Regulation of intracellular pH in proximal tubules of avian long-looped mammalian-type nephrons

In nonperfused proximal tubules isolated from chicken long-looped mammalian-type nephrons, intracellular pH (pH(i)), measured with the pH- sensitive fluorescent dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, was ~7.3 under control conditions (HEPES-buffered medium with pH 7.4 at 37°C) and was reduced to ~7.0 in response to NH₄Cl pulse. The rate of recovery of phi from this level to the resting level was 1) significantly reduced by the removal of Na⁺ from the bath, 2) significantly increased by the removal of Cl^- from the bath, 3) unchanged by the removal of both Na⁺ and Cl^- from the bath, 4) significantly reduced by the addition of either ethylisopropylamiloride or DIDS to the bath, 5) significantly increased by a high bath K⁺ concentration, and 6) unchanged by the addition of Ba²⁺ to the bath. These data suggest that both Na⁺-coupled and Cl^--coupled basolateral acid-base fluxes are involved in determining the rate of recovery of phi after acidification. The most likely ones to be important in regulating phi are a Na⁺/H⁺ exchanger and a Na⁺-coupled Cl^-/HCO₃/^- exchanger. In birds, long-looped mammalian-type nephrons resemble short- looped transitional nephrons but differ markedly from superficial loopless reptilian-type nephrons.