Thyroid Hormones and Putative Nuclear T 3 Receptors in Tissues of the Ascidian, Phallusia mammillata Cuvier

Thyroid hormones were measured by radioimmunoassay in blood plasma and in extracts (butanol/chloroform/ammonia) of pharynx, alimentary canal, and tunic of Phallusia mammillata. Other animals were injected with [ 125I]T 3 and its distribution in the same tissues was determined from 6 to 48 hr after injection. Last, the saturable binding of [ 125I]T 3 to salt-extracted nuclear proteins in the pharynx and alimentary canal was studied in vitro. T 4 was found in all tissues examined and in the same order of magnitude (2.7 to 8.4 ng/g) whereas plasma concentration was low (0.2 ng/ml). Tissue T 3 concentrations were always much lower than T 4 tissue concentrations, being highest in alimentary canal (0.8-1.1 ng/g) and very low in the tunic as well as in plasma, in which T 3 was generally below 0.02 ng/ml. The tissue distribution of [ 125I]T 3 was correlated with T 3 concentrations. Tissue/plasma ratios were approximately 10 in the alimentary canal, 5 in the pharynx, and 0.18 in the tunic. Saturable binding of T 3 to nuclear proteins in the alimentary canal and pharynx was demonstrated. The affinity ( K d ) was similar to that found in tissues from other chordates but the maximal binding capacity was much lower. The very low levels of plasma T 3 and low T 3/T 4 ratios may indicate that the endostyle releases primarily T 4 into the body fluid. On the other hand, the high levels of T 3 and the high T 3/T 4 ratios in the alimentary canal suggest that this metabolically active target tissue is the main site of the process of deiodination of T 4 into T 3, a process which has been previously shown in P. mammillata in vivo. The data indicate putative T 3 receptors in ascidian tissue, suggesting a physiological role of T 3 in protochordates.