Abstract: The water channel protein CHIP28 is responsible for the high constitutive plasma membrane permeability to water of erythrocytes, renal proximal tubule, and thin descending limb of Henle. The male reproductive tract is embryologically related to kidney and some segments, particularly the efferent ducts, exhibit a high rate of solute flux-dependent reabsorption of luminal fluid. To determine whether this could occur through water channels in the plasma membrane of reproductive tract epithelial cells, we used anti-CHIP28 antibodies to localize this protein by Western blotting and immunocytochemistry. Western blotting of proteins from efferent duct homogenate indicated the presence of CHIP28 in the efferent duct cells. By indirect immunofluorescence and protein A-gold immunolabeling, CHIP28 was localized to the brush-border and basolateral membranes of nonciliated cells. Ciliated cells in the same epithelium showed no plasma membrane staining for CHIP28. In accord with immunocytochemical findings, freeze-fracture of nonciliated efferent duct cells revealed a plasma membrane organization resembling that of renal proximal tubule cells that are rich in CHIP28. The anti-CHIP28 antibodies also stained plasma membranes of epithelial cells in the ampulla of the vas deferens, seminal vesicles, and prostate, but not the cells in seminiferous tubules, epididymis, and proximal parts of the vas deferens. Therefore, CHIP28 may be a principal mediator of the transmembrane water transport in absorptive epithelial cells of efferent ducts, as well as in epithelia of several other segments of the male reproductive tract that show both secretory and reabsorptive functions
