Journal of Lipid Research, Vol. 9, 103-109, January 1968
Copyright © 1968 by Lipid Research, Inc.
Pathways of absorption of retinal and retinoic acid in the rat
Noel H. Fidge , Tatsuji Shiratori , Jagannath Ganguly , and Dewitt S. Goodman
Department of Medicine, Columbia University College of Physicians and Surgeons, New York 10032
The chemical and anatomical pathways of absorption of dietary retinal, retinoic acid, and retinol were examined in rats containing lymph, bile, and duodenal cannulae. The experiments were designed to maintain physiological conditions to the greatest possible extent. In each rat an uninterrupted flow of bile into the duodenum was maintained by connecting the duodenal cannula to the bile duct of a second rat.
Labeled vitamin A compounds were introduced into the duodenum in very small amounts (7-14 µg) in the form of a bile-lipid mixture resembling normal intestinal contents. Under these conditions, most (70-80%) of the radioactivity recovered after the feeding of labeled retinol or retinal was found in the lymph, predominantly in saturated retinyl esters. In contrast, 92-95% of the radioactivity recovered after the feeding of labeled retinoic acid was found in the bile, and was contained in a mixture of polar metabolites, most of them more polar than free retinoic acid. Two-thirds of the small amount of radioactivity found in lymph after retinoic acid-14C feeding was in the form of free retinoic acid.
The results indicate that under normal conditions the major pathway of retinal absorption involves its reduction to retinol, which is then esterified and transported via the lymphatics in a manner similar to that of dietary retinol. A small proportion of retinal is apparently normally oxidized, and is then transported via the portal vein and excreted in the bile in a manner similar to that of dietary retinoic acid. The relative importance, in quantitative terms, of these two pathways of retinal metabolism can vary, depending on the status of the animal.
Supplementary key words vitamin A • retinal • retinoic acid • retinol • retinyl esters • rat • absorption • metabolism • intestine • lymphatics • portal vein • bile • reduction • oxidation
Submitted on August 4, 1967
Accepted on October 6, 1967
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