Fig. 16

Simplified scheme for explaining the initial results of ion substitutions and inhibition by 4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid (DIDS). In a the rates of acid loading by Cl⁻/HCO₃ ⁻ exchange and acid extrusion by Na⁺, HCO₃ ⁻-cotransport are nearly in balance and the pH is stable. In b removal of external Na⁺ reverses the direction of the Na⁺ gradient and Na⁺, HCO₃ ⁻-cotransport is acid loading until the internal Na⁺ is depleted. While both types of transport are acid loading, pH_i falls, i.e. there is cellular acidification. In c removal of external Cl⁻ reverses the direction of the Cl⁻ gradient and Cl⁻/HCO₃ ⁻ exchange is acid extruding until the internal Cl⁻ is depleted. While both types of transport are acid extruding, pH_i increases, i.e. there is cellular alkalinization. In d DIDS blocks both types of transport and there is little acid loading or extrusion and only slow if any change in pH_i