1. Counter movements of K+ and H+ across cell membranes were studied in nephrectomized KCl-loaded rats. In one group of animals, the movements of K+ and H+ were determined during and after a KCl load, and in another group, amiloride was used in order to evaluate Na+ participation in K+/H+ exchange.
2. After a KCl load at constant Pco2, 79% of infused K+ left the inulin space, half of which was in exchange for H+. As a result, blood pH fell from 7.40 ± 0.01 to 7.30 ± 0.01 (mean ± sem; P < 0.001).
3. During KCl infusion, the K+/H+ exchange ratio varied between 1.3 and 6.8, showing that the coupling ratio is not fixed.
4. Amiloride did not change blood pH and plasma [K+], but prevented the metabolic acidosis produced by the KCl load without affecting K+ entry into the non-inulin space. Therefore, K+ and H+ movements became completely dissociated.
5. The results indicate that KCl activates an amiloride-sensitive H+ extrusion from the cells. This finding is compatible with the view that Na+/H+ exchange participates in the metabolic acidosis produced by a KCl load.