Intraenterocyte distribution of absorbed lipid and effects of phosphatidylcholine

The present studies were designed to investigate the distribution of absorbed lipid in intestinal mucosal cells and to identify the chylomicron precursor pool. Rats were infused intraduodenally with glyceryl tri[9,10(n)-3H]oleate (135 mumol/h); other rats were in addition infused with 9 mumol/h of phosphatidylcholine. After 5-h infusion the proximal one-half of intestine was removed and the mucosa obtained. It was found that 50% of the radioactivity in the whole homogenate pelleted on centrifugation at 75,000 g.min. The supernatant was further fractionated by high-speed centrifugation resulting in a floating lipid layer, a supernatant, and a microsomal pellet. The results showed that these subcellular fractions had a triacylglycerol specific activity 46-52% of the infusate's specific activity. Including phosphatidylcholine in the duodenal lipid infusion increased the triacylglycerol specific activity of all subcellular fractions (70%) resulting in a specific activity approaching that of the infusate, which would be expected of chylomicron triacylglycerol. These studies demonstrate 1) that considerable mucosal lipid is distributed into a low-speed pellet, 2) that mucosal triacylglycerol specific activity can be greatly increased by including phosphatidylcholine in a lipid infusion, and 3) that despite obtaining multiple subcellular fractions, the chylomicron precursor pool could not be clearly identified in the mucosa of control rats.