Glycogen synthase regulating glycogen/glucose levels and is activated by dephosphorylation. In humans, there are two paralogous isozymes of glycogen synthase: The liver enzyme expression is restricted to the liver, whereas the muscle enzyme is widely expressed. Liver glycogen serves as a storage pool to maintain the blood glucose level during fasting, whereas muscle glycogen synthesis accounts for disposal of up to 90% of ingested glucose. The role of muscle glycogen is as a reserve to provide energy during bursts of activity. Meanwhile, the muscle isozyme plays a major role in the cellular response to long-term adaptation to hypoxia. Notably, hypoxia only induces
Glycogen many respects serves as a countersignal to insulin. In response to insulin levels being below normal (when blood levels of glucose begin to fall below the normal range), glucagon is secreted in increasing amounts and stimulates both glycogenolysis (the breakdown of glycogen) and gluconeogenesis (the production of glucose from other sources). Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally, as well. As muscle cells lack glucose-6-phosphatase, which is required to pass glucose into the blood, the glycogen they store is available solely