Insulin secretion (revision 9)

Insulin secretion is the regulated release of insulin from the beta cells of the pancreatic islets. It is triggered by glucose metabolism within the beta cell rather than by glucose binding to a receptor, and it is amplified by a second set of signals — among them the incretin hormones — that have no effect in the absence of the trigger.^[1]

The distinction between triggering and amplifying pathways is the single most important idea for understanding why GLP-1 receptor agonists rarely cause hypoglycaemia while sulfonylureas frequently do. Sulfonylureas act on the triggering pathway and initiate secretion regardless of glucose; incretins act on the amplifying pathway and can only enlarge a response that glucose has already begun.^[2]

Secretion is biphasic. A first phase lasting some ten minutes reflects release of a small pool of vesicles already docked at the membrane; a sustained second phase reflects recruitment and priming of further vesicles. Loss of first-phase secretion is among the earliest detectable abnormalities in the progression to type 2 diabetes.^[1]

Glucose enters the beta cell through GLUT1 and GLUT3 transporters and is phosphorylated by glucokinase, whose kinetics — a high K_m near 8 mM and no product inhibition — make it the rate-limiting step and the cell's effective glucose sensor. Subsequent glycolysis and mitochondrial oxidation raise the cytosolic ATP:ADP ratio.^[1]

The rise in ATP:ADP closes ATP-sensitive potassium channels. The membrane depolarises, voltage-gated calcium channels open, cytosolic calcium rises, and the calcium rise triggers exocytosis of insulin granules. Sulfonylureas close the same potassium channel pharmacologically, which is why their action is glucose-independent, and loss-of-function mutations in the channel produce congenital hyperinsulinism.^[1]

The dose-response between glucose and secretion is sigmoid, with a threshold near 5 mM and a half-maximal response near 8 mM. Below the threshold the amplifying pathways have essentially nothing to amplify — the mechanistic statement of what "glucose-dependent" means in the incretin literature.

Amplification acts on the efficiency of the exocytotic machinery rather than on the trigger. The best characterised amplifier is cAMP, generated when GLP-1 or GIP receptors couple to G_s. cAMP acts through protein kinase A and through the exchange protein Epac2, which increases the number of granules released per unit of calcium influx.^[2]

Metabolic amplification independent of cAMP also exists: at fixed calcium, raising glucose still increases secretion, an effect attributed to mitochondrially derived coupling factors including NADPH and glutamate. This pathway accounts for a substantial fraction of the total glucose response and is not fully characterised.^[1]

Amino acids, free fatty acids acting at FFAR1, and parasympathetic input all amplify. The incretin effect — the excess of the oral over the intravenous insulin response — is the integrated in vivo expression of the hormonal amplifiers.^[3]