Thyroid hormones and putative nuclear T3 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]T3 and its distribution in the same tissues was determined from 6 to 48 hr after injection. Last, the saturable binding of [125I]T3 to salt-extracted nuclear proteins in the pharynx and alimentary canal was studied in vitro. T4 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 T3 concentrations were always much lower than T4 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 T3 was generally below 0.02 ng/ml. The tissue distribution of [125I]T3 was correlated with T3 concentrations. Tissue/plasma ratios were approximately 10 in the alimentary canal, 5 in the pharynx, and 0.18 in the tunic. Saturable binding of T3 to nuclear proteins in the alimentary canal and pharynx was demonstrated. The affinity (Kd) was similar to that found in tissues from other chordates but the maximal binding capacity was much lower. The very low levels of plasma T3 and low T3/T4 ratios may indicate that the endostyle releases primarily T4 into the body fluid. On the other hand, the high levels of T3 and the high T3/T4 ratios in the alimentary canal suggest that this metabolically active target tissue is the main site of the process of deiodination of T4 into T3, a process which has been previously shown in P. mammillata in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)
Fredriksson
G
G
Laboratoire de Physiologie Générale et Comparée du Muséum, CNRS, URA 90, Paris, France.
Lebel
J M
JM
Leloup
J
J
D000818:Animals; D002467:Cell Nucleus / Q000737:chemistry*; D002816:Chordata, Nonvertebrate / Q000737:chemistry*; D004064:Digestive System / Q000737:chemistry; D008566:Membranes / Q000737:chemistry; D009928:Organ Specificity / Q000502:physiology; D010614:Pharynx / Q000737:chemistry; D011988:Receptors, Thyroid Hormone / Q000032:analysis*; D013963:Thyroid Hormones / Q000032:analysis*
D011988:Receptors, Thyroid Hormone; D013963:Thyroid Hormones