Slow inward current and action potential in cardiac Purkinje fibres. The effect of Mn plus,plus-ions.

Abstract

The slow inward current in short Purkinje fibres was studied in voltage clamp experiments. The results were as follows: 1. The slow inward current displayed as inward tail current on repolarization can be inactivated by conditioning depolarization. Thereby the outward current during the depolarization step was slightly increased. This result indicates that in the net outward current flowing during depolarization slow inward current and dynamic outward current overlap. 2. Mn++ ions caused shortening of the action potential and blocked the slow inward current elicited either by depolarization or repolarization (inward tail current). Thereby the dynamic outward current was often increased (see 1.). An effect of Mn++ ions (7–10 mM) on the outward currents (dynamic and steady-state current) was not observed. 3. The inward tail currents on repolarization were reduced by about one half when the extracellular sodium was replaced by cholinchloride. The residual inward tail current was further reduced on lowering the extracellular calcium concentration and it was increased on increasing the calcium concentration in the bathing solution. In Tyrode's solution (150 mM Na) lowering the calcium concentration strongly reduced the tail current, presumably by reducing the contribution of calcium ions to the tail current and/or by a rise in intracellular sodium, resulting in a decrease of the contribution of the sodium ions to the tail current. The effects of several combinations of reduction in both sodium and calcium ions on the tail current were not additive: the effect of sodium reduction was smaller at lower than at normal extracellular calcium concentrations. 4. The instantaneous conductance of the slow inward current was found constant at potentials negative to −25 mV but could not be determined positive to −25 mV because of large overlapping outward current. 5. The current voltage relation of the slow inward current was estimated. The current seems to be maximum between −20 mV and zero mV. 6. The inactivation of the slow inward current was slower at low and faster at high negative membrane potentials. 7. On clamping the potential during the action potential a rising outward current flowed which was not seen in presence of Mn++ ions indicating that this rising outward current is in fact due to decline of the slow inward current.