Fig. 6. Preliminary working model of Na⁺ absorption by killifish gills. Apical Na⁺ uptake occurs via NHE2 in exchange for H⁺ supplied by CA2. Active transport of Na⁺ across the basolateral surface is accomplished by Na⁺,K⁺-ATPase. Na⁺ may also leave the cell in symport with HCO₃^– through NBC1 operating in efflux mode. V-ATPase may pump H⁺ across the basolateral surface to help maintain intracellular acid–base balance.