In this study we examined the possibility that the reduced brown adipose tissue (BAT) thermogenesis in the Zucker obese rat may result from a limited capacity for enzymic conversion of thyroxine (T4) to triiodothyronine (T3) in BAT. A total of 34 lean and obese rats, ~4 mo old were divided into three treatment groups: group 1 (5 lean and 6 obese) was fed Purina rat chow for 21 days, and group 2 (5 lean and 6 obese) was fed a cafeteria diet for 21 days, and group 3 (6 lean and 6 obese) was fed Purina rat chow and maintained in the cold (8 ± 1°C) for 7 days. The lean and obese rats in all three groups of animals were matched closely for age and respective body weight. Activity of T4 5'-deiodinase was determined as the rate of T3 production from added T4 under controlled in vitro conditions. Serum T4 and T3 were determined by radioimmunoassay. The rate of T4-to-T3 conversion in BAT was similar in the lean and obese rats maintained at room temperature, whether fed rat chow or a cafeteria diet (~40-50 pmol T3/scapular BAT depot per h). However, expressed per scapular BAT depot, lean rats exposed to cold displayed about a fivefold increase in BAT T3 production (P < 0.0001), whereas only a small increase was observed in the cold-exposed obese rats. Serum T3 levels tended to be reduced in the Zucker obese rats. Our data indicate a reduced capacity for T3 production in Zucker rat BAT exposed to cold. This defect may account for the reduced tolerance of the obese animals to cold, but it does not account for their reduced diet-induced BAT thermogenesis.
|Original language||English (US)|
|Journal||American Journal of Physiology - Endocrinology and Metabolism|
|State||Published - 1987|
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