Final answer:
During intense exercise, glucagon is the key hormone that promotes glycogen breakdown in the liver, releasing glucose for energy.
Step-by-step explanation:
During intense exercise, the key hormone that promotes glycogen breakdown in the liver is glucagon. Glucagon is released by the alpha cells of the pancreas when blood glucose levels decline below normal levels. It stimulates the breakdown of glycogen into glucose in skeletal muscle cells and liver cells, in a process called glycogenolysis. Glucose can then be utilized as energy by muscle cells and released into circulation by the liver cells.