The Na+ pump current (Ip) of isolated, single rabbit cardiac Purkinje cells in Na(+)-free solution was measured at 32-34 degrees C by means of whole-cell recording. 2. The Ip amplitude was studied as a function of clamp potential (Vc) and external concentration of various monovalent cations known to activate the Na(+)-K+ pump. 3. Under conditions which strongly activated Ip the Ip-Vc curve of the cells displayed a positive slope at membrane potentials negative to -20 mV and little variation at more positive potentials. 4. The Ip-Vc relationship showed an extended region of negative slope at positive and negative potentials in solutions containing low concentrations of activator cations which caused little Ip activation. A positive slope of the Ip-Vc curve was occasionally observed at clamp potentials negative to -60 mV under these conditions. 5. The shape of the Ip-Vc relation was independent of the cation species used as external Ip activator. 6. At zero membrane potential half-maximum Ip activation (K0.5(Vc = 0 mV) occurred at 0.05 mM Tl+, 0.08 mM K+, 0.4 mM NH4+ and 1.5 mM Cs+. The Hill coefficient derived amounted to 0.9 for Tl+, 1.2 for K+, 1.04 for NH4+ and 1.5 for Cs+. 7. The concentrations of external activator cations required for half-maximum Ip activation increased with depolarization. The voltage dependence of the K0.5 values could be described by a single exponential function for clamp potentials positive to -40 mV. 8. The steepness of the function is determined by a factor alpha, indicating the apparent fraction of an elementary charge which moves in the electrical field across the sarcolemma when external monovalent cations bind to the Na(+)-K+ pump. 9. The alpha values were calculated to be 0.32 for Tl+, 0.24 for K+, 0.29 for NH4+ and 0.18 for Cs+. Possible interpretations of the alpha values are considered. 10. It is suggested that binding of external monovalent activator cations to the Na(+)-K+ pump (or a process related to the binding) is voltage dependent. This potential-dependent process determines mainly the shape of the Ip-Vc curve in cardiac Purkinje cells superfused with Na(+)-free media containing low concentrations (< K0.5(Vc = 0 mV)) of K+ or its congeners.
