HCO3 accumulation in proximal tubule: roles of carbonic anhydrase, luminal buffers, and pH.

Most of filtered bicarbonate is reabsorbed in the early proximal tubule, and the high blood-to-lumen HCO3 concentration gradient generated is then maintained in the distal proximal tubule. To determine the factor(s) that prevent reaccumulation of HCO3 in the lumen, surface proximal tubules of the rat kidney were perfused in vivo. All perfusion solutions were similar in ionic composition to late proximal tubule fluid but, instead of HCO3, contained sulfate (SO4) or N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES). Bicarbonate concentration ([HCO3]L) was measured by microcalorimetry, while collected fluid PCO2 was maintained in vitro at either renal cortical or atmospheric levels. A new single microelectrode was used to simultaneously measure PCO2 and luminal pH (pHL). With SO4 solution, pHL was 6.84 +/- 0.06, PCO2 was 50.8 +/- 5.0 mmHg, and [HCO3]L was 8.7 +/- 0.9 mM. When 10(-3) M acetazolamide (ATZ) was added to the perfusate, pHL was 7.00 +/- 0.08, PCO2 remained unchanged, [HCO3]L was 12.1 +/- 1.3 mM, and the rate of HCO3 accumulation increased by approximately 50%. When SO4 was replaced with HEPES, pHL increased to 7.18 +/- 0.05, PCO2 was unchanged, [HCO3]L was 16.5 +/- 1.4 mM, and the rate of HCO3 accumulation doubled. Because measured [HCO3]L and that calculated from pHL and PCO2 were approximately the same, the net gain of HCO3 occurred in vivo, a result of either HCO3 backflux and/or intraluminal generation from CO2. By allowing the collected fluid PCO2 to fall to near zero prior to [HCO3]L measurement, we estimate that approximately 50% of the total gain of HCO3 with all solutions was due to backflux.(ABSTRACT TRUNCATED AT 250 WORDS)