3,5,3′-Triiodothyronine receptors and thyroxine 5′-monodeiodinating activity in thyroid hormone-insensitive Amphibia

Abstract

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 (T 4) to 3,5,3′-triiodothyronine (T 3) or to the absence of nuclear TH receptors. Following injection of [ 125I]T 4, significant amounts of [ 125I]T 3, analyzed by chromatography, were detected in the serum and liver of Necturus and Rana catesbeiana frog indicating that both possesses a T 4 5′-monodeiodinating system. Nuclear binding of T 3 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 T 3 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 (MBC 1) was 2.2 ± 0.31 versus 12.9 ± 1.80 and MBC 2 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 ( K d < 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. When expressed per milligram DNA, however, the MBC was only one eighth of that in the tadpole. These findings are compatible with the view that a low receptor number may account for the lack of response of frog and Necturus to TH.