Thyroid hormone deiodination in brain, liver, gill, heart and muscle of Atlantic salmon (Salmo salar) during photoperiodically-induced parr-smolt transformation. I. Outer- and inner-ring thyroxine deiodination.

Outer-ring deiodinase (ORD) and inner-ring deiodinase (IRD) pathways for L-thyroxine (T4) were examined in the microsomes of tissues of 20-month-old Atlantic salmon induced to undergo parr-smolt transformation (PST) in late February and March by imposing a 16-hr photoperiod. Smolt external characteristics, decline in condition factor, increases in brain-somatic index, cardiac-somatic index, gill-somatic index, gill Na+/K+ ATPase activity, and food consumption developed progressively during the 5-week study. Plasma T4 (4.1 ng/ml, Week 1) rose to 13.5 and 12.5 ng/ml (Weeks 3 and 4) and fell to 7.5 ng/ml (Week 5). Plasma T3 rose from 2.1 ng/ml (Week 1) to 3.8 ng/ml (Week 2) and fell to 2.4 ng/ml (Week 5). T4ORD activity occurred in liver, heart, gill, brain, and skeletal muscle, increasing in liver and heart between Weeks 1 and 2, and in brain between Weeks 4 and 5. Only liver T4ORD activity was correlated (r = +0.946) with plasma T3 concentrations, suggesting that hepatic T4ORD may determine the plasma T3 concentration. For hepatic T4ORD the mean Km was 0.42 nM and the mean Vmax 1.2 pmols T4 converted.hr-1.mg microsomal protein-1. HPLC analysis revealed 3,3',5-T3(rT3) and 3,3'-diiodo-L-thyronine, as evidence of T4IRD activity, but only in liver and brain. The lower bound estimates of T4IRD activities to generate rT3 were 28% (liver) and 50% (brain) of the respective T4ORD activities. Brain T4IRD increased progressively during PST. In post-smolts in later October, plasma T3 concentrations and both hepatic and brain T4ORD and T4IRD activities were low, but some rT3 was produced. We conclude that during induced PST in Atlantic salmon (i) T4ORD activity increases in liver, heart, and brain, but not in gill or skeletal muscles; (ii) the hepatic T4ORD is highly correlated with plasma T3 and may therefore determine plasma T3; and (iii) there are changes in T4ORD and T4IRD in brain, which may reflect regulation of intracellular thyroid hormone metabolism in that tissue. However, it remains to be determined how changes in deiodinase activity relate to the surge in plasma T4 and the role of thyroid hormones during PST.