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H Liang, O Morin and AG Burger

Until now, most potent inhibitors of monodeiodination are iodinated, propylthiouracil being an exception. We report here studies on a new non-iodinated substance, triethylene glycol bis-3-(3-tertbutyl-4-hydroxy-5-methyl-phenyl) propionate (TK 12627 or Irganox), which is used as a very efficient antioxidant in the chemistry of plastics. The studies were performed with 23 hypothyroid rats that received Irganox in their daily food (8 mg·day−1·(100 g body wt)−1) for 3 weeks. Thyroxine (T4) metabolism was studied by implanting minipumps delivering 2.3 nmol T4·dayℒ1·(100 g body wt)ℒ1 for 1 week. On day 1 before sacrifice, another minipump containing [125I]-3,5,3′-triiodothyronine (T3, 2.6 μCi/day) and [131I]-3,3′,5′-triiodothyronine (rT3, 2.1 μCi/day) was implanted. The results showed that with Irganox treatment serum T4 concentrations were higher (p<0.05). Serum T3 concentrations markedly decreased (1.07±0.07 vs 0.65±0.04 nmol/l), accompanied by a decrease of free T3 concentrations (p<0.001). Serum rT3 concentrations increased by 50% (p<0.001). Serum thyrotropin levels were mostly unmeasurable. The plasma clearance rate decreased slightly for T4 (19%, p<0.05) and remarkably for rT3 (46.7%, p<0.001). The conversion rate of T4 to rT3 did not change. Deiodinase type I (5′D-I) activity decreased in both liver and kidney tissues by 54% and 52%, respectively, and correlated with T3 (r2 = 0.79 and 0.65, respectively). Brain deiodinase type III (5D-III) was unchanged and type II (5′D-II) was unmeasurable. The relative abundance of 5′D-I messenger ribonucleic acid (mRNA) levels decreased by 40% in liver and 42% in kidney and it correlated with serum T3 levels also (r2 = 0.71 and 0.72, respectively). The amount of hepatic spot 14 mRNA also decreased by 41% (p<0.05); however, the abundance of hepatic beta-actin mRNA was not affected by the treatment. In an additional experiment. Irganox-treated rats received 2 nmol T3 ·day−1·(100 g body wt)−1; this treatment prevented the changes in mRNA levels of 5′D-I and spot 14 but did not prevent the decrease of 5′D-I activity in either liver or kidney. In vitro studies with liver microsomes showed that Irganox is a specific non-competitive inhibitor of 5′D-I activity, with a Ki of 12 μmol/l. These studies suggest that Irganox, by inhibiting 5′D-I activity, can effect thyroid hormone monodeiodination followed by changes of mRNA levels of some thyroid hormone-dependent target genes.

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H Liang, CE Juge-Aubry, M O'Connell and AG Burger

In order to compare the effect of 3,5,3'-triiodothyroacetic acid (TRIAC) with those of triiodothyronine (T3) and thyroxine (T4), severely hypothyroid rats (n=56) were infused over 13 days with 1, 2 or 4 nmol/100 g body weight (BW) per day of T3 or 2, 4 or 8 nmol/100 g BW per day of T4 or TRIAC. The 8 nmol/100 g BW per day of T4 or TRIAC induced the same increase in resting metabolic rate, yet 4 nmol/100 g BW per day of T3 was more potent (P < 0.05). For inhibiting serum TSH levels, 2 nmol/100 g BW per day of TRIAC were significantly less active than 2 nmol/100 g BW per day of T4 or 1 nmol/100 g BW per day of T3 (TRIAC, serum TSH 35.5 +/- 5.7; T3 2.58 +/- 0.91; T4 2.12 +/- 0.59 ng/ml). At higher doses serum TSH and beta-TSH mRNA were unmeasurable. Using serum T3 levels as covariate, the action of T3 and T4 was identical on cardiac monodeiodinase type 1 (5'D1) activity and hepatic malic enzyme (Me) mRNA levels and similar for hepatic 5'D1 activity. The effect of TRIAC was compared with T3 by using increasing doses of 1, 2 and 4 nmol/100 g BW per day of T3 and 2, 4 and 8 nmol/100 g BW per day of TRIAC. ANOVA indicated that there was no major difference between the effects of the hormones since with increasing doses the response of hepatic 5'D1 mRNA levels and enzyme activity and Me mRNA remained parallel. However, when studying the effect on cardiac 5'D1 activity there was not only a difference for type of treatment (T3 > TRIAC) but this difference became greater with each increment in dose. Interestingly there was also only a small effect of TRIAC on increase in heart weight compared with T3 and T4. Brain cortex monodeiodinase type 2 (5'D2) was mainly inhibited by T4 infusions. Monodeiodinase type 3 (5'D3) was stimulated by T4, less so by TRIAC and least by T3, expressing probably the local T3 and TRIAC concentrations. In conclusion, despite apparently similar effects of TRIAC and T3 and T4 on hepatic parameters of thyroid hormone action, TRIAC differs considerably in terms of its effects on cardiac 5'D1 activity and possibly on other fundamental effects of thyroid hormones on the heart since heart weight increased significantly less with TRIAC than with T3 or T4.

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MJ Müller, CA Reynard, AG Burger, G Toffolo, C Cobelli and E Ferrannini

Müller MJ, Reynard CA, Burger AG, Toffolo G, Cobelli C, Ferrannini E. Kinetic analysis of thyroid hormone action on glucose metabolism in man. Eur J Endocrinol 1995;132:413–18. ISSN 0804–4643

Thyroid hormone action on insulin's effect on glucose kinetics was investigated with the use of a physiological three compartment model. In six healthy volunteers before and after 14 days of thyroxine treatment (300 μg/day), a bolus of [3-H3]glucose was injected and the time course of plasma radioactivity was followed closely for 150 min. Then a hyperinsulinemic (1 mU · min−1 · kg−1) and euglycemic clamp was started, and euglycemia was maintained for another 250 min. A second bolus of the tracer was then given at 240 min, and the plasma radioactivity was followed for 160 min. Insulin stimulated basal plasma glucose clearance fourfold (p < 0.001) and completely suppressed basal hepatic glucose production (p < 0.001). Concomitantly, the total distribution volume of glucose was increased by 19% (p < 0.05); this change was accompanied by about 50% expansion of the slowly exchanging glucose pool (putatively representing the insulin-dependent compartment). Thyroxine treatment increased plasma triiodothyronine by about 20% (0.1 > p > 0.05) but did not affect basal glucose turnover, insulin-stimulated plasma glucose clearance or the insulin-induced suppression of endogenous glucose output. However, thyroxine treatment blunted the insulin-induced increases in total distribution volume and the slowly exchanging pool of glucose (p = NS vs the basal state). We conclude that minor changes in plasma triiodothyronine (such as occur during overfeeding) do not interfere with the ability of insulin to stimulate the rate of disappearance of glucose or suppress endogenous glucose release; however, our data suggest that they induce finer changes in glucose kinetics, possibly reflecting acceleration or intracellular glucose degradation.

Manfred J Müller, Institut für Humanernährung und Lebensmittelkunde, Christian-Albrechts-Universität zu Kiel, Düsternbrooker Weg 17, D-24105 Kiel, Germany

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A Gorla-Bajszczak, C Siegrist-Kaiser, O Boss, AG Burger and CA Meier

OBJECTIVE: Examination of the pattern of expression of peroxisome proliferator-activated receptor (PPAR) isoforms alpha and gamma in a model of obesity. DESIGN: Examination of adipose tissue and primary adipocyte cultures from lean and obese Zucker rats at different ages (28 days and 12 weeks). METHODS: mRNA levels were measured by RNase protection assay.RESULTS: The highest levels of PPARalpha and gamma mRNA were present in brown adipose tissue (BAT), followed by liver and white adipose tissue (WAT) for the alpha and gamma subtypes, respectively, at both ages examined. PPARalpha was expressed 100-fold higher in BAT compared with WAT, and PPARgamma mRNA levels were 2-fold higher in the WAT of obese compared with lean rats. PPARalpha and gamma expression was minimal in m. soleus, although higher levels of PPARgamma were found in the diaphragm. In marked contrast to the findings in vivo, virtually no PPARalpha mRNA could be detected in BAT cultures differentiated in vitro. CONCLUSION: PPARalpha and gamma are most highly expressed in BAT in vivo. However, PPARalpha is undetectable in brown adipose cells in vitro, suggesting that the expression of this receptor is induced by some external stimuli. In addition, the expression of PPARgamma was increased in WAT from young obese animals, compatible with an early adaptive phenomenon. Finally, the presence of PPARgamma mRNA is detectable only in particular muscles, such as the diaphragm, suggesting the possibility of an influence of fiber type on its expression, although exercise did not influence the expression of PPARgamma in other skeletal muscles.