HCO3 −-dependent transient acidification induced by ionomycin in rat submandibular acinar cells

Ionomycin (IM, 5 μM), which exchanges 1 Ca2+ for 1 H+, changed intracellular pH (pHi) with Ca2+ entry into rat submandibular acinar cells. IM-induced changes in pHi consisted of two components: the first is an HCO3−-dependent transient pHi decrease, and the second is an HCO3−-independent gradual pHi increase. IM (1 μM), which activates store-operated Ca2+ channels, induced an HCO3−-dependent and transient pHi decrease without any HCO3−-independent pHi increase. Thus, a gradual pHi increase was induced by the Ca2+/H+ exchange. The HCO3−-dependent and transient pHi decrease induced by IM was abolished by acetazolamide, but not by methyl isobutyl amiloride (MIA) or diisothiocyanatostilbene disulfonate (DIDS), suggesting that the Na+/H+ exchange, the Cl−/HCO3− exchange, or the Na+-HCO3− cotransport induces no transient pHi decrease. Thapsigargin induced no transient pHi decrease. Thus, IM, not Ca2+ entry, reduced pHi transiently. IM reacts with Ca2+ to produce H+ in the presence of \( {\text{CO}}_{ 2} /{\text{HCO}}_{ 3}{^{ - }} : \, \left[ {{\text{H}} - {\text{IM}}} \right]^{ - } + {\text{ Ca}}^{ 2+ } \,+ {\text{CO}}_{ 2} \rightleftarrows \left[ {{\text{H}}-{\text{Ca}} - {\text{IM}}} \right]^{ + } \cdot {\text{HCO}}_{ 3}{^{ - } }+ {\text{H}}^{ + } \). In this reaction, a monoprotonated IM reacts with Ca2+ and CO2 to produce an electroneutral IM complex and H+, and then H+ is removed from the cells via CO2 production. Thus, IM transiently decreased pHi. In conclusion, in rat submandibular acinar cells IM (5 μM) transiently reduces pHi because of its chemical characteristics, with HCO3− dependence, and increases pHi by exchanging Ca2+ for H+, which is independent of HCO3−.