Lipoprotein lipase and uptake of chylomicron triacylglycerol and cholesterol by perfused rat mammary tissue

The role of lipoprotein lipase in the uptake of chylomicron triacylglycerol and cholesterol from blood was studied in perfused inguinal-abdominal mammary tissue of rats lactating 10-15 days. Lipoprotein lipase activity in the tissue was reduced, from 0.47 to 0.10 units/g, by removing the anterior pituitary gland from lactating rats 2 days before the experiment. Perfused mammary tissue of normal lactating rats took up 12% of the chylomicron triacylglycerol infused, whereas the tissue of hypophysectomized lactating rats took up less than 1%. About two-thirds of the triacylglycerol taken up was retained as glyceride, and the rest was hydrolyzed and released to the perfusing fluid as fatty acids and glycerol. Autoradiographic studies of perfused tissues of normal lactating rats showed that both the acyl and glycerol moieties derived from chylomicron triacylglycerol were incorporated into milk lipid droplets. Perfused mammary tissue of normal lactating rats also took up 15% of the chylomicron cholesterol infused, whereas the tissue of hypophysectomized lactating rats took up less than 1%. The findings demonstrate that chylomicron cholesterol is taken up with triacylglycerol by lactating mammary tissue, and that uptake of both lipids is markedly suppressed when lipoprotein lipase activity is low, as in tissue of hypophysectomized rats. It is proposed that uptake of triacylglycerol from chylomicrons by mammary tissue requires the action of lipoprotein lipase, while uptake of cholesterol is dependent on reduction of the triacylglycerol core, resulting from action of the enzyme on the core and uptake of lipolytic products by the tissue.