LATE NA CHANNELS IN CARDIAC-CELLS - THE PHYSIOLOGICAL-ROLE OF BACKGROUND NA CHANNELS

Two types of the late Na channels, burst and background, were studied in Purkinje and ventricular cells. In the whole-cell configuration, st eady-state Na currents were recorded at potentials (-70 to -80 mV) clo se to the normal cell resting potential. The question of the contribut ion of late Na channels to this background Na conductance was investig ated. During depolarization, burst Na channels were active for periods (up to similar to 5 s), which exceeded the action potential duration. However, they eventually closed without reopening, indicating the pre sence of slow and complete inactivation. When, at the moment of burst channel opening, the potential was switched to -80 mV, the channel clo sed quickly without reopening. We conclude that the burst Na channels cannot contribute significantly to the background Na conductance. Back ground Na channels undergo incomplete inactivation. After a step depol arization, their activity decreased in time, approaching a steady-stat e level. Background Na channel openings could be recorded at constant potentials in the range from -120 to 0 mV. After step depolarizations to potentials near -70 mV and more negative, a significant fraction of Na current was carried by the background Na channels. Analysis of the background channel behavior revealed that their gating properties are qualitatively different from those of the early Na channels. We sugge st that background Na channels represent a special type of Na channel that can play an important role in the initiation of cardiac action po tential and in the TTX-sensitive background Na conductance.