Analysis of Ca 2+ fluxes and Ca 2+ pools in pancreatic acini

Abstract

45 Ca 2+ movements have been analysed in dispersed acini prepared from rat pancreas in a quasi-steady state for 45 Ca 2+ . Carbamyl choline (carbachol; Cch) caused a quick 45 Ca 2+ release that was followed by a slower 45 Ca 2+ ‘reuptake’. Subsequent addition of atropine resulted in a further transient increase in cellular 45 Ca 2+ . 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 Ca 2+ sequestration was studied in isolated acini pretreated with saponin to disrupt their plasma membranes. In the presence of 45 Ca 2+ (1 µM), addition of ATP at 5 mM caused a rapid increase in 45 Ca 2+ uptake exceeding the control by fivefold. Maximal ATP-promoted Ca 2+ uptake was obtained at 10 µM Ca 2+ (half-maximal at 0.32 µM Ca 2+ ). In the presence of mitochondrial inhibitors it was 0.1 µM (half-maximal at 0.014 µM). 45 Ca 2+ release could still be induced by Cch but the subsequent reuptake was missing. The latter was restored by ATP and atropine caused further 45 Ca 2+ uptake. Electron microscopy showed electron-dense precipitates in the rough endoplasmic reticulum of saponin-treated cells in the presence of Ca 2+ , 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’ Ca 2+ pool and ATP-dependent Ca 2+ storage systems in mitochondria and rough endoplasmic reticulum of pancreatic acini. It is assumed that Ca 2+ is taken up into these pools after secretagogue-induced Ca 2+ release.