Resistance of hepatic glycogen to depletion in obese Zucker rats

Glycogen stores (liver and carcass) have been studied in lean and obese Zucker rats. The animals were submitted to one of three feeding conditions: ad libitum, a 48-h fast, or a 48-h fast and food ad libitum for 24 h, and to two environmental conditions, either thermoneutrality or an acute cold exposure (2 days at 4-7 degrees C). After a 2-day fast at 25 degrees C, the liver glycogen store was reduced by 45 times in the lean rats, while it was decreased by only 3 times in the obese rats. Under these conditions, the liver glycogen store was 45 times higher in the obese than in the lean rats. After 2 days in the cold, liver glycogen store was 4.4 times higher in obese rats than in lean rats. After a 2-day fast in the cold, the liver glycogen store in the obese rats was 30 times higher than in the lean rats. In comparison to fasting at thermoneutrality, fasting in the cold did not lead to a further reduction in hepatic glycogen in obese Zucker rats. The differences observed in the mobilization of the hepatic glycogen store between obese and lean rats have not been found in the mobilization of the carcass glycogen store. Drastic conditions, such as a 2-day fast in the cold, did not exhaust the glycogen store in obese Zucker rats. The present observations point out that obese Zucker rats cannot mobilize the entire hepatic glycogen store, as seen in lean control rats. The role of this abnormality in the high hyperlipogenesis that maintains the obese state is still to be evaluated.