[Na+, K+, Cl-]-cotransport function and dysfunction in different forms of primary hypertension.

We investigated the effect of an increase in cell Na+ content on outward and inward unidirectional fluxes catalyzed by the [Na+, K+, Cl-]-cotransport system in human erythrocytes (incubated in Li-Rb media). Erythrocytes with low Na+ content exhibited an uncoupled K+ efflux. The increase in cell Na+ content resulted in a more marked stimulation of outward Na+, K+ than of inward Li+, Rb+ cotransport fluxes (with stoichiometries not very different from one-to-one). These results suggest that in human erythrocytes and in nonepithelial cells with small but outwardly directed electrochemical Cl- gradients, the [Na+, K+, Cl-]-cotransport system may behave as a "second pump" by using the extra energy supplied by an additional net [K+, Cl-] efflux. The [Na+, K+, Cl-]-cotransport system (of vascular cells and/or noradrenergic endings) may play two different roles in primary hypertension: (a) "defective second pump" in some essential hypertensive patients with decreased cotransport affinity for internal Na+ and (b) "compensatory second pump" in other forms of primary hypertension where abnormalities in the Na+, K+ pump or in other ion transport systems may predispose the cell to a defective extrusion of excess cell Na+ content.