Immunolocalization of AE2 anion exchanger in rat and mouse epididymis

A low-bicarbonate concentration and an acidic pH in the luminal fluid of th e epididymis and vas deferens are important for sperm maturation. These fac tors help maintain mature sperm in an immotile but viable state during stor age in the cauda epididymidis and vas deferens. Two proton extrusion mechan isms, an Na+/H+ exchanger and an H(+)ATPase, have been proposed to be invol ved in this luminal acidification process. The Na+/H+ exchanger has not yet been localized in situ, but we have reported that H(+)ATPase is expressed on the apical membrane of apical (or narrow) and clear cells of the epididy mis. These cells are enriched in carbonic anhydrase II, indicating the invo lvement of bicarbonate in the acidification process and suggesting that the epididymis is a site of bicarbonate reabsorption. Previous unsuccessful at tempts to localize the Cl/HCO3 anion exchanger AE1 in rat epididymis did no t investigate other anion exchanger (AE) isoforms. In this report, we used a recently described SDS antigen unmasking treatment to localize the Cl/HCO 3 exchanger AE2 in rat and mouse epididymis. AE2 is highly expressed in the initial segment, intermediate zone, and caput epididymidis, where it is lo cated on the basolateral membrane of epithelial cells. The cauda epididymid is and vas deferens also contain basolateral AE2, but in lower amounts. The identity of the AE2 protein was further confirmed by the observation that basolateral AE2 expression was unaltered in the epididymis of AE1-knockout mice. Basolateral AE2 may participate in bicarbonate reabsorption and lumin al acidification, and/or may be involved in intracellular pH homeostasis of epithelial cells of the male reproductive tract.