Energy allocations to xenobiotic transport and biotransformation reactions in rainbow trout (Oncorhynchus mykiss) during energy intake restriction.

Limited energy intake may result in the down-regulation of cellular defense mechanisms, or if maintained, result in trade-offs with other physiological systems. To examine this, juvenile rainbow trout (Oncorhynchus mykiss) were fed full-rations (1.17% body weight [BW]/day), half-rations (0.59% [BW]/day), or fasted for 9 weeks followed by refeeding at full-rations. BW and liver somatic index (LSI), P-glycoprotein (P-gp), ethoxyresorufin-o-deethylase (EROD), and glutathione-S-transferase (GST) activities were measured to determine if they are maintained under limited resources. P-gp and EROD activities were maintained at baseline values in ration-restricted (P-gp: 119+/-29 pg R123/min/mg cells, EROD: 0.58+/-0.17 nmol/min/mg protein) and fasted fish (P-gp: 120+/-14 pg R123/min/mg cells, EROD: 0.47+/-0.14 nmol/min/mg protein), suggesting they may be prioritized systems during fasting. GST activity was attenuated within 6 weeks of fasting (34% decrease from control), but recoverable to baseline values after refeeding. Changes in BW and LSI of calorie-restricted (BW: 16% decrease from control; LSI: 33% decrease from baseline value) and fasted trout (BW: 38% decrease from control; LSI: 44% decrease from baseline value) suggest that resources were mobilized from body stores partly to support these systems. Condition indices and defense activities in groups also varied over time, suggesting that environmental temperature may modulate these parameters.