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Using "3C nuclear magnetic resonance spectroscopic methods we examined in vivo the synthesis of liver glycogen during the infusion of D-11-13Cjglucose and the turnover of labeled glycogen during subsequent infusion of D-11-13Cjglucose. In fasted rats the processes of glycogen synthesis and degradation were observed to occur simultaneously with the rate of synthesis > degradation leading to net glycogen synthesis. In fed rats, incorporation of infused D-[1-_3Cjglucose occurred briskly; however, over 2 h there was no net glycogen accumulated. Degradation of labeled glycogen was greater in the fed versus the fasted rats (P < 0.001), and the lack of net glycogen synthesis in fed rats was due to degradation and synthesis occurring at similar rates throughout the infusion period. There was no indication that suppression of phosphorylase a or subsequent activation of glycogen synthase was involved in modulation of the flux of tracer into liver glycogen.We conclude that in both fed and fasted rats, glycogen synthase and phosphorylase are active simultaneously and the levels of liver glycogen reached during refeeding are determined by the balance between ongoing synthetic and degradative processes. (J. Clin. Invest. 1990. 86:612-617.) Key words: liver glycogen -in vivo NMR * phosphorylase * glycogen synthase Introduction After an overnight fast the rat liver is nearly depleted of glycogen stores. Repletion begins promptly upon feeding with both glucose and gluconeogenic substrates contributing to the formation of UDP-glucose and glycogen (1-6). The factors that control the transition of the liver from the glucose-producing to the glycogen-storing mode have been extensively studied, and include release of insulin into portal blood (7), suppression of glucagon secretion (8), and a rise in portal glucose concentration (9, 10). When isolated hepatocytes are incubated with high concentrations of glucose (1 1), and when mice are given large doses of glucose parenterally (12), near complete inactivation of phosphorylase a (GPa)' precedes the activation of glycogen synthase (GSi) and the onset of glycogen

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