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 quiescent during storage in the cauda epididymidis. Previous studies have implicated a Na⁺/H⁺ exchanger (NHE) in epididymal acidification together with carbonic anhydrase (CA) and vacuolar proton adenosine triphosphatase (H⁺-ATPase). The present studies were undertaken to discover whether the NHE isoform involved is NHE-3, which is known to mediate Na⁺ and HCO₃ ^- absorption in renal tubules. Using the reverse transcription polymerase chain reaction technique (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 NHE-3 was present in the apical membranes of the epithelial principal cells and confirmed that its expression is strongest in the cauda region, decreasing towards the more proximal regions. Immunoblotting showed a similar expression pattern. These results demonstrate that NHE-3 is expressed in the rat epididymal duct with strongest expression in its cauda region. These findings are thus consistent with the possibility that NHE-3 in the epididymal duct is involved in luminal Na⁺ and/or HCO₃^- absorption, as in the renal proximal tubule, and thereby in the regulation of sperm motility and maturation.