NHE3 is the predominant isoform responsible for apical membrane Na+/H+exchange in the proximal tubule. Deletion of NHE3 by gene targeting results in an NHE3−/−mouse with greatly reduced proximal tubule[Formula: see text] absorption compared with NHE3+/+ animals (P. J. Schultheis, L. L. Clarke, P. Meneton, M. L. Miller, M. Soleimani, L. R. Gawenis, T. M. Riddle, J. J. Duffy, T. Doetschman, T. Wang, G. Giebisch, P. S. Aronson, J. N. Lorenz, and G. E. Shull. Nature Genet. 19: 282–285, 1998). The purpose of the present study was to evaluate the role of other acidification mechanisms in mediating the remaining component of proximal tubule [Formula: see text] reabsorption in NHE3−/− mice. Proximal tubule transport was studied by in situ microperfusion. Net rates of[Formula: see text] ( J HCO3) and fluid absorption ( J v) were reduced by 54 and 63%, respectively, in NHE3 null mice compared with controls. Addition of 100 μM ethylisopropylamiloride (EIPA) to the luminal perfusate caused significant inhibition of J HCO3 and J v in NHE3+/+ mice but failed to inhibit J HCO3 or J v in NHE3−/− mice, indicating lack of activity of NHE2 or other EIPA-sensitive NHE isoforms in the null mice. Addition of 1 μM bafilomycin caused a similar absolute decrement in J HCO3 in wild-type and NHE3 null mice, indicating equivalent rates of[Formula: see text] absorption mediated by H+-ATPase. Addition of 10 μM Sch-28080 did not reduce J HCO3 in either wild-type or NHE3 null mice, indicating lack of detectable H+-K+-ATPase activity in the proximal tubule. We conclude that, in the absence of NHE3, neither NHE2 nor any other EIPA-sensitive NHE isoform contributes to mediating [Formula: see text] reabsorption in the proximal tubule. A significant component of[Formula: see text] reabsorption in the proximal tubule is mediated by bafilomycin-sensitive H+-ATPase, but its activity is not significantly upregulated in NHE3 null mice.

We have previously demonstrated that formate and oxalate stimulate volume absorption in the rat proximal tubule, consistent with Cl-/formate and Cl-/oxalate exchange process across the apical membrane. To sustain Cl- absorption by these processes requires mechanisms for recycling formate and oxalate from lumen to cell. The aims of the present study were to characterize these mechanisms of formate and oxalate recycling. Proximal tubules and peritubular capillaries were simultaneously microperfused in the rat kidney in situ. Serum formate concentration was determined to be 56.5 +/- 7.7 microM. Addition of 5, 50, and 500 microM formate to both luminal and capillary perfusates significantly increased net Cl- absorption (Jcl) by 26, 26, and 46%, respectively. Jcl was stimulated 38% by 1 microM oxalate added to the perfusates. Removal of sulfate completely prevented the stimulation of Jcl by 1 microM oxalate but had no effect on the stimulation of Jcl by formate. Luminal addition of the Na+/H+ exchange inhibitor ethylisopropylamiloride completely blocked the stimulation of Jcl by 50 microM formate but had no effect on stimulation by oxalate. We conclude that physiological concentrations of formate and oxalate markedly stimulate Cl- and fluid absorption in the rat proximal convoluted tubule. Whereas formate recycling most likely involves Na+/H+ exchange in parallel with H(+)-coupled formate entry, oxalate recycling involves sodium-sulfate cotransport in parallel with sulfate/oxalate exchange.

Na+/H+ exchanger (NHE) isoforms play important roles in intracellular pH regulation and in fluid absorption. The isoform NHE3 has been localized to apical surfaces of epithelia and in some tissues may facilitate the absorption of NaCl. To determine whether the apical isoform NHE3 is present in cholangiocytes and to examine whether it has a functional role in cholangiocyte fluid secretion and absorption, immunocytochemical studies were performed in rat liver with NHE3 antibodies and functional studies were obtained in isolated bile duct units from wild-type and NHE3-/- mice after stimulation with forskolin, using videomicroscopic techniques. Our results indicate that NHE3 protein is present on the apical membranes of rat cholangiocytes and on the canalicular membrane of hepatocytes. Western blots also detect NHE3 protein in rat cholangiocytes and isolated canalicular membranes. After stimulation with forskolin, duct units from NHE3-/- mice fail to absorb the secreted fluid from the cholangiocyte lumen compared with control animals. Similar findings were observed in isolated bile duct units from wild-type mice and rats in the presence of the Na+/H+ exchanger inhibitor 5-(N-ethyl-N-isopropyl)-amiloride. In contrast, we could not demonstrate absorption of fluid from the canalicular lumen of mouse or rat hepatocyte couplets after stimulation of secretion with forskolin. These findings indicate that NHE3 is located on the apical membrane of rat cholangiocytes and that this NHE isoform can function to absorb fluid from the lumens of isolated rat and mouse cholangiocyte preparations.

NHE3−/− mice display a profound defect in proximal tubule bicarbonate reabsorption but are only mildly acidotic owing to reduced glomerular filtration rate and enhanced H + secretion in distal nephron segments. In vivo microperfusion of rat distal tubules suggests that a significant fraction of bicarbonate reabsorption in this nephron segment is mediated by NHE2. Two approaches were used to evaluate the role of distal tubule NHE2 in compensating for the proximal defect of H + secretion in NHE3 −/− mice. First, renal clearance experiments were used to assess the impact of HOE694, an inhibitor with significant affinity for NHE2, on excretion of bicarbonate in NHE3 −/− and NHE2 −/− mice. Second, in vivo micropuncture and microperfusion were employed to measure the concentration of bicarbonate in early distal tubule fluid and to measure distal bicarbonate reabsorption during a constant bicarbonate load. Our data show that HOE694 had no effect on urinary bicarbonate excretion in NHE3 +/+ mice, whereas bicarbonate excretion was higher in NHE3 −/− mice receiving HOE694. HOE694 induced a significant increase in bicarbonate excretion in mice given an acute bicarbonate load, but there was no effect during metabolic acidosis. Bicarbonate excretion was not affected by HOE694 in bicarbonate-loaded NHE2 −/− mice. In vivo micropuncture revealed that early distal bicarbonate concentration was elevated in both bicarbonate-loaded and NHE3 −/− mice. Further, microperfusion experiments showed that HOE694-sensitive bicarbonate reabsorption capacity was higher in acidotic and NHE3 null animals. We conclude that NHE2 contributes importantly to acidification in the distal tubule, and that it plays a major role in limiting urinary bicarbonate losses in states in which a high luminal bicarbonate load is presented to the distal tubule, such as in NHE3 null mice.

The absorption of NaCl in the proximal tubule is markedly stimulated by formate. This stimulation of NaCl transport is consistent with a cell model involving Cl(-)-formate exchange in parallel with pH-coupled formate recycling due to nonionic diffusion of formic acid or H(+)-formate cotransport. The formate recycling process requires H(+) secretion. Although Na(+)-H(+) exchanger isoform NHE3 accounts for the largest component of H(+) secretion in the proximal tubule, 40-50% of the rates of HCO absorption or cellular H(+) extrusion persist in NHE3 null mice. The purpose of the present investigation is to use NHE3 null mice to directly test the role of apical membrane NHE3 in mediating NaCl absorption stimulated by formate. We demonstrate that formate stimulates NaCl absorption in the mouse proximal tubule microperfused in vivo, but the component of NaCl absorption stimulated by formate is absent in NHE3 null mice. In contrast, stimulation of NaCl absorption by oxalate is preserved in NHE3 null mice, indicating that oxalate-stimulated NaCl absorption is independent of Na(+)-H(+) exchange. The virtually complete dependence of formate-induced NaCl absorption on NHE3 activity raises the possibility that NHE3 and the formate transporters are functionally coupled in the brush border membrane.