Figure 6.

Transport mechanism proposed for transbranchial NaCl absorption across the gills of hyperosmoregulators characterized by a low conductance (“tight”) epithelium (after Onken and Riestenpatt, 1998). Active, Cl⁻ independent Na⁺ absorption proceeds via epithelial, amiloride-sensitive Na⁺ channels (ENAC) in the apical membrane and the Na⁺/K⁺-ATPase in the basolateral membrane. Active, Na⁺ independent Cl⁻ absorption proceeds via apical Cl⁻/HCO⁻₃ exchangers and basolateral Cl⁻ channels. Cytoplasmic carbonic anhydrase (CA) supports both Na⁺ and Cl⁻ transport. Basolateral K⁺ channels generate a negative electrical potential in the cells. An apical V-type H⁺-pump increases intracellular HCO⁻₃, supporting transapical Cl⁻ absorption. Moreover, the electrogenic nature of the H⁺-pump hyperpolarizes the intracellular negativity, supporting the entry of Na⁺ ions across the apical membrane and the exit of Cl⁻ ions across the basolateral membrane.