H+ ion secretion in proximal tubule of low-CO2/HCO 3 − perfused isolated rat kidney

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Abstract
Acidification in proximal tubule of the isolated rat kidney, perfused in vitro, was studied by stopped-flow microperfusion techniques, using Sb microelectrodes to measure luminal pH. The kidney was perfused with mammalian Ringer's solution at pH 7.4 buffered by 20 mmol/l phosphate and containing 7.5 g/100 ml bovine albumin, equilibrated with air. Final urine pH was 6.88±0.5. Steady-state pH in proximal segments was 6.81±0.03 (n=80), and acidification half-time (t/2) 7.25±0.33 (80) s, giving a net secretory H+ ion flux of 0.51±0.05 nmol·cm−2·s−1. This flux was about 70% of “in vivo” (blood perfused kidneys). During luminal perfusion with solutions at pH 6.2, back-flux of H+ was 0.82 ±0.08 nmol·cm−2·s−1, with an alkalinizationt/2 of 6.33 ±0.34 (34) s. The difference between acidification and alkalinizationt/2 was not significant. This is compatible with a pump-leak system of H+ transport. The back flux of H from the lumen was markedly reduced in low Na+ perfused kidneys in the presence of 10−4 mol/l amiloride in the lumen, indicating that this process is mediated by the luminal Na/H exchanger. Observations in the presence of high K levels suggest that it may have also a charged component. 10−4 mol/l acetazolamide added to the kidney perfusate reduced acidification to 0.5% of control, and 10−6 mol/l SITS to 25% of control. Thus, despite the lowpCO2 (0.1–0.4 kPa, or 1–3 mm Hg), the CO2/Hco 3 − buffer system still plays an important role in tubular acidification in this preparation.