1. Erythrocyte Na+ transport (Na+ pump activity, co-transport, countertransport and passive Na+ efflux) and intracellular Na+ concentration were studied in 10 normal individuals and in 29 uraemic patients on chronic haemodialysis, before and after a haemodialysis session. Eight of them fulfilled the criteria of hypertension.
2. Normotensive patients before haemodialysis were classified in two groups: group 1 (pump –) with decreased erythrocyte Na+ pump activity, and group 2 (normal pump) with normal erythrocyte Na+ pump activity. Group 1 showed, compared with controls, a normal intracellular Na+ concentration and a decreased co-transport, but no difference in either countertransport or passive Na+ efflux. After haemodialysis this difference disappeared. Before haemodialysis, group 2 showed a high intracellular Na+ concentration, but activities of the Na+ transport systems studied were similar to those of controls. After haemodialysis, cell Na+ concentration decreased to a level not significantly different from that of controls.
3. Both before and after haemodialysis, hypertensive patients showed Na+ transport system activities and an intracellular Na+ concentration similar to those of controls.
4. Endogenous digoxin-like immunoreactivity (EDLI) and erythrocyte Na+ transport were studied in five normotensive and five hypertensive patients, before and after haemodialysis. EDLI in plasma was similar in both groups before and after haemodialysis. No correlation was found between EDLI and erythrocyte Na+ pump activity.
5. These results suggest the existence in some dialysed uraemic patients of alterations in erythrocyte Na+ fluxes, which may be corrected by haemodialysis. EDLI and erythrocyte Na+ fluxes do not seem to be markers of secondary hypertension in these patients.