The major function of the epididymis is to maintain a lumenal fluid environment, which is optimal for the maturation and, in many animals, the storage of spermatozoa. The ability of epididymal epithelial cells to alter the ionic composition of the lumenal fluid has been measured, some of the major transport proteins involved in this process have been identified, and their sites of expression have been characterized. Transepithelial transport of water and various anions and solutes take place in the epididymis.
Results from our laboratory indicate that vacuolar-type H+-ATPase is a key player in the establishment of an acidic pH in the epididymal lumen. In vitro studies on isolated rat vas deferens using inhibitors of proton secretion, such as bafilomycin, suggested that H+-ATPase is a major source of acidification. Epididymal cells that contain H+-ATPase also coexpress carbonic anhydrase type 2 (CAII), suggesting that bicarbonate transport proteins are also important in lumenal acidification. The involvement of bicarbonate transport in the acidification process was confirmed in isolated vas deferens. Epididymal cells rich in H+-ATPase share similar features to type A intercalated cells of the kidney collecting duct, which also express vacuolar-type H+-ATPase and CAII, and they play a major role in urinary acidification.