Depolarization shifts the voltage dependence of cardiac sodium channel and calcium channel gating charge movements.

In cardiac ventricular myocytes, membrane depolarization leads to the inactivation of the Na channel and Ca channel ionic currents. The inactivation of the ionic currents has been associated with a reduction of the gating charge movement ("immobilization") which governs the activation of Na channels and Ca channels. The nature of the apparent "immobilization" of the charge movement following depolarization was explored in embryonic chick ventricular myocytes using voltage protocols applied from depolarized holding potentials. It was found that although all of the charge was mobile following inactivation, the voltage dependence of its movement was shifted to more negative potentials. In addition, the shift in the distribution of the Na channel charge could be differentiated from that of the Ca channel charge on the basis of kinetic as well as steady-state criteria. These results suggest that the voltage-dependent activation of Na channel and Ca channel charge movements leads to conformational changes and charge rearrangements that differentially bias the movements of these voltage sensors, and concomitantly produce channel inactivation.