Depolarization increases the apparent affinity of the Na+-K+ pump to cytoplasmic Na+ in isolated guinea-pig ventricular myocytes

1. 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 degrees 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+. 2. Na+-K+ pump current (I-p) 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 I-p by 0.5-5 mM Na-pip(+) was clearly voltage sensitive and increased with depolarization, independently of the main intracellular cation species. 3. The apparent affinity of the Na+-K+ pump for cytoplasmic Na+ increased monotonically with depolarization. The [Na+](pip) required for half-maximal I-p activation (K-0.5 value) amounted to 5.6 mar at -100 mV and to 2 2 mM at +40 mV. 4. 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 K-0.5 values the dielectric coefficient for intracellular Na+ binding/translocation was calculated to be similar to 0.08. The voltage-dependent mechanism might add to the activation of the cardiac Na+-K+ pump during cardiac excitation.