The Endocrine Pancreas
This chapter focuses on the endocrine actions of cells within the pancreas, which are contained within dispersed clusters termed islets of Langerhans. It considers the ontogeny of the pancreatic islet and discusses the secretion and action of the major islet hormones. Genes for islet hormones are transcribed before true bud development and prior to the genes for exocrine enzymes. It is unknown whether exocrine and islet cells derive from different progenitor cells. Insulin is the predominant hormone produced within the islets of Langerhans. Insulin secretion is the dominant regulator of metabolism in the fed state; its secretion is maintained at basal levels during the fasted state. Insulin secretion in response to glucose is modulated by many factors including amino acids, other sugars, fatty acids, ketone bodies, hormones, and drugs. The insulin receptor has endogenous tyrosine kinase activity, and insulin action is thought to be mediated by an extensively studied but still fairly poorly understood cascade of phosphorylation.