EFFECTS OF PREGNANCY AND LACTATION ON PLASMA-LIPID AND LIPOPROTEIN CONCENTRATIONS, LIPOPROTEIN COMPOSITION AND POSTHEPARIN LIPASE ACTIVITIES IN SHETLAND PONY MARES

The incidence of hyperlipaemia in ponies is highest in mares in late g estation and then early in lactation. Plasma lipid and lipoprotein con centrations were measured to establish the metabolic basis for this an d the lipoprotein composition of six healthy Shetland ponies was analy sed before pregnancy, in the last six weeks of gestation and one month after foaling. In the pregnant ponies, the concentrations of choleste rol and triglyceride were significantly increased (both P < 0.05) beca use of increased concentrations of high density lipoproteins (HDL) and very low density lipoproteins (VLDL), respectively (both P < 0.05). T he VLDL were significantly enriched in triglyceride and depleted of pr otein (P < 0.05 in each case), with the majority of cholesterol in the free rather than esterified form. These changes appeared to reflect i ncreased hepatic triglyceride synthesis and VLDL secretion because the activities of lipoprotein lipase and hepatic lipase, the enzymes resp onsible for the catabolism of VLDL and their remnants, were unaltered. After foaling, the concentrations of triglyceride and VLDL decreased significantly (both P < 0.05) because of increased lipoprotein lipase activity consistent with the induction of the enzyme in mammary tissue to provide for milk fat synthesis. Plasma cholesterol and HDL concent rations remained high and non-esterified fatty acid concentrations wer e significantly increased (P < 0.05). The VLDL remained enriched in tr iglyceride but had normal cholesterol and protein compositions, althou gh the mass of phospholipids was reduced. The changes in plasma VLDL c oncentration and composition found in the pregnant ponies mimicked tho se previously reported in ponies with hyperlipaemia and suggested a me tabolic basis for the incidence of the disease in late gestation.