FIGURE 15-37 Integrated regulation of glycogenolysis by Ca²⁺ and cAMP/PKA pathways. (a) Neuronal stimulation of striated muscle cells or epinephrine binding to β-adrenergic receptors on their surfaces leads to increased cytosolic concentrations of the second messengers Ca²⁺ or cAMP, respectively. The key regulatory enzyme glycogen phosphorylase kinase (GPK) is activated by binding Ca²⁺ ions and by phosphorylation by cAMP-dependent PKA. (b) In liver cells, hormonal stimulation of two β-adrenergic receptors leads to increased cytosolic concentrations of cAMP and two other second messengers, diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃). Enzymes are marked by white boxes. (+) = activation of enzyme activity; (−) = inhibition.