Muscle glycogen synthesis in the resting phase…

A 28- year-old professional cyclist has been training for a race. His coach strongly suggests the intake of carbohydrates after his workout to ensure a muscle glycogen storage that can endure the 28-day race. The activity of muscle Glycogen synthase in the resting muscles is increased by the action of which of the following?

A. Epinephrine

B. Glucagon

C. Insulin

D. Cortisol

E. Thyroid hormone

The correct answer is- C- Insulin.

Glucose homeostasis reflects a balance between hepatic glucose production and peripheral glucose uptake and utilization. Insulin is the most important regulator of this metabolic equilibrium, but neural input, metabolic signals, and other hormones (e.g., glucagon) result in integrated control of glucose supply and utilization.

In the fasting state, low insulin levels increase glucose production by promoting hepatic gluconeogenesis and glycogenolysis and reduce glucose uptake in insulin-sensitive tissues (skeletal muscle and fat), thereby promoting mobilization of stored precursors such as amino acids and free fatty acids (lipolysis). Glucagon, secreted by pancreatic alpha cells when blood glucose or insulin levels are low, stimulates glycogenolysis and gluconeogenesis by the liver and renal medulla.

In the postprandial, the glucose load elicits a rise in insulin and fall in glucagon, leading to a reversal of these processes. Insulin, an anabolic hormone, promotes the storage of carbohydrate and fat and protein synthesis. The major portion of postprandial glucose is utilized by skeletal muscle, an effect of insulin-stimulated glucose uptake. Other tissues, most notably the brain, utilize glucose in an insulin-independent fashion.

Insulin acts on skeletal muscle to increase uptake of glucose. In the fed state, most glucose is used to synthesize glycogen, which acts as a store of glucose for use in exercise; “preloading” with glucose is used by some long-distance athletes to build up stores of glycogen.

Insulin regulates the activity of Glycogen synthase, the key regulatory enzyme of glycogen synthesis by covalent modification (reversible phosphorylation and dephosphorylation).The enzyme is activated by dephosphorylation.

Follow the link to know the details of regulation of glycogen synthesis-

http://www.namrata.co/glycogen-metabolism-power-point-presentation/

Epinephrine and glucagon promote glycogen degradation by activating phosphorylase, the key regulatory enzyme of glycogen degradation.

Cortisol stimulates gluconeogenesis, by activating regulatory enzymes of gluconeogenesis and by providing substrates. Cortisol also stimulates glycogenesis in the liver. Corticosteroids are anabolic to liver but catabolic to peripheral tissues.

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