THE INFLUENCE OF DIETARY-PROTEIN AND CALORIC CONTENT ON THYROID-FUNCTION AND HEPATIC THYROXINE 5'-MONODEIODINASE ACTIVITY IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

The influence of dietary protein and caloric intake on thyroid function of immature rainbow trout (6.5-degrees-C; 12 h L : 12 h D photocycle) fed a daily ration of 0.97% of body weight was studied. In experiment 1, trout fed four isocaloric diets (approximately 3300 kcal of estimated metabolizable energy/kg; 1 cal = 4.1855 KJ) of identical digestible carbohydrate content(15.2%) but with varying percentages of lipid (7.5-17.6%) and protein (25.6-47.5%) exhibited negligible or modest changes in plasma L-thyroxine (T4) or 3,5,3'-triiodo-L-thyronine (T3) after 7 or 35 days. After 14 days, activity of hepatic 5'-monodeiodinase (5'D), which converts T4 to the active T3 form, was unaltered. However, at 38 days the functional level (V(max)) of 5'D was directly related to the level of protein intake. Substrate affinity (K(m)) was unchanged. In experiment 2, trout fed four isocaloric diets (approximately 3575 kcal/kg) of similar lipid content (15.8%) but with varying percentages of digestible carbohydrate (3.6-54.2%) and protein (0.32-47.1%) generally showed unaltered plasma T4 after 7 or 35 days, but plasma T3 was depressed by the lowest dietary protein levels. Activity of 5'D was unaffected at 14 days, but at 38 days V(max) was directly related to the level of protein ingested; K(m) was not changed. Trout pair-fed diets of the highest protein content to achieve levels of protein intake equivalent to those of trout fed diets of lower protein content showed a depressed V(max) but no changes in K(m) or plasma T4 or T3 levels. We conclude that the level of protein intake in trout mainly determines the activity of 5'D, which is a more sensitive index of chronic thyroidal adjustment than plasma T4 or T3 levels. However, the amount of ingested protein relative to total available dietary energy and (or) the absolute intake levels of one or more nonprotein nutrients change 5'D activity, possibly by influencing the balance between protein synthesis and degradation.