Calcium oxalate crystallization in urine: role of urate and glycosaminoglycans

Treatment of calcium nephrolithiasis in the patient with hyperuricosuria

Nearly one-third of patients with calcium stones have hyperuricosuria. In vitro studies and clinical trials have investigated the relationship between uric acid and calcium stones, but the association between hyperuricosuria and calcium stone formation in patients is still being debated. Uric acid appears to cause salting out of calcium oxalate in human urine. However, the importance of this in vitro phenomenon to the proposed association is not supported in cross-sectional observational studies. A small placebo-controlled randomized clinical trial showed that allopurinol decreased the rate of recurrent calcium oxalate calculi in patients with hyperuricosuria and normocalciuria. An assessment of the effect of combination therapy of allopurinol with indapamide showed no additive effect. Allopurinol may have antioxidant effects that are responsible for its reducing calcium stone formation, which are independent of xanthine oxidase inhibition. In addition, a newer xanthine oxidoreductase inhibitor, febuxostat, may also be effective in the prevention of calcium stones, as it reduces urinary uric acid excretion.

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.

Kidney stone disease

About 5% of American women and 12% of men will develop a kidney stone at some time in their life, and prevalence has been rising in both sexes. Approximately 80% of stones are composed of calcium oxalate (CaOx) and calcium phosphate (CaP); 10% of struvite (magnesium ammonium phosphate produced during infection with bacteria that possess the enzyme urease), 9% of uric acid (UA); and the remaining 1% are composed of cystine or ammonium acid urate or are diagnosed as drug-related stones. Stones ultimately arise because of an unwanted phase change of these substances from liquid to solid state. Here we focus on the mechanisms of pathogenesis involved in CaOx, CaP, UA, and cystine stone formation, including recent developments in our understanding of related changes in human kidney tissue and of underlying genetic causes, in addition to current therapeutics.

Dissolved urate salts out calcium oxalate in undiluted human urine in vitro: implications for calcium oxalate stone genesis

Hyperuricosuria has long been documented as a predisposing factor to calcium oxalate (CaOx) stone pathogenesis. However, its mechanism is still without sound scientific foundation. Previously, we showed that hyperuricosuria, simulated by the addition of dissolved sodium urate, promotes the crystallization of CaOx. In the present study, we demonstrate that the urate's effect on the crystallization is attributable to its salting out CaOx from solution. Furthermore, analysis of urines revealed that their metastable limit decreased with increases in the product of the prevailing concentrations of calcium and urate: this has implications for CaOx stone genesis. We also outline anti-salting out strategies for future research for the prevention and/or treatment of CaOx calculi.

Hyperuricosuric calcium nephrolithiasis

Since the findings of Yü and Gutman [1], the hyperuricosuric calcium stone former is a unique clinical entity. While an impressive number of clinical and epidemiologic studies implicate hyperuricosuria in calcium stone formation, the exact physicochemical mechanism by which uric acid affects calcium oxalate crystallization has not been proven. Allopurinol decreases stone recurrences and is the drug of choice for patients with isolated HCN.

Relationship between supersaturation and calcium oxalate crystallization in normals and idiopathic calcium oxalate stone formers

BACKGROUND

In an earlier study on recurrent CaOx stone formers with no detectable abnormalities, we found that the urine of these subjects had a lower tolerance to oxalate load than controls and that the removal of urinary macromolecules with a molecular weight greater than 10,000 D improved their tolerance to oxalate.

METHODS

The effects on CaOx crystallization of reduced urinary supersaturation of calcium oxalate (CaOx), induced by night water load, were studied in 12 normal males and in 15 male OxCa stone formers who were free from urinary metabolic abnormalities. The effect of the macromolecules, purified and retrieved from the natural and diluted urine, were analyzed in a metastable solution of CaOx.

RESULTS

The water load caused an increase in urine volume (from 307 +/- 111 to 572 +/- 322 ml/8 hr, P = 0.014 in normal subjects, and from 266 +/- 92 to 518 +/- 208 ml/8 hr, P = 0.001 in the stone formers) and a concomitant reduction of the relative CaOx supersaturation (from 8.7 +/- 2.5 to 5.1 +/- 2.5 ml/8 hr, P = 0.001 in normal subjects, and from 10.4 +/- 3.5 to 5.0 +/- 2.7 ml/8 hr, P = 0.001 in the stone formers). The decrease in CaOx supersaturation was accompanied by an increase of the permissible increment in oxalate, both in normal subjects (from 43.8 +/- 10.1 to 67.2 +/- 30. 3 mg/liter, P = 0.018) and in the stone formers (from 25.7 +/- 9.4 to 43.7 +/- 17.1 mg/liter, P = 0.0001), without any significant variations of the upper limit of metastability for CaOx (from 21.6 +/- 5.3 to 20.5 +/- 4.2 mg/liter in normal subjects, and from 18.7 +/- 4.5 to 17.1 +/- 3.7 mg/liter in the stone formers). The inhibitory effect of urinary macromolecules with molecular weight greater than 10,000 Daltons did not undergo any change when the latter were recovered from concentrated or diluted urine, either in normal subjects or in the stone formers.

CONCLUSIONS

Reduced CaOx supersaturation by means of water load has a protective effect with regards to CaOx crystallization in subjects who do not present any of the common urinary stone risk factors.

Morphological analysis of renal cell culture models of calcium phosphate stone formation

Cell culture models of calcium phosphate renal stone formation were established using the MDCK cell line. Renal microliths were detected within pseudocysts in three-dimensional soft agar cultures, and were also observed in the basal region of cells lining the cell sheet, and immediately beneath domes or blisters in monolayers and collagen gel cultures. Light and scanning electron microscopy indicated that these microliths had a similar lamellated and spherical appearance to those in humans. These microliths were first detected microscopically after 21 days of culture, and were found to be composed of calcium phosphate by X-ray and micro-infrared spectroscopic analyses. These culture models may provide a powerful new tool to study the pathogenesis of renal stone diseases and/or calcium phosphate stone formation in humans and animals.

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.

Effects of urinary macromolecules on the nucleation of calcium oxalate in idiopathic stone formers and healthy controls

Urinary macromolecules have attracted great interest because of their possible role as both promoters and inhibitors of calcium oxalate (CaOx) crystallization and it remains unclear whether there is any difference, in their nucleating activity, between stone formers and controls. We selected 9 male idiopathic CaOx stone formers whose 24-h urines presented no evidence of common urinary stone risk factors such as hypercalciuria, hyperoxaluria, hyperuricosuria, hypocitraturia, hypomagnesiuria or low glycosaminoglycans excretion and 12 male controls (matched for age and body weight) whose 24-h urines did not differ from those of stone formers. The study of urinary CaOx nucleation was made in freshly voided overnight urines whose biochemical composition was almost identical in the two groups. In filtered (0.22 micron) and ultrafiltered (10 kDa) urine we performed an oxalate tolerance test to determine the permissible increment of oxalate, the oxalate level for nucleation and the permissible increment of CaOx relative supersaturation (CaOx RS). In filtered urine from stone formers the permissible increment of oxalate was lower than controls (30 +/- 10.2 vs. 46.7 +/- 9.7 mg/l, P = 0.001), the oxalate level for nucleation was lower (64.4 +/- 14.2 vs. 79.5 +/- 15.6 mg/l, P = 0.035) and the permissible increment of CaOx RS was also lower (9.71 +/- 2.59 vs. 13.39 +/- 3.62, P = 0.018). In ultrafiltered urine these differences disappeared because the removal of macromolecules in stone formers significantly enhanced the oxalate-tolerance values. The difference between the change of the oxalate permissible increment of filtered and ultrafiltered urine allowed a distinction to be made between stone formers and controls that was not feasible in other ways (7.6 +/- 5.3 vs. 3.3 +/- 5.9 mg/l, P < 0.0001). The study suggests that, in idiopathic CaOx stone formers free from common urinary risk factors of CaOx crystallization, there is an increased tendency for CaOx nucleation in urine, which is mediated by macromolecular components.

Risk factors for calculus formation in patients with renal transplants

OBJECTIVE

To investigate the risk factors for stone formation in patients with functioning renal transplants in whom renal calculi develop.

PATIENTS AND METHODS

Renal calculi developed in six of 178 patients with functioning renal transplants under current review, an incidence of 3%. Risk factors for stone formation were investigated in five of these patients and compared with a randomly selected control group of 41 transplant patients with no stone problems.

RESULTS

Patients with transplant calculi typically passed smaller volumes of significantly more concentrated and alkaline urine with greater urinary excretion of uric acid (P < 0.05). Urine calcium excretion was also increased. Crystalluria was present in three of five stone formers compared with two of 25 controls. Overall, metabolic abnormalities included hypocitraturia (75%), hyperparathyroidism (36%), hypophosphataemia (24%) and hypercalcaemia (10%). Urinary infection was common (50%) and urinary output of magnesium and phosphate was at the lower end of normal for all patients.

CONCLUSION

These results suggest a multifactorial aetiology for stone formation in renal transplant recipients. Approaches to prevention and management are discussed.

Glycosaminoglycans and other sulphated polysaccharides in calculogenesis of urinary stones

Naturally occurring glycosaminoglycans (GAGs) and other, semisynthetic, sulphated polysaccharides are thought to play an important role in urolithiasis. Processes involved in urinary stone formation are crystallization and crystal retention. Oxalate transport and renal tubular cell injury are determining factors in these processes. In this article experimental results concerning the possible mechanisms of action of GAGs and other sulphated polysaccharides are reviewed. GAGs are inhibitors of crystal growth and agglomeration and possibly also of nucleation. They can prevent crystal adherence, correct an abnormal oxalate flux and prevent renal tubular cell damage.

The scientific basis of calcium oxalate urolithiasis. Predilection and precipitation, promotion and proscription

The documentation of no other human disease threads as far into antiquity as that of urinary stones. However, despite this arcane history and the development of novel means of treating the condition, the basic mechanisms of stone formation and the identity of indicators of recurrence remain largely shrouded in uncertainty. This review is concerned with what scientific information is known about the cause and formation of calcium oxalate stones--the most common component of human uroliths. Stone pathogenesis can be broadly divided into two main processes: (1) nucleation of insoluble crystals in urine and (2) retention of those crystals within the urinary tract. The first section of the article presents the various factors that are known or surmised to influence the likelihood that crystals will nucleate within the renal collecting system, and these are considered from the perspective of both their relation to metabolic disorders and their usefulness as diagnostic and therapeutic indicators. A discussion of factors that may influence the probability that newly formed crystals will be retained within the nephron forms the second part of the review. In developing this more mechanistic aspect of the disease the epitaxy, matrix and inhibitor theories of stone formation are presented, with particular emphasis being placed on their relation to crystal nucleation, growth or aggregation, and experimental evidence both for and against the hypotheses are discussed.