Calcium oxalate nephrolithiasis: defective oxalate transport

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

Kidney stone analysis techniques and the role of major and trace elements on their pathogenesis: a review

Kidney stone disease is a polygenic and multifactorial disorder with a worldwide distribution, and its incidence and prevalence are increasing. Although significant progress has been made in recent years towards identifying the specific factors that contribute to the formation of kidney stone, many questions on the pathogenesis of kidney stones remain partially or completely unanswered. However, none of the proposed mechanisms specifically consider the role(s) of the trace elements and, consequently, the contribution of trace constituents to the pathogenesis of kidney stones remains unclear and under debate. The findings of some studies seem to support a role for some major and trace elements in the initiation of stone crystallization, including as a nucleus or nidus for the formation of the stone or simply as a contaminant of the stone structure. Thus, the analysis of kidney stones is an important component of investigations on nephrolithiasis in order to understand the role of trace constituents in the formation of kidney stones and to formulate future strategies for the treatment and prevention of stone formation and its recurrence. The aim of this review is to compare and evaluate the methods/procedures commonly used in the analysis of urinary calculi. We also highlight the role of major and trace elements in the pathogenesis of kidney stones.

Influence of nutrition on feline calcium oxalate urolithiasis with emphasis on endogenous oxalate synthesis

The prevalence of calcium oxalate (CaOx) uroliths detected in cats with lower urinary tract disease has shown a sharp increase over the last decades with a concomitant reciprocal decrease in the occurrence of struvite (magnesium ammonium phosphate) uroliths. CaOx stone-preventative diets are available nowadays, but seem to be marginally effective, as CaOx urolith recurrence occurs in patients fed these diets. In order to improve the preventative measures against CaOx urolithiasis, it is important to understand its aetiopathogenesis. The main research focus in CaOx formation in cats has been on the role of Ca, whereas little research effort has been directed towards the role and origin of urinary oxalates. As in man, the exogenous origin of urinary oxalates in cats is thought to be of minor importance, although the precise contribution of dietary oxalates remains unclear. The generally accepted dietary risk factors for CaOx urolithiasis in cats are discussed and a model for the biosynthetic pathways of oxalate in feline liver is provided. Alanine:glyoxylate aminotransferase 1 (AGT1) in endogenous oxalate metabolism is a liver-specific enzyme targeted in the mitochondria in cats, and allows for efficient conversion of glyoxylate to glycine when fed a carnivorous diet. The low peroxisomal activity of AGT1 in cat liver is compatible with the view that felids utilised a low-carbohydrate diet throughout evolution. Future research should focus on understanding de novo biosynthesis of oxalate in cats and their adaptation(s) in oxalate metabolism, and on dietary oxalate intake and absorption by cats.

Alterations in band 3 protein and anion exchange in red blood cells of renal failure patients

The precise nature of band 3 protein and its involvement in oxalate exchange in the red blood cells (RBCs) of renal failure patients has not been studied in detail. Therefore, here we studied the oxalate exchange and binding by band 3 protein in RBCs of humans with conditions of acute and chronic renal failure (ARF and CRF). The RBCs of ARF and CRF patients exhibited abnormal red cell morphology and an increased resistance to osmotic hemolysis. Further, an increase in the cholesterol content and decrease in the activities of Na(+)-K(+)-, Ca(2+)-, and Mg(2+)-ATPases of membranes were observed in the RBCs of ARF and CRF patients. A decrease in the oxalate flux was observed in the RBCs of ARF and CRF patients. The oxalate-binding activities of the RBC membranes were significantly lower in ARF (20 pmoles/mg protein) and CRF (5.3 pmoles/mg protein) patients as compared to that in the normal subjects (36 pmoles/mg protein). DEAE-cellulose and Sephadex G-200 column chromatography purification profiles revealed a distinctive shift in oxalate-binding activity of band 3 protein of RBCs of ARF and CRF patients as compared to that of the normal subjects. It was also observed from the binding studies with a fluorescent dye, eosin-5-maleimide, which specifically binds to band 3 protein, that the RBCs of ARF and CRF patients exhibited only 53 and 32% of abundance of band 3 protein, respectively, as compared to that in the RBCs of the normal subjects, thus revealing a decrease in the band 3 protein content in ARF and CRF patients. These results for the first time showed a decrease in the oxalate exchange in RBCs of patients with ARF and CRF, which was also concomitant with the low levels of abundance of band 3 protein.

Counteraction of oxalate induced nitrosative stress by supplementation of l-arginine, a potent antilithic agent

BACKGROUND

Our understanding of nitrosative stress in the process of urolithiasis is far from complete. Earlier studies carried out in our laboratory demonstrate the presence of nitrated THP in stone formers, l-arginine (l-arg) a precursor of nitric oxide (NO), attenuates the endothelial dysfunction caused by reactive nitrogen species. We investigated the role of l-arg in ethylene glycol (EG)-induced urolithic rat model and observed its antilithic and antioxidative properties.

METHODS

Hyperoxaluria was induced using 0.75% EG in drinking water. l-arg [1.25 g/kg body weight] was given orally for a period of 28 days.

RESULTS

EG-treated rats showed significant loss in body weight and increase in the activities of oxalate synthesizing enzymes such as glycollic acid oxidase in liver. Lactate dehydrogenase activity in liver and kidney was increased. The activity of the free radical producing enzyme xanthine oxidase, tissue oxalate and calcium levels were significantly increased in EG-treated rats. Depletion in the antioxidant enzymes, membrane bound ATPases and thiol status was observed in these rats. l-arg co-supplementation to EG-treated rats maintained the activities of the oxalate synthesizing enzymes and free radical producing enzymes with in the normal range. Tissue oxalate and calcium levels were also maintained near normal in l-arg treated hyperoxaluric rats. l-arg, by its cytoprotective effect, maintained the thiol status, thereby preserving the activities of the membrane bound ATPases and preventing proteinuria and subsequent weight loss in EG-treated rats.

CONCLUSION

l-arg feeding prevents the retention of calcium oxalate crystals in hyperoxaluric rats by way of protecting the renal cells from oxidative injury and also by providing a second line of defense through the normalization of the oxalate metabolism. It reduces the risk of stone formation, by curtailing free radicals and hyperoxaluria as both of them have to work in close association to form stones.

ACP Best Practice No 181: Chemical pathology clinical investigation and management of nephrolithiasis

Renal stones have afflicted humans for millennia but there is still no solution to this problem. This review discusses the laboratory and metabolic aspects of the clinical management of patients with renal stones, both primary and secondary in origin. First, non-pharmacological interventions such as increased fluid intake, decreased protein consumption, dietary changes in sodium, calcium, oxalate, potassium, purine, vitamins, and essential fatty acids are considered. Then specific pharmacological treatment to modify urine calcium, oxalate, urate, citrate, and acidity are considered. Finally, more unusual types of stone are examined.

Effects of Diet on Urine Composition of Cats With Calcium Oxalate Urolithiasis

Ten client-owned cats with calcium oxalate (CaOx) urolithiasis were evaluated to determine the effect of diet on urine CaOx saturation. Two dietary treatments were evaluated in each cat: the diet consumed just prior to urolith detection and a canned diet formulated to prevent CaOx uroliths. This study revealed that hypercalciuria is a consistent abnormality in cats with CaOx urolith formation. When urolith-forming cats consumed a diet formulated to prevent urolith formation, activity product ratios for CaOx (which estimate the degree to which urine is saturated with CaOx) were significantly lower. These results suggest that consumption of an appropriately formulated urolith-prevention diet will reduce recurrence of CaOx urolithiasis.

Oxalate binding proteins in calcium oxalate nephrolithiasis

The existence of several oxalate specific binding proteins have been demonstrated in human and rat kidney. These occur in both cortical and medullary cells and are distributed mostly in the subcellular organelles. About 1/3 of the total cellular oxalate binding was localised in the inner mitochondrial membrane while the rest was in the nucleus. The purified mitochondrial oxalate binding protein (62 kDa) was composed, with a higher molar proportion, of basic amino acids, and could accumulate oxalate on incorporation into liposomes. In the nucleus, histone H(1B) (27.5 kDa), nuclear membrane protein (68 kDa) and nuclear pore complex protein (205 kDa) were present with oxalate binding activities. In addition, a 45 kDa calcium oxalate binding protein was identified in most of the subcellular organelles. Both mitochondrial and nuclear oxalate binding proteins and calcium oxalate binding protein have shown the kinetic properties of specificity, saturability, pH and temperature dependency, energy of activation and inhibition by substrate analogues. All oxalate binding proteins were sensitive to the transport inhibitor 4'-4' diisothiocyano stilbene-2-2 disulphonic acid (DIDS), which is known to interact with the lysine moiety of the proteins. Calcium oxalate monohydrate (COM) crystals adsorbed oxalate binding proteins from human and rat kidney, and oxalate binding proteins isolated from human kidney stone matrix also exhibited the above kinetic properties. In experimental hyperoxaluria, all of the renal oxalate binding proteins showed enhanced oxalate binding activity with increased protein concentration. This enhanced oxalate binding activity was also attributed to increased lipid peroxidation, which correlated positively, and to decreased thiol status, which correlated negatively. A positive correlation was observed between the lipid peroxidation and both the oxalate binding activity of the in vitro peroxidised subcellular organelles and the purified protein. Similarly, in an in vivo hyperoxaluric condition, a negative correlation was observed between thiol content and both the oxalate binding activity of the peroxidised subcellular organelles and the purified protein. In the calcium oxalate crystal growth system, the oxalate binding proteins behaved either as promoters or inhibitors of the nucleation and aggregation of crystals. Following the peroxidation of the proteins, the degree of effect of the promoter protein was further stimulated while the degree of inhibition caused by the inhibitor protein further declined. Similar observations were duplicated with the proteins derived from hyperoxaluric rat kidney or kidney homogenate subjected to in vitro lipid peroxidation. The oxalate binding proteins were thought to modulate the crystallisation process in an hyperoxaluric condition similar to calcium specific binding protein modulators.

Does fluid amount and choice influence urinary stone formation in persons with spinal cord injury?

OBJECTIVE

To examine the effects of fluid intake and beverage type on stone formation in persons with spinal cord injury (SCI).

DESIGN

A matched case-control study.

SETTING

A rehabilitation center within a university hospital.

PATIENTS

Forty-one patients with SCI who were diagnosed with urinary stones between 1992 and 1998 (stone cases) and 171 age- and duration-matched controls were interviewed by telephone.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Urinary stones.

RESULTS

Stone cases were more likely than controls to be white, to be current smokers, to be users of indwelling catheterization, and to have a lower body mass index (kg/m(2)), worse renal function, and higher urine specific gravity (P<.10). In a multivariable model that adjusted simultaneously for other possible risk factors, the greater consumption of juice (>207mL/d) was associated with a 70% decreased risk (odds ratio=0.3; 90% confidence interval, 0.1-1.0). An interesting, but not significant (P=.15), increased risk for coffee consumption was observed. There was no association for total fluid intake.

CONCLUSION

Total fluid intake does not appear to determine stone occurrence; however, fluid type may influence stone formation in persons with SCI. Further study is required to verify these findings, thus possibly leading to an effective fluid regimen for stone prophylaxis.

Determination of urinary calcium-oxalate formation risk with BONN-Risk-Index and EQUIL applied to a family

Apart from environmental and acquired risk factors, a person's genetic predisposition may have a distinct influence on the probability of the onset of urolithiasis. To investigate the family related development of calcium oxalate, CaOx, crystallization risk, we studied urines from three generations of the same family. The paternal line has been suffering from CaOx-urolithiasis for at least two generations; no case of urolithiasis has been reported from the maternal line and the youngest generation.We applied the BONN-Risk-Index and the computer program EQUIL to determine the crystallization risk of each family member (n = 7). We clearly verified by probability calculations of the existence of the two risk groups within the family and showed that one of the siblings of the youngest generation may have inherited the stone-formation risk from its paternal relatives as this person clearly reflects a high risk pattern.

Urine specific gravity and water hardness in relation to urolithiasis in persons with spinal cord injury

STUDY DESIGN

A matched case-control study.

OBJECTIVES

To clarify the influence of urine specific gravity and drinking water quality on the formation of urinary stones in persons with spinal cord injury (SCI).

SETTING

A rehabilitation center within a university hospital.

METHODS

Between 1992 and 1998, 63 stone cases (31 kidney, 27 bladder, and five both) and 289 age-duration-matched controls were recruited from a cohort of SCI patients enrolled in an on-going longitudinal study. Data on urine specific gravity and other characteristics of study participants were retrieved from the database and medical charts. Community water supply information was provided by the Alabama Department of Environmental Management. Multivariable conditional logistic regression analysis was performed to evaluate the association with stone formation.

RESULTS

SCI individuals who had urinary stones were more likely than control subjects to use indwelling catheters and have decreased renal function. The occurrence of stones was not significantly related to gender, race, severity of injury, urinary tract infection, nor urine pH. After controlling for the potential confounding from other factors, a continuously increasing stone occurrence with increasing specific gravity was observed (P=0.05); this association was stronger for kidney (odds Ratio [OR]=1.8 per 0.010 g/cm(3)) versus bladder stones (OR=1.2) and for recurrent (OR=2.0) versus first stones (OR=1.5). Increased water hardness was not significantly associated with a decreased stone occurrence.

CONCLUSIONS

Study results suggest that maintaining urine specific gravity below a certain level might reduce the occurrence of urinary stones. This could be easily achieved by using a dipstick for self-feedback along with appropriate fluid intake. For persons with SCI who are at an increased risk of a devastating stone disease, this prophylactic approach could be very cost-effective; however, this requires further confirmation.

Risk factors for kidney stones in older women in the southern United States

BACKGROUND

The occurrence of kidney stones is disproportionate in the southern region of the United States. Risk factors for the occurrence of kidney stones in this geographic area have not been reported previously.

METHODS

The Women's Health Initiative (WHI) is an ongoing multicenter clinical investigation of strategies for the prevention of common causes of morbidity and mortality among postmenopausal women. A case-control ancillary study was conducted on 27,410 (white or black) women enrolled in the 9 southern WHI clinical centers. There were 1,179 cases (4.3%) of kidney stones at the baseline evaluation. Risk factors for stone formation were assessed in cases versus age- and race-matched control subjects.

RESULTS

Risk factors (univariate) included low dietary potassium (2,404 versus 2,500 mg/day, P = 0.006), magnesium (243 versus 253 mg/day, P = 0.003) and oxalate (330 versus 345 mg/day, P = 0.02) intake, as well as increased body mass index (28.5 versus 27.7 kg/m2, P = 0.001) and a history of hypertension (42% versus 34%, P = 0.001). A slightly lower dietary calcium intake (683 versus 711 mg/day, P = 0.04) was noted in case subjects versus control subjects, but interpretation was confounded by the study of prevalent rather than incident cases. Supplemental calcium intake >500 mg/day was inversely associated with stone occurrence.

CONCLUSION

Multivariate risk factors for the occurrence of kidney stones in postmenopausal women include a history of hypertension, a low dietary intake of magnesium, and low use of calcium supplements.

Effects of hydrochlorothiazide and diet in dogs with calcium oxalate urolithiasis

OBJECTIVE

To determine whether hydrochlorothiazide (HCTZ) reduces urinary calcium excretion in dogs with calcium oxalate urolithiasis.

DESIGN

Original study.

ANIMALS

8 dogs with calcium oxalate urolithiasis.

PROCEDURE

4 treatment protocols were evaluated in each dog (a low calcium, low protein diet designed to prevent calcium oxalate urolith formation with and without administration of HCTZ [2 mg/kg (0.9 mg/lb) of body weight, PO, q 12 h] and a maintenance diet with higher quantities of protein and calcium with and without administration of HCTZ). At the end of each 2-week treatment period, 24-hour urine samples were collected. Blood samples were collected during the midpoint of each urine collection period. Analysis of variance was performed to evaluate the effects of HCTZ and diet on urine and serum analytes.

RESULTS

Hydrochlorothiazide significantly decreased urine calcium and potassium concentration and excretion. Hydrochlorothiazide also significantly decreased serum potassium concentration. Compared with the maintenance diet, the urolith prevention diet significantly decreased urine calcium and oxalic acid concentration and excretion. Dogs consuming the urolith prevention diet had significantly lower serum concentrations of albumin and urea nitrogen.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of HCTZ decreased urine calcium excretion in dogs with a history of calcium oxalate urolith formation. The greatest reduction in urine calcium concentration and excretion was achieved when dogs received HCTZ and the urolith prevention diet. Results of this study suggest that the hypocalciuric effect of HCTZ will minimize recurrence of calcium oxalate urolith formation in dogs; however, long-term controlled clinical trials are needed to confirm the safety and effectiveness of HCTZ.

The influence of a high-oxalate/low-calcium diet on calcium oxalate renal stone risk factors in non-stone-forming black and white South African subjects

OBJECTIVE

To evaluate the influence of a high-oxalate/low-calcium diet on calcium oxalate stone risk factors in both black South Africans (who are largely immune to kidney stones) and white South Africans (in whom stones are more common).

SUBJECTS AND METHODS

Urinary and dietary variables were examined in 11 black and 11 white South African men. None of the subjects had had a kidney stone or any metabolic illness. Their normal domestic food intake was assessed using a semiquantitative food frequency questionnaire. Subjects were given a standardized high-oxalate/low-calcium diet for 3 days; 24-h urine samples were collected before the protocol and during the final day. The samples were analysed using routine modern laboratory techniques. The urine analysis data were used to calculate the Tiselius risk index and the relative urinary supersaturations of calcium oxalate, uric acid and calcium phosphate.

RESULTS

Urine analysis showed an intriguing anomaly; black subjects had significantly higher urinary pH and oxalate values than whites (6.50 vs 6.21 and 0.23 vs 0.14 mmol/24 h, respectively), while their urinary citrate was lower (1.47 vs 3.69 mmol/24 h). In addition, the Tiselius risk index and relative supersaturation of calcium oxalate were higher in black subjects. These results are contrary to those which might have been reasonably expected when comparing stone-free and stone-prone groups. After the dietary protocol, the only urinary variable which changed significantly was urinary oxalate, which increased by 57% in whites.

CONCLUSION

Factors which are conventionally used to assess stone risk (pH, oxaluria, citraturia, relative supersaturation) are not helpful in identifying why South African blacks are relatively immune to stones. We suggest that relatively lower oxalate absorption rates may be a physiological feature of this racial group.

Calcium metabolism in sarcoidosis and its clinical implications

OBJECTIVE

: To examine the clinical implications of disturbed calcium metabolism in sarcoidosis and how the pathophysiology affects management strategies.

METHODS

: The literature concerning calcium metabolism in sarcoidosis was reviewed.

RESULTS

: Dysregulated calcium metabolism is a well-recognized complication of sarcoidosis, resulting in hypercalcaemia (prevalence 5-10%), hypercalcuria (40-62%) and reduced bone density (40-55%). Extrarenal synthesis of calcitriol [1,25(OH)(2)D(3)] is central to the pathogenesis of abnormal calcium homeostasis, but alterations in parathyroid hormone (PTH) activity and the expression of PTH-related peptide have also been demonstrated. The immunosuppressive properties of calcitriol suggest that the raised levels seen in sarcoidosis could represent an adaptive response to the undefined antigen that causes sarcoidosis.

CONCLUSIONS

: The mechanisms of abnormal calcium metabolism in sarcoidosis need to be understood when treating hypercalcaemia, hypercalcuria and corticosteroid-induced osteoporosis. Studies are required to determine if the currently available therapies for osteoporosis are safe and effective in sarcoidosis.

Should dietary calcium and protein be restricted in patients with nephrolithiasis?

Renal stone disease is a painful condition that affects 1-20% of the general population. Therapy aimed at decreasing the incidence of recurrent stones includes dietary advice. Dietary considerations include intake of both calcium and protein. Calcium restriction in stone formers is not recommended because it can have adverse effects on bone and the incidence of stones. Although a high-protein diet can elevate urinary calcium, uric acid, and sulfate and decrease urinary citrate, which may alter the propensity to form stones, restriction of protein to less than the current RDA for the management of stone disease can not be recommended at this time.

Relative hyperoxaluria, crystalluria and haematuria after megadose ingestion of vitamin C

BACKGROUND

Long-term or high-dosage consumption of vitamin C may play a role in calcium oxalate kidney stone formation. The present study was undertaken to determine the biochemical and physicochemical risk factors in a male subject who developed haematuria and calcium oxalate crystalluria after ingestion of large doses of ascorbic acid for 8 consecutive days.

METHODS

Twenty-four-hour urine samples were collected before and during the ascorbic acid ingestion period as well as after the detection of haematuria. A special procedure was implemented for urine collections to allow for oxalate, ascorbate and other urinalysis. Oxalate was determined in the presence of EDTA to prevent in vitro conversion to ascorbic acid, whereas ascorbate itself was determined by manual titration in a redox method using the dye dichlorophenolindophenol. Urinalysis data were used to compute calcium oxalate relative supersaturations and Tiselius risk indices, whereas scanning electron microscopy was used to examine urinary deposits.

RESULTS

Oxalate excretion increased by about 350% during ascorbate ingestion before haematuria. Ascorbate concentrations also increased dramatically but appeared to reach a plateau maximum. Increasing calcium excretion was accompanied by decreasing potassium and phosphate values. The calcium oxalate relative supersaturation and Tiselius risk index increased during vitamin C ingestion and large aggregates of calcium oxalate dihydrate crystals were observed by scanning electron microscopy immediately after the detection of haematuria.

CONCLUSION

High percentage metabolic conversion of ascorbate to oxalate in this subject caused relative hyperoxaluria and crystalluria, the latter manifesting itself as haematuria. Clinicians need to be alerted to the potential dangers of large dose ingestion of vitamin C in some individuals.

Pathogenesis of idiopathic calcium nephrolithiasis: update 1997

Idiopathic calcium nephrolithiasis (ICN) is a frequent disease in Western countries. The physicochemical theory of lithogenesis, which explains stone formation by the precipitation, growth, and crystalline aggregation of lithogenic salts in the urine, has contributed greatly to the understanding of the pathogenesis of calcium urolithiasis. However, several aspects are still unexplained; the co-existence of familial occurrence, primary tubular dysfunctions with ICN, and anomalies in the systemic handling of oxalate and calcium led to the development of a cellular hypothesis of ICN. A number of cellular defects in the handling of ions has been reported that involves both anion and cation transport. These anomalies are probably the expression of a still unknown cellular defect in idiopathic calcium stone formers. We suggested that an anomaly in the cell membrane composition might be responsible for the complex array of cell ion flux abnormalities observed in ICN. Recently, a disorder in the n-6 polyunsaturated fatty acid series has been described; it is characterized by a lower linoleic acid content and a higher arachidonic acid concentration in both plasma and erythrocyte membrane phospholipids of renal calcium stone patients. This anomaly could cause an increased activity of ion carriers; furthermore, it may lead to increased prostaglandin synthesis and to secondary phenomena at the kidney, skeletal, and intestinal level. As a consequence, critical conditions for lithogenesis in the kidney may ensue. The data suggest a common pathogenesis for hypercalciuria and hyperoxaluria. The systemic defect in the phospholipid arachidonic acid level may be both of dietary or genetic origin; experimental data suggest that the increase in delta-6 desaturase activity, the limiting enzyme in the metabolic pathway of polyunsaturated fatty acids, might be relevant to the pathogenesis of lipid abnormalities observed in nephrolithiasis and to the pathogenesis of ICN and its related problems (at the kidney, intestinal, and bone level).

Renal injury mediated calcium oxalate nephrolithiasis: role of lipid peroxidation

The role of lipid peroxidation (LPO) in renal tubular damage mediated calcium oxalate retention was investigated in a rat model. Hyperoxaluria, without deposition of oxalate in kidney, was induced by administration of ethylene glycol (EG), a precursor of oxalate. Oxidative stress condition was produced by administration of buthionine sulfoximine (BSO), an inhibitor of glutathione biosynthesis. BSO-treated rats showed a significant (p < 0.001) increase in LPO over EG-treated rats and it was almost doubled in BSO + EG treated rats. LPO was accompanied by significant urinary excretion of renal damage marker enzymes such as gamma-glutamyl transpeptidase (gamma-GT), alkaline phosphatase (ALP) and cathepsin D, mucoproteins, and glycosaminoglycans (GAGs) in the BSO and BSO + EG groups but not in the EG group. Urinary excretion of gamma-GT (r = +0.90) (p < 0.001) and deposition of oxalate (r = +0.78) (p < 0.001) in kidney positively correlated with LPO. These results suggest that LPO initiates renal damage, thereby leading to calcium oxalate retention and stone formation.

Lack of increased urinary calcium-oxalate supersaturation in long-term kidney transplant recipients

Nephrolithiasis is uncommon after kidney transplantation. However, calcium (Ca) supplementation, which has been proposed as a treatment of post-transplant osteopenia, might increase calciuria and bolster Ca stone formation. Therefore, in 24-hour urine of 82 normocalcemic long-term renal transplant recipients (RT) and in 82 healthy subjects (HS), we assessed some Ca nephrolithiasis risk factors and the Ca-salt saturation estimated by the ion-activity product index (AP) and relative supersaturation (RS). In RT, calciuria was lower (mean +/- SD, 3.20 +/- 2.25 vs. 4.61 +/- 1.71 mmol/day; P < 0.001), urinary volume higher (2.41 +/- 0.83 vs. 1.39 +/- 0.53 liter/day; P < 0.001), oxaluria higher (419 +/- 191 vs. 311 +/- 79 mumol/day; P < 0.001) and citraturia lower (1.40 +/- 1.36 vs. 3.77 +/- 1.36 mmol/day; P < 0.001) than in HS. As a result, Ca-oxalate supersaturation was lower in RT than HS (AP, 1.07 +/- 0.69 vs. 2.07 +/- 1.13, P < 0.001; and RS, 0.62 +/- 0.26 vs. 0.94 +/- 0.21, P < 0.001), and was similar in subgroups of RT (N = 37) and HS (N = 37) matched for urinary volume, demonstrating that even without any larger urinary volume, Ca-oxalate saturation was not higher in RT than HS, and suggesting that opposite changes in Ca and oxalate in RT likely canceled their effects on lithogenic risk. In RT which had similar urinary pH and phosphate (P) than HS, Ca-P supersaturation was lower than in HS for brushite (AP, 3.25 +/- 6.67 vs. 6.01 +/- 4.85, P < 0.001; RS, -0.33 +/- 0.76 vs. 0.48 +/- 0.53, P < 0.001) and octacalcium phosphate (RS, -0.95 +/- 0.72 vs. 0.21 +/- 0.85, P < 0.001), and similar for apatite. Finally, fasting calciuria and calciuric response to a single oral Ca load were similar in RT (N = 19) and HS (N = 8). Together, these results argue strongly against a higher risk of Ca stone formation in RT than HS, even in case of Ca supplementation.

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.

Inappropriate phosphate excretion in idiopathic hypercalciuria: the key to a common cause and future treatment?

AIMS

To present experimental evidence in support of a proposed common cause for absorptive hypercalciuria, renal hypercalciuria, renal phosphate leak and enhancement of 1,25-(OH)2-vitamin D concentrations in patients presenting with renal stone disease; and to suggest further investigation with a view to new management.

METHODS

An oral calcium loading test was administered to 15 patients with renal stones and 10 normal controls in the fasting state: urine and blood were collected hourly. After the second urine sample, 400 mg calcium dissolved in water was administered orally. Serum calcium, albumin, parathyroid hormone (PTH), and phosphate were measured together with urine calcium clearance and urinary phosphate from which the TmPO4/glomerular filtration rate (GFR) ratio was calculated. Serum 1,25-(OH)2-vitamin D was measured in the first serum sample. In addition, 24 hour urine calcium results were collected retrospectively from the patients' case notes over the previous 18 months.

RESULTS

In the basal state, renal stone patients had an overall greater phosphaturia (lower TmPO4/GFR: median 1.72 compared with 2.10 in controls) and increased calcium clearance. Serum corrected calcium and PTH concentrations did not differ between the groups. After calcium loading, serum calcium and urine calcium clearance rose in both groups, with patients with renal stones experiencing a greater percentage fall in phosphaturia. In both groups TmPO4/GFR fell (greater phosphaturia) with increased serum corrected calcium, with the patients showing notably greater phosphaturia for any given calcium concentration. Patients also had notably greater phosphaturia compared with the serum calcium concentration for any given PTH value. Serum 1,25-(OH)2-vitamin D was higher in patients than controls and for any 1,25-(OH)2-vitamin D concentration phosphaturia measured against serum calcium was greater in patients than controls. 1,25-(OH)2-vitamin D did not correlate with phosphaturia relative to serum calcium concentrations within the patient and control groups.

CONCLUSIONS

It is proposed that patients with idiopathic hypercalciuria have an "inappropriately' high phosphate excretion for any given serum calcium concentration. Loss of phosphate may induce increased activation of 1,25-(OH)2-vitamin D. Some of the commonly described causes of stone formation may be manifestations of a single mechanism.

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.

Cystic transformation and calcium oxalate deposits in rete testis and efferent ducts in dialysis patients

The histological study of the testes and epididymides obtained from autopsies of 24 men with chronic renal insufficiency revealed bilateral cystic transformation of the rete testis and efferent ducts in patients who underwent hemodialysis or peritoneal dialysis, but not in patients who did not receive this treatment. The lesion was associated with an accumulation of crystalline calcium oxalate deposits in the lumen of the rete testis and efferent ducts, and in the connective tissue adjacent to these excretory ducts. The rete testis epithelium showed columnar transformation with occasional papillary proliferations. Neither atypias or mitoses were observed. In three specimens, fibrosis and giant cell reactions was also present in the rete testis at the level of crystalline deposits. In three specimens, the caput epididymidis was enlarged, and the efferent ducts showed an increase in both tubular diameter and epithelial height, irregular outline, and development of diverticula. The lesions appeared within 30 months after the onset of dialysis.

Idiopathic calcium oxalate urolithiasis and endogenous oxalate production

Despite the great effort that has gone into investigating urolithiasis, this condition still persists as one of the major ailments of the urinary tract. Calcium oxalate urolithiasis is the most common form, accounting for some 60 to 80% of total stones. This review examines the elements (i.e., urine volume and pH and urinary excretion of calcium, oxalate, citrate, urate, magnesium, pyrophosphate, and glycosaminoglycans) that give rise to idiopathic calcium oxalate urolithiasis. Treatment strategies for idiopathic calcium oxalate urolithiasis, including lithotripsy, also are discussed. Urinary oxalate excretion is a major risk factor for calcium oxalate urolithiasis, with 85 to 95% of the urinary load derived endogenously. The factors controlling endogenous oxalate production are reviewed, including pathways for the diversion of glyoxylate from oxalate production. The use of beta-aminothiols and other substances to reduce endogenous oxalate production in subjects with idiopathic calcium oxalate urolithiasis is also discussed. A review of current methodologies for the determination of urinary oxalate is also included.

Erythrocyte transmembrane flux and renal clearance of oxalate in idiopathic calcium nephrolithiasis

An anomaly in erythrocyte oxalate transport has been reported in patients with idiopathic calcium oxalate nephrolithiasis. Even if clinical and experimental evidence suggests a causal role of this cellular anomaly in calcium nephrolithiasis, a definitive answer to this fundamental question is still lacking. We approached this problem by searching for a possible relationship between the erythrocyte oxalate self-exchange anomaly and the renal clearance of this anion in stone formers. In 10 idiopathic calcium-oxalate renal stone formers, and 10 healthy subjects we evaluated the erythrocyte oxalate flux rate, and the renal fractional clearance of oxalate by a recently described enzymathic procedures for plasma oxalate determination. With respect to controls, stone formers had higher oxalate flux rate in erythrocytes, and higher oxalate renal fractional clearance with a significant direct correlation between the two parameters. These data are compatible with a membrane transport abnormality within the kidney of these stone formers, and the existence of a common defect of the oxalate transport shared by both erythrocytes and tubular renal cells. The latter may be crucial in the pathogenesis of calcium oxalate nephrolithiasis, by modifying the renal handling of oxalate.

Effect of second messenger systems on oxalate uptake in renal epithelial cells

The oxalate transport system along with protein phosphorylation appears to be deranged in stone formers. This study was undertaken to characterize in LLC-PK1 cells in culture the effect of altering specific intracellular second messenger systems on oxalate uptake. Cellular uptake experiments were performed at 37 degrees C in buffer [265 mM mannitol, 5 mM NaOH, 5 mM KOH, 10 mM Ca-EGTA, 25 mM HEPES/TRIS, pH = 7.4 or in Hank's balanced salt solution (HBSS)] containing 200 microM labeled oxalate (1-14C, 0.3 microCi). Cells were preincubated with DAG (final concentration of 100 microM), phorbol myristate acetate (10 microM), forskolin (50 microM), 8-bromo-cyclic AMP (50 microM), trifluoroperazine (20 microM) and low molecular weight heparin (1 mg/ml) for 10 min in the presence and absence of the anion transport inhibitor DIDS (100 microM) and the effect(s) on oxalate uptake at 10, 25 and 45 min incubation were determined. Chemicals (DAG, forskolin, TPA and 8-bromo-cAMP) which stimulate protein kinase A or C activity resulted in an increased uptake of oxalate while inhibitors of these systems (trifluoroperazine and low molecular weight heparin) resulted in decreased oxalate uptake. The results demonstrate that oxalate uptake in renal tubular cells is modulated by protein kinase C and A dependent mechanisms.

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.

A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones

BACKGROUND

A high dietary calcium intake is strongly suspected of increasing the risk of kidney stones. However, a high intake of calcium can reduce the urinary excretion of oxalate, which is thought to lower the risk. The concept that a higher dietary calcium intake increases the risk of kidney stones therefore requires examination.

METHODS

We conducted a prospective study of the relation between dietary calcium intake and the risk of symptomatic kidney stones in a cohort of 45,619 men, 40 to 75 years of age, who had no history of kidney stones. Dietary calcium was measured by means of a semiquantitative food-frequency questionnaire in 1986. During four years of follow-up, 505 cases of kidney stones were documented.

RESULTS

After adjustment for age, dietary calcium intake was inversely associated with the risk of kidney stones; the relative risk of kidney stones for men in the highest as compared with the lowest quintile group for calcium intake was 0.56 (95 percent confidence interval, 0.43 to 0.73; P for trend, < 0.001). This reduction in risk decreased only slightly (relative risk, 0.66; 95 percent confidence interval, 0.49 to 0.90) after further adjustment for other potential risk factors, including alcohol consumption and dietary intake of animal protein, potassium, and fluid. Intake of animal protein was directly associated with the risk of stone formation (relative risk for men with the highest intake as compared with those with the lowest, 1.33; 95 percent confidence interval, 1.00 to 1.77); potassium intake (relative risk, 0.49; 95 percent confidence interval, 0.35 to 0.68) and fluid intake (relative risk, 0.71; 95 percent confidence interval, 0.52 to 0.97) were inversely related to the risk of kidney stones.

CONCLUSIONS

A high dietary calcium intake decreases the risk of symptomatic kidney stones.