Indirect versus direct routes of hepatic glycogen synthesis.

During refeeding after a brief period of starvation, glucose carbon is deposited into hepatic glycogen by both a direct and an indirect route. In the indirect route glucose is first metabolized to 3-carbon precursors, which then transverse the gluconeogenic pathway before being deposited into glycogen. Recent studies have yielded widely different estimates of the percentage of glucose carbon that follows the indirect route. Work summarized here demonstrates that the relative contributions of glucose carbon to hepatic glycogen formation by the indirect and direct pathways are greatly dependent on experimental design, and at least in vitro, are possibly dependent on the extent of glycogen/glucose 1-P recycling. Under physiological refeeding conditions in vivo, both pathways are used, each contributing approximately 50% of the amount of carbon appearing in glycogen. The level of glucokinase activity does not appear to be responsible for poor glucose utilization in liver. Poor glucose utilization in isolated liver preparations may result from the absence of a neurophysiological feedback loop that senses the arterial/portal glucose gradient and then regulates whole liver glucose uptake.