MECHANISM OF ACIDIFICATION ALONG CORTICAL DISTAL TUBULE OF THE RAT

The cellular mechanism of luminal acidification (bicarbonate reabsorption) was studied in cortical distal tubules of rat kidney. The stopped-flow microperfusion technique was applied to early distal (ED) and late distal (LD) segments, perfused with bicarbonate Ringer solution to which specific inhibitors were added, to measure bicarbonate reabsorption [HCO3 flux (J(HCO3))]. pH and transepithelial potential difference (V-t) were recorded by double-barreled H+ exchange resin/reference (1 M KCl) electrodes. Amiloride increased stationary pH and reduced V-t in both early and late segments. Hexamethylene-amiloride (HMA), a specific Na+-H+ exchange blocker, reduced J(HCO3) in both segments (ED by 43.6 and LD by 40.3%) without affecting V-t. Benzamil, an Na+-channel blocker, reduced V-t by 75.9 in ED and 74.9% in LD but had no significant effect on acidification in both segments. The specific inhibitor of H+-ATPase, bafilomycin A(1), inhibited LD J(HCO3) at a concentration of 2 x 10(-7) M by 49%, but ED was inhibited by 24% only at 2 x 10(-6) M. Sch-28080, an inhibitor of gastric H+-K+-ATPase, reduced J(HCO3) by 35% in LD of K+-depleted rats but not in control rats and had no effect on ED. These data indicate that, in ED, bicarbonate reabsorption is mediated mostly by Na+-H+ exchange. In LD, there is evidence for contribution of Na+-H+ exchange, vacuolar H+-ATPase, and H+-K+-ATPase (in K+-depleted rats) to bicarbonate reabsorption.