Absence of glycogen cycling in cultured rat hepatocytes.
Glycogen synthesis and degradation were studied in cultured rat hepatocytes prelabeled by incubation with [14C]glucose or [14C]galactose. During prelabeling about 75% of the accumulated glycogen was synthesized from glucose and about 25% from gluconeogenic precursors. Following the labeling period, glycogen synthesis and degradation were estimated at 5 and 12.5 mM glucose and varying concentrations of insulin and glucagon. At 12.5 mM glucose and 10 nM insulin the accumulation of glycogen was comparable to in vivo values, whereas the level of radioactivity in prelabeled glycogen remained constant. Further addition of 0.1 nM glucagon resulted in constant values of both content and radioactivity of glycogen. Increasing the concentration of glucagon to 10 nM resulted in a parallel decrease of content and radioactivity in glycogen. At 5 mM glucose, 10 nM insulin, and 0.1 nM glucagon both the content and the radioactivity of glycogen were constant, whereas addition of 10 nM glucagon resulted in a parallel decrease of content and radioactivity of glycogen, which was 64% higher than that observed with 12.5 mM glucose. In the absence of insulin, prostaglandin D2 had effects similar to those of 10 nM glucagon, whereas no effects was observed in the presence of insulin. From these results and from calculated rates of glucose 6-phosphate formation, it is concluded that the rate of glycogen degradation is less than 10% of the rate of synthesis under conditions favoring glycogen accumulation. At conditions favoring glycogen degradation (10 nM insulin plus 10 nM glucagon or prostaglandin in the absence of insulin) no synthesis could be detected. Results from cells prelabeled with [14C]galactose suggested that glycogen degradation is not an absolutely ordered process, but that some random degradation takes place.