Thyroid Hormone Regulation of Extrathyroidal Iodoproteins ALAN BALSAM A N D FRANKLIN C. SEXTON
from [I25I]T3 was markedly slowed in hypothyroid animals and accelerated in intact controls rendered hyperthyroid with daily injections of T4, 8 /xg/100 g BW. Propylthiouracil (PTU) treatment of thyroidectomized rats maintained on T4, 2 /ng/100 g BW per day resulted in increased NEI25I from [I25I]T3 in plasma, kidney, and liver. The results of the foregoing investigations suggest that thyroid hormone regulates levels of iodothyronine-derived iodoproteins by influencing the rate of degradation of iodoproteins. Moreover, the observed elevation of iodoprotein levels in T4-maintained thyroidectomized animals after PTU administration appears consistent with the modification of thyroid status due to the peripheral antithyroxine effect of PTU. (Endocrinology 98: 476, 1976)
ABSTRACT. The effect of thyroid status on plasma and tissue levels of labeled nonextractable iodine (NEI) derived from the metabolism of radioiodothyronines was examined in the rat. Concentrations of radioiodoprotein were substantially elevated in plasma, kidney, and liver in thyroidectomized animals 72 h postinjection of [125I]triiodothyronine ([125I]T3). Similarly, total rat concentrations of radioactive NEI were increased (52%) 72 h after injection of [125I]T3. NE125I concentrations from [125I]T3 in plasma, kidney, and liver were diminished progressively in thyroidectomized animals maintained on increasing doses of thyroxine replacement, demonstrating that iodoprotein levels were inversely related to thyroid state. The plasma disappearance rate of radioiodoprotein
D
URING the metabolism of radioiodothyronines, labeled iodoproteins accumulate in plasma and tissues in euthyroid man and in the rat (1,2). Surks, Schwartz, and Oppenheimer have demonstrated that these radioiodoproteins are derived in extrathyroidal tissues and consist of, in part, a covalent complex of tracer iodothyronine and protein molecules (1,3,4). Although the mechanism of translocation of hormonal radioiodine to protein has not been elucidated, previous investigations have suggested a direct relationship between iodothyronine deiodination and iodoprotein formation. Galton and Ingbar (5) demonstrated that iodoprotein formation was associated with the liberation of inorganic iodide in tissue homogenates incubated with tracer thyroxine. Surks, Schwartz, and Oppenheimer (2) observed increased iodoprotein concentrations in plasma and liver after injection of labeled thyroid hormones in the rat following hepatic deiodinase induction Received July 30, 1975. Reprints: Dr. Alan Balsam, Division of Endocrinology, Beth Israel Hospital, 330 Brookline Avenue, Boston, Massachusetts 02215.
with phenobarbital. Additional determinants of iodoprotein concentrations in plasma and tissues have not been identified. The present studies demonstrate that thyroidal status modulates the levels of plasma and tissue iodoproteins by regulating the rate of iodoprotein disposal. Materials and Methods Male Sprague Dawley rats weighing 150200 g were used in all studies. Thyroidectomized (TX) rats were obtained from Charles River (Wilmington, Mass.). Animals were supplied a diet of pelleted rations (iodine content 1 jug/g ration) and drinking water ad libitum. Iodothyronines labeled in the /3 ring with either 131I or I25I were purchased from commercial sources. [I25I]triiodo-L-thyronine ([125I]T3; SA, 70-75 fiCVfig) and [125I]-L-thyroxine ([125I]T4; SA, 50-75 fiCViAg) were obtained from Abbott Laboratories (North Chicago, Illinois). [131I]triiodo-L-thyronine; ([I31I]T3; SA, 25-35 and [131I]-L-thyroxine ([131I]T4; SA, 27 were obtained from Amersham-Searle (Amersham, England). Labeled iodothyronines were diluted in 1% human serum albumin and purified by dialysis. Plasma and tissue concentrations of nonextractable iodine were assessed by the method of Surks, Schwartz, and Oppenheimer (2).
476
Downloaded from https://academic.oup.com/endo/article-abstract/98/2/476/2619612 by Leiden University / LUMC user on 15 January 2019
Veterans Administration Hospital and the Department of Internal Medicine, the University of Texas Southwestern Medical School, Dallas, Texas
EXTRATHYROIDAL NEI AND THYROID STATUS 125
enate were counted, extracted with ethanol, and reassayed to determine the concentration of NE125I in the carcass. The influence of thyroid status on plasma and tissue levels of NE125I derived from 125I-T3 was assessed in TX animals injected with L-thyroxine. Groups of 4 animals were injected SC daily with 0.5, 2.0, 4.0, and 8.0 /xg/100 g BW for 18 days. Seventy-two hours after injection of [125I]T3 the concentrations of NE125I were measured in plasma, kidney, and liver. The composite plasma disappearance rate of NE125I after injection of [125I]T3 was assessed in INT and TX animals. Plasma concentrations of NE125I were measured in animals sacrificed 3, 7, 10 and 17 days postinjection of [125I]T3. Plasma disappearance of NEI25I derived from [I25I]T3 was similarly measured in INT and hyperthyroid (injected with 8 /xg T4/100 g BW for 16 days) subgroups of 3-6 animals sacrificed 3, 6, and 9 days after injection of the tracer iodothyronine. Plasma disappearance rate of labeled NEI was analyzed using single compartment kinetics. The effect of treatment with 6-n-propylthiouracil (PTU), an agent which inhibits the monodeiodination of T4 to T3, was assessed on iodoprotein levels as follows. TX animals were divided into two groups; one group was injected daily with 2 fjLg T4 per 100 g BW and supplied 0.1% PTU EXP. 2
EXP. 1
FIG. 1. Plasma and tissue concentrations of NEI25I were measured 72 h postinjection of [I25I]T3 in experiments (EXP) 1 and 2: EXP 1, intact (INT) and thyroidectomized (TX) animals were studied; EXP 2, TX animals were rendered athyreotic by radioablation with 131I. Vertical bars denote mean ± SE and statistical comparison by Student's t test in all figures.
PLASMA KIDNEY 5- (% dose/ml (% dose/g xiO 2 ) xiO 3 ) p
from [I25I]T3 was markedly slowed in hypothyroid animals and accelerated in intact controls rendered hyperthyroid with daily injections of T4, 8 /xg/100 g BW. Propylthiouracil (PTU) treatment of thyroidectomized rats maintained on T4, 2 /ng/100 g BW per day resulted in increased NEI25I from [I25I]T3 in plasma, kidney, and liver. The results of the foregoing investigations suggest that thyroid hormone regulates levels of iodothyronine-derived iodoproteins by influencing the rate of degradation of iodoproteins. Moreover, the observed elevation of iodoprotein levels in T4-maintained thyroidectomized animals after PTU administration appears consistent with the modification of thyroid status due to the peripheral antithyroxine effect of PTU. (Endocrinology 98: 476, 1976)
ABSTRACT. The effect of thyroid status on plasma and tissue levels of labeled nonextractable iodine (NEI) derived from the metabolism of radioiodothyronines was examined in the rat. Concentrations of radioiodoprotein were substantially elevated in plasma, kidney, and liver in thyroidectomized animals 72 h postinjection of [125I]triiodothyronine ([125I]T3). Similarly, total rat concentrations of radioactive NEI were increased (52%) 72 h after injection of [125I]T3. NE125I concentrations from [125I]T3 in plasma, kidney, and liver were diminished progressively in thyroidectomized animals maintained on increasing doses of thyroxine replacement, demonstrating that iodoprotein levels were inversely related to thyroid state. The plasma disappearance rate of radioiodoprotein
D
URING the metabolism of radioiodothyronines, labeled iodoproteins accumulate in plasma and tissues in euthyroid man and in the rat (1,2). Surks, Schwartz, and Oppenheimer have demonstrated that these radioiodoproteins are derived in extrathyroidal tissues and consist of, in part, a covalent complex of tracer iodothyronine and protein molecules (1,3,4). Although the mechanism of translocation of hormonal radioiodine to protein has not been elucidated, previous investigations have suggested a direct relationship between iodothyronine deiodination and iodoprotein formation. Galton and Ingbar (5) demonstrated that iodoprotein formation was associated with the liberation of inorganic iodide in tissue homogenates incubated with tracer thyroxine. Surks, Schwartz, and Oppenheimer (2) observed increased iodoprotein concentrations in plasma and liver after injection of labeled thyroid hormones in the rat following hepatic deiodinase induction Received July 30, 1975. Reprints: Dr. Alan Balsam, Division of Endocrinology, Beth Israel Hospital, 330 Brookline Avenue, Boston, Massachusetts 02215.
with phenobarbital. Additional determinants of iodoprotein concentrations in plasma and tissues have not been identified. The present studies demonstrate that thyroidal status modulates the levels of plasma and tissue iodoproteins by regulating the rate of iodoprotein disposal. Materials and Methods Male Sprague Dawley rats weighing 150200 g were used in all studies. Thyroidectomized (TX) rats were obtained from Charles River (Wilmington, Mass.). Animals were supplied a diet of pelleted rations (iodine content 1 jug/g ration) and drinking water ad libitum. Iodothyronines labeled in the /3 ring with either 131I or I25I were purchased from commercial sources. [I25I]triiodo-L-thyronine ([125I]T3; SA, 70-75 fiCVfig) and [125I]-L-thyroxine ([125I]T4; SA, 50-75 fiCViAg) were obtained from Abbott Laboratories (North Chicago, Illinois). [131I]triiodo-L-thyronine; ([I31I]T3; SA, 25-35 and [131I]-L-thyroxine ([131I]T4; SA, 27 were obtained from Amersham-Searle (Amersham, England). Labeled iodothyronines were diluted in 1% human serum albumin and purified by dialysis. Plasma and tissue concentrations of nonextractable iodine were assessed by the method of Surks, Schwartz, and Oppenheimer (2).
476
Downloaded from https://academic.oup.com/endo/article-abstract/98/2/476/2619612 by Leiden University / LUMC user on 15 January 2019
Veterans Administration Hospital and the Department of Internal Medicine, the University of Texas Southwestern Medical School, Dallas, Texas
EXTRATHYROIDAL NEI AND THYROID STATUS 125
enate were counted, extracted with ethanol, and reassayed to determine the concentration of NE125I in the carcass. The influence of thyroid status on plasma and tissue levels of NE125I derived from 125I-T3 was assessed in TX animals injected with L-thyroxine. Groups of 4 animals were injected SC daily with 0.5, 2.0, 4.0, and 8.0 /xg/100 g BW for 18 days. Seventy-two hours after injection of [125I]T3 the concentrations of NE125I were measured in plasma, kidney, and liver. The composite plasma disappearance rate of NE125I after injection of [125I]T3 was assessed in INT and TX animals. Plasma concentrations of NE125I were measured in animals sacrificed 3, 7, 10 and 17 days postinjection of [125I]T3. Plasma disappearance of NEI25I derived from [I25I]T3 was similarly measured in INT and hyperthyroid (injected with 8 /xg T4/100 g BW for 16 days) subgroups of 3-6 animals sacrificed 3, 6, and 9 days after injection of the tracer iodothyronine. Plasma disappearance rate of labeled NEI was analyzed using single compartment kinetics. The effect of treatment with 6-n-propylthiouracil (PTU), an agent which inhibits the monodeiodination of T4 to T3, was assessed on iodoprotein levels as follows. TX animals were divided into two groups; one group was injected daily with 2 fjLg T4 per 100 g BW and supplied 0.1% PTU EXP. 2
EXP. 1
FIG. 1. Plasma and tissue concentrations of NEI25I were measured 72 h postinjection of [I25I]T3 in experiments (EXP) 1 and 2: EXP 1, intact (INT) and thyroidectomized (TX) animals were studied; EXP 2, TX animals were rendered athyreotic by radioablation with 131I. Vertical bars denote mean ± SE and statistical comparison by Student's t test in all figures.
PLASMA KIDNEY 5- (% dose/ml (% dose/g xiO 2 ) xiO 3 ) p
