25.2 : グルコース恒常性：膵島とインスリン分泌

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.