Failure of dinitrotyrosine to affect the TSH-induced acute depression of thyroidal iodide transport.

Abstract

Dinitrotyrosine (DNT), a potent inhibitor of thyroid iodotyrosine deiodinase, failed to affect the acute TSH-induced depression of the thyroid-.serum radioiodide concentration [T/S(I~)]. This finding lends support to the concept that the acute TSH-induced depression of T/S(I~) is not caused by expansion of the intrathyroidal iodide pool consequent upon increased proteolysis of thyroglobulin and deiodination of free iodotyrosines following TSH administration. (Endocrinology 88:1277,1971) T (TSH)-induced release of iodide from thyroids was reported by Becker et al. (1) in 1953 and other groups (2). Halmi et al. (3) described a biphasic effect of TSH on thyroid iodide concentration in the rat with an early depression of the thyroid: serum radioiodide ratio [T/S(I~)], 4-5 hr after injection of TSH, followed by a later rise which exceeded the pre-TSH levels. The early depression was explained by them as due to an increased exit rate for iodide (4). Rosenberg (2) and Wolff (5) considered TSH-induced hydrolysis of thyroglobulin with subsequent deiodination of iodotyrosines and expansion of the thyroid iodide pool as another possible cause. However, studies (3) in iodine-deprived rats whose thyroid glands would contain relatively small amounts of iodotyrosines showed a TSHinduced early depression of the T/S(I~), similar to that of animals not deprived of iodine. In the present experiments, the problem has been explored further by inhibiting deiodination of free intrathyroidal iodotyrosines by administration of dinitrotyrosine (DNT). Materials and Methods Experiments with in situ perfusion. To demonstrate the inhibitory effect of DNT administered in vivo on intrathyroidal deiodination of iodotyrosines, male Sprague-Dawley rats weighing 200-300 g were fed Purina laboratory chow or a low iodine diet (approximately 30 Mg I/kg, General BioReceived February 26, 1970. This investigation was supported by Grants AM-01447 and AM-05065 from the National Institute of Arthritis and Metabolic Diseases, NIH. The work was completed with support from Grant 36X0161, Medical Research VA, 1970. 1 Trainee in Endocrinology of the National Institute of Arthritis and Metabolic Diseases. Present address: Veterans Administration Hospital, Jefferson Barracks Division, St. Louis, Mo. 63125. chemicals, Inc., Chagrin Falls, Ohio). They were injected with carrier-free I~ 16—24 hr and with DNT 105-120 min before exsanguination. Sufficient I was injected ip to obtain a thyroid content of 30-60 /uCi at the time of perfusion. DNT was prepared as described below and injected sc in the doses shown in Table 1. Immediately after sacrifice, the thyroid glands were perfused in situ by the technique developed in this laboratory (6). The entering perfusate contained bovine thyrotropin (TSH) (a gift of the National Institute of Health, lot NIH-B4), 1 U/100 ml, to maintain thyroid secretion. The total radioactivity secreted per min and the relative distribution of labeled compounds did not appreciably change over the first 30 min of perfusion when TSH was added to the perfusate (6). The perfusion lasted 30-45 min. Two or 3 thyroid effluent collections were made. Only the last one is presented in Table 1. Thyroid effluent plasma, pancreatin digests of the glands removed after perfusion, and the plasma obtained during exsanguination before perfusion were analyzed by paper chromatography in butanol-ethanol-ammonia (BEA) or butanol-acetic acid-water (BAW) systems to determine the relative quantitative distribution of labeled iodoamino acids and iodide (6). Origin material was not included in the calculation of the relative distribution. The degree of inhibition of iodotyrosine deiodination was estimated by comparison of the per cent of labeled MIT and DIT in the effluent with that in the thyroid digest. The mean effluent percentage iodotyrosines of all DNT-treated rats in Table 1 (57.3+1.8 SEM) was divided by that of the digest (68.9+2.1 SEM), which was 83%. It was assumed that the inhibition is complete when the percentage iodotyrosines secreted equals that of the digested thyroglobulin. Greer and Grimm (7) achieved almost complete inhibition by this criterion when they perfused prelabeled thyroids with blood containing 10~ and 10~M DNT. Experiments involving T/S(I~) ratios. Male Sprague-Dawley rats weighing 120-150 g were fed Purina chow and given tap water ad lib. TSH was obtained from the Armour Pharmaceutical Co.,