Effect of K+ and H+ on sodium/citrate cotransport in renal brush-border vesicles

The uptake of citrate by renal brush-border vesicles, prepared according to the method of Vannier, occurs by Na+-linked cotransport. It is 'positive rheogenic', i.e., stimulated by an (inside) negative, and inhibited by an (inside) positive electrical potential. The question arises whether, besides Na+, other ions (e.g., K+ and H+) participate in the cotransport. As to K+, neither an inward nor an outward directed K+ gradient has a significant effect on the citrate movement, but at equal concentrations of K+ inside and outside, equilibrium exchange of citrate, and to a smaller extent, the Na+-linked net uptake of citrate, are significantly stimulated. This observation is consistent with a hypothetical model in which K+ acts by accelerating both the empty and the fully loaded translocator. As to H+, citrate uptake is also stimulated by decreasing extravesicular pH, an effect previously attributed to protonization of the citrate anion in the assumption that the resulting secondary citrate anion is more acceptable to the translocator site. It was found, however, that the pH effect is still apparent if the concentration of the secondary citrate is kept constant by adjusting the total citrate concentration. This is taken as an argument against the above assumption and as being consistent with H+-linked cotransport. After the overshoot peak citrate exits slowly, and even after several hours does not attain equilibrium distribution, presumably owing to trapping by vesicular calcium.