Functional evaluations of the cat epidydimis related to the expression of aquaporins and lectin-binding sites

Previous ultrastructural studies of the epithelium lining the cat epididymal duct indicated that principal cells could be involved in fluid absorption as well as glycoprotein secretion1. It is known that modifications of the surface glycoproteins occur during the maturation, capacitation and acrosome reactions of spermatozoa, these glycoconjugates most probably being of epithelial origin by a secretory process into the lumen2. Therefore, the purpose of the present work was to study the possible absorptive and secretory events taking place in the cat epididymis by 1) the immunohistochemical localization of four different isoforms of Aquaporins (AQP1, 2, 5 and 9), integral membrane water channels that facilitate rapid passive movement of water, and 2) the carbohydrate expression and localization in the epithelial lining, by means of lectin histochemistry, utilizing a panel of 12 lectins in association with sialidase treatment, as previously published3. The study was carried out on fragments of adult cat epididymides obtained through orchiectomy, fixed in neutral formalin and paraffin-embedded. AQP1-immunoreactivity, strongly evidenced at the apical surface of the ductuli efferentes in non-ciliated cells, was present in the blood vessels all along the epididymis but not in the lining epithelium. Otherwise, AQP2 was well localized at the adluminal surface of principal cells in the cauda epidydimidis and AQP9 shows immunoreactivity at the apical region of the principal cells throughout the duct. AQP5-immunoreactivity was undetectable. Lectin histochemistry revealed the following binding sites. In principal cells: 1) stereocilia showed greater reactivity with MAL II, PNA, s-PNA, s- DBA, RCA120, WGA in the cauda and no change in the staining intensity with SNA, HPA, SBA, Con A along the ductus; 2) the apical zone reacted with SBA in the whole ductus and with SNA in the cauda; 3) the Golgi zone bound UEA I along the ductus, whereas MAL II, DBA, RCA120, HPA, and WGA were bound in the caput and corpus regions. Probable apical cells reacted with s-PNA and GSA I-B4 in the cauda epididymis. Spermatozoa displayed a staining increase with MAL II, PNA, s-PNA, s- DBA, RCA120, HPA, SBA, WGA moving to cauda epididymis but no change with SNA and Con A. The regional differences in the lectin-binding pattern of the epithelium and spermatozoa of cat epididymis could be related to the production of region-specific glycoproteins implicated in the maturation and/or storage of spermatozoa. Our data provide evidence for a possible pathway in the bulk reabsorption of water throughout the cat epididymis. The abundant expression of AQP2 and AQP9 water channels represents an important pathway for transmembrane water and solute movement. Region-specific glycoproteins have also been evidenced, probably implicated in the maturation and/or storage of spermatozoa.