Analysis of Ca2+ fluxes and Ca2+ pools in pancreatic acini.

45Ca2+ movements have been analysed in dispersed acini prepared from rat pancreas in a quasi-steady state for 45Ca2+. Carbamyl choline (carbachol; Cch) caused a quick 45Ca2+ release that was followed by a slower 45Ca2+ 'reuptake'. Subsequent addition of atropine resulted in a further transient increase in cellular 45Ca2+. The data suggest the presence of a Cch-sensitive 'trigger' pool, which could be refilled by the antagonist, and one or more intracellular 'storage' pools. Intracellular Ca2+ sequestration was studied in isolated acini pretreated with saponin to disrupt their plasma membranes. In the presence of 45Ca2+ (1 microM), addition of ATP at 5 mM caused a rapid increase in 45Ca2+ uptake exceeding the control by fivefold. Maximal ATP-promoted Ca2+ uptake was obtained at 10 microM Ca2+ (half-maximal at 0.32 microM Ca2+). In the presence of mitochondrial inhibitors it was 0.1 microM (half-maximal at 0.014 microM). 45Ca2+ release could still be induced by Cch but the subsequent reuptake was missing. The latter was restored by ATP and atropine caused further 45Ca2+ uptake. Electron microscopy showed electron-dense precipitates in the rough endoplasmic reticulum of saponin-treated cells in the presence of Ca2+, oxalate and ATP which were absent in intact cells or cells pretreated with A23187. The data suggest the presence of a plasma membrane-bound Cch-sensitive 'trigger' Ca2+ pool and ATP-dependent Ca2+ storage systems in mitochondria and rough endoplasmic reticulum of pancreatic acini. It is assumed that Ca2+ is taken up into these pools after secretagogue-induced Ca2+ release.U