Immunolocalization of AE2 anion exchanger in rat and mouse epididymis

A low-bicarbonate concentration and an acidic pH in the luminal fluid of the epididymis and vas deferens are important for sperm maturation. These factors help maintain mature sperm in an immotile but viable state during storage in the cauda epididymidis and vas deferens. Two proton extrusion mechanisms, an Na(+)/H(+) exchanger and an H(+)ATPase, have been proposed to be involved 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 epididymis. These cells are enriched in carbonic anhydrase II, indicating the involvement of bicarbonate in the acidification process and suggesting that the epididymis is a site of bicarbonate reabsorption. Previous unsuccessful attempts to localize the Cl/HCO(3) anion exchanger AE1 in rat epididymis did not 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 located on the basolateral membrane of epithelial cells. The cauda epididymidis 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 luminal acidification, and/or may be involved in intracellular pH homeostasis of epithelial cells of the male reproductive tract.