Dexamethasone lowers cytosolic pH in macrophages by altering alkalinizing pH-regulatory mechanisms

The effect of dexamethasone on cytosolic pH (pH(c)) in resident mouse perit oneal macrophages was investigated using the fluorescent probe 2',7'-bis(ca rboxyethyl)-5(6)-carboxyfluorescein tetra-acetoxymethyl ester (BCECF-AM). D examethasone was found to significantly lower pH(c) and this reduction of p H(c) evolved gradually with time, was near maximal at 10 nM dexamethasone, and could be prevented by the glucocorticoid receptor antagonist RU-38486. The lower pH(c) of dexamethasone-treated cells was neither due to a reducti on of cellular buffer capacity nor to an altered regulation of pH, by Na+/H +-exchange or by acidifying Na+-independent Cl-/HCO3- exchange, as assessed by studies of pH recovery after acute acid and alkali loads, respectively. Instead, an impaired pH(c) recovery by both the H+-ATPase and the alkalini zing Na+-dependent CI-/HCO3- exchange was observed. This impairment was mos t likely not caused by an altered expression or localization of the 39-kDa subunit of the proton pump. Dexamethasone treatment caused a reduction of p H(c) also in a HCO3--containing solution, suggesting that acid extrusion by both the H+-ATPase and Na+-dependent Cl-/HCO3- exchange is important for m aintenance and regulation of macrophage resting pH(c). The lowering of macr ophage pH, might be one mechanism whereby glucocorticoids exert their anti- inflammatory effects.