NUTR SCI 332 Lecture Notes - Lecture 3: Blood Sugar, Glucose Transporter, Adipose Tissue
Hormonal Control of Blood Glucose Video
Blood glucose levels are controlled largely by the endocrine pancreas. Within the
pancreas, specialized cells of the Islets of Langerhans secrete glucose-regulating
hormones into the blood. Alpha-cells secrete glucagon: a protein hormone that maintains
blood glucose by including the release of stored glucose, called glycogen, from the liver.
Beta-cells secrete insulin, which lowers blood glucose by facilitating the movement of
glucose from blood into muscle liver and adipose tissue; and amylin, which slows the
rate at which digested glucose enters the bloodstream. Delta cells secrete somatostatin.
Somatostatin helps regulate blood glucose by decreasing gastrointestinal activity after
eating. This delays intestinal absorption of glucose into the blood to satisfy its
energy needs.
The body uses glucose, fatty acids, and other substrates as fuel. The brain relies almost
entirely on glucose for its fuel. One way for blood glucose levels to rise is by food
ingestion. Glucose not immediately used for energy is removed from the blood by the
liver, which stores it as glycogen. When blood glucose levels fall, the liver’s stored
glycogen is broken down and released into the blood as glucose in order to maintain the
blood glucose within its normal ranges. Blood glucose levels usually reflect the
difference between the amount of glucose released into circulation by the liver and the
amount of glucose removed from the blood by body cells. For health to be maintained,
blood glucose levels must be kept within very narrow parameters.
Cell membranes are impermeable to glucose, so they require a special carrier to move
glucose from the blood into the cell. Insulin is that carrier. Insulin is either rapidly bound
to peripheral tissues or destroyed by the liver or kidneys. Insulin has a half-life of
approximately 15 minutes once it is released into the general circulation. Insulin lowers
blood glucose levels in several ways:
• One way is that insulin facilitates the movement of glucose from the bloodstream
into body tissues by binding to a target cell’s membrane receptor. The insulin
activates the receptors through a protein kinase messenger system, causing an
increase in the number of glucose transporter molecules present on the outside of
the cell membrane. The glucose transporter molecules, called gluten for glucose
transporters, facilitate the diffusion of glucose into about 80% of body tissues.
Within seconds after insulin binds to a membrane receptor of a susceptible cell, the
membrane increases its uptake of glucose. This is particularly true of skeletal
muscle and adipose tissues.
• Another way is that insulin provides for the storage of glucose as glycogen in the
liver. When the supply of glucose is abundant, insulin activates the enzyme
hexokinase, which in turn phosphorylates glucose. The addition of phosphate to
the glucose molecule traps it within the liver cell. Approximately 2/3 of the glucose
that is ingested with a meal is removed from the blood and stored in the liver as
glycogen.
• Yet another way is that insulin inhibits gluconeogenesis, which is the building of
glucose from new sources. This mechanism of action is a subject of investigation,
but appears to involve the promotion of hepatic glycogen cycling as well as the
interaction of insulin with transcription factors.
• And finally, insulin also reduces blood glucose because it facilitates triglyceride
synthesis from glucose and fat cells while inhibiting the intracellular breakdown of
stored triglycerides.
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Document Summary
Blood glucose levels are controlled largely by the endocrine pancreas. Within the pancreas, specialized cells of the islets of langerhans secrete glucose-regulating hormones into the blood. Alpha-cells secrete glucagon: a protein hormone that maintains blood glucose by including the release of stored glucose, called glycogen, from the liver. Beta-cells secrete insulin, which lowers blood glucose by facilitating the movement of glucose from blood into muscle liver and adipose tissue; and amylin, which slows the rate at which digested glucose enters the bloodstream. Somatostatin helps regulate blood glucose by decreasing gastrointestinal activity after eating. This delays intestinal absorption of glucose into the blood to satisfy its energy needs. The body uses glucose, fatty acids, and other substrates as fuel. The brain relies almost entirely on glucose for its fuel. One way for blood glucose levels to rise is by food ingestion. Glucose not immediately used for energy is removed from the blood by the liver, which stores it as glycogen.
