LOCALIZATION OF THE APELIN SYSTEM IN THE OVARY AND GENITAL TRACT OF EWES SUBJECTED TO DIFFERENT NUTRITIONAL LEVELS

The present work was aimed to evaluate the presence and localization of the apelin system in the reproductive system of the ewe. Apelin is a recently discovered adipokine mainly secreted by adipose tissue and also produced by several peripheral tissues. A biological activity of apelin on female reproduction was supposed since this molecule regulates gonadotropin release and steroidogenesis [1]. Moreover, it was evidenced in ovary and uterus with differences among species [2]. In this work, apelin and its receptor (APLNR) were evaluated by immunohistochemistry and PCR at the level of ovary, oviduct and uterus. The trial was carried out on 15 ewes grazing on a seminatural pasture and slaughtered at the maximum pasture flowering (MxF, 5 ewes) and at the maximum pasture dryness (MxD, 10 ewes). Five ewes of the second group were fed with 600 gr/die/head of barley and corn (1:1) in addition to the fresh forage (Exp). Since apelin is involved in energy metabolism, sheep were fed differently to assess whether the feeding can modulate the secretion of the apelin system. Accordingly, during the grazing period, concentrations of plasma apelin were evaluated every 15 days. Experimental procedures were approved by the Ministry of Health (No. of approval 95/2018-PR). The transcript and protein for both apelin and APLNR were evidenced in all tissues evaluated. In the ovary the corpus luteum only showed a strong positivity for APLNR and a weaker staining for apelin while the other structures appeared negative. In the oviduct and uterus, both molecules were observed in the lining epithelium and in the uterine glands as uterus concerns. The highest levels of apelin and APLNR mRNA were detected in the MxD and EXP groups in the luteal phase of the estrous cycle in comparison to MxF group slaughtered during the anestrous phase. Pasture dryness had no effects on the plasma apelin levels. The present data confirm that the apelin is involved in the reproduction function also in the ewe, being differentially distributed and expressed in the ovary and genital tract. The overlapping localization of both apelin and its receptor suggest a paracrine and autocrine action of apelin likely aimed at regulating the development and function of corpus luteum and the activity of genital tract such as epithelium proliferation and secretion. In addition, it can be supposed that apelin may be secreted in the lumen of oviduct and uterus playing a nutritional role for both oocyte and embryo respectively. Differences of expression evidenced among ewe groups are likely due to the cyclic activity of tissues rather than diet influence.