Cardiac channel gating charge movements: recovery from inactivation

The nonlinear charge movements which occur during membrane depolarization of cardiac ventricular myocytes (QON) have been previously identified and separated, by kinetic and steady-state criteria, into constituent components arising from the gating of Na channels and Ca channels. In contrast, the nature and time course of the OFF charge movements (QOFF), which follow membrane repolarization have not been as clearly established. In order to address this question cardiac QOFF was studied using small-diameter, 17-day-old embryonic chick ventricular myocytes that can be rapidly and uniformly voltage-clamped. The application of brief (5.4 ms) depolarizing steps were employed to produce Na channel inactivation but little Ca channel inactivation. Following the return of the membrane potential to -100 mV QOFF, measured as the gating current termed IgOFF, displayed two kinetic components. Double exponential fits to IgOFF yielded time constants of a few tenths of a millisecond for the fast component (IgOFFfast) and of 1-2 ms for the slower component (IgOFFslow). The time course and voltage dependence for the slower component suggested that it might be linked to the inactivation, and the recovery from inactivation, of Na channels. In order to identify these kinetic components double-pulse protocols were employed in which the duration of the prepulse and the interval separating the prepulse and test pulse were varied. The time course for the decay of IgOFFslow following a brief inactivating prepulse was similar to the time course for the recovery of the Na channel QON (QNaRecov). Both IgOFFslow and QNaRecov preceded the recovery of the Na channel (ionic) current. The recovery from inactivation of both the Na current and QNa displayed a similar voltage dependence. These experiments have helped to identify the two components of cardiac IgOFF and, therefore, will facilitate the interpretation of further biophysical and pharmacological studies concerning cardiac Na channel and Ca channel gating charge movements.