Identification of two adipokines, leptin and apelin, in the abomasum of the sheep

Leptin and apelin are adipokines mainly produced by adipose tissue related to the individual's nutritional status. These molecules have been also described in the gastrointestinal tract in humans and laboratory animals suggesting a paracrine and apocrine activity. In this work, the presence and localization of leptin and apelin were investigated in the abomasum of the sheep. Samples of the abomasum (sample width 5 mm) were collected from 15 Comisana x Appenninica adult female sheep reared in a semi-natural pasture of the Italian Central Apennines. Immunohistochemistry was performed on formalin fixed and paraffin embedded sections by using a mouse monoclonal antileptin (Fitzgerald) and a rabbit polyclonal anti-apelin (Novus Biochemical) as primary antibodies. In addition, an anti-serotonin antibody (Dako) was used to identify neuroendocrine cells. Leptin and apelin were evidenced in the mucous layer of the abomasum in all evaluated samples. Both molecules were localized in the gastric glands, mainly in their lower half; in addition, apelin was detected in the lining epithelium. By morphological characteristics and localization, positive cells were labelled as principal cells. Serotonin positive neuroendocrine cells were positive to leptin and negative to apelin. All molecules were observed at the cytoplasmic cell level. Leptin and apelin detection in the gastric glands suggests a local secretion and a role of these adipokines in the paracrine regulation of the digestive function, as already attested in other species. All husbandry and experimental procedures were approved by the institutional ethics committee and the Ministry of Health [No. of approval 95/2018-PR].