Hepatic and branchial thyroid hormone deiodinase activities associated with the parr-smolt transformation of coho salmon (Oncorhynchus kisutch).
Plasma 3,5,3'-triiodo-L-thyronine (T3) 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-smolt transformation. The plasma Na+ response to a 24-hr seawater (30 ppt) challenge showed characteristic smolt 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 T3 occurred in late April and early May. Both liver and gill T4 (L-thyroxine) outer-ring 5'-monodeiodinase (T4(5)'D) activities, converting T4 to T3, fell from March to June, with no biologically meaningful overall correlation with plasma T3. However, T4(5)'D activity of gill and liver did increase in presmolts in late May, coinciding with an elevation in plasma T3. Hepatic inner-ring T4(5)D activity, converting T4 to 3,3',5'-T3 (rT3 = reverse T3), was lower in smolts than in presmolts. Activity of hepatic outer-ring T3(5)'D, which converts T3 to 3,5-T2, was negligible, but activity of hepatic inner-ring T3(5)D, which produces 3,3'-T2 from T3, was higher in early smolts than in postsmolts. We conclude that hepatic deiodinase activities of smolting coho salmon may act to maintain hepatic and plasma T3 at modest levels and that the branchial production of T3 appears to parallel that of the liver.
Sweeting
R M
RM
Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada.
Alexander
G P
GP
Eales
J G
JG
McKeown
B A
BA
D000818:Animals; D001835:Body Weight; D002851:Chromatography, High Pressure Liquid; D005880:Gills / Q000201:enzymology* / Q000254:growth & development; D007453:Iodide Peroxidase / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000201:enzymology* / Q000254:growth & development; D008862:Microsomes, Liver / Q000201:enzymology; D017684:Oncorhynchus kisutch / Q000254:growth & development* / Q000378:metabolism*; D009929:Organ Size; D012964:Sodium / Q000097:blood; D014284:Triiodothyronine / Q000097:blood; D014882:Water-Electrolyte Balance / Q000502:physiology
D014284:Triiodothyronine; D012964:Sodium; D007453:Iodide Peroxidase