Abstract
1. We have investigated interactions between intracellular pH (pH(i)) and the intracellular free calcium concentration ([Ca2+](i)) in collagenase-isolated rat lacrimal acinar cells. The fluorescent dyes fura-2 and 2',7'-bis(carboxyethyl)-5-carboxyfluorescein (BCECF) were used to measure [Ca2+](i) and pH(i) respectively. 2. Application of the weak base NH4Cl alkalinized the cytosol and caused a dose-dependant increase in [Ca2+](i). Trimethylamine (TMA) also alkalinized the cytosol and increased [Ca2+](i). The increase in [Ca2+](i) evoked by NH4Cl or TMA was much smaller than that evoked by the secretory agonist acetylcholine (ACh). 3. Application of NH4Cl also increased [Ca2+](i) in cells bathed in Ca2+-free medium, indicating that NH4Cl released Ca2+ from an intracellular pool. 4. Ammonium chloride had no effect on [Ca2+](i) in cells bathed in Ca2+-free medium if agonist-sensitive intracellular Ca2+ pools had been depleted with either ACh or the microsomal Ca2+-ATPase inhibitor 2,5-di(tert-butyl)hydroquinone. Treatment of cells with NH4Cl in Ca2+-free medium reduced the amount of Ca2+ from the same intracellular pool released by ACh. 5. Calcium release from the agonist-sensitive pool was also triggered when the cytosol was alkalinized by removing the weak acid acetate. 6. Ammonium chloride caused a modest increase in inositol phosphate production, suggesting that NH4Cl may have released stored Ca2+ via an increase in the intracellular inositol 1,4,5-triphosphate concentration. 7. The increase in [Ca2+](i) evoked by NH4Cl was not sustained even in the presence of extracellular Ca2+. In contrast, when a low dose of ACh was used to evoke intracellular Ca2+ release of similar magnitude, sustained Ca2+ entry was observed. 8. Alkalinizing the cytosol appeared to partially inhibit Ca2+ entry triggered by thapsigargin or by ACh. 9. We suggest that alkalinizing the cytoplasm in unstimulated lacrimal acinar cells can release Ca2+ from the intracellular agonist-sensitive Ca2+ pool. However, releasing stored Ca2+ via alkalinization does not appear to trigger significant Ca2+ entry, perhaps because intracellular alkalinization inhibits either the Ca2+ entry pathway or the mechanism which couples the entry pathway to store depletion.
Original language | English |
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Pages (from-to) | 601-611 |
Number of pages | 10 |
Journal | Journal of Physiology |
Volume | 499 |
Issue number | 3 |
Publication status | Published - 15 Mar 1997 |