1. The purpose of this study was to determine whether physiological changes in extracellular free [K+] cause significant changes in the Na+-K+ pump rate and intracellular free [Na+].

2. The Na+-K+ pump rate was measured in human lymphocytes by determining ouabain-sensitive 86Rb+ influx at several concentrations of K+. The Na+-K+ pump rate increased within the physiological range of extracellular free [K+] (K1/2 = 1.5 mmol/l).

3. To test the hypothesis that elevation of extracellular free [K+] reduces intracellular free [Na+] rapidly, which in turn then slows the pump rate during experimental incubations, lymphocyte intracellular free [Na+] was measured using the fluorochrome sodium-binding benzofuran isophthalate. With larger elevations of extracellular free [K+], intracellular free [Na+] dropped more rapidly. Thus previous discrepancies among determinations of K1/2 may be the result of variations in incubation times, which can skew the pump rates measured during incubations in various extracellular free [K+] values. Steady-state intracellular free [Na+] varied inversely with extracellular free [K+].

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