Glycogen breakdown (and synthesis) is regulated by hormones.

Epinephrine (also called adrenalin) and glucagon stimulate breakdown. Insulin stimulates synthesis.

Breakdown occurs as a result of a kinase cascade that arises from binding of the appropriate hormone to the appropriate cell surface receptor. Recall that a regulatory cascade is a process in which the intensity of an initial regulatory signal is amplified manyfold through a series of enzyme activations.

The cascade depicted in Figure 13.18 provides a way for cells to rapidly turn on glycogen breakdown and release of glucose. This is useful in emergency situations (e.g., the need to catch prey or the need to avoid being caught).