Glycogenolysis is primarily controlled by which hormone?

• A. Glucagon
• B. Insulin
• C. Estrogen
• D. Thyroxine

Answer: (A)

Glycogenolysis is driven mainly by the body's need to raise blood glucose, especially during fasting. When blood glucose falls, glucagon is released from the pancreas and acts on the liver. It activates a cAMP signaling pathway that turns on glycogen phosphorylase (the enzyme that breaks down glycogen) and turns off glycogen synthase, so glycogen is broken down into glucose that can be released into the bloodstream. This makes glucagon the primary hormonal trigger for glycogenolysis in the liver.

Insulin does the opposite. After a meal, insulin promotes glycogen synthesis and inhibits glycogen breakdown, so it is not the primary trigger for glycogenolysis. Estrogen and thyroxine don’t directly regulate the rapid breakdown of hepatic glycogen in this context. (Note: in muscle, glycogenolysis is more strongly influenced by epinephrine and muscle activity, not glucagon.)