Partition of circulating triglycerides between formation of milk fat and other metabolic pathways in sheep

Abstract

SUMMARYAn emulsion of [3H]triglycerides was used to investigate how circulating triglycerides are partitioned between milk fat, body fat and use as an energy source in lactating sheep. Immediately after intravenous injection of the emulsion the radioactivity in the blood was predominantly associated with the triglycerides of the fraction that contained chylomicrons and very low density lipoproteins. The radioactive triglyceride was used both as a precursor of milk fat and as a source of energy. The proportions utilized for these two purposes were determined experimentally and the amount retained in the carcass lipids was found by difference. In separate control experiments with nonlactating sheep the total radioactivity in the body lipids was determined by carcass analysis. This total together with that accounted for by oxidation amounted to 98% of the total radioactivity injected. These results established that the emulsion was a reliable model for the purpose intended.The proportion of the labelled triglyceride that appeared in milk fat declined from 45% at 17 days after lambing to 8–10% at 73 days after lambing. The proportion that was oxidized was also lower in late lactation so that retention in carcass lipids was substantially increased at that time.