Dietary restriction alters retinol and retinol-binding protein metabolism in aging rats

Recent studies reported that retinoid metabolism was influenced by long-ter m dietary restriction (DR) in rats. Because plasma retinol was decreased in rats subjected to DR, it was thought that this dietary manipulation may ha ve an effect on retinol-binding protein (RBP) metabolism. Thus, the aim of this study was to assess retinoids, RBF: and transthyretin (TTR) levels in plasma and liver of young (3 months), adult (12 months) and old (22 months) female Sprague-Dawley rats fed ad libitum (AL) or subjected to a 40% DR, e nriched (DR), or not (DR), with vitamins and minerals. Results indicate tha t hepatic total retinoid concentrations and content increased with age in a ll the groups. DR+ rats showed higher hepatic retinoid concentrations than age-matched AL and DR rats. Adult and old DR and DR+ rats exhibited signifi cantly lower plasma RBP-retinol and higher total retinoic acid levels than corresponding controls, although these parameters were not influenced by ag ing. Liver RBP levels were also decreased in DR and DR+ rats when compared to respective AL controls. There was a slight age-related decline in plasma TTR levels in DR and DR+ rats which was riot associated with modifications in liver TTR levels. Hepatic gene expression of RBP and TTR, as evaluated by Northern blot hybridization, did riot change with age or diet, suggestin g that the lower levels of plasma RBP-retinol and liver RBP in vitamin A-su fficient rats subjected to DR may reflect post-transcriptional alterations and/or accelerated degradation of hepatic RBI. The elevated plasma levels o f retinoic acid may represent an adaptive mechanism developed by DR rats to maintain retinoid-dependent functions.