Pathways of Glycogen Formation in Liver and Skeletal Muscle in Fed and Fasted Rats1

Abstract

Fasted and fed rats were given 2-C14 glucose by intraperitoneal injection. The glucose was isolated from the glycogen of liver and skeletal muscle and degraded by fermentation with Leuconostoc mesenteroides to determine the C14 incorporation into each of the C positions of the hexose. In the glucose of liver glycogen, the predominant labeling was in C-2. An appreciable migration of tracer from C-2 to C-1 was observed with a lesser randomization of isotope to the other C positions of glucose. Fasting was associated with an increase in the specific activity of C-l relative to C-2. In the glucose of skeletal muscle glycogen from fasted rats, tracer was present essentially only in C-2. In the studies with fed animals, somewhat greater randomization of isotope occurred, with C-l being more heavily labeled than C-3 through C-6. These data were interpreted in terms of the known alternative routes of glucose utilization. It is concluded that, in liver, the pentose phosphate pathway contributes to the formation of glycogen glucose and that this is more apparent in the fasted than in the fed animal. In contrast, the glycogen of skeletal muscle appears to be derived almost entirely from glucose by direct incorporation of the intact administered 6-carbon unit, either in the fed or fasted states.