Proton pump-driven cutaneous chloride uptake in anuran amphibia

AbstractKrogh introduced the concept of active ion uptake across surface epithelia of freshwater animals, and proved independent transports of Na+ and Cl− in anuran skin and fish gill. He suggested that the fluxes of Na+ and Cl− involve exchanges with ions of similar charge. In the so-called Krogh model, Cl−/HCO3− and Na+/H+ antiporters are located in the apical membrane of the osmoregulatory epithelium. More recent studies have shown that H+ excretion in anuran skin is due to a V-ATPase in mitochondria-rich (MR) cells. The pump has been localized by immunostaining and H+ fluxes estimated by pH-stat titration and mathematical modelling of pH-profiles in the unstirred layer on the external side of the epithelium. H+ secretion is voltage-dependent, sensitive to carbonic-anhydrase inhibitors, and rheogenic with a charge/ion-flux ratio of unity. Cl− uptake from freshwater is saturating, voltage independent, and sensitive to DIDS and carbonic-anhydrase inhibitors. Depending on anuran species and probably on acid/base balance of the animal, apical exit of protons is coupled to an exchange of Cl− with base (HCO3−) either in the apical membrane (γ-type of MR cell) or in the basolateral membrane (α-type MR cell). The γ-cell model accounts for the rheogenic active uptake of Cl− observed in several anuran species. There is indirect evidence also for non-rheogenic active uptake accomplished by a β-type MR cell with apical base secretion and basolateral proton pumping. Several studies have indicated that the transport modes of MR cells are regulated via ion- and acid/base balance of the animal, but the signalling mechanisms have not been investigated. Estimates of energy consumption by the H+-ATPase and the Na+/K+-ATPase indicate that the γ-cell accomplishes uptake of NaCl in normal and diluted freshwater. Under common freshwater conditions with serosa-positive or zero Vt, the K+ conductance of the basolateral membrane would have to maintain the inward driving force for Na+ uptake across the apical membrane. With the K+ equilibrium potential across the basolateral membrane estimated to −105 mV, this would apply to external Na+ concentrations down to 40–120 μmol/l. NaCl uptake from concentrations down to 10 μmol/l, as observed by Krogh, presupposes that the H+ pump hyperpolarizes the apical membrane, which would then have to be associated with serosa-negative Vt. In diluted freshwater, exchange of cellular HCO3− with external Cl− seems to be possible only if the proton pump has the additional function of keeping the external concentration of HCO3− low. Quantitative considerations also lead to the conclusion that with the above extreme demand, at physiological intracellular pH of 7.2, the influx of Cl− via the apical antiporter and the passive exit of Cl− via basolateral channels would be possible within a common range of intracellular Cl− concentrations