Figure 4.

Transport mechanism proposed for transbranchial NaCl uptake across the gills of hyperosmoregulators characterized by “leaky” gill epithelia and high transport rates (from Onken et al., 2003). Active NaCl uptake is coupled and proceeds via Na⁺/K⁺/2Cl⁻ cotransporters in the apical membrane. Absorbed Na⁺ ions are then pumped by the Na⁺/K⁺-ATPase across the basolateral membrane. The presence of basolateral and apical K⁺ channels results in a negative electrical potential inside of the cells which drives the movement of Cl⁻ ions through Cl⁻ channels in the basolateral membrane. Apical K⁺ channels are also of importance to supply the apical Na⁺/K⁺/2Cl⁻ cotransporters with K⁺ ions, and give the overall transport its electrogenic nature. The outside positive transbranchial voltage can then be used as the driving force for paracellular Na⁺ absorption to ensure equimolar NaCl absorption. In addition, active NaCl uptake proceeds via cation (Na⁺/H⁺) and anion (Cl⁻/HCO⁻₃) exchangers in the apical membrane, supported by a carbonic anhydrase that catalyzes the fast production of acid/base equivalents as intracellular substrates for the exchangers.