Which hormone activates glycogen synthase?

Glycogen synthase is the key enzyme responsible for converting glucose into glycogen, which is the stored form of glucose in the liver and muscles. Insulin plays a crucial role in promoting glycogen synthesis in response to elevated blood glucose levels, such as after a meal.

When insulin is released from the pancreas, it binds to its receptor on target cells, activating a signaling cascade that results in the dephosphorylation and activation of glycogen synthase. This process allows for the synthesis of glycogen and promotes the storage of glucose, thereby lowering blood glucose levels.

In contrast, hormones like glucagon and adrenaline primarily function to increase blood glucose levels by stimulating glycogenolysis (the breakdown of glycogen into glucose) and gluconeogenesis (the production of glucose from non-carbohydrate sources). Cortisol, a glucocorticoid, is involved in the regulation of metabolism but does not primarily activate glycogen synthase; rather, it tends to promote gluconeogenesis and has a complex role in glucose regulation.

Therefore, the activation of glycogen synthase is specifically associated with insulin, which is why it is the correct answer.