What hormone is released when muscle glycogen is depleted?

When muscle glycogen is depleted, the hormone released is glucagon. This hormone plays a crucial role in regulating blood glucose levels. As glycogen stores diminish, particularly during prolonged exercise or fasting, the pancreas secretes glucagon to stimulate the liver to convert stored glycogen into glucose, which is then released into the bloodstream.

In the context of energy metabolism, glucagon promotes gluconeogenesis and glycogenolysis in the liver, which are vital processes for maintaining blood glucose levels and supplying energy to the body, especially when muscle glycogen is insufficient. This response helps to ensure that there is a continuous supply of energy substrates available for muscular activity, particularly during extended physical exertion.

Other hormones listed, such as cortisol, insulin-like growth factor (IGF-1), and thyroxine, have different functions and roles in the body that do not directly address the depletion of muscle glycogen in the same immediate manner as glucagon. Cortisol, for example, is more involved in stress responses and may elevate blood sugars over longer periods, while IGF-1 is primarily associated with growth and anabolic processes. Thyroxine is a thyroid hormone that regulates metabolism and energy expenditure but doesn’t directly respond to glycogen levels in muscles.