LIPOPROTEIN-LIPASE ACTIVITY AND ITS RELATIONSHIP TO HIGH MILK-FAT TRANSFER DURING LACTATION IN GREY SEALS

Lipoprotein lipase regulates the hydrolysis of circulating triglycerid e and the uptake of fatty acids by most tissues, including the mammary gland and adipose tissue. Thus, lipoprotein lipase is critical for th e uptake and secretion of the long-chain fatty acids in milk and for t he assimilation of a high-fat milk diet by suckling young. In the lact ating female, lipoprotein lipase appears to be regulated such that lev els in adipose tissue are almost completely depressed while those in t he mammary gland are high. Thus, circulating fatty acids are directed to the mammary gland for milk fat production. Phocid seals serve as ex cellent models in the study of lipoprotein lipase and fat transfer dur ing lactation because mothers may fast completely while secreting larg e quantities of high fat milks and pups deposit large amounts of fat a s blubber. We measured pup body composition and milk fat intake by iso tope (deuterium oxide) dilution and plasma post-heparin lipoprotein li pase activity in six grey seal (Halichoerus grypus) mother-pup pairs a t birth and again late in the 16-day lactation period. Maternal post-h eparin lipoprotein lipase activity increased by an average of four-fol d by late lactation (P = 0.027), which paralleled an increase in milk fat concentration (from 38 to 56%; P = 0.043). Increasing lipoprotein lipase activity was correlated with increasing milk fat output (1.3-2. 1 kg fat per day) over lactation (P = 0.019). Maternal plasma triglyce ride (during fasting) was inversely correlated to lipoprotein lipase a ctivity (P = 0.027) and may be associated with the direct incorporatio n of long-chain fatty acids from blubber into milk. In pups, post-hepa rin lipoprotein lipase activity was already high at birth and increase d as total body fat content (P = 0.028) and the ratio of body fat: pro tein increased (P = 0.036) during lactation. Although pup plasma trigl yceride increased with increasing daily milk fat intake (P = 0.023), p ups effectively cleared lipid from the circulation and deposited 70% o f milk fat consumed throughout lactation. Lipoprotein lipase may play an important role in the mechanisms involved with the extraordinary ra tes of fat transfer in phocid seals.