Characterization of sustained [Ca2+]i increase in pancreatic acinar cells and its relation to amylase secretion

The sustained increase in cytosolic free Ca2+ concentration ([Ca2+]i) during maximal stimulation of rat pancreatic acini with carbamylcholine (10(-5) M) was investigated in individual acinar cells by microspectrofluorometric analysis of fura-2. After the large initial [Ca2+]i increase from intracellular stores, [Ca2+]i remained significantly elevated as long as the stimulus was applied. The amplitude of this plateau was dependent on the median Ca2+ concentration ([Ca2+]o) being 45-50 nM above prestimulation in medium with 1 mM [Ca2+]o increasing to 90 nM at 10 mM [Ca2+]o. This Ca2+ plateau was completely blocked by 2.5 mM Ni2+ and 0.25 mM La3+ but was unaffected by elevated K+ or the Ca2+ channel blocker D 600. Mn2+ was able to enter the cytosol after the cell stimulation as indicated by intracellular quenching of fura-2, indicating that acinar cells possess a Mn2(+)-permeable Ca2+ channel. Elimination of [Ca2+]o or addition of Ni2+ and Mn2+ to the medium reduced the level of sustained amylase secretion in a reversible manner under superfusion conditions. Increasing [Ca2+]i above the normal level by increasing [Ca2+]o had no effect on amylase secretion. The process for sustained Ca2+ entry was pH sensitive; decreasing extracellular pH (pHo) to 6.5-6.8 during the cell stimulation resulted in a reduction of the sustained [Ca2+]i plateau level and a decrease in sustained amylase secretion. By contrast, increasing pHo to 8.0 enhanced the level of the sustained [Ca2+]i in a Ni2(+)-sensitive manner but did not increase amylase release. Changes in cytosolic pH had only minimal effects on the sustained [Ca2+]i plateau. The results demonstrate a receptor-mediated Ca2+ entry mechanism, which results in a small increase in [Ca2+]i important in the maintenance of sustained amylase release.