Absorptive activities in the efferent ducts of adult cat studied by Aquaporin immunohistochemistry and lectin histochemistry

Ultrastrucural features of the epithelium lining the efferent ducts (ED) in the cat, as in other mammalian species, are strongly indicative of an absorptive activity taking place towards the intraluminal fluids. It is well-known that more than 95% of the fluid leaving the testis is reabsorbed by the ED, but the cell structures involved in the reabsorption processes are still a matter of debate. The purpose of the present work was to study the absorptive pathways in the ED of adult cats by means of 1) the immunohistochemical localization of different isoformes of the aquaporine family (AQPs), integral membrane water channels that facilitate rapid passive movement of water, and 2) the localization and the carbohydrate characterization of the endocytotic apparatus by means of the lectin histochemistry. The results will be interpreted in the light of ongoing electron microscopy studies. The study was carried on fragments of cat epididymides obtained at orchiectomy, fixed in neutral formalin and paraffin embedded. Immunohistochemistry examined the localization of AQPs 1, 2 and 5, whereas lectin histochemistry analyzed the glycoconjugates by a panel of 12 lectins in association with sialidase (s) treatment. AQP1-immunoreactivity was strongly evidenced at the apical surface of the ED non-ciliated cells, whereas the ciliated ones were unstained. The vasal endothelium was immunoreactive, too. No other AQP1-immunoreactivity was observed throughout the epididymal duct. Otherwise, AQP2 appeared absent in the ductuli efferentes but it was well localized in the cauda epidydimidis. AQP5-immunoreactivity was undectectable. Lectin histochemistry showed that the luminal surface and the apical region of ED non-ciliated cells contain glycans with terminal Neu5Acα2,3Galβ1,3GalNAc, Neu5acα2,3Galβ1,4GlcNAc, Galβ1,4GlcNAc, GalNAc (s-PNA, MAL II, RCA120, SBA reactivity) and with internal/terminal αMan (Con A affinity). In addition, terminal GalNAc and Neu5Acα2,6Gal/GalNAc (SNA reactivity) were present in glycans of the luminal surface and the apical zone, respectively. Ciliated cells expressed glycoconjugates only on cilia which showed terminal Neu5Acα2,3Galβ1,4GlcNAc (s-RCA120 staining), GalNAc, and internal/terminal αMan, GlcNAc (s-WGA, GSA II staining). Our data provide evidence for the involvement of different pathways in the bulk reabsorption of water and low molecular weight solutes by the non-ciliated cell of the cat ED. A combination of pathways is possible, too: AQP-mediated trans-cellular route together with fluid phase glycocalix-mediated endocytosis.