Effects of temperature on triiodothyronine plasma levels, kinetics, and hepatocyte nuclear binding in rainbow trout, Salmo gairdneri

Plasma levels of 3,5,3′-triiodo-L-thyronine (T3), plasma T3 kinetics, and properties of in vivo T3 binding to saturable hepatocyte nuclear sites were studied in fed immature rainbow trout maintained at 12 °C and then held for up to 14 d at 5, 11–12, or 19 °C. Elevation (19 °C) or depression (5 °C) of plasma T3 occurred during the first 3 h following abrupt transfer from 11 °C, but from 12 h to 7 d, plasma T3 did not differ significantly among the three temperatures. In contrast, the plasma T3 degradation rate increased fourfold from 5 to 19 °C largely because of an increased fractional rate of turnover of the plasma T3 pool. Outer-ring deiodination of T3 was negligible at 5 and 12 °C and slight at 19 °C. Temperature did not influence the proportion of [125I]T3 lost via the enterohepatic route. Uptake of [125I]T3 into the liver and liver nuclear fraction was most rapid at 19 °C, intermediate at 12 °C, and least rapid at 5 °C. Saturable nuclear binding of [125I]T3 occurred at all temperatures. The apparent affinity of T3 for hepatic nuclear sites was similar at 12 and 19 °C but lower at 5 °C; the apparent site capacity underwent no significant change with temperature. In conclusion, over the range of 5 to 19 °C there are marked increases in plasma T3 clearance, rate of T3 uptake from plasma to liver, and rate of T3 uptake by the liver nuclear fraction, but relatively small changes in plasma T3 level, proportion of T3 excreted via the enterohepatic route, and properties of the saturable T3-binding nuclear sites.