Feed restriction up-regulates uncoupling protein 3 (UCP3) gene expression in heart and red muscle tissues of gilthead sea bream (Sparus aurata L.): New insights in substrate oxidation and energy expenditure

The physiological regulation of the mitochondrial uncoupling protein 3 (UCP3) remains practically unexplored in fish and the aim of this study was to examine the effects of ration size on the regulation of UCP3 in heart, red skeletal muscle and white skeletal muscle of gilthead sea bream (Sparus aurata L.). Juvenile fish were fed at three different levels for 11 weeks: i) full ration until visual satiety (R100 group), ii) 70% of satiation (R70 group) and iii) 70% of satiation with two finishing weeks at the maintenance ration (20% of the satiation level) (R70–20 group). The thirty percent feed restriction improved fish performance, increasing feed conversion efficiency and circulating levels of insulin-like growth factor-I (IGF-I). Fish of the R70–20 group showed reduced growth and low circulating levels of IGF-I in combination with increased circulating concentrations of growth hormone and free fatty acids. Feed restriction did not alter UCP3 transcript levels in white skeletal muscle, but improved this tissue's oxidative capacity as assessed by changes in glycolytic and oxidative mitochondrial enzyme activities. In contrast, in cardiac and red skeletal muscle tissues, this dietary treatment primarily increased UCP3 mRNA expression. The respiratory control ratio of freshly isolated heart mitochondria was slightly lower in R70–20 fish than in R100 fish, which suggests that there was an increase in mitochondrial uncoupling concomitant with the enhanced UCP3 mRNA expression. Altogether, these findings highlight the different adaptive mechanism of glycolytic and highly oxidative muscle tissues for their rapid adjustment to varying feed intake.