New views on intestinal absorption of lipids in teleostean fishes: an ultrastructural and biochemical study in the rainbow trout.

Lipid absorption in rainbow trout was studied after gastric administration of [1-14C]linoleic and [1-14C]-palmitic acids. The intestinal epithelial cells were isolated at various times of absorption and the major lipid classes were isolated. Radioactivity was found primarily in the triglycerides. Blood radioactivity was measured at different times after administration of the labeled acids. It was very low until after 6 hr. After 4 hr when it was detectable, it was located essentially in the triglyceride fraction. At various times after feeding (a meal with 60% of unsaturated long chain fatty acids) the absorptive epithelium of the anterior intestine and pyloric caeca were examined by electron microscopy. Surprisingly, the esterification of fatty acids corresponded to the formation of VLDL-like particles, seen in SER, RER, Golgi apparatus, lamellar structures, intercellular space, interstitial space of lamina propria and lumen of lymphatic vessels. The respective roles of endoplasmic reticulum and Golgi apparatus in the lipoprotein synthesis are discussed. The particles are delivered in the intercellular spaces by way of the lamellar structures, whose role until now was unknown. Though the absorption of dietary triglycerides is much slower than in mammals, the mechanism does not differ fundamentally. The long chain fatty acids are esterified by the intestinal cells and transferred as VLDL-like particles to lymph. X