The content of glycogen phosphorylase and glycogen in preparations of sarcoplasmic reticulum-glycogenolytic complex is enhanced in diabetic rat skeletal muscle

Abstract

Aims/hypothesis: We have examined the effect of diabetes and pharmacological insulin treatment on the content of glycogen phosphorylase and glycogen associated with the sarcoplasmic reticulum-glycogenolytic complex from rat skeletal muscle.

Methods: Diabetes was induced in rats by streptozotocin injection. Enzymatic activities were measured using spectrophotometric methods. Glycogen phosphorylase was determined measuring the pyridoxal-5' -phosphate content and using polyacrylamide gel electrophoresis. Glycogen content was measured by enzymatic and the phenol sulfuric methods.

Results: The content of glycogen phosphorylase associated with the sarcoplasmic reticulum glycogenolytic complex gradually arises after diabetes induction. The content of glycogen phosphorylase was restored to a control value by pharmacological insulin treatment. In addition, the content of glycogen in preparations of sarcoplasmic reticulum-glycogenolytic complex of diabetic animals was also increased, whereas the content of glycogen in total muscle of diabetic rats was similar to that of the control rats. The absolute and relative amount of glycogen associated with sarcoplasmic reticulum seemed to increase in diabetic animals. These effects on the compartmentalisation of glycogen were suppressed by insulin treatment. Additionally, the rate of conversion of glycogen phosphorylase b to a, an index of the phosphorylase kinase activity, was 50 % lower in diabetic rats, increasing the dephosphorylated form of glycogen phosphorylase and, as a consequence, its association with sarcoplasmic reticulum membranes.

Conclusion/interpretation: These results suggest that under diabetic conditions, both glycogen phosphorylase and a small percentage of muscle glycogen are relocalized in the sarcoplasmic reticulum-glycogenolytic complex.