Abstract
The in vitro binding capacity of triiodothyronine (T 3) to hepatocyte nuclei was determined for lampreys ( Petromyzon marinus L.) at different phases of the life cycle. The binding of T 3 to nuclei in ammocoetes (larvae) is of high affinity ( K d = 2.9 × 10 −10 M) with a maximum binding capacity of 1.89 pg T 3 · μg DNA −1. Binding capacities of metamorphosing individuals, young parasitic adults, and upstream-migrant adults were 2.40, 0.78, and 0.12 pg T 3 · μg DNA −1, respectively. With the exception of the value obtained from the upstream migrants, the hepatocytes of lampreys have a higher binding capacity for T 3 than most other vertebrates. The decline in serum T 3 at the beginning of metamorphosis cannot be accounted for by increased binding of this hormone in the liver. It is still unclear whether T 3 plays any role in lamprey metamorphosis.
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