Absorptive activities of the efferent ducts evaluated by the immunolocalization of aquaporin water channels and lectin histochemistry in adult cats

Ultrastructural and biochemical features of efferent ducts (EDs) are indicative of an intense absorptive activity towards the luminal fluid. This function was investigated by 1) the immunohistochemical localization of different aquaporins, integral membrane water channels that facilitate rapid passive movement of water, and 2) the histochemical localization of lectins, known to have specific affinity for glycoconjugate residues. AQP1 was mostly revealed at the apical surface and adluminal cytoplasm of non-ciliated cells and to a minor extent in their lateral plasma membrane, whereas it was absent in ciliated cells. Blood vessels showed AQP1-immunoreactivity, which was present in endothelial cells of venous vessels and capillaries and around the muscular sheath of arteries. AQP9 was expressed in the apical zone of ciliated and non-ciliated cells and in the lateral cell membrane. AQP2 and AQP5 were undetectable. Lectin histochemistry showed that non-ciliated cells contain glycans with terminal Neu5Acalpha2,3Galbeta1,3GalNAc, Neu5Acalpha2,3Galbeta1,4GlcNAc, Galbeta1,4GlcNAc, GalNAc (s-PNA, MAL II, RCA120, SBA reactivity) and with internal/terminal alphaMan (Con A affinity) at the luminal surface and the apical region. In addition, non-ciliated cells expressed oligosaccharides terminating with GalNAc and Neu5Acalpha2,6Gal/GalNAc (SNA reactivity) in the luminal surface and the apical zone, respectively. Ciliated cells revealed glycoconjugates only on cilia, which showed terminal Neu5Acalpha2,3Galbeta1,4GlcNAc (s-RCA120 staining) and GalNAc, as well as internal/terminal alphaMan and GlcNAc (s-WGA, GSA II staining). 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 EDs. AQP-mediated trans-cellular route can be hypothesized, together with fluid phase endocytosis mediated by the glycocalix and a well-developed endocytotic apparatus. Epithelial ciliated cells, whose main function is the movement of luminal content, might also participate in absorptive processes to a lesser extent.