Generation of pH gradient across the rabbit collecting duct segments perfused in vitro

Abstract

Ability of pH gradient generation was examined in three segments of rabbit collecting duct, cortical collecting duct (CCD), outer medullary collecting duct (OMCD) and inner medullary collecting duct (IMCD). These segments were perfused in vitro, and the steady-state luminal pH in stop-flow condition (pHs) and the electrochemical potential difference of H+ (EH) were determined using double-barreled liquid membrane pH microelectrode punctured into the lumen. In CCD, pHs was 7.71 +/- 0.08 in normal rabbits, 7.72 +/- 0.08 in DOCA-treated rabbits and 7.27 +/- 0.05 in starved rabbits, while peritubular fluid was kept at pH 7.5 EH (positive value means H+ accumulation in the lumen above electrochemical equilibrium) was -15.3 +/- 5.2, -31.3 +/- 3.7 and 10.3 +/- 3.1 mV, respectively. Peritubular acidification (peritubular pH 6.8) by reducing HCO3-concentration decreased pHs, but increased its negativity of EH in all groups. In OMCD pHs was 6.57 +/- 0.08 in normal, 6.58 +/- 0.11 in DOCA-treated and 6.47 +/- 0.12 in starved animals. EH was 54.5 +/- 4.6, 57.7 +/- 6.8 and 64.2 +/- 6.9 mV, respectively. Peritubular acidification lowered pHs further, 5.51 +/- 0.07, 5.67 +/- 0.16 and 5.41 +/- 0.19, respectively. EH was enhanced in all groups. In IMCD pHs was 7.36 +/- 0.04 with EH being 6.8 +/- 2.9 mV, and peritubular acidification did not generate pH gradient. These data suggest that the generation of a steep acid pH gradient is mainly due to OMCD. Luminal alkalinization predominated in CCD except in starved rabbits. IMCD did not generate appreciable pH gradient.(ABSTRACT TRUNCATED AT 250 WORDS)