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The mechanism by which exogenous glucose stimulates the incorporation of hepatic glucose-6-phosphate into glycogen in fasted rats has not been clearly delineated. We gave glucose intragastrically over a 3.5-h period during which liver glycogen was deposited at linear rates. Simultaneous primed continuous infusion of 2-3H or 3-3Hglucose established that under these conditions absolute carbon flow through hepatic glucose-6-phosphatase was greatly suppressed. After 1 h, hepatic UDP-glucose and glucose-6-phosphate had fallen by 50-60% and the former remained low throughout the experiment. By contrast, glucose-6-phosphate rebounded to its initial value by 2 h and remained at this level during the subsequent hour. We interpret the data as follows. Exogenous glucose, in addition to acting as a precursor of liver glucose-6-phosphate, causes diversion of the latter away from free glucose formation and into glycogen synthesis. The fall in UDP-glucose is in accord with a glucose-induced activation of glycogen synthase, as proposed by Hers (Annu. Rev. Biochem. 1976; 45:167-89.). However, the fall-rise sequence of glucose-6-phosphate concentration constitutes the first direct evidence in vivo for simultaneous inhibition at the level of glucose-6-phosphatase.
Newgard et al. (Wed,) studied this question.