Adipose tissue metabolism and its role in adaptations to undernutrition inruminants

Changes in the amount and metabolism of adipose tissue (AT) occur in underf ed ruminants, and are amplified during lactation, or in fat animals. The fa t depot of the tail of some ovine breeds seems to play a particular role in adaptation to undernutrition; this role could be linked to its smaller adi pocytes and high sensitivity to the lipolytic effect of catecholamines. Glu cocorticoids and growth hormone probably interact to induce teleophoretic c hanges in the AT responses to adenosine and catecholamines during lactation . Fat mobilization in dry ewes is related both to body fatness and to energ y balance. The in vivo P-adrenergic lipolytic potential is primarily relate d to energy balance, whereas basal postprandial plasma non-esterified fatty acids (NEFA) are related to body fatness, and preprandial plasma NEFA is t he best predictor of the actual body lipid loss. Several mechanisms seem to be aimed at avoiding excessive fat mobilization and/or insuring a return t o the body fatness homeostatic set point. As well as providing the underfed animal with fatty acids as oxidative fuels, AT acts as an endocrine gland. The yield of leptin by ruminant AT is positively related to body fatness, decreased by underfeeding, P-adrenergic stimulation and short day length, a nd increased by insulin and glucocorticoids. This finding suggests that the leptin chronic (or acute) decrease in lean (or underfed respectively) rumi nants is, as in rodents, a signal for endocrine, metabolic and behavioural adaptations aimed at restoring homeostasis.