Body lipids and adaptation of camel to food and water shortage : new data on adipocyte size and plasma leptin

The ability of camels to cope with food or water shortage is exceptional, thanks to several mechanisms of adaptation, including mobilization of body fat reserves during underfeeding and regeneration when food is available. In camel, the adipose tissues are mainly located in the hump (external) and around the kidney (internal perirenal fat, PF) and consist mainly of cells able to store lipids (adipocytes). However, the measurement of adipocyte size was scarcely performed, and no data are available on the variations of plasma leptin, an hormone which is secreted by adipocytes in mammalian species and could play a major role in the regulation of energy homeostasis. Recent experiments showed that the mean adipocyte size is similar between hump and PF, and was 100-700 picoliters in 70 adult male or female camels, in the range of values observed in cattle and sheep. Hump or PF weights were positively correlated and better explained by adipocyte size than number. Hump adipocyte size was positively correlated to hump height-hemicircumference and to hump lipid content. Hump biopsies during experiments with different levels of food or water allowance showed that adipocyte size decreased during a 2-month underfeeding, and this decrease was more marked when camels were previously overfed, whereas adipocyte size was not affected by 3 weeks of water deprivation. However, dehydration increased fat mobilization, with an increase in plasma non-esterified fatty acids and a decrease in hump lipid content. A radioimmunoassay was developed for camel leptin, using antibodies raised against sheep leptin. Plasma leptin concentration was 2-9 ng/ml, and positively correlated to hump lipid content or adipocyte size, but less closely than in cattle. It was not affected by underfeeding nor overfeeding, contrary to what was observed in cattle and sheep. Plasma leptin increased steadily (+ 20%) during 3 weeks of water deprivation, and returned rapidly to the control level after 6 hours of rehydration. Further studies will precise the role of leptin in the adaptation of camels to desert conditions. (Résumé d'auteur)