A study of exocrine cell functions using local photolysis of caged compounds.

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Agonists evoke Ca^<2+> gradients and oscillations initiating at the trigger zone in exocrine acinar cells. Using a low affinity Ca^<2+> indicator dye, BTC,we found that agonists raised Ca^<2+> concentration as high as 10 muM at the trigger zone. Comparison with the results obtained with fura-2 suggests profuse Ca^<2+> release and Ca^<2+> domains at the trigger zone. The resulting micromolar Ca^<2+> gradients were necessary and sufficient for exocytosis of zymogen granules investigated with the capacitance measurement. In contrast, submicromolar Ca^<2+> gradients selectively gave rise to sequential activation of luminal and basal ion channels. These results emphasize critical role of the micromolar Ca^<2+> gradients in exocytosis, and prompt reinvestigation of other cells using low affinity Ca^<2+> indicators.