Action of cholecystokinin and cholinergic agents on membrane-bound calcium in dispersed pancreatic acinar cells.

In dispersed acinar cells prepared from guinea pig pancreas, cellular uptake of 45Ca was moderately rapid and reached a steady state by 60 min. At the steady state, 69% of total cellular 45Ca was membrane-bound. In acinar cells preloaded with 45Ca and then incubated with COOH-terminal octapeptide of cholecystokinin (CCK-OP) or carbamylcholine, total cellular 45Ca decreased by approximately 40% within 5-10 min and then steadily increased to control values by 60 min. Under identical conditions, membrane-bound 45Ca decreased by 40% within 5-10 min and remained constant for the duration of the incubation. Free cellular 45Ca did not change during the initial 30 min but then increased steadily to values three times those in control cells by 60 min. In cells preloaded with 45Ca and then incubated with EDTA, the loss of total cellular radioactivity stimulated by CCK-OP could be accounted for by loss of membrane-bound 45Ca. CCK-OP failed to alter total cellular uptake of 45Ca when both tracer and peptide were added at the beginning of the incubation. Under identical conditions, membrane-bound 45Ca was not altered by CCK-OP during the first 30 min of incubation but was significantly below control values after this time. The effect of CCK-OP on free cellular 45Ca was the same as in cells preloaded with the tracer. These results suggest that CCK-OP causes release of 45Ca from a membrane-bound compartment that equilibrates slowly with extracellular fluid and that the change in free cellular 45Ca is a secondary effect.