Retinol combined with retinoic acid increases retinol uptake and esterification in the lungs of young adult rats when delivered by the intramuscular as well as oral routes

The lungs require an adequate supply of vitamin A for normal embryonic development, postnatal maturation, and maintenance and repair during adult life. We have previously shown that a nutrient-metabolite combination of vitamin A admixed with a small proportion (10%) of retinoic acid (RA), referred to as VARA, acts synergistically to increase lung retinyl ester (RE) concentration in neonatal rats. A series of studies was designed to test whether VARA increases RE in adult lungs, and whether VARA is more effective than vitamin A when given by the i.m. route. Orally administered VARA increased RE in the lungs of vitamin A-marginal adult rats more than either vitamin A or RA alone (P < 0.05). In vitamin A-deficient young adult rats, lung RE was increased by VARA when administered by the i.m. route. When a tracer of ³H-retinol was added to the placebo (oil), vitamin A, and VARA doses, total ³H and ³H-RE increased in the lungs more with VARA than vitamin A alone, for oral and i.m. dosing. Nevertheless, when VARA and vitamin A were given by the oral route, they were more effective in increasing RE in the liver. Plasma retinol was increased similarly in vitamin A-deficient rats after administration of VARA and vitamin A, by either the oral or the i.m. route. Overall, VARA can increase retinol uptake and esterification in adult lungs when delivered intramuscularly as well as orally.