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 (T 3 ), plasma T 3 kinetics, and properties of in vivo T 3 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 T 3 occurred during the first 3 h following abrupt transfer from 11 °C, but from 12 h to 7 d, plasma T 3 did not differ significantly among the three temperatures. In contrast, the plasma T 3 degradation rate increased fourfold from 5 to 19 °C largely because of an increased fractional rate of turnover of the plasma T 3 pool. Outer-ring deiodination of T 3 was negligible at 5 and 12 °C and slight at 19 °C. Temperature did not influence the proportion of [ 125 I]T 3 lost via the enterohepatic route. Uptake of [ 125 I]T 3 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 [ 125 I]T 3 occurred at all temperatures. The apparent affinity of T 3 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 T 3 clearance, rate of T 3 uptake from plasma to liver, and rate of T 3 uptake by the liver nuclear fraction, but relatively small changes in plasma T 3 level, proportion of T 3 excreted via the enterohepatic route, and properties of the saturable T 3 -binding nuclear sites.