Studies in rats indicated that the major physiologic stimulus for synthesis of liver glycogen is a rise in the portal glucose concentration after ingestion of a meal. Conversely, glycogen degradation in the liver is stimulated by a rise in portal glucagon concentration. In humans, ingestion of carbohydrate lowers the concentration of circulating glucagon, whereas protein stimulates an increase in peripheral glucagon concentration. Little is known about the effects of these nutrients on glucagon concentrations in the rat. Therefore, we studied the effects of oral protein administration to 24-h-fasted rats pretreated with glucose for 2 h to test the effect of two potent but potentially opposite signals for glycogen metabolism. An increase in liver glycogen concentration was observed in fasted rats given oral glucose, as expected. Removal of glucose by the liver could not account for the glycogen synthesized, indicating that most glycogen formed was derived from gluconeogenesis. In addition, the apparent intracellular and extracellular glucose concentrations were not in equilibrium. A small amount of glucose may have been taken up against a concentration gradient. The portal glucagon was not significantly decreased. Oral protein administration to the rats pretreated with glucose resulted in a rapid and dramatic decrease in liver glycogen concentration. This was associated with an increase in the portal glucagon concentration, no change in insulin concentration, a slight increase in liver cAMP concentration, an increase in the active form of phosphorylase, and a decrease in the active form of synthase. Glycogenolysis could account for the glucose released into the circulation from the liver after protein administration.

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