Mechanism of hydrogen ion transport in the diluting segment of frog kidney

Abstract

Transepithelial H⁺ transport was studied in diluting segments of the isolated-perfused kidney ofrana esculenta. The experiments were performed in controls as well as in K⁺-adapted and Na⁺-adapted animals (exposed to 50 mmol/l KCl or NaCl, resp. for at least 3 days). Conventional and single-barreled, liquid ion-exchanger H⁺-sensitive microelectrodes were applied in the tubule lumen to evaluate transepithelial H⁺ net flux (J ^H_te ) as well as limiting transepithelial electrical and H⁺ electrochemical potential differences (PD _te ,E ^H_te ) and luminal pH at zero net flux conditions. The measurements were made in absence (control) and presence of furosemide (5·10⁻⁵ mol/l) or amiloride (10⁻³ mol/l).

E ^H_te (lumen positive vs ground) was 19±3 mV in controls, 43±3 mV in K⁺ adapted but about zero in Na⁺ adapted animals. Using the correspondingPD _te -values, steady state luminal pH of 7.63±0.05, 7.13±0.05 and 8.02±0.02 was calculated for the respective groups of animals (peritubular pH 7.80). In parallel, significant secretoryJ ^H_te (from blood to lumen) was found in controls (14±2 pmol·cm⁻²·S⁻¹) which was stimulated by K⁺ adaptation (61±8 pmol·cm⁻²·s⁻¹) but reversed in direction by Na⁺-adaptation (−8±1 pmol·cm⁻²·s⁻¹). Amiloride inhibited secretoryJ ^H_te . Elimination of the lumen positivePD _te by furosemide did not affect significantlyE ^H_te andJ ^H_te in control and K⁺ adapted animals but abolished reabsorptiveJ ^H_te in Na⁺ adapted animals.

We conclude that in frog diluting segment H⁺ secretion is an active, amiloride-sensitive, furosemide-insensitive transport process. The data are consistent with luminal Na⁺/H⁺ exchange. The activity of this system depends critically on the metabolic state of the animal.