Amiodarone antagonizes the effects of T3 at the receptor level: an additional mechanism for its in vivo hypothyroid-like effects

Amiodarone is a diiodinated benzofuran derivative that has some structural similarities to the thyroid hormones and contains two iodine atoms per molecule. It has exhibited hypothyroid-like effects that are thought to be the result of an inhibition of thyroid hormone synthesis due to iodine load, a decrease in the T4 to T3 conversion, and (or) a competitive binding for T3 receptors. The aim of this study was to determine if this third mechanism contributes to the hypothyroid-like effects of amiodarone in vivo. To do so, some characteristic features known to be influenced by hypothyroidism were determined in surgically thyroidectomized rats (n = 48), which received replacement doses of T3 (0.5 and 1.0 microgram.100 g-1.day-1) with or without amiodarone (60 mg.kg-1.day-1). Thyroidectomy produced a hypothyroid state upon which amiodarone had no detectable effects except a negative body weight gain. T3 (0.5 microgram) nearly normalized the thyroid status of the animals, but the concomitant administration of amiodarone induced hypothyroid-like effects suggesting that these effects are dependent on T3. Higher doses of T3 (1.0 microgram) produced hyperthyroid-like effects and attenuated the effects of amiodarone. Unexpectedly, amiodarone decreased T3 plasma concentrations. To determine if the effects of amiodarone were the results of a decrease in T3 plasma and myocardial concentrations or a competition with T3 for its receptors, exogenous T3 pharmacokinetics were studied in thyroidectomized rats receiving T3 (0.5 microgram) with or without amiodarone. The results suggested that amiodarone increased T3 cardiac concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of desethylamiodarone on thyroid hormone metabolism in rats: comparison with the effects of amiodarone

Desethylamiodarone is the principal metabolite of amiodarone. Amiodarone is a class III antiarrhythmic agent, which acts by lengthening repolarization in the myocardium, an effect that is identical to that produced by hypothyroidism. Amiodarone is known to alter thyroid hormone metabolism, and it has been suggested that the mechanism underlying its antiarrhythmic action is the induction of a myocardial but not generalized hypothyroidism. Since the serum levels of desethylamiodarone reach those of the parent compound during chronic amiodarone therapy, it has been suggested that at least part of amiodarone's pharmacological effects may be attributable to the additive effects of the metabolite. Therefore, we investigated the effects of desethylamiodarone on thyroid hormone metabolism and compared them with those of amiodarone in rats. We have shown that chronic treatment with desethylamiodarone decreased serum T3, markedly increased serum reverse T3 with no significant change in serum T4. These effects are similar to those of amiodarone. The data suggest that the chronic effects of amiodarone on thyroid hormone metabolism may be due at least in part to the actions of desethylamiodarone.     

Evidence for an inhibition of thyroid hormone effects during chronic treatment with amiodarone

The effects of chronic amiodarone treatment on several thyroid and cardiac function parameters were studied in 50 euthyroid patients with refractory ventricular arrhythmias, divided in responders and nonresponders according to their sensitivity to the antiarrhythmic action of the drug. No differences in the severity of cardiac disease and blood amiodarone concentrations were found in the two groups. Amiodarone induced a significant inhibition of peripheral T4 monodeiodination, more pronounced in responders compared to nonresponders. On the contrary, only in responsive patients, elevated basal and TRH-stimulated TSH levels were observed (despite serum T3 levels were not different from those in nonresponders) and the indirect indices of cardiac performance, particularly the systolic time intervals, fell in a range usually observed in the hypothyroid states. These findings suggest that amiodarone, besides the well-known inhibition of T4 to T3 conversion, also induces a partial resistance to the thyroid hormones, which is probably involved in the therapeutical effectiveness of the drug.     

Amiodarone reduces the effect of T3 on beta adrenergic receptor density in rat heart

The effect of injection of 1 mg/kg triiodothyronine on cardiac beta-adrenoceptor state was investigated in hypothyroid rats and compared to the effect in hypothyroid rats pretreated with amiodarone (200 mg/kg/day for 8 days). The Kd values of iodocyanopindolol binding to the beta-receptors were not influenced by either T3 injection or by amiodarone treatment. In the absence of amiodarone, injection of triiodothyronine resulted in a small decrease in receptor density at 6 hr, followed by an increase at 24 hr. Rats treated with amiodarone showed a similar response pattern to hormone injection (i. e. a small decrease in receptor density at 6 hr, followed by an increase at 24 hr), but the amplitude of the response was significantly reduced. Moreover, in vehicle injected rats amiodarone treatment resulted in a decrease in receptor density when rats were mildly hypothyroid, but not when rats were severely hypothyroid. It is concluded that amiodarone interferes (directly or indirectly) with thyroid hormone action in the heart.     

Amiodarone-induced changes in lipid metabolism

Hypothyroidism is a major cause of secondary hypercholesterolemia. Amiodarone treatment alters both the levels of serum lipids and thyroid hormones. We investigated whether the amiodarone-induced changes in lipid metabolism are related to the changes in thyroid hormone levels. Eighteen patients received amiodarone (31 +/- 3 g cumulative dose) for six weeks. Serum triglyceride, total-cholesterol, high density lipoprotein-cholesterol and its subfractions, apolipoproteins B and AI, and plasma post-heparin lipoprotein lipase and hepatic triglyceride lipase activities were determined. Amiodarone treatment caused significant increases in serum total-cholesterol (baseline 4.4 +/- 0.21 (SE), 6 weeks 5.12 +/- 0.26 mmol/l, P less than 0.01), in low density lipoprotein cholesterol (baseline 2.61 +/- 0.26, 6 weeks 3.36 +/- 0.21 mmol/l, P less than 0.05) and in apolipoprotein B (baseline 1.95 +/- 0.15, 6 weeks 2.26 +/- 0.13 mmol/l, P less than 0.01) concentrations. Serum high density lipoprotein and its subfractions, or apolipoprotein AI levels did not change. Plasma post-heparin lipoprotein lipase activity increased (baseline 137 +/- 21, 6 weeks 168 +/- 21 U/ml, P less than 0.01) while hepatic triglyceride lipase did not change. Amiodarone also caused an increase in serum thyroxine (baseline 110 +/- 8, 6 weeks 136 +/- 6 mmol/l, P less than 0.05), although values remained in euthyroid range. In summary, amiodarone therapy increased the concentrations of atherogenic lipoproteins in the serum similar to that seen in hypothyroidism. On the other hand the effect of amiodarone on lipoprotein lipase was opposite to that seen in hypothyroidism. Therefore, amiodarone-induced changes in lipid metabolism cannot be explained solely on the basis of the changes in circulating thyroid hormone levels.     

Epiphysis cerebri-thyroid interrelations. Distorting effect of partial thyroidectomy on the antithyroid action of epiphyseal methoxyindoles in animal experiments

The effect of pineal methoxyindoles (MI) on hypothalamus-hypophysis-thyroid gland system was studied in intact and partially thyroidectomized male Wistar rats in conditions of a short light day (winter). Melatonin administration for 10 days suppressed 131I uptake by the thyroid gland and decreased the levels of T3, T4 and thyrotropic hormone (TTH) in the blood serum of intact animals, with TTH reaction to thyroid hormone (TH) retained. 5-methoxytryptamine administration was less effective. Partial thyroidectomy distorted the direction of MI effect: melatonin and to a lesser extent 5-methoxytryptamine caused a marked normalization of a decreased TH content and an increased TTH level in the blood serum of partially thyroidectomized rats. TTH-TH reaction also corresponded to the control. A predominantly modulating character or pineal MI effect on thyroid system is suggested.     

The direct effect of the thyroid hormone on cardiac chronotropism

To establish whether thyroid hormone modifies the heart rate directly or through an action on other neuroendocrine modulators, the authors have examined several animals models differing in the plasma levels of such compounds. Induction of the hypothyroid state in rats produced a slow onset of bradycardia, which may be removed by a prolonged triiodothyronine treatment. The involvement of TSH was excluded as, by comparing thyroidectomized, hypophysectomized and cold exposed rats, the heart rate was found to vary according to the thyroid levels and not to the TSH levels. Moreover growth hormone, corticotropin and gonadotropins do not influence the heart rate, as the bradycardia induced by hypophysectomy was fully removed by triiodothyronine treatment. The lack of influence by ACTH and GnH was confirmed by treatment of thyroidectomized rats with corticosteroids or testosterone, respectively. Finally, thyroid hormone did not act on the heart rate by changing the norepinephrine output at the sympathetic nerve endings in the heart. In fact, thyroidectomy produced a more intense bradycardia than sympathectomy, and such bradycardia was equally removed by triiodothyronine treatment in thyroidectomized rats and in thyroidectomized and then sympathectomized ones. The authors suggest that the direct effect of the thyroid hormone on cardiac chronotropism is due to an early enhancement of beta-adrenoceptors, followed by a late modification of the electrophysiological properties of the myocardium.     

Functional Effects of Short-Term Treatment with Amiodarone on Thyroid Tissues of the Rabbit

The effect of short-term treatment with Amiodarone on thyroid gland tissue was studied in a group of 26 New Zealand albino rabbits. Ten rabbits were left untreated and served as controls; the remaining animals were treated with 10 mg/kg/day Amiodarone. The serum levels of serum triiodothyronine (T3), thyroxine (T4), and thyroid stimulating hormone (TSH) levels were measured at days 0 (baseline), 7, 30, and 45. The serum selenium levels were also measured, but only on days 0 and 45 of the experiment. At the end of the experiment the animals were sacrificed and the levels of selenium, T3, T4, and iodine were determined in thyroid tissue. After 30 days treatment the values of T3 were significantly lower than those of the untreated controls or the baseline levels (p < 0.001). The T4 level was significantly lower and the TSH value was significantly higher after 45 days of Amiodarone (p < 0.001). In thyroid tissue the T3, T4, and iodine levels were significantly higher in the treated group when compared to untreated controls (p < 0.05). These results show that Amiodarone induces changes in the hormone levels in both serum and thyroid tissues, as well as in the amount of iodine taken up by the thyroid gland in rabbits.     

Effects of thyroid hormones on the receptor level in estrogen target organs

The influence of thyroid hormones on the turnover of cytoplasmic estrogen receptors in the liver, kidney and uterus of intact and ovariectomized female rats was studied under in vivo conditions. Thyroidectomy had no significant effect on the receptor level in the uterus but caused a substantial reduction of the receptor content in the liver and kidney. In livers of intact and ovariectomized animals receptor values were reduced with 70 and 80%, respectively, 30 days after thyroidectomy. Substitution with triiodothyronine (T3) restored the hepatic estrogen receptor concentration in thyroidectomized rats to the preoperative level. If rats that had been both ovariectomized and thyroidectomized were substituted with thyroid hormone for the same time period, the receptor level was increased but did not reach the level seen in animals that had been ovariectomized only. The effects of thyroid hormone substitution was found to be dose dependent and paradoxical. Thus, a high dose of 50 micrograms/day of triiodothyronine given to intact animals for nine days caused a 30% reduction in the hepatic receptor content. The same level of reduction was seen in the ovariectomized rat given a hormone dose of only 1 micrograms/day. When this type of rats was treated with the higher dose of triiodothyronine the reduction in hepatic estrogen receptors was 50%. These results are discussed in relation to existing information concerning the multihormonal regulation of estrogen receptor concentration in the rat liver.     

Effects of thyroid hormone deficiency and replacement on rat hypothalamic growth hormone (GH)-releasing hormone gene expression in vivo are mediated by GH

The role of thyroid hormone and GH in the regulation of hypothalamic GH-releasing hormone (GRH) gene expression in the rat was examined after the induction of thyroid hormone deficiency by thyroidectomy. Thyroidectomy resulted in a time-dependent decrease in hypothalamic GRH content, which was significant by 2 weeks postoperatively, and a reduction in pituitary GH content to 1% of the control level by 4 weeks. In contrast, GRH secretion by incubated hypothalami under both basal and K(+)-stimulated conditions was increased after thyroidectomy. Hypothalamic GRH mRNA levels also exhibited a time-dependent increase, which was significant at 1 week and maximal by 2 weeks after thyroidectomy. Administration of antirat GH serum to thyroidectomized rats resulted in a further increase in GRH mRNA levels. T4 treatment of thyroidectomized rats for 5 days, which also partially restored pituitary GH content, lowered the elevated GRH mRNA levels. However, comparable effects on GRH mRNA levels were observed by rat GH treatment alone. These results suggest that the changes in hypothalamic GRH gene expression after thyroidectomy in the rat are due to the GH deficiency caused by thyroidectomy, rather than a direct effect of thyroid hormone on the hypothalamus, since the changes were reversible by GH alone despite persistent thyroid hormone deficiency. In addition, they further support the role of GH as a physiological negative feedback regulator of GRH gene expression.     

Effect of thyroid hormones on delayed type hypersensitivity reaction

The effect of long term administration of thyroid hormones and its deprivation on delayed type hypersensitivity (DTH) reaction to 2-4 dinitrochlorobenzene (DNCB) was studied. Animals were either pre-treated with thyroid hormones (T3 or T4) for 15 days and then subjected to DNCB skin test or the animals received thyroid hormones and simultaneously subjected to DNCB skin test. In both the cases DTH reaction was found to be increased significantly. When DNCB skin test was performed in the thyroidectomized animals, DNCB skin reaction was significantly decreased and the reaction was restored to normal following supplementation of thyroid hormones to the thyroidectomized animals. TLC and ALC were increased significantly following hormone treatment and thyroidectomized animals. TLC hand, induced significant depression in the count which was restored by hormone administration to the thyroidectomized animals.     

Modulation of hepatic level of microsomal testosterone 7 alpha-hydroxylase, P-450a (P450IIA), by thyroid hormone and growth hormone in rat liver

The effects of thyroid hormone and growth hormone on microsomal testosterone 7 alpha-hydroxylase, P-450a, were studied to understand the interaction of these hormone-mediated regulations in rats. In Western blots using anti-P-450a IgG, 1.7-fold higher content of P-450a was observed in livers of female than male adult rats, while no appreciable sex-related difference was detected in prepubertal rats and rats of 24 months of age. Treatment with n-propyl-2-thiouracil or thyroidectomy of male rats increased by 2-fold the hepatic content of P-450a, but neither regimen had a significant effect on the content in female rats. Levels of P-450a in both sexes of thyroidectomized rats were decreased by the supplementation of triiodothyronine (T3, 50 micrograms per kg, i.p. for 7 days) to levels similar to that observed in normal male rats. Hypophysectomy also caused an increase in microsomal P-450a content in male rats. Continuous infusion of human growth hormone, which mimicked the female secretion, further significantly increased the content in hypophysectomized rats to a level similar to that observed in normal female rats. In contrast, hepatic level of P-450a in hypophysectomized male and female rats was reduced by intermittent injection, which mimicked the male secretion. Clear suppression on the level of hepatic P-450a was also observed by the treatment of hypophysectomized rats with 5 or 50 micrograms/kg of T3 and of hGH-infused hypophysectomized rat with 50 micrograms/kg of T3.(ABSTRACT TRUNCATED AT 250 WORDS)     

Trace amine-associated receptor 1 (TAAR1) is activated by amiodarone metabolites

Amiodarone (Cordarone, Wyeth-Ayerst Pharmaceuticals) is a clinically available drug used to treat a wide variety of cardiac arrhythmias. We report here the synthesis and characterization of a panel of potential amiodarone metabolites that have significant structural similarity to thyroid hormone and its metabolites the iodothyronamines. Several of these amiodarone derivatives act as specific agonists of the G protein-coupled receptor (GPCR) trace amine-associated receptor 1 (TAAR(1)). This result demonstrates a novel molecular target for amiodarone derivatives with potential clinical significance.     

Late-onset thyrotoxicosis after the cessation of amiodarone

IntroductionAmiodarone is a highly effective antiarrhythmic-drug with well recognized toxic side-effects. The effects of the drug late in patients with atrial fibrillation (AF) is not well described.Methods and resultsWe present a single centre prospectively collected series of patients with thyrotoxicosis occurring late after the cessation of amiodarone. Between 2006 and 2018, 8 patients were identified with amiodarone induced thyrotoxicosis (AIT). Amiodarone was prescribed for AF in 7 patients and ventricular tachycardia in 1 patient. Mean duration of therapy was 329 [42–1092] days, mean dose of 200 ± 103.5 mg/day. Amiodarone use was short term (<140 days) in 4 of the 8 cases, with one treated for 42 days. Patients presented with symptoms including weight loss, tremors, palpitations, AF, sweats all indicative of AIT at a median of 347 [60–967] days post cessation. Thyroid function testing confirmed suppressed thyroid stimulating hormone and elevated T levels in all patients. Nuclear thyroid imaging in all cases demonstrated low uptake of iodine indicative of Type II AIT. All patients recovered following pharmaceutical treatment with Carbimazole and Prednisolone.ConclusionsWe describe a series of patients with late thyrotoxicosis after exposure to amiodarone. Our findings highlight the need for a high-index of clinical suspicion for AIT regardless of treatment duration or time after cessation of amiodarone.     

alpha-adrenergic receptor in rat heart. Effects of thyroidectomy.

The effects of thyroid status on alpha-adrenergic receptors in the rat myocardium were investigated. The potent antagonist [3H]dihydroergokryptine was used to identify alpha-adrenergic receptors in rat heart particulate and sarcolemmal fractions. Administration of triiodothyronine to thyroidectomized rats decreased specific binding to alpha-adrenergic receptors in heart particulate and sarcolemmal fractions by 41% and 45%, respectively. Scatchard analysis revealed that the cardiac sarcolemmal fraction from thyroidectomized rats contained 29.3 fmol/mg of protein, as compared with 17.0 fmol/mg of protein found in the heart preparation of thyroidectomized rats treated with triiodothyronine. The equilibrium dissociation constants for the interaction of receptors with dihydroergokryptine were similar (about 1.5 nM) in the heart sarcolemmal fractions derived from these two groups of rats. The results of this study demonstrate that thyroid hormone can regulate the number of cardiac alpha-adrenergic receptors. In addition, there appears to be a reciprocal relationship between alpha-adrenergic and beta-adrenergic receptors in the rat myocardium.     

Evaluation of the role of calcitonin deficiency in ovariectomy-induced osteopenia

Studies were carried out in rats to examine the role of calcitonin deficiency in the pathogenesis of ovariectomy-induced osteopenia. The parathyroid glands of 80 female Wistar rats were autotransplanted to their thigh muscle and the animals divided into 4 groups. Group 1 rats were sham ovariectomized, and thyroidectomized to make them calcitonin deficient; Group 2 rats were thyroidectomized, and ovariectomized to make them deficient in ovarian hormones as well; Group 3 rats were sham thyroidectomized and sham ovariectomized, and Group 4 rats were sham thyroidectomized and ovariectomized. A fifth group of rats were unoperated upon and served as controls. Thyroidectomized animals were maintained on thyroxine replacement and 11 months after ovariectomy all the animals were bled, killed and their femurs dissected out. In both the thyroid intact and thyroidectomized animals, ovariectomy decreased femur density significantly (P less than 0.01). Similarly, ovariectomy resulted in a decrease in femur calcium (P less than 0.01) in both groups of animals, and in a significant decrease in serum calcitonin (P less than 0.05) in the thyroid intact animals. We conclude from these findings that ovarian hormone deficiency can cause bone loss independently of lowering circulating calcitonin levels.     

Differential regulation of thyrotropin-releasing hormone receptor mRNA levels by thyroid hormone in vivo and in vitro (GH3 cells).

We studied the effect of thyroid status on thyrotropin-releasing hormone receptor (TRH-R) mRNA levels both in vivo and in vitro (GH3 cells) using a cloned rat TRH-R cDNA by RT-PCR. Experimental hypothyroid rats were produced by total thyroidectomy and were then killed 7 days after the operation. TRH receptor binding in the anterior pituitary and serum TSH level were elevated approximately 2-fold and 8-fold, respectively, in 7 day thyroidectomized rats. TRH-R mRNA levels in hypothyroid rats were also increased significantly compared with those of normal rats. In GH3 cells, however, no significant change of TRH-R mRNA level was observed between cultures treated with triiodothyronine (T3, 10(-9) and 10(-7) M) and the untreated group. The present data indicate that 1) the in vivo effects of thyroid status on TRH-R mRNA levels differ from the in vitro one, and that 2) the down regulation of TRH-R binding by thyroid hormone in GH3 cells may be mediated by translational or post-translational mechanisms.