The ability to develop nonshivering thermogenesis (NST) and the effect of fasting on thermogenic response to cold were studied in winter-acclimatized king penguin chicks. Metabolic rate (MR) and integrated electrical muscle activity were measured at different ambient temperatures. In cold-acclimatized (5 degrees C) fed chicks, shivering threshold temperature (STT) was 9.4 degrees C lower than lower critical temperature (LCT), indicating that NST (0.7 W/kg) occurs at moderate cold, whereas in control chicks fed and reared at 25 degrees C for 3 wk, LCT and STT were similar. Chicks reared in the cold and fasting for 3 wk or 4-5 mo (natural winter fast) developed an NST of 0.8 and 2.4 W/kg, respectively, despite the fast. In fasting chicks, the intercept of the metabolic curve with the abscissa at zero MR was far below body temperature, contrasting with the classic model for heat loss. Their low LCT indicates the capacity of a large reduction in convective conductance characteristic of diving animals and allows energy sparing in moderate cold. Below LCT, conductance reincreases progressively, leading to a steeper than expected slope of the metabolic curve and allowing preservation of a threshold temperature in the shell. These results show for the first time in a wild young bird the development of NST after cold acclimatization. Further, at the temperature of cold acclimatization, an energy-sparing mechanism is shown in response to long-term fast adaptation.

The control of uncoupling protein-1, -2 and -3 (UCP-1, UCP-2, UCP-3) mRNA levels by sympathetic innervation in rats was investigated by specific and sensitive RT-PCR assays. In rats reared at thermoneutrality (25³C), unilateral surgical sympathetic denervation of interscapular brown adipose tissue (BAT) markedly reduced the UCP-1 mRNA level (338%) as compared with the contralateral innervated BAT pad, but was without significant effect on UCP-2 and -3 mRNA levels. Cold exposure (7 days, 4³C) markedly increased UCP-1 (+180%), UCP-2 (+115%) and UCP-3 (+195%) mRNA levels in interscapular BAT. Unilateral sympathetic denervation prevented the cold-induced rise in BAT UCP-1 and UCP-2 mRNAs, but not that in BAT UCP-3 mRNA. Results were confirmed by Northern blot analysis. These data indicate a differential endocrine control of UCP-1, UCP-2 and UCP-3 gene expression in rat BAT both at thermoneutrality and during prolonged cold exposure.z 1999 Federation of European Biochemical Societies.

4. Mitochondria isolated from this differentiated tissue were less abundant than in b.a.t. of mammals. Their respiration rate was similar to the respiration rate ofwhite adipose tissue mitochondria from control rats and much lower than the b.a.t. mitochondria rate from cold-acclimated rats. It is therefore unlikely that this differentiated adipose tissue contributes to the n.s.t. observed, an n.s.t. whose capacityreached5-26 W/kg ( + 73-5 % above resting metabolic rate) in cold-acclimated ducklings.5. The role of this differentiated adipose tissue in the metabolic adaptation to cold is discussed.

SUMMARY1. The histochemical characteristics of gastrocnemius muscle were investigated in 6-week-old cold-acclimated (5 weeks, 4 TC) and glucagon-treated (5 weeks, 25 0C, 103 nmol/kg i.P. twice daily) muscovy ducklings, two groups able to develop nonshivering thermogenesis in vivo. A comparison was made with thermoneutral controls (25 'C) of the same age. All animals were fed ad libitum. Fibre type, fibre area and capillary supply have been studied. Further, a quantitative histochemical method for mitochondrial Mg2+-ATPase activity was developed to characterize the mitochondrial coupling state in situ.2. White gastrocnemius was composed of fast glycolytic (FG) and fast oxidative glycolytic (FOG) fibres, while red gastrocnemius contained FOG and slow oxidative (SO) fibres. In white gastrocnemius, the proportion of FG fibres was higher in glucagon-treated than in control or cold-acclimated ducklings. In red gastrocnemius, the proportion of SO fibres was higher in both cold-acclimated and glucagon-treated ducklings than in controls. The area of all fibres was generally lower in glucagontreated than in other ducklings.3. The capillary density was higher in both red and white components of the gastrocnemius muscle in cold-acclimated and glucagon-treated than in control ducklings, as a result of an increased number of capillaries around each fibre.4. In all fibres, except the FG type in cold-acclimated ducklings, the staining intensity of the Mg2+-ATPase reaction was higher in cold-acclimated and glucagontreated than in control ducklings whereas the staining intensity with maximal decoupling of oxidative phosphorylation by dinitrophenol was unchanged. This indicated a more loose-coupled state of mitochondria in situ in all fibres of coldacclimated ducklings, and in FOG fibres of white gastrocnemius and SO fibres of red gastrocnemius in glucagon-treated ducklings. C. DUCHAMP AND OTHERS 5. These results indicated a higher oxidative metabolism of skeletal muscle in both cold-acclimated and glucagon-treated than in control ducklings, and for most of the parameters studied, a similarity between cold acclimation and glucagon treatment. Because of the higher loose-coupled state of muscle mitochondria in cold-acclimated and glucagon-treated than in control ducklings, the higher oxidative capacity of skeletal muscle in these ducklings could be used for heat production rather than ATP synthesis and account for muscular non-shivering thermogenesis.