In vivo demonstration of the presence of specific saturable binding sites for triiodothyronine in nuclei of tadpole liver

Differences in nuclear thyroid hormone receptors among species

Hepatic nuclear thyroid hormone receptors from rat, dog, chicken, and rainbow trout were compared. Receptor affinities for 3,5,3'-triiodo-L-thyronine (T3) were similar in preparations from rat, dog, and chicken, using isolated nuclei and nuclear extracts. Rainbow trout nuclear receptor showed a lower affinity for T3. Almost half of the receptors were released into the medium with rat and chicken nuclei, and 79.7 +/- 1.1% of the receptors were released with rainbow trout nuclei, when isolated nuclei were incubated with T3 at 22 degrees for 2 hr. The affinity constant of rat liver receptor for calf thymus DNA-cellulose at 0.17 M KCl, pH 7.4, was 3.98 +/- 1.47 x 10(5) M-1, when determined using DNA-cellulose columns. The number of salt bridges involved in DNA binding of the rat receptor was 5.73 +/- 0.38. When receptor-DNA interactions were compared among species, significant differences were found, but the receptors from dog and rainbow trout liver were similar. Sephacryl S-200 column chromatography showed that chicken receptor had a Stokes radius significantly smaller than that of rat receptor. Partial proteolysis of T3-receptor complex using trypsin alpha-chymotrypsin, elastase, and papain produced distinct T3-binding fragments in different species. Our data provide evidence that nuclear thyroid hormone receptors from different species have significant structural dissimilarities.

Nuclear receptors for L-triiodothyronine in trout erythrocytes

We have developed an in vitro assay to evaluate saturable specific binding of triiodothyronine (T3) by erythrocyte (RBC) nuclei isolated from rainbow trout. Our results indicate that the nuclei contain a T3-saturable protein which binds T3 with temperature, and pH dependency, high T3 affinity (Ka = 1.6 X 10(9) M-1), and relative thyroid hormone (TH) analog affinities (TRIAC greater than T3 greater than T4 greater than rT3 greater than T2) which are characteristic of TH receptors in other vertebrates. Our estimate of the maximal T3 binding capacity (MBC) of nuclei isolated from heterogeneous populations of RBCs at different maturational stages (MBC = 3.6 fM/mg DNA; 13 sites/nucleus) was 10-100 times lower than would be expected of a TH-responsive tissue. Differential cell counts revealed that 1% of the RBCs in our trout were immature (pro-RBCs). Pro-RBCs, in contrast to mature RBCs, contain abundant heterochromatin, mitochondria, and polyribosomes, and synthesize hemoglobin. Evaluation of binding data for RBC nuclei taken from trout in which erythropoiesis was stimulated by prior bleeding indicated that MBC was directly proportional to the absolute number of pro-RBC nuclei in the incubation. Our maximum estimate of MBC for pro-RBC nuclei (458 fmol/mg DNA; 1781 sites/nucleus) falls within the range of MBC values reported for other vertebrate TH-responsive tissues. These data indicate that RBCs of rainbow trout contain a nuclear protein (putative receptor) which binds T3 with characteristics similar to the TH receptor of higher vertebrates, and that nuclear T3 binding may be diminished during RBC maturation.

Immunofluorescent detection and localization of thyroxine in blood of Rana catesbeiana from early larval through metamorphic stages

Indirect immunofluorescent staining was used to detect and localize thyroxine (T4) in blood smears from individual Rana catesbeiana tadpoles in almost every stage of larval development. Earlier radioimmunoassays revealed a surge of T4 in the blood plasma during metamorphic stages, but plasma T4 concentrations in earlier stages were either very low or below the minimal detectable limits of the assays. With the present immunofluorescent method, T4 was found in plasma of tadpoles throughout the entire larval period from early limb bud stages to the end of metamorphosis. Moreover, T4 was also found in association with cytoplasm and nuclei of red blood cells, particularly nuclei of the new population of adult red cells differentiating during metamorphic stages. In conclusion, thyroid hormone is present in both blood plasma and erythrocytes of R. catesbeiana from early through late stages of larval development.

Immersion of rainbow trout in 3,5,3'-triiodo-L-thyronine (T3): effects on plasma T3 levels and hepatic nuclear T3 binding

An immersion protocol was established for chronically administering thyroid hormone to fish. Rainbow trout were immersed in solutions of 3,5,3'-triiodo-L-thyronine (T3) at 11 degrees C for various periods. Most plasma T3 adjustment occurred by 24 hr and within 5 days consistent plasma T3 levels were observed. Availability of unoccupied hepatic nuclear T3-binding sites was assessed in two experiments by uptake of nuclear radioactivity in T3-immersed trout injected 12 hr previously with [125I]T3. At low ambient T3 levels over 92% of nuclear radioactivity was identified chromatographically and immunologically as T3, but at high ambient levels (25 micrograms/100 ml) biliary [125I]T3 metabolites may contaminate the nuclear fraction. Displacement of [125I]T3 from nuclei was first detected at 0.25 micrograms T3/100 ml water, corresponding to a plasma T3 level of 0.55 ng/ml; maximum displacement, depending on experiment, occurred at 1.25 and 3.8 micrograms T3/100 ml, corresponding to plasma T3 levels of 14 and 9 ng/ml. In conclusion, chronic physiologic T3 treatment of these trout can be achieved by immersion in T3 solutions of 0.1-5 micrograms T3/100 ml water.

3,5,3'-Triiodothyronine receptors and thyroxine 5'-monodeiodinating activity in thyroid hormone-insensitive amphibia

Adult anuran Amphibia and neotenous urodeles, such as Necturus maculosus, rarely respond to thyroid hormone (TH). In this study, the possibility was examined that this lack of response is due either to an inability to convert thyroxine (T4) to 3,5,3'-triiodothyronine (T3) or to the absence of nuclear TH receptors. Following injection of [125I]T4, significant amounts of [125I]T3, analyzed by chromatography, were detected in the serum and liver of Necturus and Rana catesbeiana frog indicating that both possesses a T4 5'-monodeiodinating system. Nuclear binding of T3 was studied in suspensions of purified hepatic nuclei and intact red blood cells (RBC). Analysis of binding data revealed that frog liver nuclei contained two sets of saturable T3 binding sets with affinities comparable to those of the two sets of sites previously demonstrated in tadpole liver nuclei (Galton and Schaafsma, 1983). However, the number of sites per nuclei was small compared to tadpole; expressed as pmol/mg DNA, the maximum binding capacity of the high affinity set of sites (MBC1) was 2.2 +/- 0.31 versus 12.9 +/- 1.80 and MBC2 was 24.9 +/- 4.5 versus 42.2 +/- 54. Receptor number in RBC nuclei was also smaller in frog than tadpole: 90 +/- 26 versus 882 +/- 56 sites/nucleus (Kd less than 10(-11) M in both groups). No comparable high affinity binding sites were detected in Necturus liver, but some sites were found in Necturus RBC. These cells contained 2111 +/- 120 sites/nucleus, more than twice the number found in tadpole.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear receptors for L-triiodothyronine in quail liver

Binding studies were conducted using in vitro-labeled quail liver nuclei to identify and characterize receptors for L-triiodothyronine (T3). Saturation binding experiments were analyzed by Scatchard analysis and indicated a single class of high-affinity, limited-capacity T3 binding sites. These receptors exhibited binding specificity as demonstrated by competition experiments between labeled T3 and unlabeled thyroid hormones or hormone analogs. Binding specificity was virtually identical in quail and rat liver nuclei. Thus, apparent differences between mammals and birds, in regard to the biological potency of T4 versus T3, are not apparent at the level of the nuclear T3 receptor.

Corticoids augment nuclear binding capacity for triiodothyronine in bullfrog tadpole tail fins

The effect of aldosterone and corticosterone on T3 binding to the nuclear fraction of the tail fin of bullfrog tadpoles was investigated in vitro. Both corticoids (10(-7) - 10(-5) M) added to the incubation medium increased T3 binding in a dose-dependent manner. The aldosterone-induced increase in T3-binding was blocked by cycloheximide, an inhibitor of protein synthesis, and actinomycin D, an inhibitor of RNA synthesis. Scatchard analysis revealed that 10(-6) M aldosterone and corticosterone increased the maximum binding capacity for T3 by 60 and 41%, respectively, and that the corticoids did not alter the value of the dissociation constant. The significance of the finding is discussed in relation to metamorphosis.

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.