A re-examination of the fate of glyceride-glycerol in neutral lipid absorption and transport.

Conventional ideas concerning the unidirectional movement of triacylglycerol from intestinal lumen to lymph with sn-2-monoacylglycerol being the major glyceride-glycerol precursor were challenged by our finding that steady state specific activities of radiolabeled triacylglycerol (glyceryl moiety) in the intestinal mucosa and lumen were greatly reduced as compared to the specific activity of intraduodenally infused triacylglycerol. Investigation of the point at which the radiolabel was diluted was performed in mesenteric lymph duct-cannulated rats with a duodenal cannula through which trioleoyl[3H]glycerol was constantly infused. Both within the bowel lumen and in the intestinal mucosa, monoacylglycerol, diacylglycerol, and triacylglycerol specific activities were 31% or less of the specific activity of the infusate; chylomicron triacylglycerol specific activity was 75%. Efflux of neutral lipid from the mucosa into the bowel lumen was directly demonstrated by finding that when 3H glucose was injected intraperitoneally during triolein infusion, luminal triacylglycerol had a higher specific activity than was present in the mucosa. We conclude that there are two pools of mucosal triacylglycerol. One is rapidly transported and derives most of its glyceride-glycerol from luminal monoacylglycerol. The second is slowly transported; it derives its glyceride-glycerol mainly from endogenous sources and may efflux back into the bowel lumen.