Evidence for a DCCD-sensitive component of proximal bicarbonate reabsorption

Abstract

To examine the possible contribution of active H+ secretion mediated by brush border enzymes to proximal tubule HCO-3 absorption, paired reperfusions of surface proximal convoluted tubules were performed with the inhibitor dicyclohexylcarbodiimide (DCCD). In control studies using a solution devoid of HCO-3 but containing 5.5 mM glucose, 1 mM DCCD had no effect on glucose or fluid (Na+) absorption, suggesting that this inhibitor did not interfere with sodium entry at the brush border or mitochondrial energy production (ATP synthesis). In experiments using a perfusion solution containing 18-25 mM HCO-3, DCCD caused a fall in absolute CO2 absorption of approximately 15% under eucapneic conditions and 30% during acute hypercapnia. One millimole per liter amiloride (an inhibitor of the passive Na+-H+ exchanger) caused a 15% inhibition of CO2 absorption during acute hypercapnia and a disproportionately large reduction in fluid (Na+) absorption. The latter was not due to cell poisoning, since 1 mM amiloride had no inhibitory effect on fluid or glucose absorption when a HCO-3-free perfusion solution was used. Addition of 1 mM DCCD to a perfusion solution containing either 10(-3) M amiloride or 10(-4) M acetazolamide caused a significant inhibition of CO2 absorption compared with amiloride or acetazolamide alone. The observations are consistent with the view that in addition to passive Na+-H+ exchange, active transport mediated by either a H+-ATPase or a redox-driven H+ pump in the brush border contributes significantly to HCO-3 absorption in the proximal tubule.

Progressive increases in luminal glucose stimulate proximal sodium absorption in normal and diabetic rats.

“…The in vivo reperfusion technique was used in which each tubule serves as its own control (12). This experimental approach obviates variations in GFR and allows precise control of delivery rate and perfused glucose concentrations.…”

“…On the day of micropuncture study, all animals were anesthetized with inactin, 100 mg/kg body wt, the trachea was intubated, and a jugular vein was cannulated with PE 50 tubing for constant infusion of Ringer's lactate solution. The left kidney was surgically exposed, cleaned of perirenal fat tissue, and immobilized in a lucite cup, as previously described (12)(13)(14). Group 1.…”

Bicarbonate transport along the loop of Henle. I. Microperfusion studies of load and inhibitor sensitivity.

“…Recently it has become apparent that, in addition to Na'-H' exchange, proton secretion along the proximal tubule can also occur by a sodium-independent mechanism, the electrogenic proton pump (10,44). Immunohistochemical (45) and biochemical studies (12,13) have shown that this transport system is distributed in varying amounts along the entire nephron including key tubule segments lining the loop of Henle.…”

“…Immunohistochemical (45) and biochemical studies (12,13) have shown that this transport system is distributed in varying amounts along the entire nephron including key tubule segments lining the loop of Henle. The properties, kinetics, and functional significance of H+-ATPase have been explored by use of H+-ATPase inhibitors: Bank et al (44) reported that proximal electrogenic proton transport was inhibited by dicyclohexyl-carbodimide, while Garg et al (12) and Ait-Mohamed et al (13) used N-ethylmaleimide to characterize the renal membrane proton ATPase. Recently, Bowman et al (46) explored the sensitivity of various membrane ATPases to bafilomycin Al, a macrolide antibiotic.…”

Axial heterogeneity of intracellular pH in rat proximal convoluted tubule.

“…This could be accomplished by lower Na, or lesser degree of inhibition of the exchanger by cytosolic OH- (26). It should be noted that other possibilities could also account for our findings: (a) a greater contribution from a luminal H+-ATPase pump (27) (existence of such pump in the proximal tubule is, however, a highly disputed matter), (b) decreased luminal cell entry of H+ or OH-exit via the operation of a HCQ symporter or Cl-/OH-antiporter (28, 29), or (c) a higher metabolic production of H'. Additional studies will be required to test these various alternatives.…”