## Abstract

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