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 m m 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 µmol (g dry muscle) −1 ; stimulated = 33.9 ± 2.3 µmol (g dry muscle) −1 ), but substantially in WT muscles (basal = 11.4 ± 0.7 µmol (g dry muscle) −1 ; stimulated = 54.2 ± 4.5 µmol (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 µ m ) 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 µ m ). These data indicate that allosteric activation of phosphorylase b accounts for the accelerated glycogenolysis in CK −/− muscle during contraction.