Chapter 51: The Endocrine Pancreas

The pancreatic islets of Langerhans contain distinct cell populations that respond to circulating glucose concentrations and autonomic nervous system inputs to modulate blood glucose levels throughout the fed and fasted states. Beta cells synthesize and release insulin in response to elevated glucose, amino acids, and gastrointestinal hormones, triggering glucose uptake in skeletal muscle and adipose tissue while suppressing hepatic glucose production. The chapter details the molecular mechanisms of glucose sensing, including the glucokinase pathway that allows beta cells to respond proportionally to physiological changes in blood glucose. Alpha cells release glucagon when glucose falls below normal ranges, activating hepatic glycogenolysis and gluconeogenesis to restore blood glucose during fasting or stress. The chapter explores the relationship between insulin secretion and the incretin effect, whereby gastrointestinal hormones amplify insulin release following oral nutrient intake, particularly through glucose-dependent insulinotropic peptide and glucagon-like peptide-1. Somatostatin-producing delta cells provide paracrine regulation by inhibiting both insulin and glucagon secretion, fine-tuning the hormonal response to metabolic demands. The chapter also addresses pathophysiological conditions including type 1 diabetes mellitus resulting from autoimmune destruction of beta cells, type 2 diabetes involving progressive insulin resistance and beta cell dysfunction, and hypoglycemia caused by excessive insulin secretion or inadequate counter-regulatory hormone release. Integration of neural, hormonal, and metabolic signals ensures appropriate pancreatic endocrine responses to maintain glucose within the narrow range required for central nervous system function and systemic energy metabolism.