Properties of the pace-maker current (if) in Purkinje fibres were studied in the presence of Ba, which by partially blocking the iK1 channel reduces K depletion during hyperpolarizing voltage-clamp pulses, and eliminates the main cause of distortion in the current time course. 2. On raising the external potassium concentration (Kb), the if fully activated current--voltage relation (if(E)) increases in the inward direction. In the range 3--36 mM--Kb and negative to -50 mV the current is inward, and no cross-over is observed. 3. In normal conditions, the reversal potential (Ef) for if lies in the voltage region positive to -50 mV, and can be observed on lowering the external sodium concentration (Nab). Ef shifts to the negative direction when Nab is decreased. Slopes ranging between 29 and 35 mV/decade are found for Nernst plots of Ef against Nab. Changing Nab in the range 140--4.4 mM causes the if(E) relation to undergo a simple shift along the voltage axis, without significant change in its slope. 4. Ef also depends on Kb, as can be observed in low Nab (35 mM), and shifts to the positive direction by about 26 mV for every 10-fold change in Kb. The fully activated slope conductance increases when Kb is increased. 5. It is concluded that Na and K both participate in carrying if. The slope of the fully activated if(E) relation increases with Kb, but is unchanged in different Nab, indicating that the channel conductance depends on Kb, but not appreciably on Nab.
