In order to investigate the possible effect of membrane potential on cytoplasmic Na+ binding to the Na+-K+ pump, we studied Na+-K+ pump current-voltage relationships in single guinea-pig ventricular myocytes whole-cell voltage clamped with pipette solutions containing various concentrations of Na+ ([Na+]pip) and either tetraethylammonium (TEA+) or N-methyl-D-glucamine (NMDG+) as the main cation. The experiments were conducted at 30 °C under conditions designed to abolish the known voltage dependence of other steps in the pump cycle, i.e. in Na+-free external media containing 20 mM Cs+.Na+-K+ pump current (Ip) was absent in cells dialysed with Na+-free pipette solutions and was almost voltage independent at 50 mM Na+pip (potential range: −100 to +40 mV). By contrast, the activation of Ip by 0.5–5 mM Na+pip was clearly voltage sensitive and increased with depolarization, independently of the main intracellular cation species.The apparent affinity of the Na+-K+ pump for cytoplasmic Na+ increased monotonically with depolarization. The [Na+]pip required for half-maximal Ip activation (K0.5 value) amounted to 5.6 mM at −100 mV and to 2.2 mM at +40 mV.The results suggest that cytoplasmic Na+ binding and/or a subsequent partial reaction in the pump cycle prior to Na+ release is voltage dependent. From the voltage dependence of the K0.5 values the dielectric coefficient for intracellular Na+ binding/translocation was calculated to be ≈0.08. The voltage-dependent mechanism might add to the activation of the cardiac Na+-K+ pump during cardiac excitation
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