INFLUENCE OF DIETARY AMINO-ACID-COMPOSITION ON THYROID-FUNCTION OF JUVENILE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

The acute (3 days) and chronic (21 days) effects of dietary protein qu ality on thyroid function and growth parameters were studied in immatu re rainbow trout held at 12-degrees-C on a LD 12:12 light cycle. Five isocaloric (approximately 17.2 MJ digestible energy/kg dry diet) and i sonitrogenous (45.1% protein) diets varying in amino acid composition were fed by hand once daily. The ration across all groups was consiste nt and varied between 1.0 and 1.5% of wet body wt. The dietary amino a cid profiles mimicked that within the rainbow trout egg protein (EP), or whole-body protein (WBP), or they were varied widely by altering th e proportions of casein and gelatin, i.e. HC/LG (high casein/low gelat in); MC/MG (medium casein/medium gelatin); LC/HG (low casein/high gela tin). Since these diets contained a chemo-attractant (FinnStim) to enh ance food intake, a sixth diet, EP/NF (EP but lacking FinnStim), was u sed to ascertain FinnStim effects. Thyroid function was assessed from plasma levels of T4 (L-thyroxine) and T3 (3,5,3'-triiodo-L-thyronine), and from the activity of the hepatic microsomal T4 5'-monodeiodinase (5'D) which converts T4 to the more active T3 form. After 3 days, plas ma T3 and T4 titres were unaffected by diet, but 5'D activity was depr essed by FinnStim and lowered in fish fed diets WBP and LC/ HG relativ e to those fed diet MC/MG. After 21 days, plasma T3 was uninfluenced b y diet, but plasma T4 was elevated in trout ingesting diet MC/MG. The 5'D activity was lower in fish fed diet WBP than in those receiving di ets MC/MG and LC/HG. Final body weight was greater for fish fed diet E P/NF than for those fed diet LC/HG. Fork length was uninfluenced by di et, but the condition factor was depressed in fish fed diet LC/HG. Fee d efficiency and percent protein deposited were depressed, whereas hep atosomatic index was elevated in fish fed diet LC/HG. It is concluded that thyroidal status, as judged primarily by hepatic 5'D activity, is influenced in trout both acutely and chronically by the profile of di etary amino acids. The diet high in gelatin content exerted the most c onsistent effects by depressing growth, condition factor, feed efficie ncy, and protein conversion into body protein, while simultaneously in creasing both the mass of the liver and its 5'D specific activity. Fur ther study is required to identify the particular amino acid (s) respo nsible for increasing hepatic 5'D potential under conditions of reduce d growth.