SUMMARY1. The effect of membrane potential and various extracellular monovalent cations on the Na+ pump current (Ip) was studied on isolated, single Purkinje cells of the rabbit heart by means of whole-cell recording.2. Ip was identified as current activated by external K+ or its congeners NH4+ and TlV. The current was blocked by dihydroouabain (1-5 x 10-4 M) over the whole range of membrane potentials tested.3. In Na+-containing solution half-maximum Ip activation (Ko.5) occurred at 0 4 mm-Tl+, 1X9 mM-K+ and 5-7 mM-NH4+ (holding potential, -20 mV). 6. The shape of the Ip-V relationship remained unchanged when the K+ concentration (5-4 mM) of the Na+-containing medium was replaced by NH4+ or TlV concentrations of similar potency to activate Ip (20 mM-NH4+ or 2 mM-Tl+).7. In Na+-free, choline-containing solution half-maximum Ip activation occurred at 0-13 mM-K+ (holding potential, -20 mV).

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SUMMARY1. The Na+ pump current (Ip) of isolated, single rabbit cardiac Purkinje cells in Na+-free solution was measured at 32-34 'C by means of whole-cell recording.2. The Ip amplitude was studied as a function of clamp potential (V,)

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