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1. When the ouabain-sensitive K influx or the ouabain-sensitive Cs influx is measured as a function of the extracellular concentration of K or Cs in Na-free solutions the resulting saturation curve at first rises more rapidly than a rectangular hyperbola, i.e. the curve is antisigmoid. 2. If the ouabain-sensitive K influx or the ouabain-sensitive Cs influx is measured in Na-free solutions at a fixed low concentration of K or Cs and at varying concentrations of Li, the influx decreases monotonically as the Li concentration rises and there is no evidence of competitive activation. 3. These findings can be accounted for by a model which proposes that there are two binding sites for K or Cs and that both the singly loaded and doubly loaded pump is capable of transport. 4. Extracellular Na changes the shape of both the K and the Cs saturation curve from antisigmoid to sigmoid. Dixon plots (1/ouabain-sensitive influx versus Na concentration at fixed K or Cs concentration) are linear at intermediate concentrations of K or Cs. 5. Na does not change the rate of K influx if the measurements are made at nearly saturating K concentrations using cells with nearly saturating internal Na concentrations. The effect of outside Na cannot therefore be explained by any mechanism which requires that Na alter the Vmax of the pump. 6. Measurement of the ouabain-sensitive Cs influx as a function of the external Cs concentration in solutions with different fixed Na concentrations results in curves which change from antisigmoid in Na-free solutions to sigmoid as the Na concentration rises. Dixon plots are linear at all but the lowest and highest Cs concentrations. 7. The resulting curves are best fit by equations which result from a model which proposes that Na acts both as a dead-end competitive inhibitor and as a heterotropic allosteric effector. Simpler models which propose either that Na acts solely as a dead-end competitive inhibitor or as a heterotropic allosteric effector do not fit as well as the more complicated model. 8. The combined competitive inhibition and allosteric effector model also describes adequately the relation between the ouabain-sensitive K influx and external K concentration measured at different external Na concentrations.

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