Insulin Secretion Mechanism
Beta cells in the islets of Langerhans release insulin in two phases. The first-phase release is rapidly triggered in response to increased blood glucose levels, and lasts about 10 minutes. The second phase is a sustained, slow release of newly formed vesicles triggered independently of sugar, peaking in 2 to 3 hours. Reduced first-phase insulin release may be the earliest detectable beta cell defect predicting onset of type 2 diabetes. First-phase release and insulin sensitivity are independent predictors of diabetes.
The description of first phase release is as follows:
• Glucose enters the β-cells through the glucose transporters, GLUT2. These glucose transporters have a relatively low affinity for glucose, ensuring that the rate of glucose entry into the β-cells is proportional to the extracellular glucose concentration (within the physiological range). At low blood sugar levels very little glucose enters the β-cells; at high blood glucose concentrations large quantities of glucose enter these cells.
• The glucose that enters the β-cell is phosphorylated to glucose-6-phosphate (G-6-P) by glucokinase(hexokinase IV) which is not inhibited by G-6-P in the way that the hexokinases in other tissues (hexokinase I – III) are affected by this product. This means that the intracellular G-6-P concentration remains proportional to the blood sugar concentration.
• Glucose-6-phosphate enters glycolytic pathway and then, via the pyruvate carboxylase reaction, into the Krebs cycle, where multiple, high-energy ATP molecules are produced by the oxidation of acetyl CoA (the Krebs cycle substrate), leading to a rise in the ATP:ADP ratio within the cell.
• An increased intracellular ATP:ADP ratio closes the ATP-sensitive SUR1/Kir6.2 potassium channel (see sulfonylurea receptor). This prevents potassium ions (K+) from leaving the cell by facilitated diffusion, leading to a buildup of intracellular potassium ions. As a result, the inside of the cell becomes less negative with respect to the outside, leading to the depolarization of the cell surface membrane.
• Upon depolarization, voltage-gated calcium ion (Ca2+) channels open, allowing calcium ions to move into the cell by facilitated diffusion.
• The cytosolic calcium ion concentration can also be increased by calcium release from intracellular stores via activation of ryanodine receptors
• The significantly increased amount of calcium ions in the cells' cytoplasm causes the release into the blood of previously synthesized insulin, which has been stored in intracellular secretory vesicles.
Source: Wikipedia
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Видео Insulin Secretion Mechanism канала Dr.Mungli
The description of first phase release is as follows:
• Glucose enters the β-cells through the glucose transporters, GLUT2. These glucose transporters have a relatively low affinity for glucose, ensuring that the rate of glucose entry into the β-cells is proportional to the extracellular glucose concentration (within the physiological range). At low blood sugar levels very little glucose enters the β-cells; at high blood glucose concentrations large quantities of glucose enter these cells.
• The glucose that enters the β-cell is phosphorylated to glucose-6-phosphate (G-6-P) by glucokinase(hexokinase IV) which is not inhibited by G-6-P in the way that the hexokinases in other tissues (hexokinase I – III) are affected by this product. This means that the intracellular G-6-P concentration remains proportional to the blood sugar concentration.
• Glucose-6-phosphate enters glycolytic pathway and then, via the pyruvate carboxylase reaction, into the Krebs cycle, where multiple, high-energy ATP molecules are produced by the oxidation of acetyl CoA (the Krebs cycle substrate), leading to a rise in the ATP:ADP ratio within the cell.
• An increased intracellular ATP:ADP ratio closes the ATP-sensitive SUR1/Kir6.2 potassium channel (see sulfonylurea receptor). This prevents potassium ions (K+) from leaving the cell by facilitated diffusion, leading to a buildup of intracellular potassium ions. As a result, the inside of the cell becomes less negative with respect to the outside, leading to the depolarization of the cell surface membrane.
• Upon depolarization, voltage-gated calcium ion (Ca2+) channels open, allowing calcium ions to move into the cell by facilitated diffusion.
• The cytosolic calcium ion concentration can also be increased by calcium release from intracellular stores via activation of ryanodine receptors
• The significantly increased amount of calcium ions in the cells' cytoplasm causes the release into the blood of previously synthesized insulin, which has been stored in intracellular secretory vesicles.
Source: Wikipedia
For REGULAR UPDATES you can consider SUBSCRIBING to this channel: https://goo.gl/eMs6rw
For short write up of theory on USMLE Biochemistry and other topics on food and nutrition, obesity, weight loss tips you may visit my site:
http://www.drmungli.com/
You can follow my Facebook page Biochemistry Made Easy: https://goo.gl/23S9Y7
checkout other awesome channels to learn biochemistry and other subjects from:
ThePenguineProf: https://goo.gl/ySNURB
Osmosis: https://goo.gl/d1zBs1
Armando: https://goo.gl/jcYwwR
Khan Academy: https://goo.gl/7YmIf9
Nucleus Medical Media: https://goo.gl/xdlqsr
Trending medical youtube channels: https://goo.gl/nUuJOL
20 useful sites for medical students: https://goo.gl/aPnc19
Biochemistry single line questions site: https://goo.gl/PFCewk
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