THE EFFECT OF ACUTE METABOLIC ALKALOSIS ON BICARBONATE TRANSPORT ALONG THE LOOP OF HENLE - THE ROLE OF ACTIVE-TRANSPORT PROCESSES AND PASSIVE PARACELLULAR BACKFLUX

The loop of Henle (LOH) reabsorbs approximately 15% of filtered HCO3- via a luminal Na+-H+ exchanger and H(+)ATPase. During acute metabolic alkalosis (AMA) induced by i.v. HCO; infusion, we have observed previo usly inhibition of LOH net HCO3- reabsorption (J(HCO3-)), which contri butes to urinary elimination of the HCO3- load and correction of the s ystemic alkalosis. To determine whether the activities of the Na+-H+ e xchanger and/or H+-ATPase are reduced during AMA, two inhibitors belie ved to be sufficiently specific for each transporter were delivered by in vivo LOH microperfusion during AMA. AMA reduced LOH J(HCO3-) from 205.0+/-10.8 to 96.2+/-11.8 pmol.min(-1) (P<0.001). Luminal perfusion with bafilomycin A(1) (10(-4) mol.l(-1)) caused a further reduction in J(HCO3-) by 83% and ethylisopropylamiloride (EIPA; 5.10(-4) mol.l(-1) ) completely abolished net HCO3- reabsorption. The combination of bafi lomycin A(1) and EIPA in the luminal perfusate was additive, resulting in net HCO3- secretion (-66.6+/-20.8 pmol.min(-1); P<0.001) and aboli shed net fluid reabsorption (from 5.0+/-0.6 during AMA to 0.2+/-1.1 nl .min(-1); P<0.001). To establish whether HCO3- secretion via luminal s tilbene-sensitive transport mechanism participates in LOH adaptation t o AMA, we added diisothiocyanato-2,2'-stilbenedisulphonate (DIDS; 10(- 4) mol.l(-1)) to the perfusate. No effect was found. However, when the same LOH were exposed to luminal DIDS for more than 10 min, the direc tion of net HCO3- movement was reversed and net HCO3- secretion occurr ed: J(HCO3-); changed from 90.6+/-8.8 to -91.9+/-34.1 pmol.min(-1); P< 0.01, an effect that was not observed in the control state (undisturbe d acid-base balance). Thus, during AMA, neither the luminal Na+-H+ exc hanger nor the H+-ATPase are noticeably suppressed. However, pharmacol ogical elimination of both transporters, as well as prolonged exposure of the tubular lumen to DIDS, induced net HCO3- secretion. This secre tory flux may reflect paracellular backflux due to the steeper blood t o lumen HCO3- concentration gradient that presumably prevails in AMA.