Intracellular pH recovery from alkalinization: Characterization of chloride and bicarbonate transport by the anion exchange system of human neutrophils

Abstract

The nature of the intracellular pH-regulatory mechanism after imposition of an alkaline load was investigated in isolated human peripheral blood neutrophils. Cells were alkalinized by removal of a DMO prepulse. The major part of the recovery could be ascribed to a Cl-/HCO3-, counter-transport system: specifically, a one-for-one exchange of external Cl- for internal HCO3-. This exchange mechanism was sensitive to competitive inhibition by the cinnamate derivative UK-5099 (Ki ∼ 1 μM). The half-saturation constants for binding of HCO3- and Cl- to the external translocation site of the carrier were ∼2.5 and ∼5.0 mM. In addition, other halides and lyotropic anions could substitute for external Cl-. These ions interacted with the exchanger in a sequence of decreasing affinities: HCO3- > Cl- ∼ NO3- ∼ Br- > I- ∼ SCN- > PAH-. Glucuronate and SO42- lacked any appreciable affinity. This rank order is reminiscent of the selectivity sequence for the principal anion exchanger in resting cells. Cl- and HCO3- displayed competition kinetics at both the internal and external binding sites of the carrier. Finally, evidence compatible with the existence of an approximately fourfold asymmetry (Michaelis constants inside > outside) between inward- and outward-facing states is presented. These results imply that a Cl-/HCO3- exchange mechanism, which displays several properties in common with the classical inorganic anion exchanger of erythrocytes, is primarily responsible for restoring the pHi of human neutrophils to its normal resting value after alkalinization.