3,5,3'-TRIIODOTHYRONINE (T-3) CLEARANCE AND T-3-GLUCURONIDE (T(3)G) APPEARANCE KINETICS IN PLASMA OF FRESH-WATER-REARED MALE TILAPIA, OREOCHROMIS-MOSSAMBICUS

Distribution and metabolism of the thyroid hormone 3,5,3'-L-triiodothy ronine (T-3) were studied in several ways to gain insights into these processes in the warm water fish tilapia Oreochromis mossambicus. Trac e doses of I-125-labeled T-3 (T-3)(1) were injected intraarterially, extraarterially, or intraperitoneally in freshwater-reared male tilapi a to explore plasma clearance kinetic responses to these different inp ut modalities. Multicompartmental analysis of the plasma clearance dat a indicated a kinetic distribution of T-3 much like that reported for the rat and human, with about 2% of total body T-3 in plasma, 5% in rapidly exchanging tissues such as kidney and liver, and 93% in slowly exchanging tissues such as muscle. However, plasma clearance rates (P CR, 5.37 mL/h.100 g body wt) and plasma appearance rates (PAR(3) = PCR x [T-3] plasma = 36.3 ng/h.100 g body wt) were quite different than t hese indices in rat and human and 5 to 50 times larger than values rep orted for rainbow trout. On a whole-body basis, normalized for body we ight, the tilapia we studied produced and accumulated much more T-3 th an rat, human, or rainbow trout. Enzymatic and chromatographic analyse s of the plasma clearance data samples indicated substantial productio n of labeled glucuronide, but not sulfate, conjugates of iodothyronine s (T(i)G) of unknown origin appearing in plasma. The T(i)G appeared be ginning a few hours postinjection, peaked at 6 hours, and yielded a pr edicted steady-state T(i)G level of 8.3% of the T-3 level in plasma. I n contrast, in published studies, no conjugates were detected in rainb ow trout plasma from 2 to 24 h after iv injection of T-3, T-4*, or re verse-T-3, although conjugates of all were present in bile. To our kn owledge, although T-3 and T-4 sulfate conjugates are present in the se ra of several mammals, this is the first quantification of iodothyroni ne glucuronides reported in blood of any species under normal conditio ns. This might have physiological significance for the tilapia, with T (3)G providing a reversible storage form of T-3 in blood, as has been suggested for sulfate conjugates of T-3 and T-4 in blood of several ma mmals. (C) 1998 Academic Press.