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In summary, the expression of various isoforms of the same V-ATPase subunit in narrow and clear cells strongly indicates that these cells are fully equipped for active proton secretion and that they possess potentially important backup mechanisms that may compensate for the lack of one or more V-ATPase subunit isoforms. This underlies the important role that the V-ATPase plays in acidifying the lumen of the epididymis, a process that is important for sperm maturation and storage. The presence of various isoforms of one given subunit in narrow and clear cells may also be important for the regulation of V-ATPase activity in the different subcellular compartments in which it is expressed (e.g., endosomes vs. plasma membrane). In addition, the localization of subunits a2 and A in intracellular organelles of both clear and principal cells indicates their potential role in regulating intracellular traffic in these cells. Finally, expression of the dl isoform in the apical membrane of principal cells in the apparent absence of other V-ATPase subunits was an unexpected finding that may indicate other, as yet unidentified role(s) for this isoform.