Thermogenesis and fatty acid composition of brown adipose tissue in rats rendered hyperthyroid and hypothyroid - With special reference to docosahexaenoic acid

The effects of hyperthyroidism and hypothyroidism on brown adipose tissue ( BAT) thermogenesis and phospholipid fatty acid composition were investigate d in rats. Chronic triiodothyronine (T-3) treatment (hyperthyroidism) incre ased the interscapular BAT pad weight, its triacylglycerol content, and its DNA content. It did not affect basal and noradrenaline-stimulated in vitro oxygen consumption of BAT expressed per mu g DNA, although it significantl y increased the oxygen consumption of the whole BAT pad. T3 treatment had l ittle effect on phospholipid content and phospholipid fatty acid compositio n. In contrast, chronic methimazole treatment (hypothyroidism) decreased th e BAT pad weight and the triacylglycerol content, but did not significantly change the DNA content in comparison with the control. It significantly de creased the noradrenaline-stimulated BAT oxygen consumption expressed per m u g DNA and per BAT pad, but did not change the basal oxygen consumption. M ethimazole treatment significantly affected phospholipid content and phosph olipid fatty acid composition. Among the major fatty acids of BAT, it decre ased docosahexaenoic acid (DHA), arachidonic acid, palmitic acid, palmitole ic acid, and oleic acid, and it increased linoleic acid, stearic acid, and eicosapentaenoic acid. A regression analysis revealed a positive relationsh ip between in vitro respiration and DHA levels in phospholipids (r=0.404, p <0.05). These results suggest that thyroid hormones have trophic action on BAT and are necessary for BAT thermogenic activity. This study also suggest s that DHA is involved in the regulation of BAT thermogenic activity, as we previously indicated.