RATION SIZE AND PROTEIN-INTAKE AFFECT CIRCULATING GROWTH-HORMONE CONCENTRATION, HEPATIC GROWTH-HORMONE BINDING AND PLASMA INSULIN-LIKE GROWTH-FACTOR-I IMMUNOREACTIVITY IN A MARINE TELEOST, THE GILTHEAD SEA BREAM (SPARUS-AURATA)

The nutritional regulation of the growth hormone liver axis has been s tudied in gilthead sea bream (Sparus aurata). In a first study, finger ling fish were fed three experimental diets with varying proportions o f protein (34, 45 and 55%). A 60% decrease in plasma growth hormone co ncentration was observed with the increase of specific growth rates an d dietary protein levels. An opposite response was observed in hepatic growth hormone-binding sites and plasma insulin-like growth factor-I immunoreactivity that would reflect the insensitivity of liver to grow th hormone action during relatively low protein intake. In a second st udy, fish were fed a commercial diet (55% protein) at different feedin g levels (0, 1.2, 2.7 and 5.5 g/(100 g body wt . d). An 84% decrease i n plasma growth hormone concentration was observed with the increase o f specific growth rates and feeding levels from 0 to 2.7 g/(100 g body wt . d). However, significantly greater growth hormone concentration was found in fish fed 5.5 g/(100 g body wt . d) when compared with fis h fed 2.7 g/(100 g body wt . d). Hepatic growth hormone-binding sites and plasma insulin-like growth factor-I immunoreactivity increased wit h the increase of feeding levels from 0 to 2.7 g/(100 g body wt . d), but these values were lower in fish-fed 5.5 g/(100 g body wt . d) than in those fed 2.7 g/(100 g body wt . d). The physiological importance of these results remains to be clarified, though probably it is a part of the mechanism that diminishes feed utilization for growth at high feeding levels. In any case, for fish of the same age, the increase of circulating growth hormone can be used as a reliable marker of impair ed growth in marine fish such as gilthead sea bream.