HEPATIC AND BRANCHIAL THYROID-HORMONE DEIODINASE ACTIVITIES ASSOCIATED WITH THE PARR-SMELT TRANSFORMATION OF COHO SALMON (ONCORHYNCHUS-KISUTCH)

Plasma 3,5,3'-triiodo-L-thyronine (T-3) concentrations, liver and gill thyroid hormone deiodinase activities, growth parameters, and hypoosmoregulatory capacity were measured for an outdoor population of freshwater, hatchery-reared yearling coho salmon prior to, during, and after the parr-smelt transformation. The plasma Na+ response to a 24-hr seawater (30 ppt) challenge showed characteristic smelt hypoosmoregulatory capacity only from mid-May to mid-June. Increases in growth parameters (body weight and length) were curbed during this time. Two peaks in plasma T-3 occurred in late April and early May. Both liver and gill T-4 (L-thyroxine) outer-ring 5'-monodeiodinase (T(4)5'D) activities, converting T-4 to T-3, fell from March to June, with no biologically meaningful overall correlation with plasma T-3. However, T(4)5'D activity of gill and liver did increase in presmolts in late May, coinciding with an elevation in plasma T-3. Hepatic inner-ring T(4)5D activity, converting T-4 to 3,3',5'-T-3 (rT(3) = reverse T-3), was lower in smelts than in presmolts. Activity of hepatic outer-ring T(3)5'D, which converts T-3 to 3,5-T-2, was negligible, but activity of hepatic inner-ring T(3)5D, which produces 3,3'-T-2 from T-3, was higher in early smelts than in postsmolts. We conclude that hepatic deiodinase activities of smelting coho salmon may act to maintain hepatic and plasma T-3 at modest levels and that the branchial production of T-3 appears to parallel that of the liver. (C) 1994 Academic Press, Inc.