Relationships between in vivo and in vitro lipid metabolism in lactating goats

Multiparous Sannen dairy goats were used to compare in vivo and in vitro lipid metabolism. Goats were infused simultaneously with a mixture of (2- 3 H)-glycerol and of (1- 14 C)-palmitic, -stearic, and -oleic acid com- plexed in plasma in early (Day 11, n = 4), mid (Day 37, n = 4), and late (Day 80, n = 3) lactation to determine in vivo lipid kinetics. Perirenal adipose tissue biopsies were obtained and used to measure in vitro lipid kinetics under both basal and norepinephrine-stimulated conditions. Plasma non-esterified fatty acid (NEFA) concentrations and NEFA entry rate were higher (P < 0.05) during early compared with mid and late lactation. Plasma glucose and glycerol concentrations and glycerol entry rate were not different between stages of lactation. Although variable, in vitro lipogenesis increased more than 10-fold (P = 0.077) between early and late lactation, whereas there were no significant differences between NEFA and glycerol release at any stage of lactation. In vitro lipid metabolism, when expressed per gram of adipose tissue, was poorly correlated with in vivo lipid. However, when in vitro NEFA release was extrapolated to the whole animal the correlations were markedly improved. These data demonstrate that the reduction in lipid reserves which occurs during early lactation is the result of increased NEFA mobilisation and reduced lipogenesis rather than increased lipolysis. Although more variable than in vivo measures of lipid metabolism, in vitro metabolism of adipose tissue can reflect metabolic status.