Potassium/proton exchange in brush-border membrane of rat ileum.

These experiments were designed to determine whether proton-driven 86Rb uptake was present in apical membrane vesicles prepared from rat ileum. The uptake of 86Rb was approximately 300 to 350% greater in the presence of a 100-fold H+ gradient than in its absence and was greater at 1, 2 and 5 minutes ("overshoot") than that at 90 minutes. Proton-driven 86Rb uptake was decreased by 20% in TMA-nitrate compared to that in TMA-gluconate. 0.3 mM amiloride did not significantly inhibit proton-driven 86Rb uptake; in contrast, proton-driven 22Na uptake was significantly inhibited by 0.3 mM amiloride by 34%. Similarly, 25 mM KCl inhibited proton-driven 86Rb uptake more than that of 22Na, while the inhibition of proton-driven 22Na uptake by 25 mM NaCl was greater than that of 86Rb. In additional studies intravesicular acidification measured by acridine orange fluorescence was demonstrated in the presence of an outwardly directed K gradient. These studies demonstrate that a proton gradient stimulates 86Rb uptake and a K gradient induces intravesicular acidification; and that these fluxes are mediated by a K/H exchange distinct from Na/H exchange which is also present in this membrane. We conclude that a specific exchange process for K/H is located in ileal apical membrane vesicles.