Evidence for net renal tubule oxalate secretion in patients with calcium kidney stones

Oxalate Homeostasis in Non-Stone-Forming Chronic Kidney Disease: A Review of Key Findings and Perspectives

Chronic kidney disease (CKD) is a significant global public health concern associated with high morbidity and mortality rates. The maintenance of oxalate homeostasis plays a critical role in preserving kidney health, particularly in the context of CKD. Although the relationship between oxalate and kidney stone formation has been extensively investigated, our understanding of oxalate homeostasis in non-stone-forming CKD remains limited. This review aims to present an updated analysis of the existing literature, focusing on the intricate mechanisms involved in oxalate homeostasis in patients with CKD. Furthermore, it explores the key factors that influence oxalate accumulation and discusses the potential role of oxalate in CKD progression and prognosis. The review also emphasizes the significance of the gut-kidney axis in CKD oxalate homeostasis and provides an overview of current therapeutic strategies, as well as potential future approaches. By consolidating important findings and perspectives, this review offers a comprehensive understanding of the present knowledge in this field and identifies promising avenues for further research.

Pathophysiology and Management of Hyperoxaluria and Oxalate Nephropathy: A Review

Hyperoxaluria results from either inherited disorders of glyoxylate metabolism leading to hepatic oxalate overproduction (primary hyperoxaluria), or increased intestinal oxalate absorption (secondary hyperoxaluria). Hyperoxaluria may lead to urinary supersaturation of calcium oxalate and crystal formation, causing urolithiasis and deposition of calcium oxalate crystals in the kidney parenchyma, a condition termed oxalate nephropathy. Considerable progress has been made in the understanding of pathophysiological mechanisms leading to hyperoxaluria and oxalate nephropathy, whose diagnosis is frequently delayed and prognosis too often poor. Fortunately, novel promising targeted therapeutic approaches are on the horizon in patients with primary hyperoxaluria. Patients with secondary hyperoxaluria frequently have long-standing hyperoxaluria-enabling conditions, a fact suggesting the role of triggers of acute kidney injury such as dehydration. Current standard of care in these patients includes management of the underlying cause, high fluid intake, and use of calcium supplements. Overall, prompt recognition of hyperoxaluria and associated oxalate nephropathy is crucial because optimal management may improve outcomes.

Primary hyperoxaluria type 1: pathophysiology and genetics

Primary hyperoxaluria type 1 (PH1) is a rare genetic form of calcium oxalate kidney stone disease. It is caused by a deficiency in the liver-specific enzyme, alanine:glyoxylate aminotransferase (AGT), a pyridoxal-5'-phosphate (PLP)-dependent enzyme involved in the metabolism of glyoxylate. The excessive endogenous synthesis of oxalate that ensues leads to hyperoxaluria, and the crystallization of the poorly soluble calcium salt of oxalate is responsible for a severe kidney stone disease, which can progress to end-stage renal disease, systemic deposition of oxalate and death. Knowledge about metabolic precursors of glyoxylate and oxalate, molecular pathology of AGT and analytical methods for diagnosis and clinical assessment have allowed a better understanding of the mechanisms underlying PH1 and opened the door to new therapeutic strategies.

Numerical characterization of astronaut CaOx renal stone incidence rates to quantify in-flight and post-flight relative risk

Changes in urine chemistry potentially alter the risk of renal stone formation in astronauts. Quantifying spaceflight renal stone incidence risk compared to pre-flight levels remains a significant challenge for assessing the appropriate vehicle, mission, and countermeasure design. A computational biochemistry model representing CaOx crystal precipitation, growth, and agglomeration is combined with a probabilistic analysis to predict the in- and post-flight CaOx renal stone incidence risk ratio (IRR) relative to pre-flight values using 1517 astronaut 24-h urine chemistries. Our simulations predict that in-flight fluid intake alone would need to increase from current prescriptions of 2.0–2.5 L/day to ~3.2 L/day to approach the CaOx IRR of the pre-flight population. Bone protective interventions would reduce CaOx risk to pre-flight levels if Ca excretion alone is reduced to <150 mg/day or if current levels are diminished to 190 mg/day in combination with increasing fluid intake to 2.5–2.7 L/day. This analysis provides a quantitative risk assessment that can influence the critical balance between engineering and astronaut health requirements.

The controversial association of gut and urinary microbiota with kidney stone formation

Nephrolithiasis (kidney stones) is one of the most common chronic kidney diseases that are typically more common among adult men comparing to adult women. The prevalence of this disease is increasing which is influenced by genetic and environmental factors. Kidney stones are mainly composed of calcium oxalate and urinary oxalate which is considered a dangerous factor in their formation. Besides diverse leading reasons in the progression of nephrolithiasis, the gut and urinary microbiome has been recognized as a major player in the development or prevention of it. These microbes produce metabolites that have diverse effects on host biological functions. Therefore, Changes in the composition and structure of the microbiome (dysbiosis) have been implicated in various diseases. The present review focuses on the roles of gut and urinary in kidney stone formation.

Contribution of Dietary Oxalate and Oxalate Precursors to Urinary Oxalate Excretion

Kidney stone disease is increasing in prevalence, and the most common stone composition is calcium oxalate. Dietary oxalate intake and endogenous production of oxalate are important in the pathophysiology of calcium oxalate stone disease. The impact of dietary oxalate intake on urinary oxalate excretion and kidney stone disease risk has been assessed through large cohort studies as well as smaller studies with dietary control. Net gastrointestinal oxalate absorption influences urinary oxalate excretion. Oxalate-degrading bacteria in the gut microbiome, especially Oxalobacter formigenes, may mitigate stone risk through reducing net oxalate absorption. Ascorbic acid (vitamin C) is the main dietary precursor for endogenous production of oxalate with several other compounds playing a lesser role. Renal handling of oxalate and, potentially, renal synthesis of oxalate may contribute to stone formation. In this review, we discuss dietary oxalate and precursors of oxalate, their pertinent physiology in humans, and what is known about their role in kidney stone disease.

Sodium Oxalate-Induced Acute Kidney Injury Associated With Glomerular and Tubulointerstitial Damage in Rats

Acute crystalline nephropathy is closely related to tubulointerstitial injury, but few studies have investigated glomerular changes in this condition. Thus, in the current study, we investigated the factors involved in glomerular and tubulointerstitial injury in an experimental model of crystalline-induced acute kidney injury (AKI). We treated male Wistar rats with a single injection of sodium oxalate (NaOx, 7 mg⋅100 g^-1⋅day^-1, resuspended in 0.9% NaCl solution, i.p.) or vehicle (control). After 24 h of treatment, food and water intake, urine output, body weight gain, and renal function were evaluated. Renal tissue was used for the morphological studies, quantitative PCR and protein expression studies. Our results revealed that NaOx treatment did not change metabolic or electrolyte and water intake parameters or urine output. However, the treated group exhibited tubular calcium oxalate (CaOx) crystals excretion, followed by a decline in kidney function demonstrated along with glomerular injury, which was confirmed by increased plasma creatinine and urea concentrations, increased glomerular desmin immunostaining, nephrin mRNA expression and decreased WT1 immunofluorescence. Furthermore, NaOx treatment resulted in tubulointerstitial injury, which was confirmed by tubular dilation, albuminuria, increased Kim-1 and Ki67 mRNA expression, decreased megalin and Tamm-Horsfall protein (THP) expression. Finally, the treatment induced increases in CD68 protein staining, MCP-1, IL-1β, NFkappaB, and α-SMA mRNA expression, which are consistent with proinflammatory and profibrotic signaling, respectively. In conclusion, our findings provide relevant information regarding crystalline-induced AKI, showing strong tubulointerstitial and glomerular injury with a possible loss of podocyte viability.

Dietary Oxalate Intake and Kidney Outcomes

Oxalate is both a plant-derived molecule and a terminal toxic metabolite with no known physiological function in humans. It is predominantly eliminated by the kidneys through glomerular filtration and tubular secretion. Regardless of the cause, the increased load of dietary oxalate presented to the kidneys has been linked to different kidney-related conditions and injuries, including calcium oxalate nephrolithiasis, acute and chronic kidney disease. In this paper, we review the current literature on the association between dietary oxalate intake and kidney outcomes.

Calcium Oxalate Nephrolithiasis and Gut Microbiota: Not just a Gut-Kidney Axis. A Nutritional Perspective

Recent studies have shown that patients with kidney stone disease, and particularly calcium oxalate nephrolithiasis, exhibit dysbiosis in their fecal and urinary microbiota compared with controls. The alterations of microbiota go far beyond the simple presence and representation of Oxalobacter formigenes, a well-known symbiont exhibiting a marked capacity of degrading dietary oxalate and stimulating oxalate secretion by the gut mucosa. Thus, alterations of the intestinal microbiota may be involved in the pathophysiology of calcium kidney stones. However, the role of nutrition in this gut-kidney axis is still unknown, even if nutritional imbalances, such as poor hydration, high salt, and animal protein intake and reduced fruit and vegetable intake, are well-known risk factors for kidney stones. In this narrative review, we provide an overview of the gut-kidney axis in nephrolithiasis from a nutritional perspective, summarizing the evidence supporting the role of nutrition in the modulation of microbiota composition, and their relevance for the modulation of lithogenic risk.

Evidence for disordered acid-base handling in calcium stone-forming patients

In stone formers (SFs) with idiopathic hypercalciuria, urine pH governs the mineral phase of stones. Calcium phosphate (CaP) SFs have higher urine pH than calcium oxalate (CaOx) SFs. Normal women have higher urine pH than men on fixed diets, accompanied by greater absorption of food alkali. Female CaP and male CaOx SFs have similar urine pH as same sex normal individuals, but male CaP and female CaOx SFs may have abnormal acid-base handling. We studied 25 normal individuals (13 men and 12 women), 17 CaOx SFs (11 men and 6 women), and 15 CaP SFs (8 men and 7 women) on fixed diets. Urine and blood samples were collected under fasting and fed conditions. Female CaOx SFs had lower urine pH and lower alkali absorption, fed, compared with normal women; their urine NH₄ was higher and urine citrate excretion lower than in normal women, consistent with their higher net acid excretion. Male CaOx SFs had higher urine citrate excretion and higher serum ultrafilterable citrate levels than normal men. Both male and female CaP SFs had higher urine pH fasting than same sex normal individuals, but only men were higher in the fed period, and there were no differences from normal in gut alkali absorption. CaP SFs of both sexes had higher urine NH₄ and lower urine citrate than same sex normal individuals. The lower urine pH of female CaOx SFs seems related to decreased gut alkali absorption, while the higher pH of CaP SFs, accompanied by higher urine NH₄ and lower urine citrate, suggests a proximal tubule disorder.

Future treatments for hyperoxaluria

PURPOSE OF REVIEW

The review of potential therapies in the treatment of hyperoxaluria is timely, given the current excitement with clinical trials and the mounting evidence of the importance of oxalate in both kidney stone and chronic kidney disease.

RECENT FINDINGS

Given the significant contribution of both endogenous and dietary oxalate to urinary oxalate excretions, it is not surprising therapeutic targets are being studied in both pathways. This article covers the existing data on endogenous and dietary oxalate and the current targets in these pathways.

SUMMARY

In the near future, there will likely be therapies targeting both endogenous and dietary oxalate, especially in subsets of kidney stone formers.

Immunity, microbiota and kidney disease

The recognition that intestinal microbiota exert profound effects on human health has led to major advances in our understanding of disease processes. Studies over the past 20 years have shown that host components, including components of the host immune system, shape the microbial community. Pathogenic alterations in commensal microorganisms contribute to disease manifestations that are generally considered to be noncommunicable, such as inflammatory bowel disease, diabetes mellitus and liver disease, through a variety of mechanisms, including effects on host immunity. More recent studies have shed new light on how the immune system and microbiota might also drive the pathogenesis of renal disorders. In this Review, we discuss the latest insights into the mechanisms regulating the microbiome composition, with a focus both on genetics and environmental factors, and describe how commensal microorganisms calibrate innate and adaptive immune responses to affect the activation threshold for pathogenic stimulations. We discuss the mechanisms that lead to intestinal epithelial barrier inflammation and the relevance of certain bacteria to the pathogenesis of two common kidney-based disorders: hypertension and renal stone disease. Limitations of current approaches to microbiota research are also highlighted, emphasizing the need to move beyond studies of correlation to causation.

Dietary oxalate and kidney stone formation

Dietary oxalate is plant-derived and may be a component of vegetables, nuts, fruits, and grains. In normal individuals, approximately half of urinary oxalate is derived from the diet and half from endogenous synthesis. The amount of oxalate excreted in urine plays an important role in calcium oxalate stone formation. Large epidemiological cohort studies have demonstrated that urinary oxalate excretion is a continuous variable when indexed to stone risk. Thus, individuals with oxalate excretions >25 mg/day may benefit from a reduction of urinary oxalate output. The 24-h urine assessment may miss periods of transient surges in urinary oxalate excretion, which may promote stone growth and is a limitation of this analysis. In this review we describe the impact of dietary oxalate and its contribution to stone growth. To limit calcium oxalate stone growth, we advocate that patients maintain appropriate hydration, avoid oxalate-rich foods, and consume an adequate amount of calcium.

Mechanism for higher urine pH in normal women compared with men

Regulation of acid-base metabolism maintains the pH of body fluids within a tight range. Urine pH (UpH) is also regulated under normal conditions. Median pH of 24-h urines is ~6, but others have noted that UpH in women is higher than men, which has been attributed to differences in diet. If true, it would help to explain the fact that calcium phosphate stones, which form at higher urine pH, are much more common in women than in men. We studied 14 normal subjects (7 men and 7 women) fed identical meals in a Clinical Research Center. Urine and blood samples were collected during fasting and after meals. UpH of women (6.74 ± 0.11) exceeded that of men (6.07 ± 0.17) fed, but not fasting, and UpH rose significantly with meals in women but not men. Serum and urine total CO₂ rose with meals in women but not men, and in women net acid excretion fell to zero during the fed period. In a general linear model adjusted for age, sex, and weight, net gastrointestinal anion uptake was the main predictor of UpH and was significantly higher in women (3.9 ± 0.6) than men (1.8 ± 0.7) in the fed period. Urine citrate, an anion absorbed by the gastrointestinal tract, was higher in women than men in the fed state, and fractional excretion of citrate was higher in women than men. The higher fed UpH in women is related to a greater absorption of food anions and raises 24-h UpH.

Idiopathic hypercalciuria and formation of calcium renal stones

The most common presentation of nephrolithiasis is idiopathic calcium stones in patients without systemic disease. Most stones are primarily composed of calcium oxalate and form on a base of interstitial apatite deposits, known as Randall's plaque. By contrast some stones are composed largely of calcium phosphate, as either hydroxyapatite or brushite (calcium monohydrogen phosphate), and are usually accompanied by deposits of calcium phosphate in the Bellini ducts. These deposits result in local tissue damage and might serve as a site of mineral overgrowth. Stone formation is driven by supersaturation of urine with calcium oxalate and brushite. The level of supersaturation is related to fluid intake as well as to the levels of urinary citrate and calcium. Risk of stone formation is increased when urine citrate excretion is <400 mg per day, and treatment with potassium citrate has been used to prevent stones. Urine calcium levels >200 mg per day also increase stone risk and often result in negative calcium balance. Reduced renal calcium reabsorption has a role in idiopathic hypercalciuria. Low sodium diets and thiazide-type diuretics lower urine calcium levels and potentially reduce the risk of stone recurrence and bone disease.

Sex differences in proximal and distal nephron function contribute to the mechanism of idiopathic hypercalcuria in calcium stone formers

Idiopathic hypercalciuria (IH) is a common familial trait among patients with calcium nephrolithiasis. Previously, we have demonstrated that hypercalciuria is primarily due to reduced renal proximal and distal tubule calcium reabsorption. Here, using measurements of the clearances of sodium, calcium, and endogenous lithium taken from the General Clinical Research Center, we test the hypothesis that patterns of segmental nephron tubule calcium reabsorption differ between the sexes in IH and normal subjects. When the sexes are compared, we reconfirm the reduced proximal and distal calcium reabsorption. In IH women, distal nephron calcium reabsorption is decreased compared to normal women. In IH men, proximal tubule calcium reabsorption falls significantly, with a more modest reduction in distal calcium reabsorption compared to normal men. Additionally, we demonstrate that male IH patients have lower systolic blood pressures than normal males. We conclude that women and men differ in the way they produce the hypercalciuria of IH, with females reducing distal reabsorption and males primarily reducing proximal tubule function.

Primary and secondary hyperoxaluria: Understanding the enigma

Hyperoxaluria is characterized by an increased urinary excretion of oxalate. Primary and secondary hyperoxaluria are two distinct clinical expressions of hyperoxaluria. Primary hyperoxaluria is an inherited error of metabolism due to defective enzyme activity. In contrast, secondary hyperoxaluria is caused by increased dietary ingestion of oxalate, precursors of oxalate or alteration in intestinal microflora. The disease spectrum extends from recurrent kidney stones, nephrocalcinosis and urinary tract infections to chronic kidney disease and end stage renal disease. When calcium oxalate burden exceeds the renal excretory ability, calcium oxalate starts to deposit in various organ systems in a process called systemic oxalosis. Increased urinary oxalate levels help to make the diagnosis while plasma oxalate levels are likely to be more accurate when patients develop chronic kidney disease. Definitive diagnosis of primary hyperoxaluria is achieved by genetic studies and if genetic studies prove inconclusive, liver biopsy is undertaken to establish diagnosis. Diagnostic clues pointing towards secondary hyperoxaluria are a supportive dietary history and tests to detect increased intestinal absorption of oxalate. Conservative treatment for both types of hyperoxaluria includes vigorous hydration and crystallization inhibitors to decrease calcium oxalate precipitation. Pyridoxine is also found to be helpful in approximately 30% patients with primary hyperoxaluria type 1. Liver-kidney and isolated kidney transplantation are the treatment of choice in primary hyperoxaluria type 1 and type 2 respectively. Data is scarce on role of transplantation in primary hyperoxaluria type 3 where there are no reports of end stage renal disease so far. There are ongoing investigations into newer modalities of diagnosis and treatment of hyperoxaluria. Clinical differentiation between primary and secondary hyperoxaluria and further between the types of primary hyperoxaluria is very important because of implications in treatment and diagnosis. Hyperoxaluria continues to be a challenging disease and a high index of clinical suspicion is often the first step on the path to accurate diagnosis and management.

Persistence of 1,25D-induced hypercalciuria in alendronate-treated genetic hypercalciuric stone-forming rats fed a low-calcium diet

Genetic hypercalciuric stone-forming (GHS) rats demonstrate increased intestinal Ca absorption, increased bone resorption, and reduced renal tubular Ca reabsorption leading to hypercalciuria and all form kidney stones. GHS have increased vitamin D receptors (VDR) at these sites of Ca transport. Injection of 1,25(OH)2D3 (1,25D) leads to a greater increase in urine (u)Ca in GHS than in control Sprague-Dawley (SD), possibly due to the additional VDR. In GHS the increased uCa persists on a low-Ca diet (LCD) suggesting enhanced bone resorption. We tested the hypothesis that LCD, coupled to inhibition of bone resorption by alendronate (alen), would eliminate the enhanced 1,25D-induced hypercalciuria in GHS. SD and GHS were fed LCD and half were injected daily with 1,25D. After 8 days all were also given alen until euthanasia at day 16. At 8 days, 1,25D increased uCa in SD and to a greater extent in GHS. At 16 days, alen eliminated the 1,25D-induced increase in uCa in SD. However, in GHS alen decreased, but did not eliminate, the 1,25D-induced hypercalciuria, suggesting maximal alen cannot completely prevent the 1,25D-induced bone resorption in GHS, perhaps due to increased VDR. There was no consistent effect on mRNA expression of renal transcellular or paracellular Ca transporters. Urine CaP and CaOx supersaturation (SS) increased with 1,25D alone in both SD and GHS. Alen eliminated the increase in CaP SS in SD but not in GHS. If these results are confirmed in humans with IH, the use of bisphosphonates, such as alen, may not prevent the decreased bone density observed in these patients.

Body fat content and distribution and urinary risk factors for nephrolithiasis

BACKGROUND AND OBJECTIVES

Obesity is associated with a higher risk of nephrolithiasis. However, it is not known whether higher body fat mass or abnormal fat distribution influences stone risk independent of dietary factors.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this cross-sectional study, non-stone-forming men with no known kidney disease and with a wide range of body weight collected a 24-hour urine specimen while consuming a fixed metabolic diet. They underwent dual-energy x-ray absorptiometry to assess body composition and fat distribution. Urinary risk factors for nephrolithiasis and urine saturation with respect to calcium oxalate and uric acid (assessed as supersaturation index [SI]) were correlated with various measures of adiposity.

RESULTS

Study participants included 21 men with a mean age of 52.1 years, mean weight of 91.1 kg, and mean total fat mass of 24.3 kg. Twenty-four-hour urine pH and SI uric acid were more closely correlated with fat mass than with lean mass or total body weight. Both 24-hour urine pH and SI uric acid were also significantly correlated with truncal fat mass but not with leg fat mass. Moreover, there was a significant negative correlation between truncal/leg fat mass and NH4(+)/net acid excretion ratio (R=-0.62; P=0.009). However, there was no significant association between SI calcium oxalate and body weight, lean mass, fat mass, trunk fat mass, or leg fat mass.

CONCLUSIONS

The association between 24-hour urine pH and SI uric acid and various measures of adiposity suggest that total body fat and trunk fat are more strongly associated with risk factors for uric acid stone formation than are total body weight and lean body mass. Under a controlled metabolic diet, adiposity is not associated with risk factors for calcium oxalate stones. Further studies are needed to confirm these findings in larger populations that include women and patients who form stones.

A test of the hypothesis that oxalate secretion produces proximal tubule crystallization in primary hyperoxaluria type I

The sequence of events by which primary hyperoxaluria type 1 (PH1) causes renal failure is unclear. We hypothesize that proximal tubule (PT) is vulnerable because oxalate secretion raises calcium oxalate (CaOx) supersaturation (SS) there, leading to crystal formation and cellular injury. We studied cortical and papillary biopsies from two PH1 patients with preserved renal function, and seven native kidneys removed from four patients at the time of transplant, after short-term (2) or longer term (2) dialysis. In these patients, and another five PH1 patients without renal failure, we calculated oxalate secretion, and estimated PT CaOx SS. Plasma oxalate was elevated in all PH1 patients and inverse to creatinine clearance. Renal secretion of oxalate was present in all PH1 but rare in controls. PT CaOx SS was >1 in all nonpyridoxine-responsive PH1 before transplant and most marked in patients who developed end stage renal disease (ESRD). PT from PH1 with preserved renal function had birefringent crystals, confirming the presence of CaOx SS, but had no evidence of cortical inflammation or scarring by histopathology or hyaluronan staining. PH1 with short ESRD showed CaOx deposition and hyaluronan staining particularly at the corticomedullary junction in distal PT while cortical collecting ducts were spared. Longer ESRD showed widespread cortical CaOx, and in both groups papillary tissue had marked intratubular CaOx deposits and fibrosis. CaOx SS in PT causes CaOx crystal formation, and CaOx deposition in distal PT appears to be associated with ESRD. Minimizing PT CaOx SS may be important for preserving renal function in PH1.

1,25(OH)₂D₃-enhanced hypercalciuria in genetic hypercalciuric stone-forming rats fed a low-calcium diet

The inbred genetic hypercalciuric stone-forming (GHS) rats exhibit many features of human idiopathic hypercalciuria and have elevated levels of vitamin D receptors (VDR) in calcium (Ca)-transporting organs. On a normal-Ca diet, 1,25(OH)2D3 (1,25D) increases urine (U) Ca to a greater extent in GHS than in controls [Sprague-Dawley (SD)]. The additional UCa may result from an increase in intestinal Ca absorption and/or bone resorption. To determine the source, we asked whether 1,25D would increase UCa in GHS fed a low-Ca (0.02%) diet (LCD). With 1,25D, UCa in SD increased from 1.2 ± 0.1 to 9.3 ± 0.9 mg/day and increased more in GHS from 4.7 ± 0.3 to 21.5 ± 0.9 mg/day (P < 0.001). In GHS rats on LCD with or without 1,25D, UCa far exceeded daily Ca intake (2.6 mg/day). While the greater excess in UCa in GHS rats must be derived from bone mineral, there may also be a 1,25D-mediated decrease in renal tubular Ca reabsorption. RNA expression of the components of renal Ca transport indicated that 1,25D administration results in a suppression of klotho, an activator of the renal Ca reabsorption channel TRPV5, in both SD and GHS rats. This fall in klotho would decrease tubular reabsorption of the 1,25D-induced bone Ca release. Thus, the greater increase in UCa with 1,25D in GHS fed LCD strongly suggests that the additional UCa results from an increase in bone resorption, likely due to the increased number of VDR in the GHS rat bone cells, with a possible component of decreased renal tubular calcium reabsorption.

Role of proximal tubule in the hypocalciuric response to thiazide of patients with idiopathic hypercalciuria

The most common metabolic abnormality found in calcium (Ca) kidney stone formers is idiopathic hypercalciuria (IH). Using endogenous lithium (Li) clearance, we previously showed that in IH, there is decreased proximal tubule sodium absorption, and increased delivery of Ca into the distal nephron. Distal Ca reabsorption may facilitate the formation of Randall's plaque (RP) by washdown of excess Ca through the vasa recta toward the papillary tip. Elevated Ca excretion leads to increased urinary supersaturation (SS) with respect to calcium oxalate (CaOx) and calcium phosphate (CaP), providing the driving force for stone growth on RP. Thiazide (TZ) diuretics reduce Ca excretion and prevent stone recurrence, but the mechanism in humans is unknown. We studied the effect of chronic TZ administration on renal mineral handling in four male IH patients using a fixed three meal day in the General Clinical Research Center. Each subject was studied twice: once before treatment and once after 4-7 mo of daily chlorthalidone treatment. As expected, urine Ca fell with TZ, along with fraction of filtered Ca excreted. Fraction of filtered Li excreted also fell sharply with TZ, as did distal delivery of Ca. Unexpectedly, TZ lowered urine pH. Together with reduced urine Ca, this led to a marked fall in CaP SS, but not CaOx SS. Since CaOx stone formation begins with an initial CaP overlay on RP, by lowering urine pH and decreasing distal nephron Ca delivery, TZ might diminish stone risk both by reducing CaP SS, as well as slowing progression of RP.

The role of Oxalobacter formigenes colonization in calcium oxalate stone disease

About 75% of urinary stones contain oxalate. As Oxalobacter formigenes is a Gram-negative anaerobic bacterium that degrades oxalate in the intestinal tract, we assessed the role of O. formigenes in oxalate metabolism by evaluating its intestinal absorption, plasma concentration, and urinary excretion. Of 37 calcium oxalate stone formers, 26 tested negative for O. formigenes and were compared with the 11 patients who tested positive. Patients provided 24-h urine samples on both a self-selected and a standardized diet. Urinary oxalate excretion did not differ significantly on the self-selected diet, but was significantly lower in O. formigenes-positive than in O. formigenes-negative patients under controlled, standardized conditions. Intestinal oxalate absorption, measured using [(13)C₂]oxalate, was similar in the patients with or without O. formigenes. Plasma oxalate concentrations were significantly higher in noncolonized (5.79 μmol/l) than in colonized stone formers (1.70 μmol/l). Colonization with O. formigenes was significantly inversely associated with the number of stone episodes. Our findings suggest that O. formigenes lowers the intestinal concentration of oxalate available for absorption at constant rates, resulting in decreased urinary oxalate excretion. Thus, dietary factors have an important role in urinary oxalate excretion. The data indicate that O. formigenes colonization may reduce the risk of stone recurrence.

Lanthanum carbonate inhibits intestinal oxalate absorption and prevents nephrocalcinosis after oxalate loading in rats

PURPOSE

Increased intestinal oxalate absorption leads to increased urinary oxalate excretion (secondary hyperoxaluria) and calcium oxalate crystal formation, contributing to nephrocalcinosis/lithiasis. Lanthanum carbonate is an intestinal phosphate binder that is orally administered to patients on dialysis to treat hyperphosphatemia. It is hypothesized that lanthanum can also bind oxalate, in addition to phosphate. We evaluated this in vitro and in vivo.

MATERIALS AND METHODS

In vitro oxalate binding was evaluated by oxalate precipitation from a solution by lanthanum. In vivo oxalate absorption kinetics and the effect of lanthanum carbonate on nephrocalcinosis development were assessed in male Sprague-Dawley® rats that received 1) 1,000 mg lanthanum carbonate and oxalate, 2) carboxymethylcellulose and oxalate or 3) carboxymethylcellulose by gavage for up to 12 hours (kinetics) or 7 days (nephrocalcinosis). Plasma and urinary oxalate concentrations were measured at several time points after gavage. The degree of nephrocalcinosis was assessed histomorphometrically on von Kossa stained sections and by measuring total calcium content in renal tissue.

RESULTS

In vitro lanthanum bound oxalate in a pH range comparable to the range of the intestine. In vivo oxalate administration in untreated animals resulted in a biphasic pattern of increased plasma oxalate levels, which was almost abolished in lanthanum treated rats. In the urine of treated rats oxaluria and calcium oxalate crystalluria were blunted. Moreover, significantly decreased nephrocalcinosis was observed compared with that in untreated rats.

CONCLUSIONS

Lanthanum carbonate is a promising agent for the future prevention/treatment of secondary hyperoxaluria.

Response to dietary oxalate after bariatric surgery

BACKGROUND AND OBJECTIVES

Bariatric surgery (BS) may be associated with increased oxalate excretion and a higher risk of nephrolithiasis. This study aimed to investigate urinary abnormalities and responses to an acute oxalate load as an indirect assessment of the intestinal absorption of oxalate in this population.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Twenty-four-hour urine specimens were collected from 61 patients a median of 48 months after BS (post-BS) as well as from 30 morbidly obese (MO) participants; dietary information was obtained through 24-hour food recalls. An oral oxalate load test (OLT), consisting of 2-hour urine samples after overnight fasting and 2, 4, and 6 hours after consuming 375 mg of oxalate (spinach juice), was performed on 21 MO and 22 post-BS patients 12 months after BS. Ten post-BS patients also underwent OLT before surgery (pre-BS).

RESULTS

There was a higher percentage of low urinary volume (<1.5 L/d) in post-BS versus MO (P<0.001). Hypocitraturia and hyperoxaluria (P=0.13 and P=0.36, respectively) were more frequent in BS versus MO patients. The OLT showed intragroup (P<0.001 for all periods versus baseline) and intergroup differences (P<0.001 for post-BS versus MO; P=0.03 for post-BS versus pre-BS). The total mean increment in oxaluria after 6 hours of load, expressed as area under the curve, was higher in both post-BS versus MO and in post-BS versus pre-BS participants (P<0.001 for both).

CONCLUSIONS

The mean oxaluric response to an oxalate load is markedly elevated in post-bariatric surgery patients, suggesting that increased intestinal absorption of dietary oxalate is a predisposing mechanism for enteric hyperoxaluria.

In vivo Drosophilia genetic model for calcium oxalate nephrolithiasis

Nephrolithiasis is a major public health problem with a complex and varied etiology. Most stones are composed of calcium oxalate (CaOx), with dietary excess a risk factor. Because of complexity of mammalian system, the details of stone formation remain to be understood. Here we have developed a nephrolithiasis model using the genetic model Drosophila melanogaster, which has a simple, transparent kidney tubule. Drosophilia reliably develops CaOx stones upon dietary oxalate supplementation, and the nucleation and growth of microliths can be viewed in real time. The Slc26 anion transporter dPrestin (Slc26a5/6) is strongly expressed in Drosophilia kidney, and biophysical analysis shows that it is a potent oxalate transporter. When dPrestin is knocked down by RNAi in fly kidney, formation of microliths is reduced, identifying dPrestin as a key player in oxalate excretion. CaOx stone formation is an ancient conserved process across >400 My of divergent evolution (fly and human), and from this study we can conclude that the fly is a good genetic model of nephrolithiasis.

Pathophysiology-based treatment of idiopathic calcium kidney stones

Idiopathic calcium oxalate (CaOx) stone-formers (ICSFs) differ from patients who make idiopathic calcium phosphate (CaP) stones (IPSFs). ICSFs, but not IPSFs, form their stones as overgrowths on interstitial apatite plaque; the amount of plaque covering papillary surface is positively correlated with urine calcium excretion and inversely with urine volume. The amount of plaque predicts the number of recurrent stones. The initial crystal overgrowth on plaque is CaP, although the stone is mainly composed of CaOx, meaning that lowering supersaturation (SS) for CaOx and CaP is important for CaOx stone prevention. IPSFs, unlike ICSFs, have apatite crystal deposits in inner medullary collecting ducts, which are associated with interstitial scarring. ICSFs and IPSFs have idiopathic hypercalciuria, which is due to decreased tubule calcium reabsorption, but sites of abnormal reabsorption may differ. Decreased reabsorption in proximal tubules (PTs) delivers more calcium to the thick ascending limb (TAL), where increased calcium reabsorption can load the interstitium, leading to plaque formation. The site of abnormal reabsorption in IPSFs may be the TAL, where an associated defect in bicarbonate reabsorption could produce the higher urine pH characteristic of IPSFs. Preventive treatment with fluid intake, protein and sodium restriction, and thiazide will be effective in ICSFs and IPSFs by decreasing urine calcium concentration and CaOx and CaP SS and may also decrease plaque formation by increased PT calcium reabsorption. Citrate may be detrimental for IPSFs if urine pH rises greatly, increasing CaP SS. Future trials should examine the question of appropriate treatment for IPSFs.

Hyperoxaluria: a gut-kidney axis?

Hyperoxaluria leads to urinary calcium oxalate (CaOx) supersaturation, resulting in the formation and retention of CaOx crystals in renal tissue. CaOx crystals may contribute to the formation of diffuse renal calcifications (nephrocalcinosis) or stones (nephrolithiasis). When the innate renal defense mechanisms are suppressed, injury and progressive inflammation caused by these CaOx crystals, together with secondary complications such as tubular obstruction, may lead to decreased renal function and in severe cases to end-stage renal failure. For decades, research on nephrocalcinosis and nephrolithiasis mainly focused on both the physicochemistry of crystal formation and the cell biology of crystal retention. Although both have been characterized quite well, the mechanisms involved in establishing urinary supersaturation in vivo are insufficiently understood, particularly with respect to oxalate. Therefore, current therapeutic strategies often fail in their compliance or effectiveness, and CaOx stone recurrence is still common. As the etiology of hyperoxaluria is diverse, a good understanding of how oxalate is absorbed and transported throughout the body, together with a better insight in the regulatory mechanisms, is crucial in the setting of future treatment strategies of this disorder. In this review, the currently known mechanisms of oxalate handling in relevant organs will be discussed in relation to the different etiologies of hyperoxaluria. Furthermore, future directions in the treatment of hyperoxaluria will be covered.