Evidence for separate pathways of chylomicron and very low-density lipoprotein assembly and transport by rat small intestine.

Previously, we demonstrated that the hydrophobic surfactant Pluronic L-81 (L-81) inhibits the intestinal formation and transport of chylomicrons (CM) but not of very low-density lipoprotein-sized (VLDL) particles. The present study was undertaken to determine whether infusion of egg lecithin results mainly in secretion of VLDL by the small intestine and whether L-81 has any effect on their formation and secretion. Intestinal fistula rats were infused intraduodenally at a rate of 3 ml/h with a lipid emulsion containing 20 mM egg lecithin and 19 mM sodium taurocholate for 8 h. This was then followed by another 8 h of infusion of a similar lipid emulsion but with 0.5 mg/h of L-81 added. Lymphatic lipid output was measured, and lymph lipoproteins were sized by use of electron microscopy. Whether L-81 was present or not, no significant difference was detected in the lymphatic triglyceride, phospholipid, or cholesterol outputs. Based on agarose gel electrophoresis, sizing of intestinal lymph lipoproteins, and also the determination of lipid in the intestinal lymph CM and VLDL as separated by ultracentrifugation, VLDL were the major lipoproteins present in lymph during the infusion of egg lecithin. Thus, intraduodenal infusion of egg lecithin in the rat results mainly in the transport of VLDL and is not affected by the administration of L-81. The results suggest that CM and VLDL are assembled separately by the enterocytes and indicate the usefulness of L-81 in further investigating the pathways and regulation of intestinal lipoprotein synthesis, assembly, and secretion.