Modulation of Na+/H+exchange activity by Cl−

Abstract

Na+/H+ exchanger (NHE) activity is exquisitely dependent on the intra- and extracellular concentrations of Na+ and H+. In addition, Cl− ions have been suggested to modulate NHE activity, but little is known about the underlying mechanism, and the Cl− sensitivity of the individual isoforms has not been established. To explore their Cl− sensitivity, types 1, 2, and 3 Na+/H+ exchangers (NHE1, NHE2, and NHE3) were heterologously expressed in antiport-deficient cells. Bilateral replacement of Cl− with nitrate or thiocyanate inhibited the activity of all isoforms. Cl− depletion did not affect cell volume or the cellular ATP content, which could have indirectly altered NHE activity. The number of plasmalemmal exchangers was unaffected by Cl− removal, implying that inhibition was due to a decrease in the intrinsic activity of individual exchangers. Analysis of truncated mutants of NHE1 revealed that the anion sensitivity resides, at least in part, in the COOH-terminal domain of the exchanger. Moreover, readdition of Cl− into the extracellular medium failed to restore normal transport, suggesting that intracellular Cl− is critical for activity. Thus interaction of intracellular Cl− with the COOH terminus of NHE1 or with an associated protein is essential for optimal activity.