Cow ventricular muscle. I. The effect of the extracellular potassium concentration on the current-voltage relationship. II. Evidence for a time-dependent outward current.

1. The effect of [K]0 on the current-voltage relationship is described. In the negative potential range the curves cross over as [K]0 is increased. At positive potentials the curves re-cross so that in low [K]0 there is more outward current than in high [K]0. 2. Chord conductance has been calculated from the current-voltage relationship and this is taken as a measure of gK. It is shown gK is a function of both the potassium driving force (EM--EK) and [K]0. 3. Current-voltage relationships obtained by the voltage clamp technique have been compared to net current-voltage relationships obtained by phase plane analysis of the action potential. [K]0 is shown to have similar effects on both. 4. The effect of [K]0 at positive potentials suggests that delayed outward current during large depolarizing voltage clamp steps is due to an activation of a time-dependent outward current and not to potassium accumulation. An analysis of current tails also suggests the presence of a time-dependent outward current. 5. In contrast delayed outward current changes during small depolarizing voltage clamp steps are probably due to potassium accumulation. 6. Evidence is presented which indicates that inward current tails following depolarizing voltage clamp steps are due to potassium accumulation.