Stoppage of Glycogenesis and “Over-Shoot” of Induction of Lipogenesis and its Related Enzyme Activities in the Liver of Fasted-Refed Rats

Abstract

To elucidate the causes of changes of carbohydrate metabolic pathways, the time course of utilization of dietary [U-¹⁴C]sucrose and induction of enzyme activities in the livers of rats were investigated. Adult male rats of BHE strain were refed after a fast of 2 days. The nutritionally complete refeeding diet contained 60% sucrose as the only source of carbohydrate. [U-¹⁴C] Sucrose was included in the diet on either day 1 or day 2, or both of refeeding. During the first day of refeeding, the radioactivity was incorporated mainly into liver glycogen which rose to over 100 mg/g. During the second day, little ¹⁴C appeared in the liver glycogen, which decreased sharply while glucose-6-phosphatase activity increased. The glycogenic pathway thus appeared to be blocked. On the other hand, ¹⁴C incorporation in the liver fat was minimal during the first day, but was quite extensive during the second day of refeeding. The enhanced lipogenesis was accompanied by large increases of the activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and NADP-malic dehydrogenase. Results clearly indicate that the carbohydrate load in the liver of intact animals was initially metabolized by the glycogenic pathway. When glycogenesis stopped, carbohydrate was metabolized differently. The enhanced incorporation of [U-¹⁴C]sucrose into liver lipids indicates an increased formation of acetyl CoA and an accelerated formation and use of NADPH, probably from increasing dehydrogenase activities. Our data suggest that the blockage of synthesis of glycogen with the continuation of carbohydrate load was a primary cause in “over-shooting” induction of hepatic dehydrogenase activities and lipogenesis.