Changes in intestinal and hepatic thyroid hormone deiodination during spontaneous metamorphosis of the sea Lamprey, Petromyzon marinus

We measured microsomal low-K-m outer-ring deiodination (ORD) and inner-ring deiodination (IRD) activities for thyroxine (T-4) and 3,5,3'-triiodothyron ine (T-3) in intestine and liver in nonmetamorphosing (undersized) larvae, immediately premetamorphic larvae, animals in stages 1-7 of metamorphosis, and immediately postmetamorphic sea lampreys (Petromyzon marinus). For inte stine: T4ORD activity was relatively low in nonmetamorphosing larvae, incre ased in premetamorphic individuals, was highest in stages 1 and 2 and was v ery low during stages 3-7; T4IRD activity was negligible until stage 3 but increased 4.7-fold through stages 3 to 7 such that T4IRD activity was 14 ti mes T4ORD activity at stage 6; T3ORD activity was undetectable; T3IRD activ ity was not measured through stages 3-7 but correlated with T4IRD activity at other stages. For liver: deiodination was only measured up to stage 2 an d in postmetamorphic animals; in contrast to intestine, T4ORD activity fell to low levels at stage 2 and was low during postmetamorphosis; T4IRD and T 3IRD activities were very low and uninfluenced by developmental stage; T3OR D activity was undetectable. We conclude that (1) deiodination activity is usually much higher in intestine than in liver (2) intestinal ORD and IRD a ctivities change reciprocally so that ORD predominates in early metamorphos is but IRD predominates in mid and late metamorphosis, and (3) changes in i ntestinal deiodination may contribute to the characteristic depression of p lasma T-4 and T-3 levels during spontaneous metamorphosis. (C) 2000 Wiley-L iss, Inc.