Efficient hepatic glycogen synthesis in refeeding rats requires continued carbon flow through the gluconeogenic pathway.

Intragastric infusion of [1-14C]glucose into awake, fasted rats at rates that produced physiological increases in the circulating glucose concentration resulted in active glycogen deposition in liver. However, degradation of this glycogen revealed extensive randomization of the label among the carbon atoms of glucose. By contrast, muscle glycogen-glucose was labeled primarily in C-1. Treatment of rats with 3-mercaptopicolinic acid, a potent inhibitor of phosphoenol-pyruvate carboxykinase, prior to [1-14C]glucose infusion reduced hepatic glycogen synthesis by 85%; this glycogen contained most of its label in C-1 of glucose. The additional infusion of unlabeled glycerol, which enters the gluconeogenic pathway distal to the 3-mercaptopicolinic acid block, reinstated hepatic glycogen synthesis, but again the label was associated almost exclusively with C-1. In all animals treated with 3-mercaptopicolinic acid, plasma lactate concentrations rose markedly, as did the rate of hepatic lipogenesis. When [1-14C]glucose was infused into pentobarbital-treated rats or administered to awake animals as a large intragastric bolus, the degree of isotopic randomization in liver glycogen-glucose was considerably reduced when compared with that seen in the awake, infused state. The data support the concept that under normal refeeding conditions the bulk of liver glycogen is formed by an indirect pathway involving the sequence glucose ----lactate----glucose-6-P----glycogen, whereas muscle glycogen is formed by the conventional, direct pathway: glucose----glucose-6-P----glycogen. They also establish that a predominantly direct mechanism can be induced in liver, but only under artificial conditions, e.g. chemical blockade of the gluconeogenic sequence, pentobarbital anesthesia, or the administration of massive glucose loads that lead to severe hyperglycemia.