Stimulation by uraemic plasma of frusemide-sensitive sodium efflux in erythrocytes from normal women

1. Reduced Na⁺, K⁺ pump activity has been reported in uraemic erythrocytes, and this has been attributed to the presence of a circulating ouabain-like inhibitor. Since we have previously reported marked suppression of Na⁺, K⁺ co-transport activity in erythrocytes from dialysis patients, we have now examined the effect of incubation with uraemic plasma on Na⁺ efflux through the Na⁺, K⁺ pump and the co-transport pathway in normal erythrocytes from both men and women.

2. The results show that Na⁺ efflux via the Na⁺, K⁺ pump is not different between men and women (3.14 ± 0.14 vs 3.68 ± 0.18 mmol h⁻¹ litre⁻¹ of cells) and is not inhibited by incubation with uraemic plasma.

3. In contrast, co-transport-mediated Na⁺ efflux is lower in erythrocytes from normal women when compared with normal men (0.17 ± 0.02 vs 0.25 ± 0.05 mmol h⁻¹ litre⁻¹ of cells, P < 0.001).

4. Moreover, incubation with uraemic plasma causes stimulation of Na⁺ efflux through the co-transport pathway in erythrocytes of healthy women (0.17 ± 0.02 vs 0.27 ± 0.03 mmol h⁻¹ litre⁻¹ of cells, P < 0.005), but not in erythrocytes of healthy men (0.27 ± 0.03 vs 0.25 ± 0.05 mmol h⁻¹ litre⁻¹ of cells).

5. Our data do not support the presence of either a Na⁺, K⁺ pump or a co-transport inhibitor in plasma from dialysed uraemic patients. Thus, the suppressed co-transport activity demonstrated in uraemic erythrocytes cannot be attributed to a circulating inhibitor, and may reflect an acquired membrane or transport defect.

6. The stimulation of co-transport by uraemic plasma from women provides a further example of sex-determined characteristics in human erythrocytes, the mechanism of which remains to be elucidated.