Fig. 7. Hypothetical model of NaCl and acid–base transport in the gills of the Atlantic stingray. Results from this and an earlier study (Piermarini and Evans, 2000) suggest that V-H⁺-ATPase and Na⁺/K⁺-ATPase occur on the basolateral cell membrane of distinct mitochondrion-rich cell types. We hypothesize that the V-H⁺-ATPase-rich cells act as base excreting cells via an apical Cl^-/HCO₃^- exchanger that would also result in Cl^- uptake. In contrast, we hypothesize that Na⁺/K⁺-ATPase-rich cells act as acid-excreting cells via an apical NHE that would also result in Na⁺ uptake. This model can explain NaCl and acid–base transport in both freshwater and seawater stingrays. For example, in freshwater animals, the gradient for NaCl entry into the cells is unfavorable, therefore ATPases would be required to establish electrochemical gradients to drive Na⁺/H⁺ and Cl^-/HCO₃^- exchange, which is supported by the greater expression and number of ATPase-rich cells found in freshwater stingray gills. In seawater animals, the gradient for NaCl entry into the cells is favorable, therefore ATPases would not be as important for driving Na⁺/H⁺ and Cl^-/HCO₃^- exchange, which is supported by the lower overall expression and number of ATPase-rich cells found in seawater-acclimated and seawater stingrays. In addition, the qualitative differences in V-H⁺-ATPase staining that we observed (see Fig. 5) may suggest that trafficking of cytoplasmic vesicles containing V-H⁺-ATPase to the basolateral membrane is enhanced in freshwater stingrays, which would enhance active proton transport across this membrane. ? indicates that the presence of the transporter needs to be demonstrated in the Atlantic stingray.

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