Effects of dietary arginine levels and carbohydrate-to-lipid ratios on mRNA expression of growth-related hormones in largemouth bass, Micropterus salmoides.

Utilizing the tissue samples and growth data collected from our two preceding researches in largemouth bass (LMB), we have investigated effects of dietary arginine (Arg) levels and carbohydrate-to-lipid (CHO/LIP) ratios on the GH, IGF-I and insulin expression in related tissues to find possible relationships between the nutrient intake, growth performance and transcript level. Hepatic IGF-I and pituitary GH mRNA levels were significantly up-regulated by lower dietary Arg levels from 1.94% to 3.01% and by higher levels from 2.76% to 3.01%, respectively, while Brockmann body (BB)-containing tissue insulin mRNA expression was not affected. Dietary CHO/LIP ratios ranging from 0.32 to 5.17 (w/w) affected pituitary GH, liver IGF-I and BB-containing tissue insulin mRNA expression in a ratio-specific pattern. The lower ratios from 0.32 to 2.36 significantly up-regulated GH and insulin transcript levels, but significantly down-regulated IGF-I transcript levels; the higher ratios did no longer exert any further effects on them. Meanwhile, two strong positive correlations (r=0.892, r=0.885) between hepatic IGF-I transcript levels and specific growth rates of tested fish were observed with varying dietary Arg levels and CHO/LIP ratios, respectively. These findings indicate that in LMB dietary Arg levels and CHO/LIP ratios regulate differentially the endocrine system of GH, IGF-I and insulin at transcription level; this system, in turn, plays a fundamental role in the regulation of the nutrient metabolism and somatic growth; and that hepatic IGF-I mRNA abundance should be a more reliable index to assess growth and nutritional fitness than the others, at least in LMB.