Erythrocyte sodium pump stimulation by ouabain and an endogenous ouabain-like factor

Cardiac glycosides inhibit the sodium pump. However, some studies suggest that nanomolar ouabain concentrations can stimulate the activity of the sodium pump. In this study, using the Na+/K+-ATPase of human erythrocytes, we compared the effect of digoxin, ouabain and an ouabain like-factor (OLF), on 86Rb uptake. Ouabain concentrations below 10-9 M significantly stimulate Rb+ uptake, and the maximal increase above base-line values is 18 ± 5% at 10-10 M ouabain. No stimulation is observed in the same conditions by digoxin. OLF behaved like ouabain, producing an activation of Rb+ flux at concentrations lower than 10-9 M ouabain equivalents (14 ± 3% at 10-10 M). Western blot analysis revealed the presence of both 1 and 3 pump isoforms in human erythrocytes. Our data confirm the analogies between OLF and ouabain and suggest that Na+/K+-ATPase activation may be related to the 3 isoform. In addition, we investigated whether ouabain at different concentrations was effective in altering the intracellular calcium concentration of erythrocytes. We found that ouabain at concentration lower than 10-9 M did not affect this homeostasis.