[LisaS]

Here is some good text from here on the process

In response to lowered blood glucose the a cells of the pancreas secrete glucagon which binds to cell surface receptors on liver and several other cells. Liver cells are the primary target for the action of this peptide hormone. The response of cells to the binding of glucagon to its cell surface receptor is the activation of the enzyme adenylate cyclase which is associated with the receptor. Activation of adenylate cyclase leads to a large increase in the formation of cAMP. cAMP binds to an enzyme called cAMP-dependent protein kinase, PKA. Binding of cAMP to the regulatory subunits of PKA leads to the release and subsequent activation of the catalytic subunits. The catalytic subunits then phosphorylate a number of proteins on serine and threonine residues. Of significance to this discussion is the PKA-mediated phosphorylation of phosphorylase kinase as shown in the diagram above. Phosphorylation of phosphorylase kinase activates the enzyme which in turn phosphorylates the b form of phosphorylase. Phosphorylation of phosphorylase-b greatly enhances its activity towards glycogen breakdown. The modified enzyme is called phosphorylase-a. The net result is an extremely large induction of glycogen breakdown in response to glucagon binding to cell surface receptors.
This identical cascade of events occurs in skeletal muscle cells as well. However, in these cells the induction of the cascade is the result of epinephrine (adrenalin) binding to receptors on the surface of muscle cells. Epinephrine is released from the adrenal glands in response to neural signals indicating an immediate need for enhanced glucose utilization in muscle, the so called fight or flight response. Muscle cells lack glucagon receptors. The presence of glucagon receptors on muscle cells would be futile anyway since the role of glucagon release is to increase blood glucose concentrations and muscle glycogen stores cannot contribute to blood glucose levels.
Regulation of phosphorylase kinase activity is also affected by two distinct mechanisms involving Ca2+ ions. The ability of Ca2+ ions to regulate phosphorylase kinase is through the function of one of the subunits of this enzyme. One of the subunits of this enzyme is the ubiquitous protein, calmodulin. Calmodulin is a calcium binding protein. Binding induces a conformational change in calmodulin which in turn enhances the catalytic activity of the phosphorylase kinase towards its substrate, phosphorylase-b. This activity is crucial to the enhancement of glycogenolysis in muscle cells where muscle contraction is stimulated by acetylcholine stimulation of neuromuscular junctions. The effect of acetylcholine release from nerve terminals at a neuromuscular junction is to depolarize the muscle cell leading to increased release of sarcoplasmic Ca2+, thereby activating phosphorylase kinase.Thus, not only does the increased intracellular calcium increase the rate of muscle contraction, it increases glycogenolysis which provides the muscle cell with the increased ATP it also needs for contraction.
The second Ca2+ ion-mediated pathway to phosphorylase kinase activation is through activation of a-adrenergic receptors by epinephrine.