No abstract

Thermogenic endurance and development of metabolic cold adaptation in birds may critically depend on their ability to synthesize and use fatty acids (FA) as fuel substrates. Hepatic lipogenesis and the capacity to oxidize FA in thermogenic tissues were measured in cold-acclimated (CA) ducklings (Cairina moschata) showing original mechanisms of metabolic cold adaptation in the absence of brown adipose tissue, the specialized thermogenic tissue of rodents. The rate of FA synthesis from [U-(14)C]glucose and from [1-(14)C]acetate, measured in incubated hepatocytes isolated from 5-wk-old thermoneutral (TN; 25 degrees C) or CA (4 degrees C) fed ducklings, was higher than in other species. Hepatic de novo lipogenesis was further increased by cold acclimation with both glucose (+194%) and acetate (+111%) as precursor. Insulin slightly increased (+11-14%) hepatic lipogenesis from both precursors in CA ducklings, whereas glucagon was clearly inhibitory (-29 to -51%). Enhanced de novo lipogenesis was associated with higher (+171%) hepatocyte activity of glucose oxidation and larger capacity (+50 to +100%) of key lipogenic enzymes. The potential for FA oxidation was higher in liver (+61%) and skeletal muscle (+29 to +81%) homogenates from CA than from TN ducklings, suggesting that the higher hepatic lipogenesis may fuel oxidation in thermogenic tissues. Present data underline the high capacity to synthesize lipids from glucose in species like muscovy ducks susceptible to hepatic steatosis. Lipogenic capacity can be further increased in the cold and may represent an important step in the metabolic adaptation to cold of growing ducklings.

Alpine marmots (Marmota marmota) were maintained on a laboratory diet, and the fatty acid composition of gonadal and subcutaneous white adipose tissues (WAT) was studied during a yearly cycle. Fatty acids (FA) released from isolated adipocytes were also identified after stimulation of in vitro lipolysis. Analysis of the FA composition of WAT depots showed that marmot WAT mainly contained monounsaturated FA (65%, mostly oleic acid, 18:1n-9) although laboratory food contained 45% of linoleic acid (18:2n-6) and only 21% of 18:1n-9. During stimulated lipolysis, saturated FA were preferentially released from isolated adipocytes whereas unsaturated FAs were retained. Despite this selective release of FA from isolated WAT cells in vitro, and despite the FA composition of the food, marmots maintained a constant FA composition in both WAT depots throughout the year. Six months of hibernation and fasting as well as an intense feeding period did not affect this composition. The potential adaptive benefit of such regulation of WAT composition, based on a high level of monounsaturated FA, might be to maintain fat with appropriate physical properties allowing animals to accommodate to and survive the wide range of body temperatures experienced during hibernation.