Intestinal absorption of radioactive long-chain saturated fatty acids by the larval dragon-fly Aeshna cyanea and co-absorptive effects of other stable fatty acids

Abstract

In contrast with unsaturated fatty acids, ingestions of long-chain saturated fatty acids by Aeshna cyanea larvae failed to induce an accumulation of lipid droplets in the intestinal absorptive cells, so that it remained questionable whether they were absorbed or not (Bauerfeind and Komnick, 1989). Therefore, we reinvestigated the question with the use of radioactive tracers, which were orally infused under two conditions: at low doses and at high doses prepared by the addition of other stable fatty acids. We found that carboxyl- 14C labelled long-chain stearic and palmitic acids, as well as the medium-chain lauric acid studied for comparison, were indeed absorbed from low doses of 7 nmol. They were incorporated predominantly into the phospholipids of the midgut wall and released into the haemolymph mainly as diacylglycerols. A substantial part was also transported as cholesterylester. The increase in total fatty acids of the infusates by the addition of 1.8 μmol stable oleic, arachidonic, myristoleic acids and lauric acid ethylester, which at high doses induce the formation of triacylglycerol droplets within the midgut enterocytes, markedly reduced the absorption rates of the radioactive saturated fatty acids and greatly favoured their incorporation into di- and triacylglycerols on the expense of the phospholipids and, at a minor part, on the expense of cholesterylester. By contrast, the addition of 1.8 μmol stable caproic acid yielded little, if any, effect on the absorption pattern of the radioactive palmitic acid. The results suggest that the co-absorption of medium- and long-chain saturated and unsaturated fatty acids is interdependent utilizing the same or similar uptake mechanisms and dose-dependent esterification pathways, while it is largely independent of and probably different from the absorption of short-chain fatty acids.