Acidification of luminal fluid by the rabbit cortical collecting tubule perfused in vitro

The ability of the rabbit cortical collecting tubule to acidify the luminal fluid was determined with double-barreled antimony pH electrodes. In Na+/K+ Ringer the tubules maintained a transepithelial voltage (VToc) of -45.4 +/- 5.5 mV (bath grounded) and a minimum luminal fluid pH of 5.93 +/- 0.11. Chronic mineralocorticoid pretreatment of the rabbits caused the VToc to become more negative (-78.7 +/- 8.2 mV) and decreased the minimum luminal fluid pH to 5.43 +/- 0.16. In most tubules (control and mineralocorticoid-pretreated) the measured pH was more acidic than could be accounted for by either the lumen-negative VToc or CO2 equilibration of the perfusion fluid. When tubules were perfused and bathed in 0 Na+/0 K+ Ringer they developed a lumen-positive VToc, which was stimulated by mineralocorticoid, was sensitive to the PCO2 of the bathing solutions, but was not dependent on Cl- in either the luminal or bath solutions. Luminal acidification in the absence of Na+ and K+ (pH = 6.05 +/- 0.12) occurred against a lumen-positive VToc of +11.5 +/- 1.9 mV. Addition of 10(-4) M ouabain to the bath of tubules studied in Na+/K+ Ringer caused the VToc to reverse polarity and the luminal fluid pH to increase. In contrast, ouabain had no effect on either the lumen-positive VToc or the minimum luminal fluid pH when added to the bath of tubules in 0 Na+/0 K+ Ringer. Bath addition of 10(-4) M acetazolamide and/or 5 X 10(-4) M 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) caused alkalinization of the luminal fluid in tubules studied in either Na+/K+ or 0 Na+/0 K+ Ringer. In 0 Na+/0 K+ Ringer, acetazolamide and SITS reduced the lumen-positive VToc to near zero. The data support the existence of a distinct acidification mechanism in the rabbit cortical collecting tubule, which is both active and electrogenic.