The effect of acute metabolic alkalosis on bicarbonate transport along the loop of Henle. The role of active transport processes and passive paracellular backflux

Abstract

The loop of Henle (LOH) reabsorbs approximately 15% of filtered HCO ⁻₃ via a luminal Na⁺-H⁺ exchanger and H⁺ATPase. During acute metabolic alkalosis (AMA) induced by i.v. HCO ⁻₃ infusion, we have observed previously inhibition of LOH net HCO ⁻₃ reabsorption$(J_{HCO_3^ - } )$, which contributes to urinary elimination of the HCO ⁻₃ load and correction of the systemic alkalosis. To determine whether the activities of the Na⁺-H⁺ exchanger and/or H⁺-ATPase are reduced during AMA, two inhibitors believed to be sufficiently specific for each transporter were delivered by in vivo LOH microperfusion during AMA. AMA reduced LOH$J_{HCO_3^ - }$ from 205.0±0.8 to 96.2±11.8 pmol · min⁻¹ (P<0.001). Luminal perfusion with bafilomycin A₁ (10⁻⁴ mol · l⁻¹) caused a further reduction in$J_{HCO_3^ - }$ by 83% and ethylisopropylamiloride (EIPA; 5.10⁻⁴ mol · l⁻¹) completely abolished net HCO ⁻₃ reabsorption. The combination of bafilomycin A₁ and EIPA in the luminal perfusate was additive, resulting in net HCO ⁻₃ secretion (−66.6±20.8 pmol · min⁻¹;P<0.001) and abolished net fluid reabsorption (from 5.0±0.6 during AMA to 0.2±1.1 nl · min⁻¹;P<0.001). To establish whether HCO ⁻₃ secretion via luminal stilbenesensitive transport mechanism participates in LOH adaptation to 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 direction of net HCO ⁻₃ movement was reversed and net HCO ⁻₃ secretion occurred:$J_{HCO_3^ - }$ 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 (undisturbed acid-base balance). Thus, during AMA, neither the luminal Na⁺-H⁺ exchanger nor the H⁺-ATPase are noticeably suppressed. However, pharmacological elimination of both transporters, as well as prolonged exposure of the tubular lumen to DIDS, induced net HCO ⁻₃ secretion. This secretory flux may reflect paracellular backflux due to the steeper blood to lumen HCO ⁻₃ concentration gradient that presumably prevails in AMA.