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 [3-3H]glucose 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.