Development-associated changes in thyroxine kinetics in juvenile salmon.

Smoltification is a transformation that occurs in some species of salmon, during which solitary fish in fresh water become schooling fish and migrate to the sea. This process is accompanied by large increases in plasma T4. T4 secretion rate and other parameters of T4 metabolism in juvenile coho salmon were estimated by applying kinetic analyses to measurements of the disappearance of injected T4 radiotracer from plasma. Studies were performed at the beginning (March) and end (May) of the increase in T4 concentration in fresh water and seawater. Early and intensive sampling permitted characterization of a very fast initial component of the T4 disappearance curve when analyses included a zero time datum derived from an independent estimate of plasma volume. The plasma volume, equal to 1.77% of body weight, was obtained by measuring the disappearance of radiolabeled albumin from the plasma in two other groups of animals in fresh water and seawater. There were 3- to 7-fold changes in T4 production, distribution, and metabolism between March and May, whereas environment (fresh water vs. seawater) had relatively minor effects on T4 kinetics. In fresh water, the T4 secretion rate was 4.48 ng/h in March and 1.50 ng/h in May. The total T4 pool size was 37.8 ng in March and 12.2 ng in May. Plasma-tissue T4 fluxes were 3- to 7-fold greater in May. Relatively less T4 was distributed in tissue in May (63% vs. 83%), and T4 spent much less time in tissue in May than in March during each pass through the tissue space (11 min vs. 3.1 h). We propose that the difference in secretion rate and a redistribution of T4 between blood and tissues contribute to both the rise and fall in the plasma T4 concentration between March and May. Changes in T4 kinetics during salmonid smoltification resemble those occurring during amphibian metamorphosis and mammalian gestation and neonatal life, and may reflect an increased requirement and an important role for thyroid hormones during periods of rapid development in vertebrates in general.