Sodium fluxes in rat red blood cells in potassium-free solutions

Summary

Unidirectional as well as net sodium fluxes were studied in rat red blood cells incubated in potassium-free sodium and sodium-substituted solutions. In the absence of ouabain the magnitude of sodium efflux in different solutions followed the sequence Na>choline>tris>Mg; in the presence of 10⁻⁴ M ouabain the sequence was choline>tris>Na>Mg. In a sodium-magnesium mixture the ouabain-sensitive sodium influx as a function of the external sodium concentration followed more or less an S-shaped curve; at high external sodium there was good agreement with the efflux values, but below 90mM-Na all efflux points were above the influx ones. Both ouabain-insensitive Na influx and efflux were stimulated by external Na following a linear relationship though with different slopes. In net flux experiments these cells were able to extrude sodium against an electrochemical gradient in K-free ouabain Na−Mg and Na-choline mixture solutions. In K-free-Na-free magnesium media the ouabain-sensitive sodium loss increased proportionally to the square of the internal sodium, whereas the ouabaininsensitive loss went to saturation. In K-free sodium solutions the net Na gain was reduced as internal Na increased and was unaffected by ouabain. These results, plus the changes in the sodium influx/net Na gain ratio and in the rate constant for Na efflux when internal Na was modified, are consistent with the existence of a facilitated diffusion system for sodium movements which contributes, together with leakage, to the net Na gain in K-free sodium solutions.