Messenger Role of Calcium in Function of Pancreatic Acinar Cells

Enzyme secretion from the exocrine pancreas is elicited by a) cholinergic stimulants, b) hormones belonging to the family of pancreozymin, c) some amphibian peptides such as bombesin, eledoisin, and physalaemin, and d) secretin and vasoactive intestinal polypeptide. Whereas the mechanism of the group d hormones in stimulating enzyme secretion involves adenosine 3',5'-cyclic monophosphate, the others seem to use a common pathway involving Ca2+ as intracellular messenger and probably guanosine 3',5'-cyclic monophosphate as modulator of their action. Their effects can be ascribed to two processes. One pathway involves release of Ca2+ from an intracellular store that is most likely located in the plasma membrane. This phase is independent of extracellular Ca2+ and leads to a rise of guanosine 3',5'-cyclic monophosphate. The other pathway is characterized by an increased permeability of the plasma membrane for Ca2+ and is necessary for sustained secretion. Both pathways lead to an increase cytosolic-free Ca2+ concentration. Ca2+ is either directly involved in fusion of zymogen granules with the luminal cell membrane or triggers events that lead to exocytosis. Furthermore, augmented cytosolic-free calcium concentration a) increased the plasma membrane permeability for Na+, Cl-, and K+, which leads to depolarization of the cell, and b) induces uncoupling of neighboring acinar cells.