Frogs are faced with various osmoregulatory problems, such as compensation of salt and water loss or metabolic acidification. Being exposed both to air and to pond water of low salinity in their natural habitat, the epithelium of the frog skin serves as one of the major organs for body fluid homeostasis. For years, the frog skin has been the guiding model for ion transport processes in animal cells energized by a Na(+)-motive force. Meanwhile, however, it was demonstrated that under natural conditions Na+ uptake is electrically coupled to active H+ secretion, mediated by an electrogenic H+ pump. A proton-motive force generated at the apical membrane of the mitochondria-rich cells (MR cells) energizes Na+ entry via apical Na+ channels. The basolateral Na+/K+ P-ATPase then pumps Na+ out of the cell into the body fluid. Thus, there are two pumps functioning in series, both involved in transepithelial Na+ transport. Our recent investigations provided conclusive evidence that the H+ pump of the frog skin is an H+ V-ATPase. In transport studies, Na+ absorption and H+ secretion were blocked by micromolar concentrations of bafilomycin A1 or concanamycin A, two highly specific inhibitors of H(+)-V-ATPases. Using immunofluorescence microscopy, H(+)-V-ATPase-like immunoreactivity was found in MR cells in the region of their apical membrane foldings and intracellularly in the apical portion of the cell at so far unidentified locations. Besides the definition of its molecular nature, these results also confirmed the localization of the H+ pump in the apical membrane of the MR cells. These cells were already candidates for H(+)-V-ATPase localization mostly from correlations between their morphological features and their epithelial H+ secretion capacity. So far, there is evidence for only one type of MR cell serving both H+ and HCO3- secretion through an apical Cl-/HCO3- antiporter. H(+)-V-ATPase-mediated H+ secretion and thus Na+ absorption can be modulated by complementary mechanisms. Changes in intracellular H+ concentration linked to the animal's acid-base status will directly influence H+ V-ATPase activity. Acute acidification increases H+ current, probably as a result of the insertion of H(+)-V-ATPase-bearing vesicles by exocytotic processes, while alkalization causes the reverse effects. Chronic metabolic acidosis induces an increase in MR cell number in response to hormonal signals.

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