Ouabain-Uninhibited Sodium Transport in Human Erythrocytes. EVIDENCE AGAINST A SECOND PUMP

Michael J. Dunn

doi:10.1172/JCI107227

Published March 1, 1973 - More info

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Abstract

Others have concluded that a second Na “pump” (active Na outflux) exists in human erythrocytes. This second pump was said to be ouabain-insensitive, unlike the classic ouabain-sensitive Na-K pump. An alternative explanation is that “pump II” is Na exchange diffusion. These hypotheses were examined in the present experiments, utilizing ²²Na influx and outflux measurements, net Na fluxes, and ATPase determinations. Ouabain-uninhibited Na outflux was reduced 0.58±0.05 mmol/liter cells per h when extracellular Na (Na_o) was replaced by Mg. Ethacrynic acid or furosemide produced similar decrements of outflux (0.50 mmol) in the presence of ouabain and Na_o. However, these diuretics had minimal inhibitory effects on outflux in the absence of Na_o suggesting that they inhibited principally the Na_o-dependent outflux. Whereas this ouabain-uninhibited portion of outflux was dependent on Na_o, it was independent of K_o. Contrary to expectations, Na influx did not change when intracellular Na was altered. No uphill, net Na transport (ouabain-uninhibited) could be demonstrated under a variety of circumstances. Furosemide at high concentrations inhibited ATPase, reducing both ouabain-sensitive and ouabain-insensitive enzyme at 1.0 mM concentration while showing no effect on ATPase at 0.05-0.1 mM concentration. The effects of furosemide on ATPase and on Na flux were dissociable on a dose-response curve. Energy depletion for 22 h practically eliminated the Na_o-dependent, diuretic-inhibited Na outflux. Activation energies and temperature coefficients for the diuretic-inhibited outflux were one-half the values for the classic ouabain-inhibited pump. These data are interpreted as evidence against a second Na pump. Exchange diffusion accounts adequately for most of these observations; however, the ouabain-insensitive fluxes may be complex and composed of several processes.

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