Immunohistochemical detection of apelinergic system in the abomasum of sheep

The growing summer drought stress given by global warming is pre-empting the moment of the maximum productivity of pasture and shortening the availability period of fresh forage for the animals with an adequate nutritional value to ensure animal productivity [2]. The adipokines are molecules mainly produced by adipose tissue related to the individual's nutritional status [3]. One of them is the apelin (AP), which with its receptor (APJ), has been detected in the gastrointestinal tract [1, 4] and it takes action in a series of physiological processes including energy metabolism and nutritional status. This work is a part of a wider project about the expression of AP and APJ in the digestive system of the sheep under different nutritional levels (No. of approval 95/2018-PR); here, the apelinergic system was investigated through immunohistochemistry in the abomasum of 15 Comisana x Appenninica adult female sheep. The flock was free to graze on the pasture from June to the pasture maximum flowering (MxF group) feeding on fresh forage. Throughout the period between the maximum pasture flowering (MxF) and the maximum pasture dryness (MxD), the MxD group has been grazing on pasture feeding only on fresh forage, while the experimental group (Exp) received also a feed supplementation of 600 g/day/head of barley and corn. Samples of the abomasum were collected for each group, fixed in formalin and embedded in paraffin wax. Dewaxed sections were microwaved for antigen retrieval. The primary antibodies, a rabbit polyclonal anti-AP (Novus Biochemical) and a rabbit polyclonal anti-APJ (Abnova), were diluted in PBS 1:200 and 1:400 respectively and incubation was performed overnight at room temperature. The immunological reaction was detected with the ABC kit (VectorLaboratories) and visualized with diaminobenzidine. Preliminary data show the presence of AP and APJ in all analyzed samples suggesting an autocrine and paracrine action of the molecule. The positive cells have been found in the lining epithelium of the mucous layer and in the gastric glands mainly localized in their basal third. The comparisons performed among the three groups evidenced a lower intensity of immunopositivity in the MxD group for both AP and APJ. The detection of both molecules in the mucous layer of the abomasum and their different staining in the three analyzed groups suggests a role of the apelinergic system in the abomasum function regulation in the sheep. This role could be influenced by diet.