URINARY-EXCRETION OF THYROID-HORMONES IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Urinary excretion of the thyroid hormones (TH) L-thyroxine (T-4), and 3,5,3'-triiodo-L-thyronine (T-3) and their derivatives was studied in urinary-catheterized rainbow trout (mean wt, 227 g) fasted for 2 days under a 12-hr L:1Zhr D photocycle in running water (12 degrees). Cathe terized trout were intracardiac-injected with [I-125]T-4 (T-4) or [I- 125]T-3 (T-3) and urine was collected as 2-hr fractions over 48 hr. T he I-125 corresponding to I-, TH conjugates (sulfates and glucuronides ), or TH was separated by LH-20 column chromatography and HPLC. Urine production (daily mean, 72.5 ml/kg/day) was lowest during the scotopha se and doubled at the start of photophase, causing acute fluctuations in excretion of I-125-labeled materials, and implying dependence on gl omerular filtration rate. During the first 48 hr, 8.2% of T-4 (I-, 3. 6%; TH conjugates, 1.7%; TH, 2.9%) and 6.7% of injected T-3 (I-, 1.8% ; TH conjugates, 2:5%; TH, 2.4%) was excreted in urine; 32.6% (T-4) a nd 26.4% (T-3) was in the gall bladder; 45.5% (*T-4) and 45.7% (*T-3) were in the remaining carcass; and 13.7% (T-4) and 23.7% (*T-3) were lost via other routes. We extrapolate that about 15% of T-4-injected and 12% of T-3-injected total radioactivity would be excreted ultima tely in urine, with 8.4% (T-4) and 9.0% (*T-3) as TH. or their conjug ates. Neither a T-4 nor a T-3 challenge (20 ng/g) influenced the amoun t of radioactive loss in urine over 48 hr. We conclude that the urine is a significant route for excretion of TH and their conjugates, and t hat urinary TH loss depends to a large extent on the rate of urine pro duction. (C) 1994 Academic Press, Inc.