Binding of triiodothyronine to hepatocyte nuclei from sea lampreys, Petromyzon marinus L., at various stages of the life cycle.

The in vitro binding capacity of triiodothyronine (T3) to hepatocyte nuclei was determined for lampreys (Petromyzon marinus L.) at different phases of the life cycle. The binding of T3 to nuclei in ammocoetes (larvae) is of high affinity (Kd = 2.9 X 10(-10) M) with a maximum binding capacity of 1.89 pg T3 X micrograms DNA-1. Binding capacities of metamorphosing individuals, young parasitic adults, and upstream-migrant adults were 2.40, 0.78, and 0.12 pg T3 X micrograms DNA-1, respectively. With the exception of the value obtained from the upstream migrants, the hepatocytes of lampreys have a higher binding capacity for T3 than most other vertebrates. The decline in serum T3 at the beginning of metamorphosis cannot be accounted for by increased binding of this hormone in the liver. It is still unclear whether T3 plays any role in lamprey metamorphosis.