Role of calcium in pancreatic acinar cell secretion

Early studies provided indirect evidence of an intracellular messenger role for Ca2+ in pancreatic stimulus-secretion coupling. Recent development of Ca2+-selective fluorescent indicators has now allowed direct measurement of the cytoplasmic free Ca2+ concentration [( Ca2+]i). Using quin 2, resting [Ca2+]i values of 90-160 nM have been determined in the pancreatic acinar cells of mouse, guinea pig and rat. Upon stimulation with carbamylcholine (CCh) or cholecystokinin (CCK), [Ca2+]i increased within seconds by 4- to 8-fold. Studies of amylase release and the [Ca2+]i increase revealed a stoichiometric relationship at submaximal CCh concentrations although amylase release at supramaximal secretagogue concentrations decreased while peak [Ca2+] remained elevated. The failure of Ca2+ ionophores to stimulate full amylase release in proportion to the rise in Ca2+, along with the short duration of the increase in [Ca2+]i induced by secretagogues, suggest that Ca2+ is not the sole determinant of amylase release. Studies of amylase release and protein phosphorylation suggest that 1,2-diacylglycerol produced by the secretagogue-induced breakdown of phosphatidylinositol-4,5-bisphosphate (PIP2) also mediates secretion and may act synergistically with Ca2+ to alter the phosphorylation of structural and regulatory acinar cell proteins.