Modulation of Na+/H+ exchange activity by Cl{-}

1 Cell Biology Program, Hospital for Sick Children, Toronto M5G 1X8, and 2 Department of Surgery, Toronto Hospital and University of Toronto, Toronto, Ontario M5G 1L7; and 3 Department of Physiology, McGill University, Montreal, Quebec, Canada H3G 1Y6 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. antiport; type 1 Na + /H + exchanger; anion dependence; osmotic activation; volume regulation