Glycogen depletion rather than glucose 6-Pincrements controls early glycogen recovery in human cultured muscle

In glycogen-containing muscle, glycogenesis appears to be controlled by glucose 6-phosphate (6-P) provision, but after glycogen depletion, an autoinhibitory control of glycogen could be a determinant. We analyzed in cultured human muscle the contribution of glycogen depletion versus glucose 6-P in the control of glycogen recovery. Acute deglycogenation was achieved by engineering cells to overexpress glycogen phosphorylase (GP). Cells treated with AdCMV-MGP adenovirus to express 10 times higher active GP showed unaltered glycogen relative to controls at 25 mM glucose, but responded to 6-h glucose deprivation with more extensive glycogen depletion. Glycogen synthase (GS) activity ratio was double in glucose-deprived AdCMV-MGP cells compared with controls, despite identical glucose 6-P. The GS activation peak (30 min) induced by glucose reincubation dose dependently correlated with glucose 6-P concentration, which reached similar steady-state levels in both cell types. GS activation was significantly blunted in AdCMV-MGP cells, whereas it strongly correlated, with an inverse relationship, with glycogen content. An initial (0-1 h) rapid insulin-independent glycogen resynthesis was observed only in AdCMV-MGP cells, which progressed up to glycogen levels approximately 150 micrograms glucose/mg protein; control cells, which did not deplete glycogen below this concentration, showed a 1-h lag time for recovery. In summary, acute deglycogenation, as achieved by GP overexpression, caused the activation of GS, which inversely correlated with glycogen replenishment independent of glucose 6-P. During glycogen recovery, the activation promoted by acute deglycogenation rendered GS effective for controlling glycogenesis, whereas the transient activation of GS induced by the glucose 6-P rise had no impact on the resynthesis rate. We conclude that the early insulin-independent glycogen resynthesis is dependent on the activation of GS due to GP-mediated exhaustion of glycogen rather than glucose 6-P provision.