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 86 Rb 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 α 1 and α 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 α 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. Copyright © 2006 John Wiley & Sons, Ltd.