3,5,3′-Triiodothyronine (T3) Clearance and T3-Glucuronide (T3G) Appearance Kinetics in Plasma of Freshwater-Reared Male Tilapia,Oreochromis mossambicus

• Published in: General and comparative endocrinology 1998-08, Vol.111 (2), p.123-140
• Main Authors: DiStefano, Joseph J., Ron, Benny, Nguyen, Thuvan T., Weber, Gregory M., Grau, E.Gordon
• Format: Article
• Language: English
• Subjects: Animals; bile; Bile - metabolism; Chromatography, Gel; Chromatography, High Pressure Liquid; compartmental model; conjugates; gallbladder; Glucuronates - metabolism; glucuronides; Infusions, Intra-Arterial; Injections, Intraperitoneal; intestine; kidney; liver; Male; Pilot Projects; plasma kinetics; production; Radioimmunoassay; sulfates; thyroid hormone; Tilapia - blood; Triiodothyronine - administration & dosage; Triiodothyronine - analogs & derivatives; Triiodothyronine - metabolism; Triiodothyronine - pharmacokinetics; Water
• ISSN: 0016-6480; 1095-6840
• DOI: 10.1006/gcen.1998.7084

Distribution and metabolism of the thyroid hormone 3,5,3′-l-triiodothyronine (T3) were studied in several ways to gain insights into these processes in the warm water fish tilapiaOreochromis mossambicus.Trace doses of125I-labeled T3(T*3)1were injected intraarterially, extraarterially, or intraperitoneally in freshwater-reared male tilapia to explore plasma clearance kinetic responses to these different input modalities. Multicompartmental analysis of the plasma clearance data indicated a kinetic distribution of T*3much like that reported for the rat and human, with about 2% of total body T*3in plasma, 5% in rapidly exchanging tissues such as kidney and liver, and 93% in slowly exchanging tissues such as muscle. However, plasma clearance rates (PCR, 5.37 mL/h·100 g body wt) and plasma appearance rates (PAR3= PCR×[T3] plasma=36.3 ng/h·100 g body wt) were quite different than these indices in rat and human and 5 to 50 times larger than values reported for rainbow trout. On a whole-body basis, normalized for body weight, the tilapia we studied produced and accumulated much more T3than rat, human, or rainbow trout. Enzymatic and chromatographic analyses of the plasma clearance data samples indicated substantial production of labeled glucuronide, but not sulfate, conjugates of iodothyronines (TiG) of unknown origin appearing in plasma. The TiG appeared beginning a few hours postinjection, peaked at 6 hours, and yielded a predicted steady-state TiG level of 8.3% of the T3level in plasma. In contrast, in published studies, no conjugates were detected in rainbow trout plasma from 2 to 24 h after iv injection of T*3, T*4, or reverse-T*3, although conjugates of all were present in bile. To our knowledge, although T3and T4sulfate conjugates are present in the sera of several mammals, this is the first quantification of iodothyronine glucuronides reported in blood of any species under normal conditions. This might have physiological significance for the tilapia, with T3G providing a reversible storage form of T3in blood, as has been suggested for sulfate conjugates of T3and T4in blood of several mammals.