Intracellular pH (pH_i) regulation in proximal tubules of avian loopless and long-looped nephrons in HCO₃^- medium

Previous data indicated that basolateral regulation of proximal tubule pH_i differed between avian nephron types in the absence of HCO₃^-. Therefore, we evaluated regulation of pH_i, measured with fluorescent dye BCECF, in chicken loopless reptilian-type (lrt) and long-looped mammalian-type (l-lmt) nephrons in HEPES (20 mM)/HCO₃^- (5 mM)-buffered medium. In both nephron types, proximal pH_i was ∼7.1 under control circumstances (pH 7.4; 37°C) and increased to ∼7.4 with addition of NH₄Cl (20 mM) and then decreased to ∼6.8 with its removal. Intrinsic buffering capacity was 88-91 mM H⁺/pH U, basolateral NH₃ flux was 3.2-3.6 nmol cm^-2 s^-1, and basolateral NH₃ permeability was 6.4-7.4 X 10^-3 cm s^-1. Also, in both nephron types control rate of recovery (dpH_i/dt) from acid level (5.2-6.6 X 10^-3 pH U/s) and resting pH_i were reduced by Na⁺ removal. Na⁺/H⁺ exchange inhibitor EIPA (100 μM) and anion exchange inhibitor DIDS (100 μM) decreased dpH_i/dt in proximal tubules of l-lmt but not lrt nephrons. Removal of Cl^-, addition of Ba²⁺ (5 mM), or presence of high K⁺ (75 mM) had no effect on dpH_i/dt in either nephron type. In l-lmt nephrons, these data support presence of a basolateral Na⁺/H⁺ exchanger and possibly a Na⁺-HCO₃^--CO₃^2- cotransporter, but other acid/base transporters must account for regulation in lrt nephrons.