The Na+ and K+ electrochemical gradients across cell membranes are believed to be maintained by the action of a Na+, K+-pump. In human erythrocytes this pump exchanges internal Na+ for external K+ in approximately a 1.5 ratio. Thus, when Na+-loaded/K+-depleted erythrocytes are incubated in physiological conditions they tend to recover their original low Na+/high K+ content. Surprisingly, in erythrocytes from healthy donors the net Na+ extrusion/K+ influx ratio exceeds the 1.5 ratio predicted for Na+, K+-pump-mediated fluxes whereas it is similar to this value in erythrocytes from essential hypertensive patients and some of their descendants. We now report that this difference is due to the presence of a Na+, K+-co-transport system in normal erythrocytes which extrudes both internal Na+ and K+ and is functionally deficient in erythrocytes of essential hypertensive patients and some of their descendants. No difference in passive Na+ permeability could be detected between normotoensive and hypertensive subjects.

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