Basolateral regulation of pH(i) in proximal tubules of avian loopless and long-looped nephrons in bicarbonate

In isolated, nonperfused chicken proximal tubules from both loopless reptilian-type and long-looped mammalian-type nephrons, resting intracellular pH (pH(i)), measured with pH-sensitive fluorescent dye 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF), was ~7.1 under control HCO₃^- conditions [20 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)/5 mM HCO₃^-buffered medium with pH 7.4 at 37°C] and was reduced to ~6.8 in response to NH₄Cl pulse. The rate of recovery of pH(i) (dpH(i)/dt) from this level to the resting level in proximal tubules from both nephron types was (1) significantly reduced by the removal of Na⁺ or both Na⁺ and Cl^- from the bath, and (2) unaffected by the removal of Cl^- from the bath or the presence of a high K⁺ concentration or Ba²⁺ in the bath. In proximal tubules from long-looped mammalian-type, but not loopless reptilian-type, nephrons, dpH(i)/dt was significantly reduced by the addition of either 5-(N-ethyl-N-isopropyl) amiloride (EIPA) or 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) to the bath. These data suggest that a Na⁺/H⁺ exchanger and most likely a Na⁺-dependent Cl^-/HCO₃^- exchanger are involved in basolateral regulation of pH(i) in mammalian-type nephrons whereas none of the commonly identified basolateral acid-base transporters appear to be involved in regulation of pH(i) in reptilian-type nephrons.