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 produc ts of the uterine endometrium in the uterine fluid of pigs, are assumed to be of importance for early embryonic development. While uterine RBP has bee n widely characterized, little information is available on the metabolism o f vitamin A itself or other specific binding proteins or nuclear receptors in the uterus of pigs. In the present study, the content and distribution o f vitamin A in uterine tissue of pigs during early gestation (Days 14-30) w ere examined macroscopically and microscopically via autofluorescence and H PLC. In addition, the distribution of specific proteins involved in vitamin A metabolism at the cellular and nuclear level was investigated. Macroscop ically, the yellowish-greenish autofluorescence characteristic of vitamin A was observed in uterine endometrium. Microscopy showed that the autofluore scence was associated with glandular and surface epithelium of the endometr ium. 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 de creased with the progress of gestation. This decrease was paralleled by a d ecrease in vitamin A content of endometrium and myometrium. In general, vit amin 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 w as retinol, representing more than 90% of total vitamin A. These results sh ow for the first time that the yellowish-greenish autofluorescence in the p ig uterus can be attributed to vitamin A. Differences in the form of vitami n A present in endometrium and myometrium might point to differences in met abolism. In the myometrium, vitamin A might be stored, and in the endometri um, vitamin A is present primarily as retinol-the form in which it is secre ted into the uterine fluid.