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Glycogenolytic, noninsulin-like effects of vanadate on rat hepatocyte glycogen synthase and phosphorylase
Abstract:
Vanadate inactivated rat hepatocyte glycogen synthase and activated glycogen phosphorylase in a dose- and time-dependent manner. These effects were observed in hepatocytes from both fasted as well as fed rats. When rat hepatocytes were preincubated with [32P]phosphate and then with vanadate, and the 32P-labeled glycogen synthase was specifically immunoprecipitated, it was observed that vanadate stimulated the phosphorylation of the 88,000-dalton subunit of glycogen synthase. All of the phosphate was located in the same two CNBr fragments of the enzyme which are phosphorylated by glucagon and other glycogenolytic hormones. In cells incubated in a calcium-depleted medium, vanadate was still able to inactivate glycogen synthase but its effects on phosphorylase were essentially lost. These results demonstrate that, in the hepatocyte, vanadate exerts opposite effects than in the adipocyte and skeletal muscle, where vanadate has an insulin-like action.
Insights
Vanadate inactivates rat liver glycogen synthase and activates glycogen phosphorylase, impacting glucose metabolism. These vanadate effects in hepatocytes differ from its insulin-like actions in other tissues.
Area of Science:
- Biochemistry
- Cell Biology
- Endocrinology
Background:
- Vanadate is known to have insulin-like effects in adipocytes and skeletal muscle.
- The impact of vanadate on liver metabolism, specifically glycogen synthesis and breakdown, requires further elucidation.
Purpose of the Study:
- To investigate the effects of vanadate on glycogen synthase and glycogen phosphorylase activities in rat hepatocytes.
- To determine the mechanism by which vanadate influences these key enzymes in glucose homeostasis.
Main Methods:
- Incubation of rat hepatocytes with vanadate.
- Measurement of glycogen synthase and glycogen phosphorylase activities.
- Phosphorylation studies using [32P]phosphate and immunoprecipitation of glycogen synthase.
Main Results:
- Vanadate inactivated glycogen synthase and activated glycogen phosphorylase in a dose- and time-dependent manner in rat hepatocytes.
- Vanadate stimulated the phosphorylation of the 88,000-dalton subunit of glycogen synthase, with phosphate located in specific CNBr fragments.
- Vanadate's effect on glycogen phosphorylase was lost in calcium-depleted medium, while glycogen synthase inactivation persisted.
Conclusions:
- Vanadate exerts distinct effects on rat hepatocytes compared to adipocytes and skeletal muscle.
- In hepatocytes, vanadate's actions on glycogen metabolism are contrary to its known insulin-like effects in other tissues.