Handling of oxalate by the rat kidney

Oxalate as a potent promoter of kidney stone formation

Kidney stones are among the most prevalent urological diseases, with a high incidence and recurrence rate. Treating kidney stones has been greatly improved by the development of various minimally invasive techniques. Currently, stone treatment is relatively mature. However, most current treatment methods are limited to stones and cannot effectively reduce their incidence and recurrence. Therefore, preventing disease occurrence, development, and recurrence after treatment, has become an urgent issue. The etiology and pathogenesis of stone formation are key factors in resolving this issue. More than 80% of kidney stones are calcium oxalate stones. Several studies have studied the formation mechanism of stones from the metabolism of urinary calcium, but there are few studies on oxalate, which plays an equally important role in stone formation. Oxalate and calcium play equally important roles in calcium oxalate stones, whereas the metabolism and excretion disorders of oxalate play a crucial role in their occurrence. Therefore, starting from the relationship between renal calculi and oxalate metabolism, this work reviews the occurrence of renal calculi, oxalate absorption, metabolism, and excretion mechanisms, focusing on the key role of SLC26A6 in oxalate excretion and the regulatory mechanism of SLC26A6 in oxalate transport. This review provides some new clues for the mechanism of kidney stones from the perspective of oxalate to improve the understanding of the role of oxalate in the formation of kidney stones and to provide suggestions for reducing the incidence and recurrence rate of kidney stones.

The role of NHE3 (Slc9a3) in oxalate and sodium transport by mouse intestine and regulation by cAMP

Intestinal oxalate transport involves Cl⁻/HCO₃⁻ exchangers but how this transport is regulated is not currently known. NHE3 (Slc9a3), an apical Na⁺/H⁺ exchanger, is an established target for regulation of electroneutral NaCl absorption working in concert with Cl⁻/HCO₃⁻ exchangers. To test whether NHE3 could be involved in regulation of intestinal oxalate transport and renal oxalate handling we compared urinary oxalate excretion rates and intestinal transepithelial fluxes of ¹⁴C‐oxalate and ²²Na⁺ between NHE3 KO and wild‐type (WT) mice. NHE3 KO kidneys had lower creatinine clearance suggesting reduced GFR, but urinary oxalate excretion rates (µmol/24 h) were similar compared to the WT but doubled when expressed as a ratio of creatinine. Intestinal transepithelial fluxes of ¹⁴C‐oxalate and ²²Na⁺ were measured in the distal ileum, cecum, and distal colon. The absence of NHE3 did not affect basal net transport rates of oxalate or sodium across any intestinal section examined. Stimulation of intracellular cAMP with forskolin (FSK) and 3‐isobutyl‐1‐methylxanthine (IBMX) led to an increase in net oxalate secretion in the WT distal ileum and cecum and inhibition of sodium absorption in the cecum and distal colon. In NHE3 KO cecum, cAMP stimulation of oxalate secretion was impaired suggesting the possibility of a role for NHE3 in this process. Although, there is little evidence for a role of NHE3 in basal intestinal oxalate fluxes, NHE3 may be important for cAMP stimulation of oxalate in the cecum and for renal handling of oxalate.

Characterization of renal NaCl and oxalate transport in Slc26a6-/- mice

The apical membrane Cl^-/oxalate exchanger SLC26A6 has been demonstrated to play a role in proximal tubule NaCl transport based on studies in microperfused tubules. The present study is directed at characterizing the role of SLC26A6 in NaCl homeostasis in vivo under physiological conditions. Free-flow micropuncture studies revealed that volume and Cl^- absorption were similar in surface proximal tubules of wild-type and Slc26a6^-/- mice. Moreover, the increments in urine flow rate and sodium excretion following thiazide and furosemide infusion were identical in wild-type and Slc26a6^-/- mice, indicating no difference in NaCl delivery out of the proximal tubule. The absence of an effect of deletion of SLC26A6 on NaCl homeostasis was further supported by the absence of lower blood pressure in Slc26a6^-/- compared with wild-type mice on normal or low-salt diets. Moreover, raising plasma and urine oxalate by feeding mice a diet enriched in soluble oxalate did not affect mean blood pressure. In contrast to the lack of effect of SLC26A6 deletion on NaCl homeostasis, fractional excretion of oxalate was reduced from 1.6 in wild-type mice to 0.7 in Slc26a6^-/- mice. We conclude that, although SLC26A6 is dispensable for renal NaCl homeostasis, it is required for net renal secretion of oxalate.

Hyperoxaluric rats do not exhibit alterations in renal expression patterns of Slc26a1 (SAT1) mRNA or protein

Little is known about oxalate transport in renal epithelia under basal conditions, let alone in hyperoxaluria when the capacity for renal oxalate excretion is increased. Sulfate anion transporter 1 (SAT1, Slc26a1) is considered to be a major basolateral anion-oxalate exchanger in the proximal tubule and we hypothesized its expression may correlate with urinary oxalate excretion. We quantified changes in the renal expression of SAT1 mRNA and protein in two rat models, one with hyperoxaluria (HYP) and one with renal insufficiency (HRF) induced by hyperoxaluria. The hyperoxaluria observed in the HYP group could not simply be ascribed to changes in SAT1 mRNA or protein abundance. However, when hyperoxaluria was accompanied by renal insufficiency, significant reductions in SAT1 mRNA and protein were detected in medullary and papillary tissue. Together, the results indicate that transcriptional modulation of the SAT1 gene is not a significant component of the hyperoxaluria observed in these rat models.

Cholinergic signaling inhibits oxalate transport by human intestinal T84 cells

Urolithiasis remains a very common disease in Western countries. Seventy to eighty percent of kidney stones are composed of calcium oxalate, and minor changes in urinary oxalate affect stone risk. Intestinal oxalate secretion mediated by anion exchanger SLC26A6 plays a major constitutive role in limiting net absorption of ingested oxalate, thereby preventing hyperoxaluria and calcium oxalate urolithiasis. Using the relatively selective PKC-δ inhibitor rottlerin, we had previously found that PKC-δ activation inhibits Slc26a6 activity in mouse duodenal tissue. To identify a model system to study physiologic agonists upstream of PKC-δ, we characterized the human intestinal cell line T84. Knockdown studies demonstrated that endogenous SLC26A6 mediates most of the oxalate transport by T84 cells. Cholinergic stimulation with carbachol modulates intestinal ion transport through signaling pathways including PKC activation. We therefore examined whether carbachol affects oxalate transport in T84 cells. We found that carbachol significantly inhibited oxalate transport by T84 cells, an effect blocked by rottlerin. Carbachol also led to significant translocation of PKC-δ from the cytosol to the membrane of T84 cells. Using pharmacological inhibitors, we observed that carbachol inhibits oxalate transport through the M(3) muscarinic receptor and phospholipase C. Utilizing the Src inhibitor PP2 and phosphorylation studies, we found that the observed regulation downstream of PKC-δ is partially mediated by c-Src. Biotinylation studies revealed that carbachol inhibits oxalate transport by reducing SLC26A6 surface expression. We conclude that carbachol negatively regulates oxalate transport by reducing SLC26A6 surface expression in T84 cells through signaling pathways including the M(3) muscarinic receptor, phospholipase C, PKC-δ, and c-Src.

Glyoxylate is a substrate of the sulfate-oxalate exchanger, sat-1, and increases its expression in HepG2 cells

BACKGROUND & AIMS

Hyperoxaluria is a major problem causing nephrolithiasis. Little is known about the regulation of oxalate transport from the liver, the main organ for oxalate synthesis, into the circulation. Since the sulfate anion transporter-1(sat-1) is present in the sinusoidal membrane of hepatocytes and translocates oxalate, its impact on increased oxalate synthesis was studied.

METHODS

Sat-1 expressing oocytes were used for cis-inhibition, trans-stimulation, and efflux experiments with labelled sulfate and oxalate to demonstrate the interactions of oxalate, glyoxylate, and glycolate with sat-1. HepG2 cells were incubated with oxalate and its precursors (glycine, hydroxyproline, glyoxylate, and glycolate). Changes in endogenous sat-1 mRNA-expression were examined using real-time PCR. After incubation of HepG2 cells in glyoxylate, sat-1 protein-expression was analysed by Western blotting, and sulfate uptake into HepG2 cells was measured. RT-PCR was used to screen for mRNA of other transporters.

RESULTS

While oxalate and glyoxylate inhibited sulfate uptake, glycolate did not. Sulfate and oxalate uptake were trans-stimulated by glyoxylate but not by glycolate. Glyoxylate enhanced sulfate efflux. Glyoxylate was the only oxalate precursor stimulating sat-1 mRNA-expression. After incubation of HepG2 cells in glyoxylate, both sat-1 protein-expression and sulfate uptake into the cells increased. mRNA-expression of other transporters in HepG2 cells was not affected by glyoxylate treatment.

CONCLUSIONS

The oxalate precursor glyoxylate was identified as a substrate of sat-1. Upregulated expression of sat-1 mRNA and of a functional sat-1 protein indicates that glyoxylate may be responsible for the elevated oxalate release from hepatocytes observed in hyperoxaluria.

Urolithiasis and hepatotoxicity are linked to the anion transporter Sat1 in mice

Urolithiasis, a condition in which stones are present in the urinary system, including the kidneys and bladder, is a poorly understood yet common disorder worldwide that leads to significant health care costs, morbidity, and work loss. Acetaminophen-induced liver damage is a major cause of death in patients with acute liver failure. Kidney and urinary stones and liver toxicity are disturbances linked to alterations in oxalate and sulfate homeostasis, respectively. The sulfate anion transporter-1 (Sat1; also known as Slc26a1) mediates epithelial transport of oxalate and sulfate, and its localization in the kidney, liver, and intestine suggests that it may play a role in oxalate and sulfate homeostasis. To determine the physiological roles of Sat1, we created Sat1-/- mice by gene disruption. These mice exhibited hyperoxaluria with hyperoxalemia, nephrocalcinosis, and calcium oxalate stones in their renal tubules and bladder. Sat1-/- mice also displayed hypersulfaturia, hyposulfatemia, and enhanced acetaminophen-induced liver toxicity. These data suggest that Sat1 regulates both oxalate and sulfate homeostasis and may be critical to the development of calcium oxalate urolithiasis and hepatotoxicity.

Oxalate in renal stone disease: the terminal metabolite that just won't go away

The incidence of calcium oxalate nephrolithiasis in the US has been increasing throughout the past three decades. Biopsy studies show that both calcium oxalate nephrolithiasis and nephrocalcinosis probably occur by different mechanisms in different subsets of patients. Before more-effective medical therapies can be developed for these conditions, we must understand the mechanisms governing the transport and excretion of oxalate and the interactions of the ion in general and renal physiology. Blood oxalate derives from diet, degradation of ascorbate, and production by the liver and erythrocytes. In mammals, oxalate is a terminal metabolite that must be excreted or sequestered. The kidneys are the primary route of excretion and the site of oxalate's only known function. Oxalate stimulates the uptake of chloride, water, and sodium by the proximal tubule through the exchange of oxalate for sulfate or chloride via the solute carrier SLC26A6. Fecal excretion of oxalate is stimulated by hyperoxalemia in rodents, but no similar phenomenon has been observed in humans. Studies in which rats were treated with (14)C-oxalate have shown that less than 2% of a chronic oxalate load accumulates in the internal organs, plasma, and skeleton. These studies have also demonstrated that there is interindividual variability in the accumulation of oxalate, especially by the kidney. This Review summarizes the transport and function of oxalate in mammalian physiology and the ion's potential roles in nephrolithiasis and nephrocalcinosis.

The effect of L-arginine methyl ester on indices of free radical involvement in a rat model of experimental nephrocalcinosis

The aim of this study was to test the effect of L: -arginine methyl ester (L-Arg) on indices of free radical involvement in a rat model of experimental nephrocalcinosis. Twenty-eight Sprague-Dawley rats were randomized into four groups of seven. The first group (G1), the sham-control group received pure distilled drinking water. The second group (G2) received drinking water containing 0.7% ethylene glycol (EG) in distilled water for 3 weeks. The third group (G3) received drinking water containing 0.7% EG in distilled water for 3 weeks and L-Arg was administered for 3 weeks. The fourth group (G4) received drinking water containing 0.7% EG in distilled water for 3 weeks and L-NAME was administered for 3 weeks. Urine and aortic blood was collected to determine some parameters. The kidneys were also removed for histological examination. The increase in blood urea nitrogen, serum creatinine, K(+), Mg(2+ )and uric acid were mild in group 3 compared with the groups 2 and 4. The urinary concentrations of Na(+), K(+), Mg(2+) and uric acid were noticed to be similar among the groups. However, Ca(2+ )and oxalate excretion were significantly higher in groups 2, 3 and 4 than in group 1. The mean values of SOD, CAT and GSH-Px values were significantly increased in group 3 when compared to groups 2 and 4. Presence of aggregated urinary crystals was clearer in experimental groups compared to group 1. The tubular dilatation, epithelial degeneration and lymphocytic infiltration were significantly found in groups 2 and 4. Mild tissue damage was observed in L-Arg-pretreated rats. Under polarized light microscope intense crystals in the cortex and medulla were observed in the kidney of group 2 and 4 and moderate crystals were noticed in group 3. In conclusion, L-Arg supplementation may decrease free radicals and tubulary membrane injury in nephrocalcinosis due to infiltrating leukocytes and decreased antioxidant enzyme activities in rats fed with EG diet.

[Calcium oxalate stones and hyperoxaluria. What is certain? What is new?]

Approximately 4 million Germans suffer from stone disease. In the majority of cases (70-75%) it is calcium oxalate. Its pathophysiology is complex and comprises disorders such as hypercalciuria, hyperoxaluria, hypocitraturia, hyperuricosuria, and hypomagnesuria. These biochemical changes in urine are well known as "classic" risk factors of calcium oxalate stone formation. However, studies in the last decade showed that calcium oxalate stones are strongly related with other diseases or disorders such as overweight, hypertension, or a lack of oxalate-degrading bacteria in the gut. The evidence for these "new" risk factors in the literature is very strong. It is particularly important in regard to effective treatment and aftercare of patients with calcium oxalate stones to be familiar with both the "classic" and the new risk factors.

The impact of dietary oxalate on kidney stone formation

The role of dietary oxalate in calcium oxalate kidney stone formation remains unclear. However, due to the risk for stone disease that is associated with a low calcium intake, dietary oxalate is believed to be an important contributing factor. In this review, we have examined the available evidence related to the ingestion of dietary oxalate, its intestinal absorption, and its handling by the kidney. The only difference identified to date between normal individuals and those who form stones is in the intestinal absorption of oxalate. Differences in dietary oxalate intake and in renal oxalate excretion are two other parameters that are likely to receive close scrutiny in the near future, because the research tools required for these investigations are now available. Such research, together with more extensive examinations of intestinal oxalate absorption, should help clarify the role of dietary oxalate in stone formation.

Lipid peroxidation and its correlations with urinary levels of oxalate, citric acid, and osteopontin in patients with renal calcium oxalate stones

OBJECTIVES

To determine whether lipid peroxidation plays a role in patients with calcium oxalate kidney stones and to determine the correlation of lipid peroxidation with tubular damage and the major urinary risk factors. We also used the isoenzymes of glutathione S-transferase (GST) to examine which parts of the renal tubules were injured in patients with renal stones.

METHODS

This clinical study included two study groups. Group 1 included 32 normal volunteers, and group 2 included 32 patients with calcium oxalate kidney stones. A 24-hour urine sample was collected from each subject, and the levels of Ca, P, Mg, oxalate, citrate, N-acetyl-beta-glucosaminidase (NAG), beta-galactosidase (GAL), alphaGST, piGST, osteopontin (OPN), thiobarbituric acid-reactive substances (TBARS), and malondialdehyde (MDA) were examined.

RESULTS

Hyperoxaluria, hypocitraturia, and low urinary OPN were the major abnormalities found in the patients with stones. Elevated urinary alphaGST, NAG, and GAL were also noted in the patients with stones; however, urinary piGST showed no statistically significant difference compared with the controls. Urinary TBARS and MDA had statistically significant correlations with alphaGST, GAL, NAG, Ca, and oxalate, but had no correlation with piGST, citrate, OPN, Mg, and P. Urinary citrate had a negative, linear, and statistically significant correlation with alphaGST, GAL, and NAG.

CONCLUSIONS

Lipid peroxidation correlated with hyperoxaluria and renal tubular damage, indicating that hyperoxaluria can induce tubular cell injury and that this injury may be due to the production of free radicals in patients with calcium oxalate stones. Renal tubular damage in patients with stones may be limited to the proximal tubules.

Concentration gradient of oxalate from cortex to papilla in rat kidney

BACKGROUND

The kidney eliminates the major fraction of plasma oxalate. It is well known that oxalate is freely filtered by glomeruli and secreted by the proximal tubules. However, the renal handling of oxalate in distal nephrons, which is considered as playing an important role in stone formation, remains obscure.

METHODS

At 15-180 min after intravenous injection of 14C-oxalate to rats, the intrarenal localization of radioactivity was quantitatively measured by the radioluminographic method using a bioimaging analyzer. Tissue radioactivity was compared with plasma, and urinary radioactivities were measured by a liquid scintillation counter. The control study was conducted with 14C-inulin.

RESULTS

The radioactivity of 14C-oxalate in the papilla was 10 times greater than in the cortex and eight times greater than in the medulla 180 min after injection when almost no radioactivity was present in the urine. In contrast, the radioactivity of 14C-inulin was nine times less in the papilla than in the cortex at the same time.

CONCLUSION

Oxalate remains in the renal papilla for an extended period. This accumulation of oxalate may be attributed to calcium oxalate crystal fixation along the deep nephron which is considered to be the first step of stone formation.

Effects of sulfate and chloride on three separate oxalate transporters reconstituted from rabbit renal cortex

Understanding the mechanism of sulfate-dependent, oxalate-stimulated chloride reabsorption in the mammalian proximal tubule is complicated by the presence of multiple oxalate and sulfate transport pathways. Accordingly, we developed a method of reconstituting functional oxalate transport from the rabbit renal cortex so that the individual transporters might be examined. Solubilized microvillus membrane proteins were separated by hydroxyapatite chromatography and then reconstituted into proteoliposomes. Two peaks of oxalate/oxalate exchange activity were observed. Sulfate (10 mM) cis-inhibits oxalate transport in the early peak by 93% and in the later peak by 41%. In contrast, 20 mM chloride inhibits oxalate/oxalate exchange by only 32% in the early peak but inhibits oxalate exchange by 70% in the later peak. Oxalate-stimulated sulfate uptake was observed in the early fractions but not in the later fractions. These data are consistent with the recovery of the sulfate/oxalate exchanger in the early hydroxyapatite fractions and the chloride/oxalate exchanger in the later fractions. The basolateral membrane sulfate/oxalate exchanger was also reconstituted. The reconstituted basolateral and apical membrane sulfate/oxalate exchangers demonstrate nearly identical patterns of substrate specificities. However, 98% of apical sulfate/oxalate exchange activity is lost following exposure to octylglucoside at room temperature, whereas the basolateral sulfate/oxalate exchange activity was reduced 67% (P < 0.05). In conclusion, functional reconstitution of solubilized membrane proteins demonstrates that apical membrane chloride/oxalate exchange and sulfate/oxalate exchange are mediated by different transport proteins. Apical and basolateral sulfate/oxalate exchange may also represent transport on two separate exchangers.

Oxalate transport and calcium oxalate renal stone disease

Hyperoxaluria is considered to play a crucial role in calcium oxalate (CaOx) renal stone disease. The amount of oxalate excreted into the urine depends on intestinal absorption, endogenous production, renal clearance and renal tubular transport. Since a primary disorder has not been found so far in most CaOx stone formers and since oxalate is freely filtered at the glomerulus, most studies are presently focussed on alterations in epithelial oxalate transport pathways. Oxalate can be transported across an epithelium by the paracellular (passive) and transcellular (active) pathway. Oxalate transport across cellular membranes is mediated by anion-exchange transport proteins. A defect in the structure of these transport proteins could explain augmented transcellular oxalate transport. Little is known about the physiological regulation of oxalate transport. In this review cellular transport systems for oxalate will be summarized with special attention for the progress that has been made to study oxalate transport in a model of cultured renal tubule cells. Better understanding of the physiological processes that are involved in oxalate transport could yield information on the basis of which it might be possible to design new approaches for an effective treatment of CaOx stone disease.

Cell cultures and nephrolithiasis

While the physical chemistry of stone formation has been intensively studied during the last decade, it has become clear that the pathophysiology of renal stone disease cannot be explained by crystallization processes only. In recent years, evidence has emerged that the cells lining the renal tubules can have an active role in creating the conditions under which stones may develop. Since it is difficult to study these mechanisms in vivo, cultured renal tubular cells have become increasingly popular for the study of physiological and cell biological processes that are possibly linked to stone disease. In this paper, we discuss the possible contribution of cellular processes such as transepithelial oxalate transport and crystal--cell interaction to the formation of renal stones. Experimental studies that have been performed with cultured renal cells to elucidate the mechanisms involved in these processes will be summarized.

Intratubular crystallization events

Can urolithiasis start as an intratubular event? Under severe hyperoxaluric conditions in animal models at least crystal formation can. Recently models have been presented that assess the chances of crystal formation under more normal conditions. These models describe changes in fluid composition as this passes through the nephron, these conditions being simulated in in vitro experiments. It appears that under naturally occurring intratubular conditions calcium-salt crystallization takes place within the time tubular fluid normally spends in the nephron. Precipitation starts with a calcium-phosphate phase under conditions found in the thin lambs. This crystalline phase then (partly) dissolves when collecting duct conditions are used, thereby inducing formation of calcium oxalates. Under these conditions the latter increase in size by way of crystal growth and agglomeration. Large particle formation and cell adhesion can eventually result in particle retention and subsequent stone formation. Viewing urolithiasis as originally an intratubular event has consequences for in vitro experiments and treatments, which are discussed in this paper.

Oxalate transport and calcium oxalate renal stone disease

Hyperoxaluria is considered to play a crucial role in calcium oxalate (CaOx) renal stone disease. The amount of oxalate excreted into the urine depends on intestinal absorption, endogenous production, renal clearance and renal tubular transport. Since a primary disorder has not been found so far in most CaOx stone formers and since oxalate is freely filtered at the glomerulus, most studies are presently focussed on alterations in epithelial oxalate transport pathways. Oxalate can be transported across an epithelium by the paracellular (passive) and transcellular (active) pathway. Oxalate transport across cellular membranes is mediated by anion-exchange transport proteins. A defect in the structure of these transport proteins could explain augmented transcellular oxalate transport. Little is known about the physiological regulation of oxalate transport. In this review cellular transport systems for oxalate will be summarized with special attention for the progress that has been made to study oxalate transport in a model of cultured renal tubule cells. Better understanding of the physiological processes that are involved in oxalate transport could yield information on the basis of which it might be possible to design new approaches for an effective treatment of CaOx stone disease.

The efficacy of (L)-2-oxothiazolidine-4-carboxylate (OTC) and (L)-cysteine in reducing urinary oxalate excretion

The effects of orally administered (L)-cysteine and (L)-2-oxothiazolidine-4-carboxylate (OTC) on urinary oxalate excretion were investigated in male Porton rats, as (L)-cysteine has been shown to form an adduct with glyoxylate in vitro. Feeding of OTC (204 +/- 1 mg. per day) for 5 days increased urinary cyst(e)ine, p < 0.001; sulphate, p < 0.001; phosphate, p < 0.05; and calcium, p < 0.05; and decreased urinary pH, p < 0.001. In addition, OTC feeding significantly decreased urinary oxalate excretion when compared with controls, p < 0.05 (delta OTC-delta Control -4.26 +/- 1.55 nmol./day/gm.). In the 5-day period after cessation of OTC feeding, all urinary parameters returned to control levels. At the completion of this recovery period there were no significant differences in any of the clinically significant plasma parameters. When fed for 22 days (191 +/- 3 mg. per day) OTC decreased urinary oxalate compared with controls, p < 0.05 (delta OTC-delta Control -9.47 +/- 4.24 nmol./day/gm.). Other urinary parameters (uric acid, magnesium, calcium, phosphate, creatinine, pH and volume) were not significantly altered by OTC feeding. Again, at the completion of this feeding period there were no significant differences in any of the clinically significant plasma parameters. (L)-cysteine feeding for 5 days (184 +/- 10 mg. per day) increased urinary sulphate, p < 0.001; and magnesium, p < 0.05, and decreased urinary pH, p < 0.001. In addition, (L)-cysteine feeding did not significantly change urinary oxalate excretion when compared with the controls (delta(L)-Cysteine-delta Control -2.94 +/- 2.14 nmol./day/gm.). However, at the completion of this feeding period, plasma urate, p < 0.02; and glucose, p < 0.05, were decreased, and plasma potassium, p < 0.01, was increased. these results indicate that orally administered OTC is effective in reducing urinary oxalate excretion without altering plasma biochemistry. It is suggested that (L)-cysteine-glyoxylate adduct formation is the mechanism by which OTC reduces urinary oxalate excretion through a reduction in endogenous oxalate production.

Oxalate transport in a line of porcine renal epithelial cells--LLC-PK1 cells

The present studies examined oxalate handling in LLC-PK1 cells, an epithelial cell line of porcine origin. These cells appear to express transport systems for oxalate, as evidenced by the fact that uptake was saturable, time dependent and sensitive to the anion transport inhibitor DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid). Oxalate uptake in these cells was also affected by the presence of certain inorganic anions (Cl-, SO4(2-), or HCO3-) but not by organic anions (para-aminohippurate, urate, malate, phenylsuccinate, succinate). This uptake was Na independent and unaffected by changes in membrane potential but was affected by external pH, with acidic pH stimulating and alkaline pH inhibiting oxalate accumulation. These findings suggest that LLC-PK1 cells express oxalate transporters similar to those observed in the mammalian renal cortex. Further studies using these cells may prove useful in defining the conditions that govern transcellular oxalate flux in renal epithelial cells.

Effect of polyamines on glutamate dehydrogenase within permeabilized kidney-cortex mitochondria and isolated renal tubules of rabbit

The effect of polyamines on glutamate dehydrogenase [L-glutamate: NAD(P) oxidoreductase (deaminating) [EC 1.4.1.3]) activity has been studied in both permeabilized kidney-cortex mitochondria and isolated renal tubules of rabbit. Spermidine was the most potent inhibitor of glutamate synthesis in permeabilized mitochondria resulting in about 80% decrease of the enzyme activity at 5 mM concentration. Putrescine, alpha-monofluoromethylputrescine (MFMP) and (R,R)-delta-methyl-alpha-acetylenic-putrescine (MAP) were more efficient than spermine. The inhibitory action of polyamines was potentiated by an elevated NADH content in the reaction mixture. Increasing concentrations of either NH4Cl, KCl or NaCl in the incubation medium resulted in a decrease of polyamine-induced inhibition of the enzyme activity, indicating that monovalent cations can compete with polyamines for the binding site at glutamate dehydrogenase. The inhibitory action of spermidine on glutamate synthesis was abolished by 2 mM ADP or 10 mM L-leucine, allosteric activators of the enzyme, as well as on the addition of either oxalate or sulphate at 20 mM concentrations. Spermidine did not affect glutamate formation when NADH was substituted by NADPH, suggesting an importance of the NADH binding to the inhibitory site of the enzyme for a decrease of reductive amination of 2-oxoglutarate by polyamine. Although spermidine did not influence glutamate deamination in the presence of NAD+, it stimulated this process by about 70% when NAD+ was substituted by NADP+. In the presence of ADP the stimulatory effect of polyamine was not significant. The data indicate that in permeabilized rabbit kidney-cortex mitochondria the effect of polyamines on both glutamate formation and glutamate deamination via the reaction catalysed by glutamate dehydrogenase is dependent upon the coenzyme utilized by the enzyme. In the presence of NADH their inhibitory effect on the glutamate formation may be alleviated by allosteric activators of the enzyme, and concentrations of potassium, sodium, sulphate and oxalate. In isolated rabbit renal tubules incubated with 5 mM methionine sulfoximine and aminooxyacetate, in order to inhibit glutamine synthetase and aminotransferases, respectively, 5 mM spermidine decreased glutamate formation by about 30%, while putrescine and spermine did not significantly diminish the enzyme activity. In the presence of octanoate glutamate formation was reduced by about 30% by naturally occurring polyamines as well as MFMP and MAP, indicating that under these conditions NADH rather than NADPH is utilized as the coenzyme. In view of these data it is possible to suggest that polyamines may be of importance to control glutamate dehydrogenase activity under physiological conditions.

Urinary excretion of oxalate by patients with renal hypercalciuric stone disease. Effect of chronic treatment with hydrochlorothiazide

Hydrochlorothiazide is employed to reduce calcium excretion in patients with urinary stone disease secondary to renal leak hypercalciuria. Because the drug also has been reported to be a competitive inhibitor of oxalate excretion by the renal tubules, we sought to determine whether chronic use indeed affected the amount of oxalate excreted. Patients taking hydrochlorothiazide 50 mg daily did not have a statistically significant reduction in twenty-four-hour urinary oxalate on their customary diets (pretreatment 37 +/- 3 mg/day [mean +/- S.E.M.; N = 22]; at one year 36 +/- 3 mg/day [N = 22]; at two years 37 +/- 3 mg/day [N = 16]). In 12 patients who voluntarily collected twelve-hour urine specimens after dinner on the third day of a low-oxalate diet and again the next day after a 1 g oxalate load, hydrochlorothiazide had no significant effect on oxalate excretion (19 +/- 2.3 mmol oxalate/mol creatinine on hydrochlorothiazide versus 20.6 +/- 2.6 mmol off the drug after low oxalate meal; 50 +/- 7.8 mmol/mol creatinine on hydrochlorothiazide versus 56.2 +/- 7.5 mmol off the drug after an oxalate load). As expected, there was a significant reduction in urinary calcium excretion and thus of calcium oxalate urinary saturation during hydrochlorothiazide administration. Hydrochlorothiazide by itself is not sufficient to reduce oxalate excretion in patients with renal leak hypercalciuria.

The evaluation of urinary protein patterns in a stone-forming animal model using two-dimensional polyacrylamide gel electrophoresis

The exact role of urinary proteins in kidney stone formation remains an area of controversy. Some investigators believe that urinary proteins are selectively incorporated within urinary calculi and as such have an active role in stone formation. Other investigators believe that urinary proteins are nonspecifically adsorbed into urinary crystals and thus have only a passive role in stone formation. In the current investigation a previously described stone-forming animal model (hyperoxaluric rat) was utilized along with an animal model for renal tubular injury (gentamicin nephrotoxicity) in an effort to clarify the role of urinary proteins in kidney stone formation. Urine specimens were collected before and after the induction of stone formation and after the induction of renal tubular injury. The purified proteins from each urine specimen were separated in two dimensions by electrophoresis resulting in a characteristic "map" of protein spots for each urine specimen. All animals in the stone-forming group had pathologic evidence of early stone formation (diffuse intranephronic calculosis). Early stone formation was consistently associated with a reduction in the excretion of low molecular weight urinary proteins (30,000 dalton and less than 20,000 dalton ranges). Alcian blue staining confirmed the presence of matrix within clumps of intranephronic calcium oxalate crystals. Renal tubular injury was associated with an increase in the excretion of low molecular weight proteins (approximate 20,000 dalton range) consistent with classical tubular proteinuria. These results suggest that low molecular weight urinary proteins are selectively incorporated within the crystalline structure of the stone early during its formation.

Hyperoxaluria or hypercalciuria in nephrolithiasis: the importance of renal tubular functions

The role of the kidney in states of hyperoxaluria and hypercalciuria was investigated in seven patients with hyperoxaluria after jejunoileal bypass (JIB) and six patients with idiopathic hypercalciuria (IHC). Eight apparently healthy persons formed a control group. Besides hyperoxaluria, the patients with JIB displayed an elevated plasma concentration of oxalate and the oxalate clearance was increased and higher than creatinine clearance, indicating a net tubular secretion of oxalate. The JIB patients had lower 24-h urinary excretions of calcium, phosphate, magnesium and citrate and higher serum parathyroid hormone (PTH) than controls, indicating increased secretion of PTH to compensate for calcium malabsorption. IHC patients exhibited increased fasting urinary calcium even though their serum values were similar to those in the controls. These results indicate a reduced tubular calcium reabsorption, which was most pronounced in patients with highest PTH values. We conclude that hyperoxaluria in JIB patients is associated both with intestinal hyperabsorption and with enhanced tubular secretion of oxalate, and that in some patients with IHC hypercalciuria is due to reduced tubular reabsorption of calcium.

Oxalate:OH exchange across rat renal cortical brush border membrane

We demonstrated the presence of oxalate:OH exchange in rat renal brush border membrane. Transient concentrative uptake of oxalate ("overshoot") was observed in the presence of an inside alkaline (pH = 8.5 inside, 6.5 outside) pH gradient, but this pH gradient-stimulated oxalate uptake was abolished by 1 mM DIDS, indicating that DIDS-sensitive and -insensitive oxalate uptake mechanisms were present. The DIDS-sensitive oxalate uptake was temperature-dependent and saturable with a Km of 0.0365 mM and a Vmax of 1.38 nmol.30 second-1.mg protein-1. In addition, oxalate was transported into the osmotically active internal space. In the presence of the pH gradient, a change in transmembrane potential had no effect on pH gradient-stimulated oxalate uptake. Oxalate was exchanged for OH and this exchange was sensitive to inhibition by DIDS. Inhibition by DIDS, furosemide and probenecid facilitated the distinction of oxalate:OH exchange from formate:Cl and oxalate:Cl exchange. In preparation of our brush border membrane vesicles, no apparent SO4:HCO3 exchange was present. These data indicate that oxalate:OH exchange occurs on the brush border membrane.

Multiorgan crystal deposition following intravenous oxalate infusion in rat

Deposition of calcium oxalate is responsible for the pathologic manifestations of oxalosis and may contribute to multiorgan dysfunction in uremia and to the progression of renal damage after renal failure is established. We have developed a rat model of oxalosis using a single intravenous injection of sodium oxalate, 0.3 mmol./kg. body weight, in rats. Polarized light microscopy and section freeze-dry autoradiography were used to identify 14C-oxalate within the renal parenchyma and in extrarenal organs. 14C-oxalate crystals under three mu in length were identified within one min. of injection in proximal tubule lumens. Section freeze-dry autoradiography showed occasional minute crystals within glomeruli, heart, lung and liver at one hr. In contrast to concentrative cellular uptake demonstrated in rat renal cortical slices in vitro, intracellular accumulation of 14C-oxalate could not be detected in vivo. Within the first 24 hr., renal oxalate retention reached a maximum of 25 +/- 4 per cent of the injected dose/gm. kidney compared to a maximum of only 7 +/- 3 per cent/gm. kidney after intraperitoneal administration. Although less than one per cent dose/gm. kidney remained after one week, crystal fragments were scattered throughout the cortex and medulla, often surrounded by foci of interstitial nephritis. The retention of crystals in kidney and other body organs following i.v. oxalate provides a model of oxalosis which stimulates pathophysiologic events in a variety of clinical situations characterized by transiently or persistently elevated serum oxalate.

Oxalate removal by hemodialysis in end-stage renal disease

Because of mounting evidence of precipitation of calcium oxalate in the soft tissues of patients with end-stage renal disease (ESRD) on maintenance hemodialysis, the plasma oxalate concentrations and calculated dialysis removal of oxalate were studied in seven patients without evidence of either primary or absorption hyperoxaluria prior to ESRD. A reversed-phase high-pressure liquid chromatographic method was developed to quantitate serum oxalate. Mean value +/- SE in four healthy controls was 28 +/- 5 mumol/L, and in the seven patients it was 187 +/- 15 mumol/L predialysis and 89 +/- 11 mumol/L postdialysis. Oxalate deposition in the soft tissues of ESRD patients is the consequence of sustained hyperoxalemia. Oxalate removal by dialysis was calculated from the four-hour oxalate clearance. Since the ionic radii of phosphate and oxalate are similar, total oxalate clearance was calculated midpoint of dialysis. Mean oxalate removal/dialysis was 3.01 +/- 0.283 mmol. On a daily basis this value was 1.645 +/- 0.155 mmol, which is about threefold the normal oxalate excretion rate. It is not significantly different from the excretion rate in absorption oxalurias but is less than that in primary hyperoxaluria. Therefore, it is concluded that hyperoxalemia in ESRD results from loss of renal excretion, failure of hemodialysis to remove enough oxalate to maintain a normal serum concentration, and increased intestinal absorption of oxalate and/or increased endogenous production.

Concentration profiles of calcium and oxalate in urine, tubular fluid and renal tissue--some theoretical considerations

This paper analyzes some aspects of the pathophysiology of urolithiasis. It is emphasized that a better understanding of factors contributing to stone formation can only be gained when the primary nucleation site is identified. Three compartments are considered in which supersaturation as a precondition for stone formation could be present: urine in the urinary tract, tubular fluid from the glomerulus down to the duct of Bellini, and the interstitium of the medulla. From calculations based on micropuncture data it becomes apparent that the oxalate concentration in the tubular fluid at the bend of Henle's loop is 1 or 2 orders of magnitude lower than in the duct of Bellini and that the oxalate concentration maximum invariably must be located in the final urine. The calculation of a tubular concentration profile of oxalate shows, that the probability of intra luminal crystal formation is even less likely for plasma oxalate values of 2-3 microM as compared to 1.2 microM, which therefore should be the correct value. The time necessary for the growth of crystals up to a critical size which can obstruct tubules or ureter is not available in the urinary tract nor in the tubules. However, in the medullary interstitium, where solute concentration is highest, nearly unlimited time for crystal growth is available due to the fact, that in this compartment convective flow is very low. It is concluded that the interstitium of the inner medulla has the best chances to function as the primary nucleation site where particles can be formed of a size which subsequently can obstruct the urinary tract.

Oxalate metabolism and renal calculi

Changes in oxalate excretion (together with changes in urinary volume) constitute the most important factors in altering the probability of renal stone formation. However, investigations on oxalate metabolism have been sparse, perhaps because of the lack of an accurate method for measuring oxalate in biologic fluids. Available data clearly implicate increased urinary oxalate excretion as the etiological factor in stone formation in two groups of patients--those with primary hyperoxaluria and those with gastrointestinal malabsorption. Evidence for the existence of hyperoxaluria in the patient with the "garden" variety of calcium oxalate stones is less persuasive.

Intrarenal distribution of oxalic acid, calcium, sodium, and potassium in man

The renal papilla in man has been shown to contain a high concentration of oxalate (5 . 5 +/- 0 . 8 mmole/kg wet weight, mean +/- SEM, n = 7 kidneys), and that there is a significant concentration gradient between oxalate in the papilla and that of the medulla (0 . 4 +/- 0 . 08, P < 0 . 05) and the cortex (0 . 3 +/- 0 . 06, P < 0 . 05). Significant calcium and sodium gradients between renal papilla and medulla and cortex were confirmed and parallel that of oxalate. Potassium showed a significant decrease in the papilla (33 . 1 +/- 0 . 9) as compared to the medulla (42 . 1 +/- 1 . 9 P < 0 . 05). The concentrations of oxalate and calcium in the papilla were respectively 25-fold and 6-fold higher than the urinary concentration of oxalate and calcium. It is concluded that these high concentrations of oxalate and calcium in the renal papilla are related to the formation of Randall's plaques and may be an essential factor in the pathogenesis of renal stones which is still far from clear.