Insulin secretion (revision 19)
Old revision·20:33, 12 Jul 2025·MicrodoseMagnus
| Insulin secretionBeta-cell physiology | |
|---|---|
Glucose-stimulated secretion follows a sigmoid dose-response with a threshold near 5 mM. | |
| Cell type | Pancreatic islet beta cell |
| Threshold | ≈5 mM glucose |
| Pattern | Biphasic; superimposed 5–10 min pulses |
| Topic infobox · conventions | |
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]
The triggering pathway
[edit]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.
Amplifying pathways
[edit]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 Gs. 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]
Pulsatility and measurement
[edit]Insulin is secreted in pulses of five to ten minutes' period, superimposed on the biphasic pattern. Pulsatility is degraded early in type 2 diabetes and in first-degree relatives of affected people. Because the liver extracts insulin in a pulse-dependent manner, degraded pulsatility alters hepatic insulin exposure disproportionately to any change in mean concentration.[1]
Secretion is measured indirectly. Peripheral insulin concentration understates secretion because of first-pass hepatic extraction, so C-peptide — co-secreted equimolar with insulin and not extracted by the liver — is the preferred analyte for quantifying secretory rate. Deconvolution of C-peptide kinetics gives a secretion profile; simpler indices such as HOMA-B and the insulinogenic index are cruder but require only a fasting or a two-point sample.
Anyone comparing published secretion data should check which measure was used. Fasting insulin, HOMA-B, C-peptide deconvolution and clamp-derived first-phase measurements are not interchangeable, and a difference between studies is as likely to be methodological as physiological.[4]
Failure in type 2 diabetes
[edit]Beta-cell secretory failure, not insulin resistance alone, is what converts impaired glucose tolerance into diabetes. Cross-sectional data indicate a substantial loss of functional beta-cell capacity by the time of diagnosis, though estimates vary widely with the method used and post-mortem mass measurements do not track functional estimates closely.[1]
Whether the loss is of cell number, of function, or of differentiated identity — beta cells dedifferentiating to a progenitor-like state — is actively debated, and the possibility of dedifferentiation is what makes partial functional recovery after substantial weight loss biologically plausible. Recovery of first-phase secretion has been reported in remission studies following large weight loss, which argues against irreversible loss of mass as the whole explanation.[2] See Beta cell function for the clinical assessment of this capacity.
See also
References
- ^ a b c d e f g Rorsman P, Braun M. "Regulation of insulin secretion in human pancreatic islets." Annual Review of Physiology 75:155–179 (2013). DOI:10.1146/annurev-physiol-030212-183754. PMID 22974438.
- ^ a b c Drucker DJ. "Mechanisms of action and therapeutic application of glucagon-like peptide-1." Cell Metabolism 27(4):740–756 (2018). PMID 29617641.
- ^ Campbell JE, Drucker DJ. "Pharmacology, physiology, and mechanisms of incretin hormone action." Cell Metabolism 17(6):819–837 (2013). PMID 23684623.
- ^ Bergman RN, Ader M, Huecking K, Van Citters G. "Accurate assessment of beta-cell function: the hyperbolic correction." Diabetes 51 Suppl 1:S212–S220 (2002). PMID 11815482.