Distribution of vitamin A, retinol-binding protein, cellular retinoic acid-binding protein I, and retinoid X receptor beta in the porcine uterus during early gestation.

Retinol and retinol-binding protein (RBP), among the major secretory products of the uterine endometrium in the uterine fluid of pigs, are assumed to be of importance for early embryonic development. While uterine RBP has been widely characterized, little information is available on the metabolism of vitamin A itself or other specific binding proteins or nuclear receptors in the uterus of pigs. In the present study, the content and distribution of vitamin A in uterine tissue of pigs during early gestation (Days 14-30) were examined macroscopically and microscopically via autofluorescence and HPLC. In addition, the distribution of specific proteins involved in vitamin A metabolism at the cellular and nuclear level was investigated. Macroscopically, the yellowish-greenish autofluorescence characteristic of vitamin A was observed in uterine endometrium. Microscopy showed that the autofluorescence was associated with glandular and surface epithelium of the endometrium. In these structures, immunoreactive RBP was localized, as was cellular retinoic acid-binding protein I. Retinoid X receptor beta was observed in the nucleus of myometrium and endometrium. The intensity of fluorescence decreased with the progress of gestation. This decrease was paralleled by a decrease in vitamin A content of endometrium and myometrium. In general, vitamin A concentration in the endometrium was higher than in the myometrium (P < 0.01). In the myometrium, if present at all, vitamin A was found almost exclusively as retinyl esters. In the endometrium, the dominant fraction was retinol, representing more than 90% of total vitamin A. These results show for the first time that the yellowish-greenish autofluorescence in the pig uterus can be attributed to vitamin A. Differences in the form of vitamin A present in endometrium and myometrium might point to differences in metabolism. In the myometrium, vitamin A might be stored, and in the endometrium, vitamin A is present primarily as retinol-the form in which it is secreted into the uterine fluid.