Key Points

• One to two million highly vascularized and richly innervated islets comprise the endocrine pancreas. These islets contain insulin-secreting beta cells, glucagon-secreting alpha cells, and somatostatin-secreting delta cells.

• Glucagon is a small, single-chain peptide hormone that is cleared rapidly from the circulation. Its physiologic role is to stimulate hepatic production of metabolic fuels and increase blood concentrations of glucose and ketone bodies.

• Glucagon secretion: (1) Alpha cells respond directly to blood glucose concentrations. Glucagon secretion is maximal at low blood glucose concentrations and inhibited at high glucose concentrations; (2) hypoglycemia sensed by the central nervous system activates parasympa-thetic and sympathetic (including epinephrine and norepinephrine from the adrenal medulla) stimulation of glucagon secretion; (3) increased concentrations of amino acids in blood and the gut hormone cholecystokinin stimulate glucagon secretion; and (4) somatostatin, insulin, ketone bodies, and free fatty acids inhibit glucagon secretion.

• Insulin is a small, two-chain peptide hormone that is cleared rapidly from the circulation. The principal physiologic action of insulin is to promote storage of metabolic fuels.

• Insulin increases transport of glucose in adipose tissue, thereby promoting fatty acid storage as triglycerides, inhibition of free fatty acid release, and increased triglyceride synthesis.

• Insulin increases glucose transport and glyco-gen synthesis in muscle and inhibits the production of glucose and ketone bodies in the liver.

• Insulin binds to a surface receptor and activates an intrinsic receptor tyrosine kinase. Tyrosine phosphorylation of insulin receptor substrate

Essential Medical Physiology, Third Edition

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