AGE-DEPENDENT GLYCOLYSIS AND GLUCONEOGENESIS ENZYME-ACTIVITIES IN STARVED-REFED RATS

Food intake, plasma glucose, insulin (I) and glucagon (G), hepatic glycogen and fructose 2,6-bisphosphate (F-2, 6-P2) and liver glucokinase, glucose 6-phosphate (G6-Pase), 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6-PF-2 kinase/F-2, 6-P2ase), pyruvate kinase (PK-L) and phosphoenolypyruvate carboxykinase (PEPCK) activities were measured in 2 and 22-month-old rats before 3 d starvation and after 2, 4, 6, 24 and 48 h refeeding a high carbohydrate (HC, 74 % w/w) diet. Expressed per 100 g of body weight, the food intake of old rats was 55 % lower than that of young rats and the amount of carbohydrate absorbed hourly during the first 6 h of refeeding was 2.4-fold higher in young than in old rats. During the first 6 h of refeeding plasma glucose increased 2-fold and returned to normal values after 24 h in young rats, while plasma glucose did not change during refeeding in old rats. In young rats [I] fell by 85 % after starvation and returned to normal values 2 h after refeeding. [I] was higher in old than in young rats; it decreased by 40 % after starvation and returned to the basal value 4 h after refeeding. No marked changes were observed in plasma [G] in both groups. No difference was observed in hepatic glycogen in the two groups, while F-2, 6-P2 was higher in old than in young rats. In young rats, the opposite changes in liver glucokinase and G6-Pase activities occuring after starvation and during refeeding were observed, while no changes were observed in old rats. 6-PF-2 kinase and F-2, 6-P2ase activities were lower in old than in young rats. During 4 to 24 h after refeeding the kinase/phosphatase ratio was higher in old than in young rats. PK-L activity was 2-fold higher in young (basal and 24 to 48 h after refeeding) than in old rats. After starvation and during the first 2 h of refeeding PEPCK activity was higher in young than in old rats. These results suggest that the age-related alteration in induction and/or deinduction of hepatic enzymes involved in glycolysis and gluconeogenesis may be due to a spontaneous decrease in the carbohydrate intake and to a decrease sensitivity of insulin (insulin resistance).