18.14
Insulin Secretory Vesicles
Insulin secretory vesicles release insulin to stimulate blood glucose uptake and regulate carbohydrate metabolism. When the blood glucose levels incre…
Increases in blood glucose stimulate pancreatic beta-islet cells to synthesize proinsulin, an insulin precursor. Newly synthesized proinsulin enters the trans-Golgi network, where it is present in a diffuse form inside immature secretory vesicles.
As the vesicles fuse together and mature, enzymes cleave proinsulin to form active insulin, which concentrates inside the vesicles, forming dense core vesicles.
High blood glucose also causes a glucose-induced calcium influx through a calcium channel. Calcium triggers a signaling cascade that eventually activates protein kinase C or PKC.
PKC enables reorganization of the actin network inside the cell to translocate the insulin containing secretory vesicles. The secretory vesicle then fuses with the plasma membrane to release large amounts of insulin in response to the increased blood glucose.
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Q1: How does increased blood glucose trigger insulin secretion in pancreatic beta cells?
Elevated blood glucose enters pancreatic beta-islet cells and is metabolized, increasing ATP concentration. This ATP closes potassium channels, causing membrane depolarization and opening voltage-gated calcium channels. Calcium influx triggers a signaling cascade activating protein kinase C, which reorganizes the actin network and enables insulin secretory vesicles to translocate and fuse with the plasma membrane for insulin release.
Q2: What is the role of proinsulin in insulin secretory vesicle formation?
Pancreatic beta cells synthesize proinsulin, an insulin precursor that enters the trans-Golgi network in immature secretory vesicles. As vesicles fuse and mature, enzymes cleave proinsulin into active insulin, which concentrates inside the vesicles, forming dense core vesicles ready for regulated secretion in response to high blood glucose.
Q3: What is the kiss-and-run model of exocytosis in insulin secretion?
The kiss-and-run model describes how insulin secretory vesicles fuse with the plasma membrane, opening a fusion pore to release insulin. The pore then closes, and the vesicle returns to the cytoplasm without full membrane integration. This mechanism allows rapid, controlled insulin release while preserving vesicle integrity for reuse.
Q4: How does insulin regulate blood glucose after secretion?
Released insulin stimulates glucose uptake by cells throughout the body, lowering blood glucose levels. Insulin also suppresses gluconeogenesis, the production of new glucose by the liver. Together, these actions maintain glucose homeostasis and regulate carbohydrate metabolism in response to dietary intake.
Q5: What role does calcium play in insulin secretory vesicle trafficking?
Glucose-induced calcium influx through voltage-gated calcium channels triggers a signaling cascade that activates protein kinase C. PKC enables reorganization of the actin cytoskeleton, allowing insulin-containing secretory vesicles to translocate toward and fuse with the plasma membrane, facilitating insulin release in response to elevated blood glucose.
Q6: What regulatory molecules control insulin secretion from pancreatic beta cells?
Insulin secretion is regulated by hormonal and neuronal signals including epinephrine, acetylcholine, and somatostatin. These molecules modulate the secretory response of pancreatic beta-islet cells to blood glucose changes, allowing fine-tuned regulation of insulin release and maintaining glucose homeostasis throughout the body.
Q7: How do defects in insulin secretory vesicles contribute to diabetes mellitus?
Diabetes mellitus can result from defects in insulin secretory vesicle biogenesis, their fusion with the plasma membrane, or the exocytosis process itself. These disruptions impair insulin secretion and release, preventing adequate glucose uptake by cells and dysregulating blood glucose levels.