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

Abstract

In voltage clamped neonatal rat heart cells a transient current is observed during depolarizing potential steps, which was identified as slow inward current (I _si) by its range of activation, by its reversal potential of approximately +50 mV and by its sensitivity to D600 or low external Ca²⁺. ThisI _si 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. Thef _∞ curve ofI _si 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 stateI _si. A maximum was expected around −30 mV. Steady stateI _si appears to be present indeed in steady state current voltage relations, as the relative minimum at −30 mV in such relations is abolished by 5·10⁻⁷ g/ml D600. Currents tails during hyperpolarizing steps from prepulse potentials near 0 mV are potential dependent in a way expected whenI _si contributes to these current tails by a decrease in inactivation. Moreover, the current tails are diminished by D600 or Co²⁺. Consequenses of steady stateI _si are discussed.