BACKGROUND INWARD CURRENT IN VENTRICULAR AND ATRIAL CELLS OF THE GUINEA-PIG

Atrial and ventricular myocytes were exposed to Ca2+- and K+-free solu tions containing blockers of gated channel and exchange currents. Repl acement of external sodium by large organic cations revealed a backgro und sodium current i(b,Na). In atrial cells, the average conductance w as 5.0 pS pF-1. In ventricular cells the conductance was 2.3 pS pF-1. Together with previous results, these figures reveal a strong gradient of background current density: sinus > atrium > ventricle. Replacemen t of sodium with inorganic cations showed that the channel selectivity behaves like an Eisenman group III/IV sequence, in agreement with pre vious results. The permeability of the channel to TMA was found to be pH dependent, suggesting that protonation of the channel is a factor d etermining permeation in addition to ionic size. The values of g(b,Na) obtained from these experiments are very similar to those assumed in computer modelling of cardiac cell electrical activity.