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Glycogen synthesis stimulation by adenylate cyclase inhibition in rat epididymal adipocytes
Abstract:
The effects of adenylate cyclase inhibition on the transport of glucose and fructose and their incorporation into glycogen were investigated in order to assess the extent to which lowered cAMP levels can take part in the various components of glycogen synthesis regulation in isolated rat epididymal adipocytes. The dose-response characteristics of (R)-N-(2-phenylisopropyl)adenosine (PIA), a potent and specific adenylate cyclase inhibitor, on glycogen synthesis were compared with those effectively inhibiting lipolysis, a measure of functional cAMP levels. PIA had no effect on basal glucose or fructose transport but stimulated glucose and fructose incorporation into glycogen. Their respective incorporation was 10 and 69% of that achieved in the presence of insulin. These effects of PIA were shown to be in part the result of increased glycogen synthase I activity. PIA was 20% as effective as insulin in this action. Thus, were insulin to lower cAMP levels and/or inhibit cAMP-dependent protein kinase, this action would be irrelevant to glucose transport but would contribute to the stimulation of glycogen metabolism. However, an additional mechanism(s) involving neither increased glucose transport nor lowered cAMP levels is required to account for the full action of insulin. Fat cells in the absence of medium glucose and in the presence of 10(-7) M PIA and adenosine deaminase constitute a system functionally depleted of cAMP where this mechanism can be studied in isolation.
Insights
Adenylate cyclase inhibition stimulates glycogen synthesis in rat fat cells by increasing glycogen synthase I activity. This suggests lowered cyclic AMP (cAMP) levels partially mediate insulin
Area of Science:
- Biochemistry
- Cellular Metabolism
- Endocrinology
Background:
- Insulin regulates glucose metabolism and glycogen synthesis in adipocytes.
- Cyclic AMP (cAMP) levels are critical in cellular signaling pathways.
- Adenylate cyclase activity influences intracellular cAMP concentrations.
Purpose of the Study:
- To investigate the role of adenylate cyclase inhibition and lowered cAMP levels in regulating glucose and fructose transport and glycogen synthesis.
- To determine the extent to which reduced cAMP contributes to glycogen synthesis regulation.
- To compare the effects of adenylate cyclase inhibition with insulin's effects on adipocyte metabolism.
Main Methods:
- Isolated rat epididymal adipocytes were used.
- (R)-N-(2-phenylisopropyl)adenosine (PIA), a specific adenylate cyclase inhibitor, was administered.
- Glucose and fructose transport and incorporation into glycogen were measured.
- Glycogen synthase I activity was assessed.
Main Results:
- PIA stimulated glucose and fructose incorporation into glycogen, reaching 10% and 69% of insulin's effect, respectively.
- PIA did not affect basal glucose or fructose transport.
- The stimulatory effect of PIA on glycogen synthesis was partly due to increased glycogen synthase I activity.
- PIA was 20% as effective as insulin in increasing glycogen synthase I activity.
Conclusions:
- Lowering cAMP levels contributes to the stimulation of glycogen metabolism, independent of glucose transport.
- Insulin's full effect on adipocytes involves mechanisms beyond glucose transport and cAMP reduction.
- A system depleted of cAMP was established for studying alternative insulin signaling pathways.