Spontaneous luminal disequilibrium pH in S3 proximal tubules. Role in ammonia and bicarbonate transport

Calcium Transport in the Kidney and Disease Processes

Calcium is a key ion involved in cardiac and skeletal muscle contractility, nerve function, and skeletal structure. Global calcium balance is affected by parathyroid hormone and vitamin D, and calcium is shuttled between the extracellular space and the bone matrix compartment dynamically. The kidney plays an important role in whole-body calcium balance. Abnormalities in the kidney transport proteins alter the renal excretion of calcium. Various hormonal and regulatory pathways have evolved that regulate the renal handling of calcium to maintain the serum calcium within defined limits despite dynamic changes in dietary calcium intake. Dysregulation of renal calcium transport can occur pharmacologically, hormonally, and via genetic mutations in key proteins in various nephron segments resulting in several disease processes. This review focuses on the regulation transport of calcium in the nephron. Genetic diseases affecting the renal handling of calcium that can potentially lead to changes in the serum calcium concentration are reviewed.

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.

Proximal tubule specific knockout of the Na⁺/H⁺ exchanger NHE3: effects on bicarbonate absorption and ammonium excretion

The existing NHE3 knockout mouse has significant intestinal electrolyte absorption defects, making this model unsuitable for the examination of the role of proximal tubule NHE3 in pathophysiologic states in vivo. To overcome this problem, we generated proximal convoluted tubule-specific KO mice (NHE3-PT KO) by generating and crossing NHE3 floxed mice with the sodium-glucose transporter 2 Cre transgenic mice. The NHE3-PT KO mice have >80 % ablation of NHE3 as determined by immunofluorescence microscopy, western blot, and northern analyses, and show mild metabolic acidosis (serum bicarbonate of 21.2 mEq/l in KO vs. 23.7 mEq/l in WT, p < 0.05). In vitro microperfusion studies in the isolated proximal convoluted tubules demonstrated a ∼36 % reduction in bicarbonate reabsorption (J_HCO3 = 53.52 ± 4.61 pmol/min/mm in KO vs. 83.09 ± 9.73 in WT) and a ∼27 % reduction in volume reabsorption (J_v = 0.67 ± 0.07 nl/min/mm in KO vs. 0.92 ± 0.06 nl/min/mm in WT) in mutant mice. The NHE3-PT KO mice tolerated NH₄Cl acid load well (added to the drinking water) and showed NH₄ excretion rates comparable to WT mice at 2 and 5 days after NH₄Cl loading without disproportionate metabolic acidosis after 5 days of acid load. Our results suggest that the Na⁺/H⁺ exchanger NHE3 plays an important role in fluid and bicarbonate reabsorption in the proximal convoluted tubule but does not play an important role in NH₄ excretion.

Luminal alkalinization attenuates proteinuria-induced oxidative damage in proximal tubular cells

A highly acidic environment surrounds proximal tubular cells as a result of their reabsorption of HCO(3)(-). It is unclear whether this luminal acidity affects proteinuria-induced progression of tubular cell damage. Here, we investigated the contribution of luminal acidity to superoxide (O(2)(·-)) production induced by oleic acid-bound albumin (OA-Alb) in proximal tubular cells. Acidic media significantly enhanced OA-Alb-induced O(2)(·-) production in the HK-2 proximal tubular cell line. Simultaneous treatment with both OA-Alb and acidic media led to phosphorylation of the intracellular pH sensor Pyk2. Highly phosphorylated Pyk2 associated with activation of Rac1, an essential subcomponent of NAD(P)H oxidase. Furthermore, knockdown of Pyk2 with siRNA attenuated the O(2)(·-) production induced by cotreatment with OA-Alb and acid. To assess whether luminal alkalinization abrogates proteinuria-induced tubular damage, we studied a mouse model of protein-overload nephropathy. NaHCO(3) feeding selectively alkalinized the urine and dramatically attenuated the accumulation of O(2)(·-)-induced DNA damage and proximal tubular injury. Overall, these observations suggest that luminal acidity aggravates proteinuria-induced tubular damage and that modulation of this acidic environment may hold potential as a therapeutic target for proteinuric kidney disease.

Ammonia production and secretion by S3 proximal tubule segments from acidotic mice: role of ANG II

ANG II has potent effects on ammonia production and secretion rates by the proximal tubule and is found in substantial concentrations in the lumen of the proximal tubule in vivo. Because our previous studies demonstrated that acid loading enhanced the stimulatory effects of ANG II on ammonia production and secretion by S2 proximal tubule segments, we examined the effect of ANG II on ammonia production and secretion by isolated, perfused S3 segments from nonacidotic control mice and acidotic mice given NH4Cl for 7 days. In the absence of ANG II, ammonia production and secretion rates were no different in S3 segments from acidotic and control mice. By contrast, when ANG II was present in the luminal perfusion solution, ammonia production and secretion rates were stimulated, in a losartan-inhibitable manner, to a greater extent in S3 segments from acidotic mice. Ammonia secretion rates in S3 segments were largely inhibited by perfusion with a low-sodium solution containing amiloride in the presence or absence of ANG II. These results demonstrated that isolated, perfused mouse S3 proximal tubule segments produce and secrete ammonia, that NH4Cl-induced acidosis does not affect the basal rates of ammonia production and secretion, and that ANG II, added to the luminal fluid, stimulates ammonia production and secretion to a greater extent in S3 segments from acidotic mice. These findings suggest that S3 segments, in the presence of ANG II, can contribute to the enhanced renal excretion that occurs with acid loading.

Carbonic anhydrase IV is expressed in H(+)-secreting cells of rabbit kidney

Carbonic anhydrase (CA) IV is a membrane-bound enzyme that catalyzes the dehydration of carbonic acid to CO(2) and water. Using peptides from each end of the deduced rabbit CA IV amino acid sequence, we generated a goat anti-rabbit CA IV antibody, which was used for immunoblotting and immunohistochemical analysis. CA IV was expressed in a variety of organs including spleen, heart, lung, skeletal muscle, colon, and kidney. Rabbit kidney CA IV had two N-glycosylation sites and was sialated, the apparent molecular mass increasing by at least 11 to approximately 45 kDa in the cortex. Medullary CA IV was much more heavily glycosylated than CA IV from cortex or any other organ, such modifications increasing the molecular mass by at least 20 kDa. CA IV was expressed on the apical and basolateral membranes of proximal tubules with expression levels on the order of S2 > S1 > S3 = 0. Because CA IV is believed to be anchored to the apical membrane by glycosylphosphatidylinositol, the presence of basolateral CA IV suggests an alternative mechanism. CA IV was localized on the apical membranes of outer medullary collecting duct cells of the inner stripe and inner medullary collecting duct cells, as well as on alpha-intercalated cells. However, CA IV was not expressed by beta-intercalated cells, glomeruli, distal tubule, or Henle's loop cells. Thus CA IV was expressed by H(+)-secreting cells of the rabbit kidney, suggesting an important role for CA IV in urinary acidification.

Mechanism of apical and basolateral Na(+)-independent Cl-/base exchange in the rabbit superficial proximal straight tubule

The present study was undertaken to determine the magnitude and mechanism of base transport via the apical and basolateral Na(+)-independent Cl-/base exchangers in rabbit isolated perfused superficial S2 proximal tubules. The results demonstrate that there is an apical Na(+)-independent Cl-/base exchanger on both membranes. HCO3- fails to stimulate apical Cl-/base exchange in contrast to the basolateral exchanger. Inhibition of endogenous HCO3- production does not alter the rate of apical Cl-/base exchange in Hepes-buffered solutions. Both exchangers are inhibited by H2DIDS and furosemide; however, the basolateral anion exchanger is more sensitive to these inhibitors. The results indicate that the apical and basolateral Cl-/base exchangers differ in their transport properties and are able to transport base equivalents in the absence of formate. The formate concentration in rabbit arterial serum is approximately 6 microM and in vitro tubule formate production is < 0.6 pmol/min per mm. Formate in the micromolar range stimulates Jv in a dose-dependent manner in the absence of a transepithelial Na+ and Cl- gradient and without a measurable effect on Cl(-)-induced equivalent base flux. Apical formic acid recycling cannot be an important component of any cell model, which accounts for formic acid stimulation of transcellular NaCl transport in the rabbit superficial S2 proximal tubule. We propose that transcellular NaCl transport in this nephron segment is mediated by an apical Na+/H+ exchanger in parallel with a Cl-/OH- exchanger and that the secreted H+ and OH- ions form H2O in the tubule lumen.

Weak acid permeability of a villous membrane: formic acid transport across rat proximal tubule

Chloride/formate exchange, in parallel with Na+/H+ exchange and nonionic diffusion of H2CO2, has been proposed as a mechanism of electroneutral transcellular Cl- reabsorption by the proximal tubule. However, the measured brush border H2CO2 permeability of the rat proximal tubule is at least an order of magnitude too low to support sufficient H2CO2 recycling. To investigate the possibility that an unstirred layer within the brush border might depress the measured H2CO2 permeability, we constructed a mathematical model of a villous membrane. Axial fluxes along villous and intervillous spaces were specified by Nernst-Planck diffusion equations. Model parameters were set to achieve agreement with ion and water fluxes measured in the rat proximal tubule. The equations were solved numerically to generate steady-state concentration profiles in the villous and intervillous spaces. An apparent brush border H2CO2 permeability was determined by perturbing luminal [H2CO2] and calculating the change in H2CO2 flux. Overall, the ratio of apparent brush border H2CO2 permeability to cell membrane H2CO2 permeability was greater than 90%. Contributing to the small decrease in apparent permeability are finite diffusion coefficients, folding of the membrane, and acidification of the luminal solution. An approximate analysis of this system shows the critical parameters of brush border formate transport to be the actual membrane H2CO2 permeability, and the diffusion coefficients of HCO2- and HCO3-. Nevertheless, decreasing the diffusion coefficients by one order of magnitude failed to depress apparent brush border H2CO2 permeability by more than an additional 25%. We conclude that although permeability is systematically underestimated across a villous membrane, unstirred layer effects in the brush border are still too small to resolve the discrepancy between the measured value of H2CO2 permeability and the value needed to allow recycling.

Electrophysiology of ammonia transport in renal straight proximal tubules

To test for electrogenic transport of ammonium ions in straight proximal renal tubules, isolated perfused tubules have been exposed to peritubular ammonium ions during continuous recording of cell membrane potential. As a result, 20 mmol/liter NH4+ leads to a rapid, reversible depolarization of the cell membrane by 9.0 +/- 0.3 mV (N = 86). This depolarization is not significantly affected by 10 mmol/liter barium or 0.1 mmol/liter amiloride on both sides of the epithelium, but is significantly blunted by omission of extracellular bicarbonate and CO2 (3.8 +/- 0.4 mV, N = 9), by 1 mmol/liter acetazolamide (4.3 +/- 0.3 mV, N = 11), by 1 mmol/liter peritubular amiloride (4.3 +/- 1.1 mV, N = 7), by 1 mmol/liter SITS (5.7 +/- 0.4 mV, N = 6), and by replacement of extracellular sodium with choline (4.7 +/- 0.5 mV, N = 8). In the presence of both amiloride (1 mmol/liter) and acetazolamide (1 mmol/liter) in the bath, the NH4+ induced depolarization is completely abolished. Furthermore, the combined omission of bicarbonate and addition of 10 mmol/liter barium eliminates the NH4+ induced depolarization. About 50% of the depolarization can be explained by enhanced electrogenic bicarbonate exit due to the intracellular alkalosis. The other 50% is explained by amiloride and barium sensitive electrogenic entry of NH4+ into the cell.

Reclamation of filtered bicarbonate

Approximately 85% of the filtered bicarbonate load is reabsorbed in the proximal convoluted tubule. Transport in this segment displays saturation kinetics, and exhibits a higher capacity for reabsorption in the earliest portion. Reclamation of bicarbonate is highly regulated in the proximal tubule: an increase in luminal [HCO3-], flow rate and arterial PCO2 increase, while alkalinization of the peritubular surface inhibits bicarbonate absorption. Angiotensin II also appears to regulate bicarbonate transport, especially in the S1 segment. The majority of the filtered bicarbonate load which escapes reabsorption in the proximal tubule is reabsorbed in the thick ascending limb of Henle's loop. Bicarbonate reclamation in this segment is enhanced by luminal [HCO3-] and furosemide, and by chronic metabolic acidosis and increased dietary sodium intake. Amiloride, AVP and glucagon inhibit absorption in the thick ascending limb.

Localization of membrane-associated carbonic anhydrase type IV in kidney epithelial cells

Rat carbonic anhydrase (CA) IV was purified by affinity chromatography and used to produce a specific antiserum in rabbits for immunolocalization studies in rat kidney. CA IV was localized in apical plasma membranes of the proximal convoluted tubule and the thick ascending limb of Henle. Both of these segments are involved in bicarbonate reabsorption in the rat. Immunofluorescent staining of the brush border was faint in the S1 segment, greatest in the S2 segment, and absent from the S3 segment of the proximal tubule. CA IV was also detected in the basolateral plasma membrane of proximal-tubule and thick-ascending-limb epithelial cells by immunofluorescence and immunoelectron microscopy. In the proximal tubule, an extracellular membrane CA had been previously suggested on the basis of electrophysiological studies. CA IV was not detected in intercalated cells of the collecting ducts. These cells contain, in contrast, abundant cytosolic CA II. Thus, the distribution of CA IV is quite distinct from that of CA II; it corresponds with the localization of an isoenzyme(s) that did not stain with antibodies against CA II but that was revealed by histochemical-staining procedures. We conclude that the apical CA IV is the luminal CA responsible for bicarbonate reabsorption in the proximal tubule and the thick ascending limb in the rat kidney. These studies also suggest that CA IV plays a role in bicarbonate transport across the basolateral plasma membrane in these two segments of the rat nephron.

Microelectrode characterization of the basolateral membrane of rabbit S3 proximal tubule

The purpose of this study was to characterize the basolateral membrane of the S3 segment of the rabbit proximal tubule using conventional and ion-selective microelectrodes. When compared with results from S1 and S2 segments, S3 cells under control conditions have a more negative basolateral membrane potential (Vbl = -69 mV), a higher relative potassium conductance (tK = 0.6), lower intracellular Na+ activity (ANa = 18.4 mM), and higher intracellular K+ activity (AK = 67.8 mM). No evidence for a conductive sodium-dependent or sodium-independent HCO3- pathway could be demonstrated. The basolateral Na-K pump is inhibited by 10(-4) M ouabain and bath perfusion with a potassium-free (0-K) solution. 0-K perfusion results in ANa = 64.8 mM, AK = 18.5 mM, and Vbl = -28 mV. Basolateral potassium channels are blocked by barium and by acidification of the bathing medium. The relative K+ conductance, as evaluated by increasing bath K+ to 17 mM, is dependent upon the resting Vbl in both S2 and S3 cells. In summary, the basolateral membrane of S3 cells contains a pump-leak system with similar properties to S1 and S2 proximal tubule cells. The absence of conductive bicarbonate pathways results in a hyperpolarized cell and larger Na+ and K+ gradients across the cell borders, which will influence the transport properties and intracellular ion activities in this tubule segment.

Basolateral membrane Na+/H+ antiport, Na+/base cotransport, and Na+-independent Cl-/base exchange in the rabbit S3 proximal tubule

The basolateral membrane Na+ and Cl(-)-dependent acid-base transport processes were studied in the isolated perfused rabbit S3 proximal straight tubule. Intracellular pH (pHi) was measured with 2'7'-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) and a microfluorometer coupled to the tubule perfusion apparatus. Reduction of basolateral HCO3- from 25 to 5 mM caused pHi to decrease at a rate of 0.81 pH/min. Approximately 50% of this rate was Na+-dependent, 30% Cl(-)-dependent and 20% Na+ and Cl(-)-independent. Two basolateral Na+-dependent acid base transport pathways were detected: (a) an amiloride-sensitive Na+/H+ antiporter and (b) a stilbene-sensitive Na+/base cotransporter. No evidence was found for a Na+-dependent Cl-/base exchanger. The Cl(-)-dependent component of basolateral base efflux was mediated by a stilbene-sensitive Na+-independent Cl-/base exchange pathway. The results suggest that the acid base transport pathways of the basolateral membrane of the S3 proximal tubule differ from more proximal nephron segments.

Apical and basolateral Na+/H+ exchange in the rabbit outer medullary thin descending limb of Henle: role in intracellular pH regulation

The present study was designed to investigate the apical and basolateral transport processes responsible for intracellular pH regulation in the thin descending limb of Henle. Rabbit thin descending limbs of long-loop nephrons were perfused in vitro and intracellular pH (pHi) was measured using BCECF. Steady-state pHi in HEPES buffered solutions (pH 7.4) was 7.18 +/- 0.03. Following the removal of luminal Na+, pHi decreased at a rate of 1.96 +/- 0.37 pH/min. In the presence of luminal amiloride (1 mM), the rate of decrease of pHi was significantly less, 0.73 +/- 0.18 pH/min. Steady-state pHi decreased 0.18 pH units following the addition of amiloride (1 mM) to the lumen (Na+ 140 mM lumen and bath). When Na+ was removed from the basolateral side of the tubule, pHi decreased at a rate of 0.49 +/- 0.05 pH/min. The rate of decrease of pHi was significantly less in the presence of 1 mM basolateral amiloride, 0.29 +/- 0.04 pH/min. Addition of 1 mM amiloride to the basolateral side (Na+ 140 mM lumen and bath) caused steady-state pHi to decrease significantly by 0.06 pH units. When pHi was acutely decreased to 5.87 +/- 0.02 following NH4Cl removal (lumen, bath), pHi failed to recover in the absence of Na+ (lumen, bath). Addition of 140 mM Na+ to the lumen caused pHi to recover at a rate of 2.17 +/- 0.59 pH/min. The rate of pHi recovery was inhibited 93% by 1 mM luminal amiloride. When 140 mM Na+ was added to the basolateral side, pHi recovered only partially at 0.38 +/- 0.07 pH/min. Addition of 1 mM basolateral amiloride inhibited the recovery of pHi by 97%. The results demonstrate that the rabbit thin descending limb of long-loop nephrons possesses apical and basolateral Na+/N+ antiporters. In the steady state, the rate of Na+-dependent H+ flux across the apical antiporter exceeds the rate of Na+-dependent H+ flux via the basolateral antiporter. Recovery of pHi following acute intracellular acidification is Na+ dependent and mediated primarily by the luminal antiporter.

Calcium and cyclic adenosine monophosphate as second messengers for vasopressin in the rat inner medullary collecting duct

Vasopressin increases both the urea permeability and osmotic water permeability in the terminal part of the renal inner medullary collecting duct (terminal IMCD). To identify the second messengers that mediate these responses, we measured urea permeability, osmotic water permeability, intracellular calcium concentration, and cyclic AMP accumulation in isolated terminal IMCDs. After addition of vasopressin, a transient rise in intracellular calcium occurred that was coincident with increases in cyclic AMP accumulation and urea permeability. Half-maximal increases in urea permeability and osmotic water permeability occurred with 0.01 nM vasopressin. The threshold concentration for a measurable increase in cyclic AMP accumulation was approximately 0.01 nM, while measurable increases in intracellular calcium required much higher vasopressin concentrations (greater than 0.1 nM). Exogenous cyclic AMP (1 mM 8-Br-cAMP) mimicked the effect of vasopressin on urea permeability but did not produce a measurable change in intracellular calcium concentration.

CONCLUSIONS

(a) Cyclic AMP is the second messenger that mediates the urea permeability response to vasopressin in the rat terminal IMCD. (b) Vasopressin increases the intracellular calcium concentration in the rat terminal IMCD, but the physiological role of this response is not yet known.

Effects of potassium on ammonia transport by medullary thick ascending limb of the rat

Renal ammonium excretion is increased by potassium depletion and reduced by potassium loading. To determine whether changes in potassium concentration would alter ammonia transport in the medullary thick ascending limb (MAL), tubules from rats were perfused in vitro and effects of changes in K concentration within the physiological range (4-24 mM) were evaluated. Increasing K concentration from 4 to 24 mM in perfusate and bath inhibited total ammonia absorption by 50% and reduced the steady-state transepithelial NH+4 concentration gradient. The inhibition of total ammonia absorption was reversible and occurred when K replaced either Na or N-methyl-D-glucamine. Increasing K concentration in the luminal perfusate alone gave similar inhibition of total ammonia absorption. At 1-2 nl/min per mm perfusion rate, increasing K concentration in perfusion and bathing solutions had no significant effect on transepithelial voltage. With either 4 or 24 mM K in perfusate and bath, an increase in luminal perfusion rate markedly increased total ammonia absorption. Thus, both potassium concentration and luminal flow rate are important factors capable of regulating total ammonia transport by the MAL. Changes in systemic potassium balance may influence renal ammonium excretion by affecting NH+4 absorption in the MAL and altering the transfer of ammonia from loops of Henle to medullary collecting ducts.

Apical Na+/H+ antiporter and glycolysis-dependent H+-ATPase regulate intracellular pH in the rabbit S3 proximal tubule

The apical transport processes responsible for proton secretion were studied in the isolated perfused rabbit S3 proximal tubule. Intracellular pH (pHi) was measured with the pH dye, 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. Steady state pHi in S3 tubules in nominally HCO3(-)-free solutions was 7.08 +/- 0.03. Removal of Na+ (lumen) caused a decrease in pHi of 0.34 +/- 0.06 pH/min. The decrease in pHi was inhibited 62% by 1 mM amiloride (lumen) and was unaffected by 50 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (lumen) and Cl- removal (lumen, bath). After a brief exposure to 20 mM NH4Cl, pHi fell by approximately 0.7 and recovered at a rate of 0.89 +/- 0.15 pH/min in the nominal absence of Na+, HCO3-, organic anions, and SO4(2-) (lumen, bath). 1 mM N,N'-dicyclohexylcarbodiimide (lumen), 1 mM N-ethylmaleimide (lumen), 0.5 mM colchicine (bath), and 0.5 mM iodoacetic acid (lumen, bath) inhibited the Na+-independent pHi recovery rate by 73%, 55%, 77%, and 86%, respectively, whereas 1 mM KCN (lumen, bath) did not inhibit pHi recovery. Reduction of intracellular, but not extracellular chloride, also decreased the Na+-independent pHi recovery rate. In conclusion, the S3 proximal tubule has an apical Na+/H+ antiporter with a Michaelis constant for Na+ of 29 mM and a maximum velocity of 0.47 pH/min. S3 tubules also possess a plasma membrane H+-ATPase that can regulate pHi, has a requirement for intracellular chloride, and utilizes ATP derived primarily from glycolysis.