Voltage Dependence of Cardiac Excitation–Contraction Coupling

Abstract

Excitation–contraction coupling in cardiac myocytes occurs by Ca 2+ -induced Ca 2+ release, where L-type Ca 2+ current evokes a larger sarcoplasmic reticulum (SR) Ca 2+ release. The Ca 2+ -induced Ca 2+ release amplification factor or gain (SR Ca 2+ release/ I Ca ) is usually assessed by the V m dependence of current and Ca 2+ transients. Gain rises at negative V m , 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 =NP o × i Ca (where NP o is the number of open channels), and NP o and i Ca both depend on V m . To assess how i Ca and NP o separately influence Ca 2+ -induced Ca 2+ release, we measured I Ca and junctional SR Ca 2+ release in voltage-clamped rat ventricular myocytes using “Ca 2+ spikes” (confocal microscopy). To vary i Ca alone, we changed [Ca 2+ ] o rapidly at constant test V m (0 mV) or abruptly repolarized from +120 mV to different V m (at constant [Ca 2+ ] o ). To vary NP o alone, we altered Ca 2+ channel availability by varying holding V m (at constant test V m ). Reducing either i Ca or NP o 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 NP o and reduced redundant Ca 2+ 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 Ca 2+ 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 V m , reduced i Ca must explain reduced SR Ca 2+ release.