Nutritional regulation of insulin-like growth factor-I mRNA expression in salmon tissues.

In salmonids, nutritional insufficiency leads to retarded growth and reduced hepatic GH receptors, but increased circulating GH levels. To understand the endocrine mechanism underlying the retarded growth in starved fish better, we investigated the effect of food deprivation and refeeding on circulating levels of GH and insulin, as well as insulin-like growth factor-I (IGF-I) mRNA expression in different tissues of juvenile coho salmon (Oncorhynchus kisutch). Deprivation of food for 2-4 weeks resulted in cessation of growth and a significant decrease in condition factor (an indicator of fish body shape). No difference in circulating insulin or glucose levels were found between starved and fed fish, whereas starvation increased the plasma GH levels. After 4 weeks of starvation, the plasma GH level rose to 9 ng/ml, which was four times as high as that of the fed fish. In spite of elevated circulating GH, hepatic IGF-I mRNA levels were significantly reduced after 4 weeks of starvation. No significant difference in IGF-I mRNA levels of fed and starved fish was found in other tissues, including kidney, spleen, ovary, gill filament and gut. Two weeks of refeeding significantly increased hepatic IGF-I mRNA levels and growth and reduced plasma GH levels. These results suggest that food deprivation primarily reduces IGF-I mRNA expression in the liver which results, most probably, in a decline in systemic IGF-I levels and consequently leads to the retarded growth of salmon.