Aquaporin 9 expression along the male reproductive tract

Fluid movement across epithelia lining portions of the male reproductive tr act is important for modulating the luminal environment in which sperm matu re and reside, and for increasing sperm concentration. Some regions of the male reproductive tract express aquaporin (AQP) 1 and/or AQP2, but these tr ansmembrane water channels are not detectable in the epididymis. Therefore, we used a specific antibody to map the cellular distribution of another AQ P, AQP9 (which is permeable to water and to some solutes), in the male repr oductive tract. AQP9 is enriched on the apical (but not basolateral) membra ne of nonciliated cells in the efferent duct and principal cells of the epi didymis (rat and human) and vas deferens, where it could play a role in flu id reabsorption. Western blotting revealed a strong 30-kDa band in brush-bo rder membrane vesicles isolated from the epididymis. AQP9 is also expressed in epithelial cells of the prostate and coagulating gland where fluid tran sport across the epithelium is important for secretory activity. However, i t was undetectable in the seminal vesicle, suggesting that an alternative f luid transport pathway may be present in this tissue. Intracellular vesicle s in epithelial cells along the reproductive tract were generally poorly st ained for AQP9. Furthermore, the apical membrane distribution of AQP9 was u naffected by microtubule disruption. These data suggest that AQP9 is a cons titutively inserted apical membrane protein and that its cell-surface expre ssion is not acutely regulated by vesicular trafficking. AQP9 was detectabl e in the epididymis and vas deferens of 1-wk postnatal rats, but its expres sion was comparable with adult rats only after 3-4 wk. AQP9 could provide a route via which apical fluid and solute transport occurs in several region s of the male reproductive tract. The heterogenous and segment-specific exp ression of AQP9 and other aquaporins along the male reproductive tract show n in this and in our previous studies suggests that fluid reabsorption and secretion in these tissues could be locally modulated by physiological regu lation of AQP expression and/or function.