H+V-ATPase-dependent luminal acidification in the kidney collecting duct and the epididymis/vas deferens: Vesicle recycling and transcytotic pathways

Many vertebrate transporting epithelia contain characteristic 'mitochondria -rich' cells that express high levels of a vacuolar proton-pumping ATPase ( H+V-ATPase) on their plasma membrane and on intracellular vesicles. In the kidney cortex, A-cells and B-cells are involved in proton secretion and bic arbonate secretion, respectively, in the distal nephron and collecting duct . A-cells have an H+V-ATPase on their apical plasma membrane and on intrace llular vesicles, whereas the cellular location of the H+V-ATPase can be api cal, basolateral, bipolar or diffuse in B-cells. The rat epididymis and vas deferens also contain a distinct population of H+V-ATPase-rich epithelial cells. These cells are involved in generating a low luminal pH, which is in volved in sperm maturation and in maintaining sperm in an immotile state du ring their passage through the epididymis and vas deferens, In both kidney and reproductive tract, H+V-ATPase-rich cells have a high rate of apical me mbrane recycling, H+V-ATPase molecules are transported between the cell sur face and the cytoplasm in vesicles that have a well-defined 'coat' structur e formed of the peripheral V-1 subunits of the H+V-ATPase. In addition, we propose that B-type intercalated cells have a transcytotic pathway that ena bles them to shuttle H+V-ATPase molecules from apical to basolateral plasma membrane domains. This hypothesis is supported by data showing that A-cell s and B-cells have different intracellular trafficking pathways for LGP120, a lysosomal glycoprotein. LGP120 was found both on the basolateral plasma membrane and in lysosomes in B-cells, whereas no LGP120 was detectable in t he plasma membrane of A-cells. We propose that the 'polarity reversal' of t he H+V-ATPase in B-intercalated cells is mediated by a physiologically regu lated transcytotic pathway that may be similar to that existing in some oth er cell types.