Inhibitory Effect Of Iodide Research Articles

Effect of Iodide on Human Choriogonadotropin, Sodium-Iodide Symporter Expression, and Iodide Uptake in BeWo Choriocarcinoma Cells

Active placental transport of maternal iodide by the thyroidal sodium iodide symporter (NIS) provides an essential substrate for fetal thyroid hormone synthesis. NIS is expressed in trophoblast and is regulated by human choriogonadotropin (hCG). In thyroid, iodide down-regulates expression of several genes including NIS. Placentas of iodine-deficient rats demonstrate up-regulation of NIS mRNA, suggesting a role for iodide in regulating placental NIS. The objectives were to examine effects of iodide on expression of NIS and hCG in BeWo choriocarcinoma cells. Gene expression was studied by quantitative real-time PCR. Effects on NIS protein expression were assessed by Western blotting. Functional activity of NIS was measured by (125)I uptake. Expression of hCG protein was assessed by immunoassay of secreted hormone. Iodide inhibited NIS mRNA and membrane protein expression as well as (125)I uptake, which were paralleled by decreased betahCG mRNA expression and protein secretion. Iodide had no effects on pendrin expression. Addition of hCG increased NIS mRNA expression. This effect was partially inhibited by addition of iodide. The inhibitory effects of iodide on NIS mRNA expression were abolished by propylthiouracil and dithiothreitol. We conclude that expression of placental NIS is modulated by maternal iodide. This may occur through modulation of hCG effects on NIS and hCG gene expression.

Kinetic-photometric determination of iodide based on its inhibitory effect on the bromate oxidation of Victoria Blue 4-R.

A new sensitive kinetic method has been developed for the determination of trace amounts of I- over the range of 50-300 ng cm(-3). The method was based on the inhibitory effect of iodide on the oxidation of Victoria Blue 4-R by KBrO3. The reaction is monitored spectrophotometrically at 596.3 nm. Method development includes optimization of reagent concentration and temperature. The kinetic parameters of the reaction were reported and a rate equation was suggested. The effects of certain foreign ions upon the reaction rate were determined for the assessment of the selectivity of the method. The proposed method was successfully applied to the determination of iodide in real samples. The new developed method was found to have fairly good selectivity, sensitivity, simplicity and rapidity.

The G2M arrest caused by iodide is unrelated to the effects of iodide at adenylate cyclase.

Previously, we have shown that the iodide was able to inhibit TSH induced thyrocyte proliferation by arresting the cell cycle at G0G1 and G2M, suggesting that the iodide may be exerting its effects through more than the TSH-adenylate cyclase-cAMP system. To confirm the effects of iodide on the adenylate cyclase (AC) system, forskolin- and dibutyryl-cyclic-AMP (dBcAMP)-stimulated FRTL5 thyroid cells were exposed to inhibitory concentrations of iodide and the resultant effects on the cell cycle were compared to the effects observed with TSH, using flow cytometric DNA analysis. Forskolin stimulated the proliferation of FRTL5 cells in a dose-dependent manner. Cell numbers rose from baseline by 169 +/- 4% to peak at 10 microM forskolin. Interestingly, 100 microM forskolin inhibited cell proliferation, causing cell numbers to fall by approximately 50%. Iodide inhibited forskolin-induced proliferation to baseline levels. However, the pattern of cell cycle perturbation was different to that with TSH-stimulated cells. There were no differences in the proportion of cells in G0G1 between forskolin alone and forskolin + NaI, while there was a marked fall in the proportion of cells in S phase, indicating possible partial arrest at G0G1. Furthermore, there was a marked accumulation of cells in G2M over and above that found with TSH + NaI, indicating arrest at G2M. dBcAMP maximally stimulated cell numbers to rise from baseline by 125% with 1 mM dBcAMP. Again, higher concentrations of the mitogen had an inhibitory effect on proliferation. The addition of NaI inhibited dBcAMP stimulated cell proliferation.

Transforming growth factor-β1 synthesis in human thyroid follicular cells: differential effects of iodide and plasminogen on the production of latent and active peptide forms

The present study has investigated the relative levels and interconversion of latent and active forms of transforming growth factor-beta 1 (TGF-beta 1) in human thyroid follicular cell cultures derived from sporadic non-toxic goitres. Northern blotting of RNA extracted from 72-h cultures revealed a 2.5 kb mRNA transcript hybridizing with a cDNA probe for latent TGF-beta 1, the intensity of which was doubled in cells exposed to NaI (10 mumol/l). Radioreceptor assay of follicular cell-conditioned medium for TGF-beta 1 content confirmed a similar enhancing effect of iodide. The endogenous active component of TGF-beta 1 present in conditioned medium represented only a minor fraction of the total TGF-beta 1 content, this fraction was not enhanced by exposure of follicular cells to iodide. The low level of endogenous active TGF-beta 1 in medium conditioned by either control or iodide-treated cells was confirmed by immunoadsorption with a precipitating antiserum against active TGF-beta 1, when such cells failed to show a reversal of the iodide-induced decrease in [methyl-3H]thymidine incorporation into trichloroacetic acid-precipitable material. In contrast to the inhibitory effect of iodide on de novo DNA synthesis, which appeared not to reflect an increase in active TGF-beta 1, the inhibitory effects of plasminogen (100 mg/l) or plasmin (2000 U/l) on [methyl-3H]thymidine incorporation into thyroid cells were reversible by TGF-beta 1 immunoadsorption. This provides evidence that the plasmin-mediated inhibition of DNA synthesis in thyroid follicular cells may be attributed to the growth-regulating action of an increased level of activated TGF-beta 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that the inhibitory effects of iodide on thyroid cell proliferation are due to arrest of the cell cycle at G0G1 and G2M phases.

The thyroid gland is unique in its ability to respond to ambient levels of iodine to autoregulate thyroid function and, possibly, thyroid cell proliferation. Although the inhibitory effects of iodide on thyroid cell proliferation have been previously reported, the exact mechanism and site of action of iodide on cellular proliferation events are poorly understood. Our initial experiments established the optimal cell plating density and timing to achieve exponential cell growth of FRTL5 thyroid cells, and subsequent studies using flow cytometric DNA analysis established the normal cell cycle kinetics of FRTL5 thyroid cell proliferation. FRTL5 cells were then exposed to graded concentrations of sodium iodide to establish whether the inhibitory effects of iodide are mediated through specific cell cycle events. We observed that increasing concentrations of iodide inhibited FRTL5 thyroid cell proliferation. Analysis of the cell cycle revealed two specific effects of iodide on cell cycle kinetics. The first was an arrest of cells in G0G1, evidenced by an accumulation of cells in this phase and a concomitant reduction in the percentage of cells in the S-phase. The second was an arrest of cells in the G2M phase of the cycle. G0G1 and G2M arrest occurred within 24 h and then reached a plateau. Iodide exposure did not increase the number of cells undergoing necrosis. The addition of methimazole at two concentrations (0.2 and 2 mM) to cells exposed to 100 mM NaI prevented the accumulation of cells in G2M, but did not abolish the accumulation of cells in G0G1 or the reduction in cell number. These results indicate that the inhibitory effects of iodide on FRTL5 thyroid cell proliferation are mediated by its action at two critical regulating points of the cell cycle, G0G1 and G2M. It appears that organified iodine may mediate the cell cycle arrest in the G2M phase, whereas inorganic iodide may be responsible for the inhibitory effects at G0G1.

Evidence that the inhibitory effects of iodide on thyroid cell proliferation are due to arrest of the cell cycle at G0G1 and G2M phases

Evidence that the inhibitory effects of iodide on thyroid cell proliferation are due to arrest of the cell cycle at G0G1 and G2M phases

Thyroid hormone synthesis in thyroglobulin secreted by porcine thyroid cells cultured on porous bottom chambers. Effect of iodide.

The long-term iodination of thyroglobulin secreted into the apical medium of thyroid cells cultured as monolayers on porous bottom chambers reached 5.87 +/- 1.66 atoms of iodine/mol thyroglobulin after 11 days incubation in the presence of TSH (0.1 mU/ml) and iodide (0.5 microM) in the basal medium. This iodinated thyroglobulin contained thyroid hormones (T3 + T4) which involved 22.7% of the thyroglobulin iodine content. The iodoamino acid content was, in residues per mole, 2.2 +/- 0.35 for monoiodotyrosine, 0.74 +/- 0.04 for diiodotyrosine, 0.23 +/- 0.04 for T4, and 0.098 +/- 0.02 for T3. Kinetic studies showed that a minimal level of iodination (2.05 +/- 0.26 atoms iodine/mol thyroglobulin) was necessary for hormonogenesis. A maximal level of iodination and hormonogenesis was obtained with 0.5 microM iodide added daily to the basal medium. In these conditions, hormonogenesis efficiency reached about 40% (a value close to this one observed in vivo). Above 0.5 microM iodide, both iodination and T4 synthesis were inhibited (28.3% and 73.9%, respectively, for 1 microM iodide). Our culture system makes it possible to demonstrate that this high iodide concentration in the basal medium did not increase apical iodide concentration above 10 microM but decreased apical thyroglobulin concentration. The inhibitory effect of iodide on hormonogenesis cannot be due to a competition with tyrosine residues of thyroglobulin for their binding to thyroperoxidase although it could be related, at least in part, to a decrease in protein synthesis.

Thyroid hormone synthesis in thyroglobulin secreted by porcine thyroid cells cultured on porous bottom chambers. Effect of iodide

The long-term iodination of thyroglobulin secreted into the apical medium of thyroid cells cultured as monolayers on porous bottom chambers reached 5.87 +/- 1.66 atoms of iodine/mol thyroglobulin after 11 days incubation in the presence of TSH (0.1 mU/ml) and iodide (0.5 microM) in the basal medium. This iodinated thyroglobulin contained thyroid hormones (T3 + T4) which involved 22.7% of the thyroglobulin iodine content. The iodoamino acid content was, in residues per mole, 2.2 +/- 0.35 for monoiodotyrosine, 0.74 +/- 0.04 for diiodotyrosine, 0.23 +/- 0.04 for T4, and 0.098 +/- 0.02 for T3. Kinetic studies showed that a minimal level of iodination (2.05 +/- 0.26 atoms iodine/mol thyroglobulin) was necessary for hormonogenesis. A maximal level of iodination and hormonogenesis was obtained with 0.5 microM iodide added daily to the basal medium. In these conditions, hormonogenesis efficiency reached about 40% (a value close to this one observed in vivo). Above 0.5 microM iodide, both iodination and T4 synthesis were inhibited (28.3% and 73.9%, respectively, for 1 microM iodide). Our culture system makes it possible to demonstrate that this high iodide concentration in the basal medium did not increase apical iodide concentration above 10 microM but decreased apical thyroglobulin concentration. The inhibitory effect of iodide on hormonogenesis cannot be due to a competition with tyrosine residues of thyroglobulin for their binding to thyroperoxidase although it could be related, at least in part, to a decrease in protein synthesis.

Iodide induces transforming growth factor beta 1 (TGF-beta 1) mRNA in sheep thyroid cells.

We examined TGF-beta mRNA levels in primary sheep thyroid cell cultures to determine whether the inhibitory effects of iodide on thyroid cells could be explained by an induction of TGF-beta mRNA and if this induction was mediated by iodine organification. Thyroid cells were incubated with TSH and five additives (insulin, somatostatin, growth hormone, transferrin, and glycyl-L-histidyl-L-lysin) for 2-3 weeks and then were exposed to sodium iodide (NaI) or 1-methylimidazole-2-thiol (methimazole, MMI), or both for 72 h. Iodide at 10(-6) M and 10(-4) M significantly increased the amount of TGF-beta mRNA as determined by Northern blot analysis with a rat TGF-beta 1 cDNA probe. This increase in TGF-beta 1 mRNA was abolished by the addition of methimazole, an inhibitor of organification. These data indicate that the effects of iodide on thyroid growth and function may be mediated by a process that involves organification of iodide and increases in TGF-beta 1 mRNA levels.

Iodide autoregulation of functional and morphological differentiation events in the FRTL-5 rat thyroid cell strain

The role of cyclic AMP (cAMP) attenuation in mediating the autoregulatory actions of iodide on thyroid cell iodide uptake and surface morphological responses to TSH was investigated in the rat thyroid cell strain FRTL-5. Preincubation of cells for 6 h with up to 1 mmol sodium iodide/l led to a progressive reduction in both accumulation of cAMP and iodide uptake responses to TSH. Forskolin-mediated accumulation of cAMP and iodide uptake responses were similarly reduced after preincubation with iodide, whilst the iodide accumulation response to dibutyryl cAMP (dbcAMP) was unaffected. The inhibitory effects of iodide on TSH or forskolin-responsive iodide accumulation were not seen if preincubation was limited to 3 h, and were also abolished by the thionamide drug methimazole (1 mmol/l). Medium containing 1 mumol iodide/l prevented the appearance of the surface microvilli and pseudopodia normally observed after re-addition of TSH or forskolin, although cytoplasmic retraction was still apparent under such conditions. In contrast, iodide was without effect on the ability of dbcAMP (1 mmol/l) to induce cytoplasmic retraction and the formation of microvilli and pseudopodia. Inclusion of 1 mmol sodium perchlorate/l together with iodide during preincubation failed to prevent or reduce the suppression by iodide of either iodide uptake or surface morphological differentiation, suggesting that both aspects of autoregulation may involve surface actions of organified iodide. These observations indicate that in FRTL-5 cells, autoregulation by iodide of both the functional and surface morphological actions of TSH principally reflects the attenuating activities of organified iodide on intracellular cAMP generation.

The inhibitory effect of iodide on growth of rat thyroid (FRTL-5) cells.

The effect of iodide on growth of rat thyroid cells (FRTL-5) was studied. TSH-stimulated cell growth was inhibited by iodide in a concentration-dependent manner, and an effect of iodide was detected at 10(-6) mol/l. KClO4 or 1-methylimidazole-2-thiol blocked the effect of iodide, suggesting that iodide uptake and its organification are required to produce the inhibitory effect of iodide on cell growth. Iodide not only decreased TSH-stimulated cAMP production in FRTL-5 cells but also cell growth induced by cAMP. These observations suggest that iodide inhibits TSH-stimulated growth of the cells by attenuating cAMP production and also by acting on the step(s) distal to cAMP generation. The inhibitory effect of iodide was also seen in growth stimulated by insulin, insulin-like growth factor-I or 12-O-tetradecanoyl phorbol 13-acetate, suggesting multiple sites of action of iodide in the process of growth of FRTL-5 cells.

Regulation of differentiated thyroid function by iodide: preferential inhibitory effect of excess iodide on thyroid hormone secretion in sheep thyroid cell cultures.

Primary cultures of sheep thyroid cells have been used to study the inhibitory effects of iodide on thyroid function. Under the influence of TSH, iodide was concentrated with a cell to medium ratio of 20. When thyroid hormone secretion was measured from cells cultured without addition of exogenous iodide, preferential T3 secretion was evident. The optimum iodide concentration for T4 and T3 synthesis and secretion was 10(-6) M. Prior exposure to 10(-5) M or more iodide decreased subsequent iodide transport in a concentration-dependent manner compared to that in cells acutely exposed to iodide. Although cell to medium ratios were decreased, intracellular iodide concentrations continued to rise with increasing external iodide concentrations, and iodide available for thyroid hormone synthesis was not in limited supply. Iodide concentrations of 10(-4) M or greater inhibited iodothyronine synthesis and thyroid hormone secretion, assessed by both assay of trichloroacetic acid-insoluble Na125I activity in cells and RIA of T4 and T3 in the medium and cell layer. An intermediate concentration of 10(-5) M iodide had a marked inhibitory effect on T4 and T3 secretion, but iodothyronine formation on thyroglobulin was only slightly affected. Our results suggest a preferential inhibitory effect of elevated iodide concentrations on thyroid hormone secretion. The adaptive advantages of this selective inhibition would allow storage of iodothyronines in times of iodide sufficiency while maintaining euthyroidism.

Involvement of cyclic AMP, iodide and metabolites of arachidonic acid in the regulation of cell proliferation of isolated porcine thyroid follicles

Experiments with primary cultures of isolated porcine thyroid follicles were performed in serum-free well-defined medium to investigate different pathways that may be involved in the regulation of thyroid cell growth. The incorporation of [ 3H]thymidine into DNA within 72 h was about 25-fold with fetal calf serum (FCS, 1%), 20-fold with epidermal growth factor (EGF, 1 ng/ml) and 3.5-fold with insulin (10 μg/ml) as compared to controls. Bovine TSH significantly reduced the basal and insulin-induced growth rate at concentrations of 10 −6 to 10 −4 U/ml and 10 −4U/ml, respectively. Forskolin stimulated cyclic AMP accumulation in thyroid cells and significantly reduced FCS-, EGF- or insulin-induced growth. In contrast, a 2- to 7-fold increase in FCS-, insulin- or EGF-induced growth rate was found, when cyclic AMP formation was inhibited by 2',5'-dideoxyadenosine (DDA). Iodide was stimulatory at low concentrations (1 μM) and inhibitory at higher concentrations (40–80 μM) on FCS-induced growth rate. The inhibitory effect of iodide was blocked by propylthiouracil (PTU), indicating that an iodinated compound is responsible for this effect. Indomethacin, a cyclooxygenase inhibitor, did not inhibit EGF- and insulin-induced growth up to a concentration of 100 μM. However, nordihydroguaiaretic acid (NDGA) and BW-755C, which are lipoxygenase inhibitors, strongly inhibited the growth of thyroid cells at micromolar concentrations. These data clearly show that (1) bovine TSH is not a growth factor for isolated thyroid cells in vitro, (2) thyroid cell proliferation, induced by FCS, EGF and insulin is under, negative control of cyclic AMP. (3) Iodide controls dose-dependently thyroid cell growth by iodinated metabolites, probably modulating 2 different pathways: (a) at low iodide concentrations, an iodinated compound enhances the growth rate by inhibition of cyclic AMP formation, and (b) at high concentrations, iodide diminishes the growth rate by inhibiting the response to growth factors. (4) Metabolite(s) of lipoxygenase appear to be involved in intracellular signal transduction evoked by growth factors in thyroid cells.

Studies on the mechanism of acute inhibition of thyroglobulin hydrolysis by iodine.

Iodine in excess is known to acutely inhibit thyroidal secretion. In the present study we have characterized the time course of the iodine effect in vitro and investigated the underlying mechanisms. Labelled thyroid glands were cultured in vitro in medium containing mononitrotyrosine, an inhibitor of iodotyrosine deiodinase. The rate of hydrolysis of labelled thyroglobulin was measured as the proportion of labelled iodotyrosines and iodothyronines recovered at the end of culture and was used as an index of thyroidal secretion. Thyrotrophin (TSH) administered in vivo acutely stimulated the rate of thyroglobulin hydrolysis. Addition of NaI to the culture medium acutely inhibited both basal and TSH-stimulated thyroglobulin hydrolysis. The effect of iodide was demonstrable after 2 h, maximal after 6 h and was not reversible upon removal of iodide. Iodide abolished the dibutyryl cAMP induced stimulation of thyroglobulin hydrolysis. Iodide required organic binding of iodine for its effect but new protein or RNA synthesis was not necessary. The inhibitory effects of iodide and lysosomotrophic agents such as NH4Cl and chloroquin on thyroglobulin hydrolysis were additive suggesting different sites of action. Iodide added in vitro altered the distribution of label in prelabelled thyroglobulin in a way that suggested increased coupling in the thyroglobulin molecule. These data indicate that 1) the iodide effect occurs progressively over a 6 h period, 2) continued presence of iodide is not necessary once the inhibition is established, 3) iodide exerts its action primarily at a post cAMP, prelysosomal site and 4) the effect requires organic binding of iodine, but not new RNA or protein synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-induced acceleration of calcium efflux from mouse thyroid: evidence for inhibition by excess iodide.

We investigated the effect of TSH on calcium transport in mouse thyroid, as well as the influence of iodide thereupon. Thyroid lobes were incubated in Krebs-Ringer bicarbonate buffer containing [45Ca2+] and the time-dependent uptake of [45Ca2+] by the lobes was observed. In the presence of 0.5 mU/ml TSH, the uptake of [45Ca2+] was significantly depressed at early phases of incubation (from 20 to 40 min). Similarly, (Bu)2cAMP (DBC) depressed the [45Ca2+] uptake. The efflux of calcium was also studied by using thyroid lobes preloaded with [45Ca2+]. In the presence of 0.5 mU/ml TSH, [45Ca2+] release from the lobes was doubled in comparison with the control lobes incubated without TSH. DBC similarly accelerated [45Ca2+] release from the lobes. The acute administration of excess iodide to mice fed a low iodine diet inhibits the TSH-induced adenylate cyclase activation in thyroids. The acceleration of calcium efflux induced by TSH was completely abolished by the acute administration of excess iodide in thyroids obtained from animals fed a low iodine diet. Similarly, DBC-induced acceleration of calcium efflux was inhibited by pretreatment with excess iodide. However, the inhibitory effect of iodide on TSH- or DBC-induced acceleration of calcium efflux was not observed in thyroids obtained from mice fed a regular diet. Inhibition of TSH-induced acceleration of calcium efflux by iodide was diminished by treatment of mice with methimazole before iodide. These results suggest that 1) TSH accelerates calcium efflux from the thyroid as a result of accumulation of cAMP in the thyroid, and that 2) iodide inhibits calcium efflux by inhibition of TSH-induced adenylate cyclase activation and by inhibition of the mechanism(s) which is (are) activated by cAMP.

Effect of potassium iodide and perchlorate on the secretion of thyroid hormones

The effect of potassium iodide and perchlorate was studied on parameters characterizing the secretion of thyroid hormones: the cyclic AMP level in the thyroid gland tissue and the number of intracellular colloid drops during stimulation with thyroid hormones. Excess of iodide was shown to depress these indices whereas perchlorate prevented the inhibitory effect of iodide. The results support the view that the adenylate-cyclase system of the thyroid gland is sensitive to the organic iodine concentration. Excess of iodide evidently reduces the ability of perchlorate to affect its concentration in the gland, so that the process of organification of iodine and the secretion of thyroid hormones are maintained at the optimal level.

Iodide Induced Suppression of Thyrotropin-Stimulated Adenosine 3´,5´-Monophosphate Production in Cat Thyroid Slices

Cat thyroid slices were studied to investigate their responsiveness to thyrotropin stimulation of cyclic AMP accumulation. Ovine and bovine thyrotropin, in the presence of 2.5 mM aminophylline, induced a dose-dependent increase in the cyclic AMP content of cat thyroid tissue. Half-maximal stimulation of cyclic AMP accumulation was obtained at a thyrotropin concentration of 1-2 mU/ml. The maximal effect of thyrotropin was observed at 10 mU/ml, and was associated with a mean 77 +/- 19-fold increase in thyroidal cyclic AMP. Preincubation of cat thyroid tissue for 2 h with 50 micron NaI resulted in an impairment in the subsequent ability of thyrotropin to enhance cyclic AMP accumulation, without altering the level of cyclic AMP in tissues not exposed to the hormone. Preincubation alone was without effect on thyrotropin stimulation of cyclic AMP, and the inhibitory effect of iodide was prevented by addition of 3 mM methimazole to the preincubation medium. In addition, the time course of thytrotropin stimulation of cyclic AMP accumulation in cat thyroid slices was not significantly altered by the preincubation with excess iodide. These studies provide additional evidence that excess iodide inhibits the adenylate cyclase-cyclic AMP system in thyroid tissue.

Cultured Thyroid Cell Adenosine 3′,5′-Cyclic Monophosphate Response To Thyrotropin: Loss and Restoration of Sensitivity to Iodide Inhibition

Unlike in all other thyroid preparations, exposure of dog thyroid cells in long-term monolayer culture to iodide (10(-7) to 10(-3) M for up to 19 h did not blunt the subsequent adenosine 3', 5'-cyclic monophosphate (cAMP) response to thyrotropin (TSH) stimulation. This lack of effect of iodide was observed even when confluent thyroid cells were "follicularized" by the action of TSH in the culture medium. Preincubation of these cells in thyroxine (T4) and triiodothyronine (T3) was similarly without effect on the subsequent cAMP response to TSH. Study of thyroid cells during the early phase of primary culture demonstrated that inhibition by iodide (10(-4) M) of the cAMP response to TSH occurred after 7 h but was lost after 48 h of cell culture. This inhibitory effect of iodide was prevented by the inclusion of methimazole in the preincubation medium. As with iodide-insensitive cells, T4 and T3 were without effect on the cAMP response to TSH in iodide-sensitive thyroid cells. Exposure of iodide-insensitive thyroid cells to iodide-containing medium obtained after 2 h of incubation with dog thyroid slices, as well as to medium enriched with the 100,000 g supernatant fraction of homogenates prepared from these thyroid slices, did not restore the inhibitory action of iodide. However, iodide-sensitivity of the cAMP response to TSH was restored by preincubation of iodide-insensitive cells in 10(-4) M iodide plus an H2O2-generating system (glucose-glucose oxidase). These data suggest that T4 and T3 are not organic iodine inhibitors of the thyroid cAMP response to TSH. In addition, they provide evidence against the existence of a soluble, freely diffusible, organic iodine inhibitor of thyroid adenylate cyclase. The loss of sensitivity to iodide inhibition of adenylate cyclase that occurs in thyroid cells shortly after initiation of primary culture appears to be related to a defect in the cellular organification mechanism, possibly the H2O2-generating system.

Is TSH-Stimulated Thyroid Hormone Release Inhibited by Iodide?1

The effect of iodide on TSH-stimulated thyroid hormone release in the McKenzie bioassay was studied. When mice were given KI (0.01 or 10 mg/ml in drinking water) for 24 hr before TSH, thyroid 125I release in response to TSH expressed as the percent increase in serum 125I (2 hr value/0 hr value) was significantly inhibited, an effect in good agreement with previously reported results. However, the absolute increase in serum 125I (2 hr value-0 hr value) was not changed by Kl-treatment. In Kl-treated mice, the concentration of 125I present in the serum before TSH injection was significantly higher than that in the control mice. Methimazole (0.02 mg/ml in drinking water) abolished the inhibitory effect of iodide on the percent increase in serum 125I in response to TSH by abolishing the increase in the concentration of serum 125I before TSH. Changes in serum 125I-PBI were essentially the same as those in serum 125I. These results suggest that 1) the inhibitory effect of iodide on the percent increase in serum ...

TSH- or LATS-stimulated thyroid hormone release is inhibited by iodide.

The inhibitory effect of iodide on thyroid release of 125I-iodine was examined using McKenzie's bioassay method. Iodide did not inhibit the action of TSH or LATS when injected simultaneously with or just before injection of TSH or LATS. A significant inhibitory effect on hormone release was observed if the animals were treated with KI for 24 hr before TSH or LATS was administered. The ratio of 127I-PBI to 127I-TI (total iodine) was decreased in the groups receiving KI. The ratio of 125I-PBI to 125I-TI was not changed. From calculations of the specific activity of thyroid PBI (ratio of 125I-PBI/127I-PBI), any dilution effect of newly bound 127I in the thyroid was found to be negligible. Excess iodide directly blocks the release of thyroid hormone caused by TSH or LATS. (Endocrinology 84: 1305, 1969)