An apical membrane Na+/H+ exchanger isoform, NHE-3, is present in the rat epididymal epithelium

An acidic milieu is required for sperm maturation and for keeping sperm qui escent during storage in the cauda epididymidis. Previous studies have impl icated a Na+/H+ exchanger (NHE) in epididymal acidification together with c arbonic anhydrase (CA) and vacuolar proton adenosine triphosphatase (H+-ATP ase). The present studies were undertaken to discover whether the NHE isofo rm involved is NHE-3, which is known to mediate Na+ and HCO3- absorption in renal tubules. Using the reverse transcription polymerase chain reaction t echnique (RT-PCR), Northern blot analysis and in situ hybridization, NHE-3 mRNA was detected mainly in the cauda epididymis and to a lesser extent in other regions of the epididymis. Immunohistochemical studies showed that NH E3 was present in the apical membranes of the epithelial principal cells an d confirmed that its expression is strongest in the cauda region, decreasin g towards the more proximal regions. Immunoblotting showed a similar expres sion pattern. These results demonstrate that NHE-3 is expressed in the rat epididymal duct with strongest expression in its cauda region. These findin gs are thus consistent with the possibility that NHE-3 in the epididymal du ct is involved in luminal Na+ and/or HCO3- absorption, as in the renal prox imal tubule, and thereby in the regulation of sperm motility and maturation .