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

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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 te H ) as well as limiting transepithelial electrical and H+ electrochemical potential differences (PD te ,E te H ) 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 te H (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 te H (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 te H . Elimination of the lumen positivePD te by furosemide did not affect significantlyE te H andJ te H in control and K+ adapted animals but abolished reabsorptiveJ te H 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.