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)

Abstract

The nutritional regulation of the growth hormone liver axis has been studied in gilthead sea bream (Sparus aurata). In a first study, fingerling fish were fed three experimental diets with varying proportions of protein (34, 45 and 55%). A 60% decrease in plasma growth hormone concentration was observed with the increase of specific growth rates and 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 growth hormone action during relatively low protein intake. In a second study, fish were fed a commercial diet (55% protein) at different feeding levels (0, 1.2, 2.7 and 5.5 g/(100 g body wt·d). An 84% decrease in plasma growth hormone concentration was observed with the increase of 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 fish fed 2.7 g/(100 g body wt·d). Hepatic growth hormone-binding sites and plasma insulin-like growth factor-I immunoreactivity increased with 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 impaired growth in marine fish such as gilthead sea bream.