An overview of pancreatic exocrine secretion

Genes for all proteins have encoded in their DNA sequences, information that specifies where these proteins will localize within the cell. Nascent translation products of transcripts of these genes, that possess a specific NH2-terminal signal sequence, are able to translocate into a specialized membranous conducting system called the endoplasmic reticulum (ER), or can be incorporated directly into a target organelle (i.e. mitochondrion). Nascent polypeptides lacking this signal sequence remain in the cytosol. Once segregation into the ER has occurred each protein appears to migrate at a characteristic rate to a connecting organelle; the Golgi Complex. Here, enzymatic modifications of these proteins determines the organelle with which each will eventually become associated. In secretory tissues such as the exocrine pancreas, many different proteins are directed into specialized secretory structures called zymogen granules. These granules are maintained in a "ready-to-release" state by steroid hormones. In the absence of estrogens and glucocorticoids the zymogen granules disappear. Physiologically, secretion from the exocrine pancreas is brought about by parasympathetic nerve stimulation, or by the gut hormone cholecystokinin (CCK). Interaction of acetylcholine or CCK with specific receptors on pancreatic acinar cells initiates the process of exocytosis; that is, fusion of the zymogen granule membrane with the plasma membrane resulting in extracellular release of the contents of the secretory granule.