Role of physiological HCO3- buffer on intracellular pH and histamine release in rat peritoneal mast cells

The purpose of this study was to examine how intracellular pH (pH(i)) regulation and histamine release are affected by HCO3- in rat peritoneal mast cells. The pH(i) was measured using the pH-sensitive dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), We observed a pH(i) of 6.88+/-0.012 (n=24) in resting mast cells exposed to a HEPES buffer (pH 7.4), but a sustained drop of 0.21 pH units to 6.67+/-0.015 (n=23) when we exposed the mast cells to a HEPES/HCO3- buffer equilibrated at all time with 5% CO2 (pH 7.4). This fall in pHi is inhibited by the carbonic anhydrase inhibitor dichlorphenamide and is Naf-independent, indicating the involvement of Na+-independent Cl-/HCO3- exchange activity. Furthermore removal of external Cl- in the presence but not in the absence of HCO3- reversed the Cl-/HCO3- exchange and induced an alkaline load. The recovery from this alkaline load was dependent on external Cl- but independent of Na+. Both the alkalinization and the recovery were inhibited by the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). In addition, Cl-36(-) uptake measurements confirm the presence of a Cl-/HCO3- exchanger. Histamine release stimulated by antigen and compound 48/80 was substantially reduced in the presence of HEPES/HCO3- buffer (pH(o) 7.4, pH(i) 6.66). Histamine release was increased, however, when pH(i) was clamped to 6.66 in HCO3--free media (pH(o) 6.9). We conclude that: (1) Na+- independent Cl-/HCO3- exchange determines steady-state pH(i) in rat peritoneal mast cells; and (2) the reduction in histamine release observed in the presence of HCO3- is not due to its effect on pH(i) per se, but rather on other changes in ion transport.