Renal bicarbonate reabsorption in the rat. I. Effects of hypokalemia and carbonic anhydrase

NBCe1-A is required for the renal ammonia and K+ response to hypokalemia

Hypokalemia increases ammonia excretion and decreases K⁺ excretion. The present study examined the role of the proximal tubule protein NBCe1-A in these responses. We studied mice with Na⁺-bicarbonate cotransporter electrogenic, isoform 1, splice variant A (NBCe1-A) deletion [knockout (KO) mice] and their wild-type (WT) littermates were provided either K⁺ control or K⁺-free diet. We also used tissue sections to determine the effect of extracellular ammonia on NaCl cotransporter (NCC) phosphorylation. The K⁺-free diet significantly increased proximal tubule NBCe1-A and ammonia excretion in WT mice, and NBCe1-A deletion blunted the ammonia excretion response. NBCe1-A deletion inhibited the ammoniagenic/ammonia recycling enzyme response in the cortical proximal tubule (PT), where NBCe1-A is present in WT mice. In the outer medulla, where NBCe1-A is not present, the PT ammonia metabolism response was accentuated by NBCe1-A deletion. KO mice developed more severe hypokalemia and had greater urinary K⁺ excretion during the K⁺-free diet than did WT mice. This was associated with blunting of the hypokalemia-induced change in NCC phosphorylation. NBCe1-A KO mice have systemic metabolic acidosis, but experimentally induced metabolic acidosis did not alter NCC phosphorylation. Although KO mice have impaired ammonia metabolism, experiments in tissue sections showed that lack of ammonia does impair NCC phosphorylation. Finally, urinary aldosterone was greater in KO mice than in WT mice, but neither expression of epithelial Na⁺ channel α-, β-, and γ-subunits nor of H⁺-K⁺-ATPase α₁- or α₂-subunits correlated with changes in urinary K⁺. We conclude that NBCe1-A is critical for the effect of diet-induced hypokalemia to increase cortical proximal tubule ammonia generation and for the expected decrease in urinary K⁺ excretion.

H+-ATPase B1 subunit localizes to thick ascending limb and distal convoluted tubule of rodent and human kidney

The vacuolar-type H⁺-ATPase B1 subunit is heavily expressed in the intercalated cells of the collecting system, where it contributes to H⁺ transport, but has also been described in other segments of the renal tubule. This study aimed to determine the localization of the B1 subunit of the vacuolar-type H⁺-ATPase in the early distal nephron, encompassing thick ascending limbs (TAL) and distal convoluted tubules (DCT), in human kidney and determine whether the localization differs between rodents and humans. Antibodies directed against the H⁺-ATPase B1 subunit were used to determine its localization in paraffin-embedded formalin-fixed mouse, rat, and human kidneys by light microscopy and in sections of Lowicryl-embedded rat kidneys by electron microscopy. Abundant H⁺-ATPase B1 subunit immunoreactivity was observed in the human kidney. As expected, intercalated cells showed the strongest signal, but significant signal was also observed in apical membrane domains of the distal nephron, including TAL, macula densa, and DCT. In mouse and rat, H⁺-ATPase B1 subunit expression could also be detected in apical membrane domains of these segments. In rat, electron microscopy revealed that the H⁺-ATPase B1 subunit was located in the apical membrane. Furthermore, the H⁺-ATPase B1 subunit colocalized with other H⁺-ATPase subunits in the TAL and DCT. In conclusion, the B1 subunit is expressed in the early distal nephron. The physiological importance of H⁺-ATPase expression in these segments remains to be delineated in detail. The phenotype of disease-causing mutations in the B1 subunit may also relate to its presence in the TAL and DCT.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Metabolic acidosis in the first 14 days of life in infants of gestation less than 26 weeks

Extremely immature newborns develop a self-limiting normal anion gap metabolic acidosis in early life. This study examined the natural history of this acidosis in a population of infants of gestation less than 26 weeks in the first 14 days of life. The acidosis was maximal on day 4 with a mean base deficit of 10.6 mmol/l and had resolved in 90 % of infants by day 11. Dopamine usage was the only independent predictor of the acidosis. Its use was associated with a greater degree of acidosis.

CONCLUSION

Extremely preterm infants experience a self-limiting normal anion gap metabolic acidosis in the first 2 weeks of life which is consistent with renal tubular immaturity.

Proteomic identification of alterations in metabolic enzymes and signaling proteins in hypokalemic nephropathy

Hypokalemic nephropathy caused by prolonged K(+) deficiency is associated with metabolic alkalosis, polydipsia, polyuria, growth retardation, hypertension, and progressive tubulointerstitial injury. Its pathophysiology, however, remains unclear. We performed gel-based, differential proteomics analysis of kidneys from BALB/c mice fed with high-normal-K(+) (HNK), low-normal-K(+) (LNK), or K(+)-depleted diet for 8 wk (n = 6 in each group). Plasma K(+) levels were 4.62 +/- 0.35, 4.46 +/- 0.23, and 1.51 +/- 0.21 mmol/L for HNK, LNK, and KD mice, respectively (p < 0.0001; KD vs. others). With comparable amounts of food intake, the KD mice drank significantly more water than the other two groups and had polyuria. Additionally, the KD mice had growth retardation, metabolic alkalosis, markedly enlarged kidneys, renal tubular dilation, intratubular deposition of amorphous and laminated hyaline materials, and tubular atrophy. A total of 33 renal proteins were differentially expressed between the KD mice and others, whereas only eight proteins were differentially expressed between the HNK and LNK groups, as determined by quantitative intensity analysis and ANOVA with Tukey's post hoc multiple comparisons. Using MALDI-MS and/or quadrupole-TOF MS/MS, 30 altered proteins induced by K(+)-depletion were identified as metabolic enzymes (e.g., carbonic anhydrase II, aldose reductase, glutathione S-transferase GT41A, etc.), signaling proteins (14-3-3 epsilon, 14-3-3 zeta, and cofilin 1), and cytoskeletal proteins (gamma-actin and tropomyosin). Some of these altered proteins, particularly metabolic enzymes and signaling proteins, have been demonstrated to be involved in metabolic alkalosis, polyuria, and renal tubular injury. Our findings may lead to a new road map for research on hypokalemic nephropathy and to better understanding of the pathophysiology of this medical disease when the functional and physiological significances of these altered proteins are defined.

A mathematical model of rat distal convoluted tubule. I. Cotransporter function in early DCT

A model of rat early distal convoluted tubule (DCT) is developed in conjunction with a kinetic representation of the thiazide-sensitive NaCl cotransporter (TSC). Realistic constraints on cell membrane electrical conductance require that most of the peritubular Cl(-) reabsorption proceeds via a KCl cotransporter,along with most of the K(+) recycled from the Na-K-ATPase. The model tubule reproduces the saturable Cl(-) reabsorption of DCT but not the micropuncture finding of linear Na(+) flux in response to load, more likely a feature of late DCT (CNT). As in proximal tubule, early DCT HCO(3)(-) reabsorption is mediated by a luminal Na(+)/H(+) exchanger (NHE), but in contrast to proximal tubule, the DCT exchanger is operating closer to equilibrium. In the model DCT, two consequences of the lesser driving force for NHE exchange are an acidic cytosol and wider swings in NHE flux with perturbations of luminal composition. Variations in luminal NaCl provide a challenge to cell volume, which can be blunted by volume dependence of the KCl cotransporter. Cell swelling can also be induced by increases in peritubular K(+) concentration. In this case, volume-dependent inhibition of TSC could provide volume homeostasis that also enhances distal Na(+) delivery, and ultimately enhances renal K(+) excretion. In the model DCT, proton secretion is blunted by peritubular HCO(3)(-), so that there is little contribution by this segment to the maintenance of metabolic alkalosis. During alkalosis, the model predicts that increasing luminal NaCl concentration enhances NHE flux, so that these calculations provide no support for a role of early DCT in recovery from Cl(-) depletion alkalosis.

NHE2-mediated bicarbonate reabsorption in the distal tubule of NHE3 null mice

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.

Altered expression of renal NHE3, TSC, BSC-1, and ENaC subunits in potassium-depleted rats

The purpose of this study was to examine whether hypokalemia is associated with altered abundance of major renal Na+ transporters that may contribute to the development of urinary concentrating defects. We examined the changes in the abundance of the type 3 Na+/H+ exchanger (NHE3), Na+ - K+-ATPase, the bumetanide-sensitive Na+ - K+ - 2Cl- cotransporter (BSC-1), the thiazide-sensitive Na+ - Cl- cotransporter (TSC), and epithelial sodium channel (ENaC) subunits in kidneys of hypokalemic rats. Semiquantitative immunoblotting revealed that the abundance of BSC-1 (57%) and TSC (46%) were profoundly decreased in the inner stripe of the outer medulla (ISOM) and cortex/outer stripe of the outer medulla (OSOM), respectively. These findings were confirmed by immunohistochemistry. Moreover, total kidney abundance of all ENaC subunits was significantly reduced in response to the hypokalemia: alpha-subunit (61%), beta-subunit (41%), and gamma-subunit (60%), and this was confirmed by immunohistochemistry. In contrast, the renal abundance of NHE3 in hypokalemic rats was dramatically increased in cortex/OSOM (736%) and ISOM (210%). Downregulation of BSC-1, TSC, and ENaC may contribute to the urinary concentrating defect, whereas upregulation of NHE3 may be compensatory to prevent urinary Na+ loss and/or to maintain intracellular pH levels.

A numerical model of acid-base transport in rat distal tubule

The purpose of this study is to develop a numerical model that simulates acid-base transport in rat distal tubule. We have previously reported a model that deals with transport of Na(+), K(+), Cl(-), and water in this nephron segment (Chang H and Fujita T. Am J Physiol Renal Physiol 276: F931-F951, 1999). In this study, we extend our previous model by incorporating buffer systems, new cell types, and new transport mechanisms. Specifically, the model incorporates bicarbonate, ammonium, and phosphate buffer systems; has cell types corresponding to intercalated cells; and includes the Na/H exchanger, H-ATPase, and anion exchanger. Incorporation of buffer systems has required the following modifications of model equations: new model equations are introduced to represent chemical equilibria of buffer partners [e.g., pH = pK(a) + log(10) (NH(3)/NH(4))], and the formulation of mass conservation is extended to take into account interconversion of buffer partners. Furthermore, finite rates of H(2)CO(3)-CO(2) interconversion (i.e., H(2)CO(3) &rlharr; CO(2) + H(2)O) are taken into account in modeling the bicarbonate buffer system. Owing to this treatment, the model can simulate the development of disequilibrium pH in the distal tubular fluid. For each new transporter, a state diagram has been constructed to simulate its transport kinetics. With appropriate assignment of maximal transport rates for individual transporters, the model predictions are in agreement with free-flow micropuncture experiments in terms of HCO reabsorption rate in the normal state as well as under the high bicarbonate load. Although the model cannot simulate all of the microperfusion experiments, especially those that showed a flow-dependent increase in HCO reabsorption, the model is consistent with those microperfusion experiments that showed HCO reabsorption rates similar to those in the free-flow micropuncture experiments. We conclude that it is possible to develop a numerical model of the rat distal tubule that simulates acid-base transport, as well as basic solute and water transport, on the basis of tubular geometry, physical principles, and transporter kinetics. Such a model would provide a useful means of integrating detailed kinetic properties of transporters and predicting macroscopic transport characteristics of this nephron segment under physiological and pathophysiological settings.

Transepithelial electrochemical gradients in the proximal convoluted tubule during potassium depletion in the rat

1. In order to examine the electrochemical gradient for potassium reabsorption across the S2 segment of the proximal convoluted tubule, transepithelial potential differences and transepithelial potassium concentrations were measured in anaesthetized potassium-replete and potassium-depleted rats. 2. Potassium-depleted rats were markedly hypokalaemic (plasma potassium, 1.4 +/- 0.1 vs. 4.1 +/- 0.1 mmol l-1 in potassium-replete rats) and had a significantly reduced muscle potassium content. In confirmation of previous reports, glomerular filtration rate was slightly reduced, while fractional reabsorption in the proximal convoluted tubule was enhanced. 3. In potassium-replete animals, the transepithelial potential difference (PD) at the late proximal convoluted tubule was +2.1 +/- 0.3 mV (lumen positive) and the tubular fluid to plasma ultrafiltrate concentration ratio for potassium (TFK/UFK) at the same site was 1. 03 +/- 0.01. In potassium-depleted rats, there was a striking reversal of the transepithelial PD (to -4.0 +/- 0.4 mV), while the TFK/UFK was increased to 1.19 +/- 0.03. 4. The data from both potassium-replete and potassium-depleted animals are consistent with accumulating evidence that potassium reabsorption in the proximal convoluted tubule is passive in nature and depends partly on diffusion down an electrochemical gradient.

Upregulation of H+-ATPase in the distal nephron during potassium depletion: structural and functional evidence

In the present study, we have investigated the effects of dietary potassium depletion on the activity and distribution of the H+-ATPase in the distal nephron of the Sprague-Dawley rat. H+-ATPase activity was assessed from the change in transepithelial potential difference (Vte) in response to bafilomycin A1 during perfusion of the late distal tubule in vivo, with solutions containing inhibitors of known ion channels. Bafilomycin A1 caused a negative deflection in Vte in control animals, an effect that was significantly enhanced during potassium depletion (P < 0.01). The distribution of H+-ATPase within the population of intercalated cells was assessed using a specific monoclonal antibody (E11). Hypokalemia was associated with a highly significant redistribution of the staining pattern (P < 0. 001), with an increase in the percentage of cells displaying immunoreactivity in the apical membrane. These results indicate that dietary potassium depletion increases electrogenic H+-ATPase activity in the rat distal tubule; this may be associated with increased insertion of pumps into the apical membrane.

Na/H exchange and H-K ATPase increase distal tubule acidification in chronic alkalosis

We examined whether H(+)-ATPase, H(+)-K(+)-ATPase, and or Na+/H+ exchange mediates increased distal tubule acidification in animals with chronic metabolic alkalosis using pharmacological inhibitors of these H+ transporters in in vivo-perfused tubules of anesthetized rats. Chronic metabolic alkalosis was induced with furosemide followed by minimum electrolyte diet and HCO3 drinking water. The reduction in net HCO3 reabsorption was greater in distal tubules of alkalotic compared to control animals perfused with Schering 28080 to inhibit H(+)-K(+)-ATPase (-6.4 +/- 0.9 vs. -1.4 +/- 0.5 pmol/mm.min-1, P < 0.02) and with EIPA to inhibit Na+/H+ exchange (-11.1 +/- 1.7 vs. -6.6 +/- 0.9 pmol/mm.min-1, P < 0.01) but was similar in distal tubules of alkalotic and control animals perfused with bafilomycin to inhibit H(+)-ATPase. The greater reduction of distal tubule net HCO3 reabsorption in alkalotic compared to control animals induced by EIPA was eliminated by systemic infusion of the endothelin receptor antagonist bosentan (-4.6 +/- 0.7 vs. -4.4 +/- 0.7 pmol/mm.min-1, P = NS) but the greater reduction induced by Schering 28080 persisted. Urine endothelin-1 (ET-1) excretion was higher in animals with maintained alkalosis (164.5 +/- 23.7 vs. 76.6 +/- 10.8 fmol/day, P < 0.03), but decreased following KCl repletion to a value (86.7 +/- 10.0 fmol/day, P < 0.02 vs. respective before-KCl value) that was not different from that for KCl-repleted control animals (79.9 +/- 8.7 fmol/day, P = NS vs. KCl-repleted alkalotic animals). The data support that augmented distal tubule acidification in alkalotic animals is due to increased H(+)-K(+)-ATPase and Na+/H+ exchange activity, the latter stimulated by endogenous endothelins.

Endothelin-1 increases rat distal tubule acidification in vivo

Because endothelin receptor inhibition blunts increased distal tubule acidification induced by dietary acid, we examined whether endothelin-1 (ET-1) increases acidification of in vivo perfused distal tubules of anesthetized rats. ET-1 was infused intraaortically (1.4 pmol x kg(-1) x min[-1]) into control animals and into those with increased distal tubule HCO3 secretion induced by drinking 80 mM NaHCO3 solution for 7-10 days. ET-1 increased distal tubule acidification in both control and NaHCO3 animals. Increased acidification in control animals was mediated by increased distal tubule H+ secretion (23.7+/-2.2 vs. 18.7 +/- 1.7 pmol x mm(-1) x min(-1), P < 0.05) with no changes in HCO3 secretion. By contrast, ET-1 increased distal tubule acidification in NaHCO3 animals predominantly by decreasing HCO3 secretion (-9.5 +/- 1.0 vs. -18.7 +/-1.8 pmol x mm(-1) x min(-1), P < 0.001) with less influence on H+ secretion. When indomethacin was infused (83 microg x kg(-1) x min[-1]) to inhibit synthesis of prostacyclin, an agent previously shown to increase HCO3 secretion in the distal tubule, ET-1 increased distal tubule H+ secretion in both control (24.3 +/-2.2 vs. 15.7 +/- 1.6 pmol x mm(-1) x min(-1), P < 0.02) and NaHCO3 (20.0 +/- 2.0 vs. 13.6 +/- 1.4 pmol x mm(-1) x min(-1), P < 0.05) without affecting HCO3 secretion. The data show that ET-1 increases distal tubule acidification in vivo and can do so by increasing H+ secretion and by decreasing HCO3 secretion when the latter is augmented by dietary NaHCO3.

Renal tubular lithium reabsorption in potassium-depleted rats

1. In order to identify the tubular sites responsible for the reduced fractional excretion of lithium (FELi) during potassium depletion, free-flow micropuncture was performed in anaesthetized rats that had been fed a low potassium (low-K+) diet or a control diet for 5-6 days. FELi in low-K+ rats was 0.09 +/- 0.02, compared with 0.25 +/- 0.01 in control animals. 2. Fractional water reabsorption in proximal convoluted tubules was enhanced in potassium-depleted rats. However, fractional lithium reabsorption was not. Consequently, the tubular fluid-to-plasma lithium concentration ratio at the late proximal convoluted tubule was raised in the low-K+ animals (1.50 +/- 0.03 vs. 1.18 +/- 0.02; P < 0.001). 3. Fractional lithium delivery to the early distal tubule in low-K+ rats (0.31 +/- 0.01) was similar to that in control animals (0.30 +/- 0.02). However, whereas in control rats there was no significant difference between early and late distal tubular deliveries of lithium, late distal fractional lithium delivery in the low-K+ group was reduced markedly (to 0.10 +/- 0.01). 4. Treatment of potassium-depleted rats with amiloride had no effect on lithium reabsorption in the proximal convoluted tubule or loop of Henle. However, fractional lithium delivery to the end of the distal tubule was increased slightly (to 0.15 +/- 0.02; P < 0.05) and FELi was increased substantially (to 0.22 +/- 0.01; P < 0.001). 5. It is concluded that two factors contribute to the reduced FELi seen in potassium-depleted rats: lithium reabsorption in the superficial distal tubules and amiloride-sensitive lithium reabsorption in the collecting ducts. The data also suggest heterogeneity with respect to lithium handling between superficial and deep nephrons during potassium depletion.

Endogenous endothelins mediate increased distal tubule acidification induced by dietary acid in rats

We examined if endogenous endothelins mediate the decreased HCO3 secretion and increased H+ secretion in in vivo-perfused distal tubules of rats fed dietary acid as (NH4)2SO4. Animals given (NH4)2SO4 drinking solution had higher endothelin-1 addition to renal interstitial fluid than those given distilled H2O (480+/-51 vs. 293+/-32 fmol g kidney wt(-1) min(-1), respectively, P < 0.03). (NH4)2SO4-ingesting animals infused with bosentan (10 mg/kg) to inhibit A- and B-type endothelin receptors had higher HCO3 secretion than baseline (NH4)2SO4 animals (-4.7+/-0.4 vs. -2.4+/-0.3 pmol mm(-1) min(-1), P < 0.01), but (NH4)2SO4 animals given a specific inhibitor of A-type endothelin receptors (BQ-123) did not (-2.0+/-0.2 pmol mm(-1) min(-1), P = NS vs. baseline). H+ secretion was lower in bosentan-infused compared with baseline (NH4)2SO4 animals (27.7+/-2.5 vs. 43.9+/-4.0 pmol mm(-1) min(-1), P < 0.03), but that for BQ-123-infused (NH4)2SO4 animals was not (42.9+/-4.2 pmol mm(-1) min(-1), P = NS vs. baseline). Bosentan had no effect on distal tubule HCO3 or H+ secretion in control animals. The data show that dietary acid increases endothelin-1 addition to renal interstitial fluid and that inhibition of B- but not A-type endothelin receptors blunts the decreased HCO3 secretion and increased H+ secretion in the distal tubule of animals given dietary acid. The data are consistent with endogenous endothelins as mediators of increased distal tubule acidification induced by dietary acid.

Secretion of HCO3-/OH- in cortical distal tubule of the rat

Secretion of bicarbonate has been described for distal nephron epithelium and attributed to apical Cl-/HCO3- exchange in beta-intercalated cells. We investigated the presence of this mechanism in cortical distal tubules by perfusing these segments with acid (pH 6) 10 mM phosphate Ringer. The kinetics of luminal alkalinization was studied in stationary microperfusion experiments by double-barreled pH (ion-exchange resin)/1 M KCl reference microelectrodes. Luminal alkalinization may be due to influx (into the lumen) of HCO3- or OH-, or efflux of H+. The magnitude of the Cl-/HCO3- exchange component was measured by perfusing the lumen with solutions with or without chloride, which was substituted by gluconate. This component was not different from zero in control and alkalotic (chronic plus acute) Wistar rats. Homozygous Brattleboro rats (BRB), genetically devoid of antidiuretic hormone, were used since this hormone has been shown to stimulate H+ secretion, which could mask bicarbonate secretion. In these rats, no evidence for Cl-/HCO3- exchange was found in control BRB and in early distal segments of alkalotic animals, but in late distal tubule a significant component of 0.14 +/- 0.033 nmol/cm2.sec was observed, which, however, is small when compared to the reabsorptive flow found in control Wistar rats, of 0.95 +/- 0.10 nmol/cm2.sec. In addition, 5 x 10(-4) M SITS had no effect on distal bicarbonate reabsorption in controls as well as on secretion in alkalotic Wistar and Brattleboro rats, which is compatible with the absence of effect of this drug on the apical Cl-/HCO3- exchange in other tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Bicarbonate transport along the loop of Henle. II. Effects of acid-base, dietary, and neurohumoral determinants

The loop of Henle contributes to renal acidification by reabsorbing about 15% of filtered bicarbonate. To study the effects on loop of Henle bicarbonate transport (JHCO3) of acid-base disturbances and of several factors known to modulate sodium transport, these in vivo microperfusion studies were carried out in rats during: (a) acute and chronic metabolic acidosis, (b) acute and chronic (hypokalemic) metabolic alkalosis, (c) a control sodium diet, (d) a high-sodium diet, (e) angiotensin II (AII) intravenous infusion, (f) simultaneously intravenous infusion of both AII and the AT1 receptor antagonist DuP 753, (g) acute ipsilateral mechanicochemical renal denervation. Acute and chronic metabolic acidosis increased JHCO3; acute metabolic alkalosis significantly reduced JHCO3, whereas chronic hypokalemic alkalosis did not alter JHCO3. Bicarbonate transport increased in animals on a high-sodium intake and following AII administration, and the latter was inhibited by the AII (AT1) receptor antagonist DuP 753; acute renal denervation lowered bicarbonate transport. These data indicate that bicarbonate reabsorption along the loop of Henle in vivo is closely linked to systemic acid-base status and to several factors known to modulate sodium transport.

Renal bicarbonate reabsorption in the rat. IV. Bicarbonate transport mechanisms in the early and late distal tubule

Bicarbonate transport was studied in vivo by separate microperfusion experiments of early and late distal tubules. Total CO2 was measured by microcalorimetry and fluid absorption by 3H-inulin. Significant bicarbonate absorption was observed in all experimental conditions. Bicarbonate transport was load-dependent upon increasing the luminal bicarbonate concentration from 15 to 50 mM in both early and late distal tubule segments and remained constant at higher concentrations at a maximum rate of 100-110 pmol/min per mm. At low lumen bicarbonate concentrations (15 mM), higher rates of bicarbonate absorption were observed in early (32.9 +/- 4.57 pmol/min per mm) as compared to late distal tubules (10.7 +/- 3.1 pmol/min per mm). Amiloride and ethyl-isopropylamiloride both inhibited early but not late distal tubule bicarbonate absorption whereas acetazolamide blocked bicarbonate transport in both tubule segments. Fluid absorption was significantly reduced in both tubule segments by amiloride but only in early distal tubules by ethyl-isopropylamiloride. Substitution of lumen chloride by gluconate increased bicarbonate absorption in late but not in early distal tubules. Bafilomycin A1, an inhibitor of H-ATPase, inhibited late and also early distal tubule bicarbonate absorption, the latter at higher concentration. After 8 d on a low K diet, bicarbonate absorption increased significantly in both early and late distal tubules. Schering compound 28080, a potent H-K ATPase inhibitor, completely blocked this increment of bicarbonate absorption in late but not in early distal tubule. The data suggest bicarbonate absorption via Na(+)-H+ exchange and H-ATPase in early, but only by amiloride-insensitive H+ secretion (H-ATPase) in late distal tubules. The study also provides evidence for activation of K(+)-H+ exchange in late distal tubules of K depleted rats. Indirect evidence implies a component of chloride-dependent bicarbonate secretion in late distal tubules and suggests that net bicarbonate transport at this site results from bidirectional bicarbonate movement.

Effect of acute changes in glomerular filtration rate on Na+/H+ exchange in rat renal cortex

Studies were undertaken in Munich-Wistar rats to assess the influence of changes in filtered bicarbonate (FLHCO3), induced by changes in GFR, on Na+/H+ exchange activity in renal brush border membrane vesicles (BBMV). Whole-kidney and micropuncture measurements of GFR, FLHCO3, and whole-kidney and proximal tubule HCO3 reabsorption (APRHCO3) were coupled with BBMV measurements of H+ gradient-driven 22Na+ uptake in each animal studied. 22Na+ uptake was measured at three Na+ concentration gradients to allow calculation of Vmax and Km for Na+/H+ exchange. GFR was varied by studying animals under conditions of hydropenia, plasma repletion, and acute plasma expansion. The increase in GFR, FLHCO3, and APRHCO3 induced by plasma administration correlated directly with an increase in the Vmax for Na+/H+ exchange in BBMV. The Km for sodium was unaffected. In the plasma-expanded rats, the Vmax for Na+/H+ exchange was 22% greater than in the hydropenic rats (P less than 0.025) whereas APRHCO3 was 86% greater (P less than 0.001). These results indicate that increases in FLHCO3, induced by acute increases in GFR, stimulate Na+/H+ exchange activity in proximal tubular epithelium. This stimulation is a mechanism which can, in part, account for the delivery dependence of proximal bicarbonate reabsorption.

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

We microperfused the loop of Henle (LOH) to assess its contribution to urine acidification in vivo. Under control conditions (Na HCO3- = 13 mM, perfusion rate approximately 17 nl/min-1) net bicarbonate transport (JHCO3-) was unsaturated, flow- and concentration-dependent, and increased linearly until a bicarbonate load of 1,400 pmol.min-1 was reached. Methazolamide (2 x 10(-4) M) reduced JHCO3 by 70%; the amiloride analogue ethylisopropylamiloride (EIPA) (2 x 10(-4) M) reduced JHCO3 by 40%; neither methazolamide nor EIPA affected net water flux (Jv). The H(+)-ATPase inhibitor bafilomycin A1 (10(-5) M) reduced JHCO3 by 20%; the Cl- channel inhibitor 5-nitro-2'-(3-phenylpropylamino)-benzoate (2 x 10(-4) M) and the Cl(-)-base exchange inhibitor diisothiocyanato-2,2'-stilbenedisulfonate (5 x 10(-5) M), had no effect on fractional bicarbonate reabsorption. Bumetanide (10(-6) M) stimulated bicarbonate transport (net and fractional JHCO3-) by 20%, whereas furosemide (10(-4) M) had no effect on bicarbonate reabsorption; both diuretics reduced Jv. In summary: (a) the LOH contributes significantly to urine acidification. It normally reabsorbs an amount equivalent to 15% of filtered bicarbonate; (b) bicarbonate reabsorption is not saturated; (c) Na(+)-H+ exchange and an ATP-dependent proton pump are largely responsible for the bulk of LOH bicarbonate transport.

Augmented bicarbonate reabsorption by both the proximal and distal nephron maintains chloride-deplete metabolic alkalosis in rats

Whether augmented bicarbonate reabsorption by renal tubular epithelium contributes to the maintenance of chloride-deplete metabolic alkalosis is not clear. This study used free-flow micropuncture to investigate bicarbonate reabsorption by surface nephron segments in a rat model of diuretic-induced alkalosis compared to control. The proximal and distal nephron of the alkalotic animals had higher values for both delivered load to and absolute reabsorption from these segments. The proximal tubules of alkalotic and control animals had similar values for the slopes of the linear regression of delivered load vs. reabsorption and for the bicarbonate tubular fluid to plasma (TF/P) ratio at the late proximal tubule. By contrast, the corresponding analysis for the distal segment of alkalotic animals revealed a greater slope (0.98 vs. 0.81, P less than 0.003) and a smaller bicarbonate TF/P ratio at the late distal tubule (0.10 vs. 0.16, P less than 0.006). The data indicate that augmented bicarbonate reabsorption by both the proximal and distal nephron contributes to maintaining the alkalosis of this model. The data suggest primary stimulation of bicarbonate reabsorption in the distal nephron and load-dependent reabsorption in the proximal tubule.

Renal bicarbonate reabsorption in the rat. III. Distal tubule perfusion study of load dependence and bicarbonate permeability

Using continuous microperfusion techniques, we studied the load dependence of bicarbonate reabsorption along cortical distal tubules of the rat kidney and their bicarbonate permeability. Net bicarbonate transport was evaluated from changes in tracer inulin concentrations and total CO2 measurements by microcalorimetry. Bicarbonate permeability was estimated from the flux of total CO2 along known electrochemical gradients into bicarbonate-and chloride-free perfusion solution containing 10(-4) M acetazolamide. Transepithelial potential differences were measured with conventional glass microelectrodes. Significant net bicarbonate reabsorption occurred at luminal bicarbonate levels from 5 to 25 mM, and at perfusion rates from 5 to 30 nl/min. Bicarbonate reabsorption increased in a load-dependent manner, both during increments in luminal bicarbonate concentration or perfusion rate, reaching saturation at a load of 250 pmol/min with a maximal reabsorption rate of approximately 75 pmol/min.mm. Rate of bicarbonate reabsorption was flow dependent at luminal concentrations of 10 but not at 25 mM. During chronic metabolic alkalosis, maximal rates of reabsorption were significantly reduced to 33 pmol/min.mm. The bicarbonate permeability was 2.32 +/- 0.13 x 10(-5) cm/s in control rats, and 2.65 +/- 0.26 x 10(-5) cm/s in volume-expanded rats. Our data indicate that at physiological bicarbonate concentrations in the distal tubule passive bicarbonate fluxes account for only 16-21% of net fluxes. At high luminal bicarbonate concentrations, passive bicarbonate reabsorption contributes moderately to net reabsorption of this anion.

Effect of amphotericin B on renal tubular acidification in the rat

Amphotericin B, a polyene antibiotic known to induce cation-selective pore formation in biological cell membranes, was given to rats by peritoneal injection (10 mg/kg for 21-26 days) or added to luminal perfusates (2 x 10(-5) M). Kinetics of tubular acidification and alkalinization after perfusion with alkaline or acid phosphate Ringer's solution was studied by means of double barrelled antimony/reference microelectrodes in cortical distal tubules. Stationary pH increased both in early and late distal segments. Acidification and alkalinization half-times decreased markedly from 15-18 s to 6-8 s, a value similar to that found in proximal tubule. Net H-ion secretion rates as well as H-ion back-flux approximately doubled after Amphotericin B. Apparent H-ion permeability of distal tubule epithelium measured during perfusion of lumen and peritubular capillaries with phosphate Ringer's solutions doubled both in early and late segments. These data show that amphotericin B produces a distal acidification defect which impairs formation of normal transepithelial pH gradients by increasing H-ion back-flux without reducing rates of net H-ion secretion.

Localization of a proton-pumping ATPase in rat kidney

The distribution of vacuolar H+ATPase in rat kidney was examined by immunocytochemistry using affinity-purified antibodies against the 31-, 56-, and 70-kD subunits of the bovine kidney proton pump. Proximal convoluted tubules were labeled over apical plasma membrane invaginations, and in the initial part of the thin descending limb, apical and basolateral plasma membranes were moderately stained. Thick ascending limbs and distal convoluted tubules were apically stained although the intensity was greater in the distal convoluted tubule. Collecting duct principal cells were virtually unlabeled, but intercalated cells had intense staining with an apical, basolateral or diffuse pattern in the cortex, and exclusively apical staining in the medulla. These results (a) show the presence of an H+ATPase in the apical plasma membrane of the proximal tubule that may contribute to H+ transport in this segment; (b) provide direct evidence that the intercalated cell contains most of the H+ATPase detectable in the collecting duct, supporting its proposed role in H+ transport; (c) demonstrate that subpopulations of cortical intercalated cells have opposite polarities of an H+ATPase, consistent with the presence of both proton- and bicarbonate-secreting cells; and (d) suggest a role for the H+ATPase in acid/base regulation or H+ transport in segments other than the collecting duct and the proximal tubule.

Secretion of bicarbonate by rat distal tubules in vivo. Modulation by overnight fasting

We have performed microperfusion studies on distal tubule bicarbonate reabsorption (JtCO2) of fed and fasted rats to extend our previous observations of in vivo bicarbonate secretion and to resolve certain discrepancies between free-flow and microperfusion data. When rats are fasted overnight, as in previous free-flow studies, distal tubule microperfusion with a 28-mM tCO2 solution results in significant JtCO2 (53 +/- 6 pmol.min-1.mm-1) at normal flow and increases briskly (91 +/- 16 pmol.min-1.mm-1) with bicarbonate load. This response is not influenced by the addition of other normal tubular fluid constituents. However, when normally fed rats are used, as in our previous microperfusion studies, distal tubule JtCO2 is not different from zero when a 28-mM tCO2 solution is perfused at normal flow rates but becomes negative (-54 +/- 13 pmol.min-1.mm-1) at high flow rates, which indicates the existence of bicarbonate secretion against a concentration gradient. Alkali loading of fasted rats also elicits bicarbonate secretion at high flow. These results demonstrate for the first time that normal feeding or alkali loading can induce bicarbonate secretion in a mammalian nephron segment in vivo, and resolves previous discrepancies between free-flow and microperfusion data.

Effects of antidiuretic hormone on urinary acidification and on tubular handling of bicarbonate in the rat

Paired micropuncture experiments were carried out in plasma-replete volume-expanded rats to examine the acute effects of 1-desamino-8-D-arginine vasopressin (dDAVP) on urinary acidification and tubular handling of bicarbonate and chloride. No effect was detected on the fractional absorption of water, total CO2, and chloride at end-proximal and early distal sites of superficial nephrons in intact animals; dDAVP, however, inhibited the fractional absorption of total CO2 in Henle's loop while stimulating that of chloride in thyroparathyroidectomized (TPTX) somatostatin-infused rats. In the distal tubule accessible to micropuncture, net total CO2 secretion was observed during hypotonic volume expansion, which reversed to net total CO2 absorption during dDAVP infusion in intact Wistar rats. Marked stimulation of urinary acidification occurred in all animals as attested by a fall in urine pH and bicarbonate excretion. Net acid excretion almost doubled in intact rats. We conclude that (a) antidiuretic hormone (ADH) inhibits fractional bicarbonate absorption in the thick ascending limb while stimulating that of chloride at least in TPTX somatostatin-infused rats, and (b) ADH stimulates proton secretion (or inhibits bicarbonate secretion) in the distal tubule and cortical collecting ducts, which leads to enhanced urinary acidification.

Renal bicarbonate reabsorption in the rat. II. Distal tubule load dependence and effect of hypokalemia

We studied two groups of rats acutely loaded with bicarbonate, control rats on a standard diet and rats kept on a K-free diet for 3 wk. Compared with controls, K-depleted rats had reduced fractional excretion of bicarbonate despite their elevated filtered bicarbonate load. Distal bicarbonate reabsorption increased in K-depleted rats. In the presence of almost identical early distal bicarbonate loads (481 +/- 40 pmol/min in controls and 444 +/- 50 pmol/min in K depletion), distal bicarbonate reabsorption was significantly enhanced in K depletion (247 +/- 17 pmol/min) as compared with controls (179 +/- 18 pmol/min). These values are significantly different from each other, and both are severalfold higher than bicarbonate reabsorption in nonloaded conditions. In conclusion, distal bicarbonate reabsorption is load dependent, and distal bicarbonate reabsorption is stimulated in K depletion.

Mechanisms of renal tubular acidification

Both bicarbonate retrieval from the filtrate as well as the net excretion of acid depend upon hydrogen ion secretion by the tubular epithelium. Hydrogen ion secretion is mediated either by sodium-hydrogen exchange, an electroneutral and secondary active process, or by hydrogen ion secretion, a directly electrogenic and primary active process. Extrusion of hydrogen ions across the apical cell membrane is accompanied by electrogenic bicarbonate transfer across the basolateral cell membrane. Both luminal and peritubular pH exert a strong influence upon acidification by altering the gradient against which hydrogen transport or base exit occur. In the distal nephron, both hydrogen ion secretion and bicarbonate secretion may occur. These transport operations have been shown to be mediated by subgroups of intercalated cells in which hydrogen pumps and bicarbonate-chloride exchange processes are located either in the apical or basolateral cell membranes. Regulation of acidification involves several factors: the rate of luminal buffer delivery, sodium and chloride delivery, the luminal and peritubular pH and pCO2, the electrical potential, mineralocorticoids and the state of the potassium balance.