Single channel and whole cell sodium currents in heart cells

We studied TTX-sensitive Na channels in dissociated single ventricular cells from neonatal rats using the patch clamp method for single channel and whole cell recording. In both cases, slowly inactivating or window currents were observed that decayed in a biexponential fashion. Customary models of Na current kinetics such as the Hodgkin-Huxley model attribute activation to a process that is much faster than inactivation. The model of Aldrich, Corey and Stevens(1), says that inactivation is fast and final and activation is dispersed. We found activation too associated, deactivation too quick, and reopenings too frequent to accept this model for cardiac Na channels. We also found that the predominant set of single Na channels had one open state and two inactivated states. Rarely, a second set of Na channels having about 2/3 the conductance and mean open time of the predominant set was found.