What is the major effect of high levels of circulating glucagon?

High levels of circulating glucagon primarily stimulate glycogenolysis, which is the breakdown of glycogen to glucose. Glucagon is a hormone produced by the alpha cells of the pancreas and plays a crucial role in maintaining blood glucose levels, especially during fasting states or periods of low carbohydrate intake. When blood glucose levels drop, glucagon is released, prompting the liver to convert stored glycogen into glucose, which is then released into the bloodstream to increase blood sugar levels.

This process is essential for providing energy to the body, particularly to glucose-dependent tissues like the brain and red blood cells. Glucagon also promotes gluconeogenesis (the production of glucose from non-carbohydrate sources) and lipolysis (the breakdown of fats), but the immediate and direct major effect of glucagon is the enhancement of glycogenolysis in the liver.

While insulin secretion, glucogenesis, and fat storage are relevant to glucose metabolism and energy storage, they are not directly stimulated by glucagon. Instead, glucagon counteracts the effects of insulin, which promotes glycogen storage and fat storage, thus underlining its primary role in increasing blood glucose levels through glycogenolysis.