Section of pancreas of dog. X 250.

Spherical clusters of cells throughout the pancreas, called the islets of Langerhans, produce and secrete the peptide hormones insulin (produced by beta cells) and glucagon (produced by alpha cells), which regulate blood glucose levels, as well as the hormone somatostatin, which inhibits the secretion of both insulin and glucagon. Insulin lowers blood glucose (lowers cAMP levels in signal transduction), speeding the conversion of glucose into glycogen (as well as the conversion of amino acids into protein). To raise glucose levels, alpha cells secrete glucagon, which brings about glycogenolysis (cAMP is raised in target cells). Somatostatin is secreted by delta cells.

In response to chemoreceptors monitoring blood glucose levels in the hypothalamus, parasympathetic neurons extending from the hypothalamus stimulate beta cells (producing insulin and lowering blood glucose) if blood glucose is too high; sympathetic neurons stimulate alpha cells if blood glucose is too low, as well as causing beta cells to reduce insulin secretion.