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 investigated in rats. Chronic triiodothyronine (T3) treatment (hyperthyroidism) increased 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 microg DNA, although it significantly increased the oxygen consumption of the whole BAT pad. T3 treatment had little effect on phospholipid content and phospholipid fatty acid composition. In contrast, chronic methimazole treatment (hypothyroidism) decreased the BAT pad weight and the triacylglycerol content, but did not significantly change the DNA content in comparison with the control. It significantly decreased the noradrenaline-stimulated BAT oxygen consumption expressed per microg DNA and per BAT pad, but did not change the basal oxygen consumption. Methimazole treatment significantly affected phospholipid content and phospholipid fatty acid composition. Among the major fatty acids of BAT, it decreased docosahexaenoic acid (DHA), arachidonic acid, palmitic acid, palmitoleic acid, and oleic acid, and it increased linoleic acid, stearic acid, and eicosapentaenoic acid. A regression analysis revealed a positive relationship 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 suggests that DHA is involved in the regulation of BAT thermogenic activity, as we previously indicated.