Effects of prolonged treatment of lactating goats with bovine somatotropin on aspects of adipose tissue and liver metabolism

Adipose tissue metabolism and its role in adaptations to undernutrition in ruminants

Changes in the amount and metabolism of adipose tissue (AT) occur in underfed ruminants, and are amplified during lactation, or in fat animals. The fat 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 adipocytes and high sensitivity to the lipolytic effect of catecholamines. Glucocorticoids and growth hormone probably interact to induce teleophoretic changes in the AT responses to adenosine and catecholamines during lactation. Fat mobilization in dry ewes is related both to body fatness and to energy balance. The in vivo beta-adrenergic lipolytic potential is primarily related to energy balance, whereas basal postprandial plasma non-esterified fatty acids (NEFA) are related to body fatness, and preprandial plasma NEFA is the best predictor of the actual body lipid loss. Several mechanisms seem to be aimed at avoiding excessive fat mobilization and/or insuring a return to 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, beta-adrenergic stimulation and short day length, and increased by insulin and glucocorticoids. This finding suggests that the leptin chronic (or acute) decrease in lean (or underfed respectively) ruminants is, as in rodents, a signal for endocrine, metabolic and behavioural adaptations aimed at restoring homeostasis.

Effect of insulin and prolactin on acyltransferase activities in MAC-T bovine mammary cells

The enzymatic activities of sn-glycerol-3-phosphate acyltransferase and lysophosphatidate acyltransferase were investigated in microsomal fractions prepared from MAC-T cells from bovine mammary gland and from FTO-2B cells from rat liver. In both cell lines, sn-glycerol-3-phosphate acyltransferase exhibited similar rates of palmitate and oleate incorporation. However, lysophosphatidate acyltransferase activity in MAC-T cells had a 2.8-fold greater rate of palmitate incorporation than of oleate incorporation. In FTO-2B cells, there was a 1.4-fold greater rate of oleate incorporation than of palmitate incorporation. FTO-2B and MAC-T cells displayed acyltransferase activities that were consistent with liver and mammary tissues, respectively. The acyltransferases were examined from FTO-2B and MAC-T cells that were treated with insulin and prolactin. Insulin suppressed both acyltransferase activities in FTO-2B cells, and prolactin had a stimulatory effect; however, these effects were very small. In contrast, insulin and prolactin had a stimulatory effect on both acyltransferase activities in MAC-T cells; prolactin elicited the largest effect. Treatment of MAC-T cells with cycloheximide inhibited the stimulatory effect of prolactin on acyltransferase activities.