The presence and localization of apelin in the sheep abomasum: impact of diets characterized by different chemical composition

The apelinergic system is a complex system including the apelin peptide (AP), initially isolated in 1998 from bovine stomach homogenates and its receptor, named APJ receptor. The apelin peptide is extensively expressed in several human and laboratory animal organs (heart, lung, brain, mammary gland). Between the different roles hypothesize for apelin there are also the control of blood pressure and the stimulation of drinking behavior in rats. Recently its expression was also evidenced in the basal glandular portion of the stomach in laboratory animals, allowing to hypothesize its intervention in the control of acid secretion. No data concerning its presence and distribution in the abomasums of the sheep are present at the moment. So, we decided to test the presence and distribution of apelin in the abomasums of the sheep and the possible existence of a variability as a consequence of the different water content of the diet.The experiment was conducted using 50 sheep fed on Apennine semi-natural pasture for two experimental periods: in the first period the animals were fed for 45 days on the pasture at the maximum of its flowering; in the second one the animals were fed on the same pasture until its maximum dryness but the half of them daily received adiet integration of cereals. At the end of each experimental period, some animals were regularly slaughtered at the local abattoir, the abomasum specimens were immediately removed and some of them processed for routine tissue-embedding preparation while other for molecular biology. The immunohistochemical reaction was visualized on 5 µm serial sections, using a primary rabbit polyclonal antibody (anti-AP), the avidin-biotin-complex and the DAB as the chromogen. The immunohistochemical study showed a peculiar immunoreaction for AP in the abomasum of the animals examined. In particular, a immunopositive reaction for AP was evident in the cells of the basal third of the tubular glands and they were mainly of the closed type, with an oval or round shape and contained many perinuclear granules. The immunopositive reactions didn’t evidence any difference both in the localization and in the number of the positive cells, between the different groups. Immuno-positivity for AP was not observed in any other histological structure or in the sections utilized as negative controls.These results allow us to conclude that AP is present in the glandular tissue of the abomasums of the sheep, as observed in laboratory animals, with a peculiar cytoplasmatic localization and to hypothesize that AP is involved in the control of gastric secretion, probably via APJ receptor. The AP expression in the stomach doesn’t seem to be influenced by the water content of the diets with a positivity that appears to be localized in the same glandular portion between the different animal groups.