induced Ca2+ release, where L-type Ca2+ current evokes a larger sarcoplasmic reticulum (SR) Ca2+ release. The Ca2+ -induced Ca2+ release amplification factor or gain (SR Ca2+ release/I-Ca) is usually assessed by the V-m dependence of current and Ca2+ transients. Gain rises at negative Vm, as does single channel I-Ca (i(Ca)), which has led to the suggestion that the increases of i(Ca) amplitude enhances gain at more negative V-m. However, I-Ca = NPo x i(Ca) (where NPo is the number of open channels), and NPo and i(Ca) both depend on V-m. To assess how i(Ca) and NPo separately influence Ca2+ -induced Ca2+ release, we measured I-Ca and junctional SR Ca2+ release in voltage-clamped rat ventricular myocytes using Ca2+ spikes (confocal microscopy). To vary i(Ca) alone, we changed [Ca2+](o) rapidly at constant test V-m (0 mV) or abruptly repolarized from +120 mV to different V-m (at constant [Ca2+](o)). To vary NPo alone, we altered Ca2+ channel availability by varying holding V-m (at constant test V-m). Reducing either i(Ca) or NPo alone increased excitation-contraction coupling gain. Thus, increasing i(Ca) does not increase gain at progressively negative test V-m. Such enhanced gain depends on lower NPo and reduced redundant Ca2+ channel openings (per junction) and a consequently smaller denominator in the gain equation. Furthermore, modest i(Ca) (at V-m = 0 mV) may still effectively trigger SR Ca2+ release, whereas at positive V-m (and smaller i(Ca)), high and well-synchronized channel openings are required for efficient excitation - contraction coupling. At very positive Vm, reduced iCa must explain reduced SR Ca2+ release.