The cell membrane of sheep Purkinje fibers hyperpolarizes transiently on returning to K containing media after several minutes in K free solution. To analyse this 'K activated response' voltage clamp experiments and measurements of the internal Na activity (aiNa) are performed in fibres bathed in solution containing 0.2-2 mM BaCl2. Compared to the response in Tyrode solution the transient hyperpolarization beyond the resting potential is increased in Ba containing media. The response is blocked by 10(-4) M dihydroouabain. During the response, in Ba treated fibres, aiNa and a transient outward current decline with the same time constant at all clamp potentials tested. The transient outward current is most probably due to a temporary increase in electrogenic Na pumping. Both the amplitude and the time constant of the pump current show little voltage dependence. The electrogenic fraction of the active Na efflux is estimated to be about 39% and is independent of aiNa. Ba ions facilitate the analysis of the pump current in voltage clamped fibres because K depletion is reduced and changes of the I-V relationship by K depletion are minimized. It is concluded that activation of the electrogenic Na pump is mainly responsible for the K activated response of fibres in Ba containing media.

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