Contraction-mediated glycogenolysis in mouse skeletal muscle lacking creatine kinase:: the role of phosphorylase b activation

Skeletal muscle that is deficient in creatine kinase (CK-/-) exhibits accelerated glycogenolysis during contraction. Understanding this phenomenon could provide insight into the control of glycogenolysis during contraction. Therefore, glycogen breakdown was investigated in isolated extensor digitorum longus CK-/- muscle. Muscles were stimulated to produce repeated tetani for 20 s in the presence of sodium cyanide to block mitochondrial respiration. Accumulation of lactate after stimulation was similar in wild-type (WT) and CK-/- muscles, whereas accumulation of glucose-6-phosphate was twofold higher in CK-/- muscles, indicating greater glycogenolysis in CK-/- muscles. Total phosphorylase activity was decreased by almost 30% in CK-/- muscle (P < 0.001). Phosphorylase fractional activity (-/+ 3.3 mm AMP) was similar in both groups in the basal state (about 10%), but increased to a smaller extent in CK-/- muscles after stimulation (39 +/- 4% vs. 52 +/- 4% in WT, P < 0.05). Inorganic phosphate, the substrate for phosphorylase, increased marginally in CK-/- muscles after stimulation (basal = 25.3 +/- 2.2 mumol (g dry muscle)(-1); stimulated = 33.9 +/- 2.3 mumol (g dry muscle)(-1)), but substantially in WT muscles (basal = 11.4 +/- 0.7 mumol (g dry muscle)(-1); stimulated = 54.2 +/- 4.5 mumol (g dry muscle)(-1)). Kinetic studies of phosphorylase b (dephosphorylated enzyme) from muscle extracts in vitro demonstrated higher relative activities in CK-/- muscles (60-135%) in response to low AMP concentrations (up to 50 muM) in both the basal state and after stimulation (P < 0.05), whereas no differences in activity between CK-/- and WT muscles were observed at high AMP concentrations (> 100 muM). These data indicate that allosteric activation of phosphorylase b accounts for the accelerated glycogenolysis in CK-/- muscle during contraction.