Chapter 49 - Structure–function Relationships in the Pancreatic Acinar Cell

Abstract

Abstract The pancreatic acinar cell synthesizes, stores, and secretes the enzymes and enzyme precursors required for the digestion of dietary proteins, carbohydrates, and lipids. To meet the daily needs for digestion, the acinar cell has the highest average rates of protein synthesis in the body. Enzymes are stored in zymogen granules that are localized to the apical region of the cell. Signals generated by food in the intestine reach the acinar cell by neural and hormonal routes stimulate the secretion of enzymes from zymogen granules by exocytosis into the pancreatic duct. The acinar cell has been a model system for examining mechanisms of protein synthesis and export. Thus, nascent digestive enzyme proteins are transported from the endoplasmic reticulum to the Golgi complex, segregated from lysosomal enzymes, and then directed to zymogen granules by vesicular transport in a time-dependent and vectorial manner. The exocrine pancreas has two major physiologic functions: it supplies the enzymes and enzyme precursors (zymogens) that are needed for digesting dietary lipids, carbohydrates, and proteins and secretes a bicarbonate-rich fluid that neutralizes acidic gastric secretions and thus provides the correct pH for intestinal digestion by pancreatic enzymes. Two major cell types form the exocrine pancreas; acinar cells and duct cells. The chapter focuses on the acinar cell which synthesizes, stores, and secretes digestive enzymes while duct cells secrete chloride and bicarbonate. Reasons to focus on the acinar cell include its critical physiologic function and its history as the model for the scientific studies that described the steps responsible for regulating protein synthesis and export and cell signaling. Thus, after electron microscopy was developed, cell biologists first visualized acinar cell organelles, they often then determined their cellular function by studying this cell. Although this review is largely based on the data from rodent acinar cells, limited data from human acinar cells suggest they will exhibit the same fundamental responses.