The positive dynamic current and its inactivation properties in cardiac Purkinje fibres.

1. The positive dynamic current in sheep cardiac Purkinje fibres was studied using ;voltage clamp' technique. The major portion of this current was carried by Cl(-) ions.2. A pre-step in membrane potential changed the response of this current to a subsequent depolarization in a way that is similar to the Na inactivation process described by Hodgkin & Huxley (1952a, b).3. Recovery of this current after its activation could be described reasonably well by a single exponential process, and the time course of the recovery varied with membrane voltage.4. The relation of steady-state inactivation of the current to membrane voltage showed a sigmoid shape, varying from 100% at membrane potential of -30 mV to 0% at -80 mV. The membrane potential at 50% inactivation was -53 mV.5. The Cl(-) current may play a role in the shape of the Purkinje fibre action potential, but exact definition depends on determining the voltage and time dependence of all the ionic currents that flow during the plateau.