Protein synthesis in trout liver is stimulated by both feeding and fasting

The response of protein synthesis in the liver of the rainbow trout to feeding and fasting was investigated in 3 experiments. In the first experiment, the fractional rate of protein synthesis (k s ), %/day) appeared to cycle with daily feeding events being increased by 46%, 123%, and 72% at 1.5h, 3h, and 6h, after a meal. In Experiment 2, liver protein synthesis fell progressively with fasting to a basal level at 4d which was only 20% of the value at 3h after feeding. Liver weight (hepatosomatic index, HSI, % body weight), total RNA and total protein also fell gradually. Between 4d and 6d, both the RNA/protein ratio and the rate of protein synthesis were significantly increased (11% and 74%). At this time, however, there was also a large loss of liver protein suggesting a concomitant increase in protein breakdown. In the last experiment, when trout were pre-fed a low ration (0.6%/d for 2 wks, LR group), the HSI and liver total RNA and protein were largely unaffected by the 6d fast (i.e., relative to the body weight). In this group, however, protein synthesis at 3h was significantly higher than in fish pre-fed a high ration (1.5%/d, HR group). In addition, at 6d after feeding, protein synthesis had increased back to fed levels in the LR group only. It is concluded that protein synthesis in the liver of the trout is influenced both by feeding events and by ration size and also by the degree to which the trout is fasted.