Fig. 4. Pathway by which insulin might stimulate glycogen synthesis through PP1 in the liver. Insulin is thought to activate cyclic-AMP phosphodiesterase, thereby lowering cyclic-AMP and leading to decreased levels of active phosphorylase (phosphorylase a). This relieves the phosphorylase a allosteric inhibition of G_L-PP1c, increasing glycogen synthase phosphatase activity. Insulin also increases the levels of G_L protein and mRNA.