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Effects of Retinoic Acid on the Mobilization of Vitamin A from the Liver in Rats^1,2,

Nutrition Program, Division of Biological Health, The Pennsylvania State University, University Park, Pennsylvania 16802

The effect of short- (3 days) and long-term (24 or 25 days) feeding of retinoic acid (RA) on plasma levels of retinol and on the ability of the hepatic system to mobilize retinol was studied in three groups of normally growing rats maintained on vitamin A-deficient diets. Chylomicron-containing [³H]retinyl esters (RE) (4.87 µg retinol equivalents) were injected into the jugular vein on day 24 or 25, and plasma levels of labeled and total retinol and RE were monitored for 5 hours post-dosing. Total vitamin A in the liver was determined in half the rats after 5 hours. Radioactivity was determined separately in the plasma, liver, intestine and feces, and in the remainder of the carcasses. The remaining animals were fed their respective predosing diets for an additional 6 days. Radioactivity in urine was monitored daily for the 6 days and determined in each of the above-noted tissues at the end of 6 days. Both short- and long-term feeding of RA lowered plasma levels of vitamin A; this effect was not due to impairment of mobilization from the hepatic system since labeled RE disappearance and retinol appearance curves were identical for all dietary groups, but rather, the effect was due to sparing of endogenous hepatic reserves. The data suggest that direct partial provision of tissue needs for vitamin A by RA modified the feedback control mechanism that, in the absence of dietary RA, regulates mobilization and/or release of retinol bound to its carrier protein from endogenous stores.

KEY WORDS: • retinoic acid • liver vitamin A • mobilization • tissue requirements

¹ Supported in part by NIH Grant AM 16578.

² Presented in part at the Annual Meeting of the Federation of American Societies for Experimental Biology, Chicago, Illinois, 1977; Abstract: Keilson, B., Lewis, K. C., Underwood, B. A. & Loerch, J. D. (1977) Effects of liver stores, dietary intake, and tissue utilization of vitamin A in the regulation of plasma vitamin A levels in rats. Federation Proc. 36 (Abstr. 4567), 1135.

³ To whom reprint requests should be addressed. Current address: Department of Nutrition and Food Science. Massachusetts Institute of Technology, Room 20A-207, Cambridge, Massachusetts 02139.

Manuscript received 4 January 1978.

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