The effect of subthreshold potentials on the membrane current in cardiac Purkinje fibres.

1. In sodium-containing solutions, up to 60% of the slow outward current in Purkinje fibres can be activated by depolarizations which are too small to activate the fast sodium conductance responsible for the depolarization phase of the action potential. This result reinforces the view that the slow outward current results from slow activation of the potassium conductance rather than from slow inactivation of the sodium conductance.2. These subthreshold changes in membrane conductance are probably responsible for the dependence of the action potential and pace-maker potential durations on small changes in membrane potential in the subthreshold region.