Lithium-sodium countertransport: physiological moorings for red cell transport disorders in hypertension

Erythrocyte lithium-sodium countertransport, a mode of ouabain-insensitive monovalent cation metabolism, is increased in human essential hypertension, but no pathophysiological link to hypertension has yet been established. Similarities between red cell lithium-sodium countertransport and renal proximal tubular sodium-hydrogen ion exchange suggest a possible role for a countertransport analogue in the control of proximal fluid reabsorption. To test this hypothesis, we measured red cell countertransport and renal lithium clearance, a reliable measure of proximal tubular sodium reabsorption, in normotensives and hypertensives. We found that lithium clearance is (a) inversely correlated with red cell countertransport, (b) decreased in essential hypertensives (reflecting increased proximal sodium reabsorption), and (c) lower in normotensive subjects with a family history of hypertension than in those without such a history. The mechanism by which enhanced proximal tubular sodium reabsorption is related to essential hypertension is unknown, but in addition to its connection to lithium clearance, red cell countertransport is correlated with total peripheral resistance and diastolic compliance of the left ventricle. It is therefore possible that alterations of transport demonstrated in red cells reflect generalized membrane characteristics shared by cardiovascular and renal tissues. Measurements of lithium clearance provide a useful tool for further characterizing the control of renal proximal tubular fluid reabsorption in hypertension. Studies using these measurements may help to explain mechanisms of salt sensitivity and differential responses to antihypertensive therapies as well as yield insights into the heritable basis of human hypertension.