Slow inward current in aggregates of neonatal rat heart cells and its contribution to the steady state current-voltage relationship.

In voltage clamped neonatal rat heart cells a transient current is observed during depolarizing potential steps, which was identified as slow inward current (Isi) by its range of activation, by its reversal potential of approximately +50 mV and by its sensitivity to D600 or low external Ca2+. This Isi activates too fast to be detected by the present methods, which implies that activation is completed within milliseconds. The time constant of inactivation was weakly potential dependent and less than 30 ms between -40 mV and +20 mV. The f infinity curve of Isi had a sigmoidal shape with 90% and 10% values near -50 mV and -10 mV respectively, half maximum was at -25 mV. From double pulse experiments an estimate was obtained of the potential dependence and amplitude of steady state Isi. A maximum was expected around -30 mV. Steady state Isi appears to be present indeed in steady state current voltage relations, as the relative minimum at -30 mV in such relations is abolished by 5 X 10(-7) g/ml D600. Currents tails during hyperpolarizing steps from prepulse potentials near 0 mV are potential dependent in a way expected when Isi contributes to these current tails by a decrease in inactivation. Moreover, the current tails are diminished by D600 or Co2+. Consequences of steady state Isi are discussed.