Tissue distribution and metabolism of newly absorbed vitamin A in the rat

Abstract

Chylomicrons containing newly absorbed labeled vitamin A were injected intravenously into normal intact rats, and the tissue distribution of radioactivity was observed for several days. Chylomicrons were used so that the vitamin could be administered physiologically, in the form in which it is normally absorbed. The total recovery of lipid-soluble radioactivity in the entire animal varied from 92% after 17 min to 63% after 24 hr to 56% after 6 days. At all times approximately two-thirds of the recovered radioactivity was found in the liver. Substantial amounts of radioactivity were also found in the kidneys and in the total depot fat, and small but significant amounts of labeled vitamin A were found in the plasma, small intestine, lungs, and adrenals. After 8 hr, labeled retinyl esters predominated in all tissues except plasma, with small amounts of labeled retinol also being present. Quantitatively significant amounts of labeled retinal or retinoic acid were not observed. The composition of labeled retinyl esters was remarkable in showing a consistent predominance of saturated esters in all tissues. Marked differences were, however, seen in the relative amounts of labeled retinyl palmitate and stearate in different tissues.During the first 24 hr 3.7% of the injected radioactivity was excreted as expired CO2, 3.5% was excreted as water-soluble compounds in the urine, and 8.7% was excreted in the bile. The biliary metabolites apparently consisted of a heterogeneous mixture of polar compounds, some of which were present as glucuronic acid conjugates.