NA-VOLTAGE RELATIONSHIPS OF RABBIT CARDIAC PURKINJE-CELLS IN NA+-FREESOLUTION( PUMP CURRENT)

1. The Na+ pump current (I(p)) of isolated, single rabbit cardiac Purk inje cells in Ne-free solution was measured at 32-34-degrees-C by mean s of whole-cell recording. 2. The I(p) amplitude was studied as a func tion of clamp potential (V(c)) and external concentration of various m onovalent cations known to activate the Na+-K+ pump. 3. Under conditio ns which strongly activated I(p) the I(p)-V(c) curve of the cells disp layed a positive slope at membrane potentials negative to -20 mV and l ittle variation at more positive potentials. 4. The I(p)-V(c) relation ship showed an extended region of negative slope at positive and negat ive potentials in solutions containing low concentrations of activator cations which caused little I(p) activation. A positive slope of the I(p)-V(c) curve was occasionally observed at clamp potentials negative to - 60 mV under these conditions. 5. The shape of the I(p)-V(c) rela tion was independent of the cation species used as external I(p) activ ator. 6. At zero membrane potential half-maximum I(p) activation (K0.5 (Vc=0mv)) 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 acti vator cations required for half-maximum I(p) activation increased with depolarization. The voltage dependence of the K0.5 values could be de scribed by a single exponential function for clamp potentials positive to - 40 mV. 8. The steepness of the function is determined by a facto r alpha, indicating the apparent fraction of an elementary charge whic h moves in the electrical field across the sarcolemma when external mo novalent cations bind to the Na+-K+ pump. 9. The a values were calcula ted to be 0.32 for Tl+, 0.24 for K+, 0.29 for NH4+ and 0.18 for Cs+. P ossible 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 dependen t. This potential-dependent process determines mainly the shape of the I(p)-V(c) curve in cardiac Purkinje cells superfused with Na+-free me dia containing low concentrations (< K0.5(Vc-0mv)) of K+ or its congen ers.