25.2: Glucose Homeostasis: Pancreatic Islets and Insulin Secretion

The pancreatic islets comprising only 1%-2% of the volume are highly vascularized and innervated mini-organs. They contain five endocrine cell types, including β cells that secrete insulin, which is synthesized as a single polypeptide chain, preproinsulin, processed to proinsulin, and finally to insulin and C-peptide. This process is complex and regulated, involving the Golgi complex, the endoplasmic reticulum, and the secretory granules of the β cell.

Insulin and C-peptide are co-secreted in equimolar quantities. While insulin has a half-life of 5-6 minutes due to extensive hepatic clearance, C-peptide, with no known physiological function or receptor, has a half-life of about 30 minutes. The β cell also synthesizes and secretes Islet Amyloid Polypeptide (IAPP) or amylin, a peptide that affects gastrointestinal motility and the speed of glucose absorption. Pramlintide, a drug used in diabetes treatment, mimics the action of IAPP.

Insulin secretion is tightly regulated to maintain stable blood glucose concentrations during fasting and feeding. This regulation is achieved by interacting with various nutrients, gastrointestinal hormones, pancreatic hormones, and autonomic neurotransmitters. Glucose, amino acids, fatty acids, and ketone bodies promote insulin secretion. GLUT1 mediates glucose uptake in human β cells, leading to glucose phosphorylation and ATP production. As a result, the K_ATP channel closes, promoting Ca²⁺ influx and insulin exocytosis. Glucagon from α cells counteracts insulin, maintaining glucose homeostasis.

Both adrenergic and cholinergic nerves richly innervate the islets. Stimulation of α₂ adrenergic receptors inhibits insulin secretion, whereas β₂ adrenergic receptor agonists and vagal nerve stimulation enhance release. Various conditions such as hypoglycemia, hypoxia, exercise, and severe burns that activate the sympathetic branch of the autonomic nervous system suppress insulin secretion by stimulating α₂ adrenergic receptors.

The pancreas comprises highly vascularized and innervated pancreatic islets, encompassing α, β, δ, PP, and ε endocrine cells.

Amongst them, β cells produce and secrete insulin, facilitating excess glucose uptake from the blood.

Initially, preproinsulin synthesized in the endoplasmic reticulum cleaves and folds into proinsulin, which translocates to the Golgi apparatus and gets enclosed in vesicles. Here, enzymatic clipping generates insulin and C-peptide.

Insulin secretion is triggered by glucose, which enters the human β cells via the GLUT1, undergoes glucokinase-mediated phosphorylation forming glucose-6-phosphate, which enters the glycolytic pathway.

The resultant  ATP elevation inhibits the K_ATP channel, depolarizing the cell membrane.

This triggers voltage-dependent Ca²⁺ channel opening, raising cytosolic Ca²⁺ levels and promoting vesicular exocytosis and insulin release.

Conditions like hypoglycemia, hypoxia, exercise, and severe burns stimulate the α₂ adrenoceptor-G_i protein coupling, lowering cAMP levels and insulin secretion.