Background inward current in ventricular and atrial cells of the guinea-pig

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Abstract
Atrial and ventricular myocytes were exposed to Ca$^{2+}$- and K$^{+}$-free solutions containing blockers of gated channel and exchange currents. Replacement of external sodium by large organic cations revealed a background sodium current i$_{\text{b,Na}}$. In atrial cells, the average conductance was 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. Replacement of sodium with inorganic cations showed that the channel selectivity behaves like an Eisenman group III/IV sequence, in agreement with previous results. The permeability of the channel to TMA was found to be pH dependent, suggesting that protonation of the channel is a factor determining permeation in addition to ionic size. The values of g$_{\text{b,Na}}$ obtained from these experiments are very similar to those assumed in computer modelling of cardiac cell electrical activity.