Effects of Retinoic Acid on the Mobilization of Vitamin A from the Liver in Rats

Abstract

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.