Mechanisms regulating cytosolic pH (pHi) in adherent resident mouse macrophages have been characterized by use of the pH-sensitive fluorescent probe 2′,7′-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Na+/H+ exchange was activated after an acid load of the macrophage cytosol. However, when Na+/H+ exchange was the only pHi-regulatory mechanism operative, recovery did not proceed beyond a pHi of approx. 6.6. The mechanisms found to be operative at physiological pHi levels were alkalinizing Na(+)-dependent and acidifying Na(+)-independent Cl-/HCO3- exchangers and a H(+)-ATPase further characterized in the accompanying paper [Tapper & Sundler (1992) Biochem. J. 281, 245-250]. Acid extrusion via Na+/Cl-/HCO3- exchange was demonstrated by the dependence on external Na+ and HCO3- and on internal Cl- and by the sensitivity to 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulphonic acid (SITS) and 4,4′-di-isothiocyanatostilbene-2,2′-disulphonic acid (DIDS). By monitoring pHi changes upon Cl- removal and re-addition, the pH-dependence and sensitivity to SITS were found to differ for the alkalinizing and the acidifying Cl-/HCO3- exchangers.

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