Effects of the replacement of chloride by methylsulphate on the membrane currents in frog atrial trabeculae

The effect of the replacement of extracellular chloride by methylsulphate was investigated on isolated frog atrial trabeculae using the voltage clamp technique. In methylsulphate solution, the following results were obtained: 1. After a transient depolarization of the resting potential, the action potential was lengthened and the diastolic depolarization rate of the repetitive activity was reduced. 2. In voltage clamp conditions, the current-voltage relationships and the steady-state inactivation curves of both early and secondary inward currents were unaffected. 3. The study of the evolution of the instantaneous backgroundd current with membrane potential showed a decrease of the current values. 4. The first component of the time-dependent outward currents was unaffected but the second component was reduced without change of its time-constant value. Assuming the absolute value of the chloride equilibrium potential at about -30 mV, it was suggested that the lengthening of the action potential in methylsulphate solution might be explained by the decrease of an influx of chloride ions which should participate in the repolarization. The reduction of the diastolic depolarization rate might be due to the decrease of an outflux of chloride ions which should be involved in the inward background currents unmasked by the deactivation of the time-dependent outward currents.