Modulatory effect of HCO3- on rat mast cell exocytosis: cross-talks between bicarbonate and calcium.

HCO-3 modulation of histamine release and its relationship with the Ca2+ signal were studied in serosal rat mast cells. Histamine release was induced by Ca2+ mobilizing stimuli, namely compound 48/80, thapsigargin, Ca2+ chelators, ionophore A23187, and PMA and ionophore A23187 in a HCO-3-buffered medium or a HCO-3-free medium. The presence of HCO-3 reduced histamine release by 48/80, Ca2+ chelators, A23187, and PMA/A23187, but increased histamine release induced by thapsigargin. Histamine release by PMA was significantly higher in a HCO-3-free medium than in a HCO-3-free medium, as it was the PMA potentiation of histamine release by A23187. Ca2+]i changes induced by these drugs were measured in fura-2-loaded mast cells. In thapsigargin and EGTA or BAPTA preincubated mast cells Ca2+]i increase was higher in a HCO-3-buffered medium than in a HCO-3-free medium in the presence of Ca2+. On the contrary, in compound 48/80 and PMA/A23187 activated mast cells the Ca2+]i increase is the same both in the presence and in the absence of HCO-3. The effect of HCO-3 on histamine release in serosal rat mast cells depends on the stimulus, but it is not related to the presence of Cl-. In thapsigargin-stimulated mast cells the effect of HCO-3 on histamine release may be related to the Ca2+ signal, but in compound 48/80, EGTA, and PMA/A23187-activated mast cells there is no relationship between intracellular Ca2+ and the inhibitory effect of HCO-3 on histamine release. Additionally, the PKC pathway is implicated in the inhibitory effect of HCO-3 on histamine release, the higher the chelation of calcium rendering the higher the enhancement of the response after adding calcium in the absence of HCO-3.