EFFECT OF AGE ON INDUCTION OF HEPATIC PHOSPHOENOLPYRUVATE CARBOXYKINASE BY FASTING

This study examines the effect of age on the induction of the rate-lim iting enzyme in gluconeogenesis, phosphoenolpyruvate carboxykinase (PE PCK), in response to fasting and refeeding in male Fischer 344 rats ag ed 3-18 mo. The rats were fasted for 30 h to increase the activity of PEPCK and subsequently were refed for 24 h to lower activity toward ba sal levels. PEPCK activity increased 2.2-fold in the 3-mo-old rats and 2.3-fold in the 18-mo-old rats during the 30-h fast. Therefore PEPCK induction during the 30-h fast was not altered with age. Similarly, re feeding resulted in a significant decrease in PEPCK activity at all ag es. After the 24-h refeeding period, the rats were fasted a second tim e, and the time course of induction from the basal refed level was mea sured. In the young rats (6 mo), the activity of PEPCK increased rapid ly from 18.12 +/- 1.61 to 42.66 +/- 5.94 U/g protein (P < 0.01) within 8 h of fasting. However, in the 18-mo-old rats, the initiation of the induction of PEPCK activity was delayed, and, after 12 h, PEPCK activ ity had increased from 17.34 +/- 1.34 to only 32.50 +/- 3.21 U/g prote in (P < 0.01). Furthermore, the rate of induction appears to be decrea sed in the older animals. The activity after 24 h of fasting was equiv alent in all four age groups (ranging from 44.72 +/- 5.38 at 3 mo to 4 0.18 +/- 5.42 U/g protein at 18 mo). Therefore, the time course of the adaptation of PEPCK activity in response to fasting is altered with a ge in these animals, although the magnitude of the induction is not co mpromised, at least until after 18 mo of age.