Nucleation Of Calcium Oxalate Research Articles

Increased calcium oxalate crystal nucleation and aggregation by peroxidized protein of human kidney stone matrix and renal cells.

Kidney stone matrix protein fractions eluted from DEAE cellulose column showed increased oxalate binding activity and had negative correlation with reduced thiol content. Fraction I (eluted in Tris-HCl, pH 7.4) and fraction 3 (0.3 M NaCl in buffer) showed nucleation and aggregation-promoting properties while fraction 2 (0.05 M NaCl in buffer) showed an inhibitory effect in an in vitro crystallization system. On peroxidation, fractions 1 and 3 showed a further increase in the promoting effect whereas fraction 2 showed a reduction in the inhibitory effect of nucleation and aggregation of calcium oxalate crystals. Protein peroxidation was negatively correlated with the inhibitory activity of the protein on calcium oxalate nucleation and aggregation. A similar promoting effect of nucleation and aggregation was seen with mitochondria and nucleus after peroxidation. These studies suggested that peroxidation of protein or tissue had greater influence on the nucleation and aggregation property of calcium oxalate crystal growth.

Nucleation and aggregation of calcium oxalate in whole urine; spectrophotometric sedimentation analysis: a new approach to study the aggregation of calcium oxalate dihydrate.

Spectrophotometric and scanning electron microscopic (SEM) studies of oxalate-induced crystallization have been performed in whole urine with and without continuous magnetic stirring and before and after millipore filtration of urine. With continuous stirring, preferential nucleation was observed and this followed second order kinetics. Important crystal aggregation only occurred after an oxalate load above 1 mmol/l and without stirring. Under these conditions and at an ionic calcium concentration of 2 mmol/l, single crystals and aggregates of calcium oxalate dihydrate and monohydrate of well defined sizes were produced. Single dehydrates, their aggregates and the other particles could be distinguished by their significantly different sedimentation rates. From sedimentation curves an aggregation ratio for calcium oxalate dihydrate (aggregated/total dihydrate particles) was extrapolated. Millipore filtration removing important urinary macromolecules increased this aggregation ratio as well as the size of the aggregates on SEM pictures.

Citrate determines calcium oxalate crystallization kinetics and crystal morphology-studies in the presence of Tamm-Horsfall protein of a healthy subject and a severely recurrent calcium stone former.

The aim of this study was to measure the effects of normal (nTHP) and abnormal stone former Tamm-Horsfall protein (SF-THP) on calcium oxalate (CaOx) nucleation and aggregation as well as on crystal morphology, in presence or absence of citrate. Nucleation and aggregation of CaOx crystals from a supersaturated, stirred solution (200 mM NaCl, 10 mM Na-acetate, pH 5.70, 5 mM Ca and 0.5 mM Ox) were studied by spectrophotometric time-course measurements of OD at 620 nm (OD(620)). Measured parameters were induction time t(I) (time to induce formation of detectable particles), S(N), (slope of increase of OD(620), mainly due to crystal nucleation), and S(A), (slope of decrease of OD(620) after equilibrium has been reached, due to crystal aggregation). Effects of citrate, nTHP and SF-THP on these parameters were measured, and scanning electron microscopy (SEM) was performed. At 1.5, 2.5 and 3.5 mM, citrate increased t(I) and inhibited crystal nucleation (by 78-87%) as well as aggregation (by 63-70%), and smaller CaOx crystals (length/width ratio 1.7+/-0.1) than under standard conditions (length/width 3.9+/-0.5) were visible (P<0.001). Normal THP at 30 and 40 mg/l inhibited crystal nucleation and, more strongly, aggregation (inhibition 76-81%). SEM revealed a decrease in length/width ratio to 2.6+/-0.4 (P=0.051 vs standard conditions) and less aggregation than without nTHP. At all concentrations tested, SF-THP reduced t(I) (P=0.0001 vs standard conditions) and promoted aggregation (inhibition -48 to -33%); crystals were elongated with a length/width ratio of 4.3+/-0.6 (P<0. 05 vs nTHP). When simultaneously present with nTHP, citrate enhanced the inhibitory effects of nTHP, producing the smallest (length/width 1.5+/-0.1) and least aggregated crystals. Finally, 3.5 mM citrate turned promotory SF-THP into a crystallization inhibitor with abundant small and clustered, but not aggregated crystals. Citrate appears to be the main determinant of CaOx crystallization rates and crystal morphology in the presence of nTHP as well as SF-THP. Its effects appear to predominate over those of THP, since even promotory SF-THP is turned into a crystallization inhibitor in the presence of citrate. This re-emphasizes at a morphological level what has been concluded from functional as well from clinical studies, namely that citrate is needed in urine at equimolar concentrations to calcium in order to prevent the formation of large crystal aggregates in presence of abnormal THP.

Membranes and their constituents as promoters of calcium oxalate crystal formation in human urine.

We have proposed that membranes of cellular degradation products are a suitable substrate for the nucleation of calcium oxalate (CaOx) crystals in human urine. Human urine is generally metastable with respect to CaOx. To demonstrate that cellular membranes present in the urine promote nucleation of CaOx we removed these substrates by filtration or centrifugation and induced crystallization by adding sodium oxalate, before and after filtration or centrifugation. In a separate experiment, membrane vesicles isolated from rat renal tubular brush border were added into the filtered or centrifuged urine before crystal induction. Crystals were counted using a particle counter. Urine, the pellet, and retentate were analyzed for the presence of membranes, lipids, and proteins. Lipids were further separated into different classes, identified, and quantified. Both filtration and centrifugation removed lipids, proteins, and membrane vesicles, causing a reduction in lipid and protein contents of the urine. More crystals formed in whole than in filtered or centrifuged urine. The number of crystals significantly increased when filtered urine was supplemented with various urinary components such as the retentate and phospholipids, which are removed during filtration. We also determined the urinary metastable limit with respect to CaOx. Filtration and centrifugation were associated with increased metastable limit which was reduced by the addition of membrane vesicles. These results support our hypothesis that urine normally contains promoters of CaOx crystal formation and that membranes and their constituents are the most likely substrate for crystal nucleation in the urine.

Crystallization during volume reduction of solutions with a composition corresponding to that in the collecting duct: the influence of hydroxyapatite seed crystals and urinary macromolecules.

To examine the effect of hydroxyapatite (HAP) seed crystals and urinary macromolecules on the crystallization under conditions similar to those in the collecting duct, we evaporated 100 ml samples of salt solutions with an ion composition assumed to correspond to that in the collecting duct without and with HAP seed crystals. The crystallization in seeded solutions was assessed both with and without dialysed urine (dU). After evaporation the number and volume of crystals were recorded in a Coulter Multisizer and the crystal morphology examined with scanning electron microscopy (SEM) and X-ray crystallography. Addition of HAP crystals was apparently followed by an approximately 15-20% increase in heterogeneous nucleation of calcium oxalate (CaOx). In these experiments SEM and X-ray crystallography showed a high percentage of CaOx in the precipitate. In samples reduced to 40-69 ml, addition of dU to the collecting duct solution containing HAP seed resulted in a greater mean (SD) number of crystals; 3895 (1841) in samples with dU and 1785 (583) in samples without. This was mainly explained by an increased mean (SD) number of small crystals. The mean crystal volume was 17.8 (1.1) and 34.3 (9.1) in samples reduced to 40 69 ml with and without dU, respectively. This might reflect the inhibitory effect of dU on the growth and/or aggregation of the CaOx-CaP precipitate or a promoted nucleation resulting in a large number of small crystals. It is concluded that calcium phosphate formed above the collecting duct might induce heterogeneous nucleation of CaOx at lower levels of the renal collecting system, and that urinary macromolecules are powerful modifiers of these processes.

Direct measurement of calcium oxalate nucleation with a laser probe.

To understand the genesis of urinary stones different crystallization models have been developed to simulate the crystallization processes that occur at the beginning of the stone formation. In this study a laser probe working on the principle of the measurement of back-scattered light was mounted on a batch crystallization model, in which calcium oxalate nucleation was induced by titration of an artificial urine with ammonium oxalate. With the laser probe the particle numbers and particle size distributions in the range from 1-250 microm can be measured in a single experiment. Measurement is performed directly at the place of crystal development and, therefore, falsification of the results (by dilution, mechanical transport or isolation, temperature gap, multiple measurements, mathematical calculations after indirect measurements, etc.) is prevented. In a basic urine, containing 7.5 mmol/l calcium (4.13 mmol/l free calcium), nucleation of calcium oxalate appears at a concentration of 0.58 mmol/l oxalate. At the start of nucleation the particle size rises to 7 microm. After addition of citrate and magnesium in concentrations up to 12 mmol/l the metastable limit is clearly shifted up to 1.50 mmol/l oxalate. Both findings are in good agreement with the literature. The laser crystallization model has proved to be useful for the testing of inhibitors and promoters of urinary stone formation.

Nucleation of calcium oxalate crystals by albumin: Involvement in the prevention of stone formation

Urine is supersaturated in calcium oxalate, which means that it will contain calcium oxalate crystals that form spontaneously. Their size must be controlled to prevent retention in ducts and the eventual development of a lithiasis. This is achieved, in part, by specific inhibitors of crystal growth. We investigated whether promoters of crystal nucleation could also participate in that control, because for the same amount of salt that will precipitate from a supersaturated solution, increasing the number of crystals will decrease their average size and facilitate their elimination. Albumin was purified from commercial sources and from the urine of healthy subjects or idiopathic calcium stone formers. Its aggregation properties were characterized by biophysical and biochemical techniques. Albumin was then either attached to several supports or left free in solution and incubated in a metastable solution of calcium oxalate. Kinetics of calcium oxalate crystallization were determined by turbidimetry. The nature and efficiency of nucleation were measured by examining the type and number of neoformed crystals. Albumin, one of the most abundant proteins in urine, was a powerful nucleator of calcium oxalate crystals in vitro, with the polymers being more active than monomers. In addition, nucleation by albumin apparently led exclusively to the formation of calcium oxalate dihydrate crystals, whereas calcium oxalate monohydrate crystals were formed in the absence of albumin. An analysis of calcium oxalate crystals in urine showed that the dihydrate form was present in healthy subjects and stone formers, whereas the monohydrate, which is thermodynamically more stable and constitutes the core of most calcium oxalate stones, was present in stone formers only. Finally, urinary albumin purified from healthy subjects contained significantly more polymers and was a stronger promoter of calcium oxalate nucleation than albumin from idiopathic calcium stone formers. Promotion by albumin of calcium oxalate crystallization with specific formation of the dihydrate form might be protective, because with rapid nucleation of small crystals, the saturation levels fall; thus, larger crystal formation and aggregation with subsequent stone formation may be prevented. We believe that albumin may be an important factor of urine stability.

In Situ Analysis of Europium Calcium Oxalate Crystallization Using Luminescence Microspectroscopy

Luminescence microspectroscopy (LMS) is described here as an in situ approach that combines the spatial resolution and imaging capabilities of optical microscopy with the structural sensitivity of rare earth ion spectroscopy. In this work, LMS, laser-induced luminescence spectroscopy, and conventional Raman spectroscopy were used to analyze the effect of Eu3+ admixtures on calcium oxalate crystallization. This new combination of techniques was used to establish a link between crystal morphology, phase, and impurity ion bonding environments for single crystals in contact with their aqueous growth solution. Distinct, spatially resolved Eu3+ luminescence spectra were measured for 1−20 μm crystals separated by less than 10 μm. Results show that micromolar quantities of Eu3+ significantly inhibit calcium oxalate nucleation in static, low ionic strength (∼2 mM) supersaturated solutions at room temperature. Eu3+ stabilizes and incorporates into the dihydrate phase of calcium oxalate and the bipyramidal morpholog...

Crystallization of calcium oxalate in minimally diluted urine

Crystallization of calcium oxalate was studied in minimally diluted (92%) urine using a mixed suspension mixed product crystallizer in series with a Malvern particle sizer. The crystallization was initiated by constant flow of aqueous sodium oxalate and urine into the reaction vessel via two independent feed lines. Because the Malvern cell was in series with the reaction vessel, noninvasive measurement of particle sizes could be effected. In addition, aliquots of the mixed suspension were withdrawn and transferred to a Coulter counter for crystal counting and sizing. Steady-state particle size distributions were used to determine nucleation and growth kinetics while scanning electron microscopy was used to examine deposited crystals. Two sets of experiments were performed. In the first, the effect of the concentration of the exogenous sodium oxalate was investigated while in the second, the effect of temperature was studied. Calcium oxalate nucleation and growth rates were found to be dependent on supersaturation levels inside the crystallizer. However, while growth rate increased with increasing temperature, nucleation rates decreased. The favored phases were the trihydrate at 18°C, the dihydrate at 38° and the monohydrate at 58°C. The results of both experiments are in agreement with those obtained in other studies that have been conducted in synthetic and in maximally diluted urine and which have employed invasive crystal counting and sizing techniques. As such, the present study lends confidence to the models of urinary calcium oxalate crystallization processes which currently prevail in the literature.

Growth of calcium oxalate monohydrate at phospholipid Langmuir monolayers

Calcium oxalate monohydrate crystals have been nucleated from metastable solutions at Langmuir monolayers of the phospholipids dipalmitoylphosphatidylglycerol (DPPG), dipalmitoylphosphatidylserine and dipalmitoylphosphatidylcholine and the fatty acid arachidic acid. The phospholipid monolayers were used as model systems for domains of pure lipid in cellular media as part of investigations of their potential role in the nucleation of calcium oxalate in the urinary tract. Crystal formation was monitored at the air/water interface using Brewster angle microscopy and in transferred films using SEM and TEM. For each Langmuir monolayer, it was observed that nucleation is heterogeneous and is selective with respect to the orientation and morphology of the precipitated crystals with up to 90% of crystals growing with the ( 1 0 1 ̄ ) face oriented towards the monolayer interface. The selectivity is attributed to calcium binding at the lipid monolayer favoring formation of the calcium-rich ( 1 0 1 ̄ ) face. The behavior at each monolayer was similar, although a higher rate of crystal formation was observed at the anionic DPPG interface.

Stone prevention: why so little progress? *

Despite intensive research the knowledge of stone pathogenesis, which is the basis of every rational stone metaphylaxis, has remained rather scanty. Epidemiology shows that stone formation in most patients is only a sporadic event, probably resulting from a coincidence of different factors. The hypercalciuria, hypocitraturia, hyperuricosuria and hyperoxaluria frequently found in calcium stone formers can be influenced therapeutically and, in affluent societies, seem to be the result of protein over-consumption. These four factors favour crystallization processes in urine. However, urine is normally protected from nucleation, growth and aggregation of calcium minerals by crystallization inhibitors. In urine, crystallization of calcium oxalate can only be induced by an extreme supersaturation, a deficient inhibitor activity and promoters of crystallization. To form a stone, crystals have to be retained in the urinary collecting system. Two mechanisms of retention are discussed: large crystal aggregates trapped in collecting ducts of renal papillae, or a pre-existing calcification of the papilla (mainly calcium phosphate) that may be responsible for growth of an initially fixed particle to a concretion large enough to become symptomatic. An excessive oxalate intake combined with a low calcium consumption can produce marked hyperoxaluria. In the animal model, hyperoxaluria induces not only calcium oxalate crystallization but also papillary damage and incrustations. Hypercalciuria at a low pH favours the aggregation of calcium oxalate, and at a high pH the crystallization of calcium phosphate, a promoter of heterogeneous nucleation of calcium oxalate. All these factors and further complex phenomena mentioned in this paper have to be taken in account to perform rational stone metaphylaxis.

Calcium Phosphate/Calcium Oxalate Crystal Association in Urinary Stones: Implications for Heterogeneous Nucleation of Calcium Oxalate

Most stones contain more than one type of crystals, and some combinations, such as calcium phosphate/calcium oxalate, are more common than others. Epitaxy between the crystals has been suggested to play a role in growth of such stones. The specific aim of this study is to investigate the involvement of calcium phosphate in crystallization of calcium oxalate. Twenty calcium oxalate stones or stone fragments were examined using various microscopic techniques, including scanning, transmission and back-scattered electron microscopy. Similarly, calcium oxalate stones induced on a plastic foreign body implanted inside urinary bladders of laboratory rats were also investigated. Examination of the interface between calcium phosphate and calcium oxalate crystals was emphasized. Close association between crystals of calcium phosphate and calcium oxalate were found in both the human and rat stones. All crystals examined were associated with an organic matrix on the surface and contained copious amounts of organic material within the crystalline entities. Interface between the crystals also appeared to be occupied by organic matrix. Results of this and other studies from our laboratory indicate that epitaxy between various crystals, even though theoretically possible, appears unlikely in vivo. The appearance of specific crystalline combinations in stones is probably a result of the urinary environment being conducive for crystallization of those components. Heterogeneous nucleation of calcium oxalate is most probably induced by biological elements, including membranous cellular degradation products.

Poly(ethyleneimine) Inhibition Effect on Calcium Oxalate Crystallization

It is shown that in contrast to poly(N,N'-dimethylaminoethyl methacrylate), polyethyleneimine has the most significant inhibitory effect on the calcium oxalate (CO) crystallization among the polymer inhibitors tested up to now. This inhibition effect is complex: on the CO nucleation, crystal growth rate and on the crystallite aggregation

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 μm) 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.

Tamm-Horsfall mucoprotein reduces promotion of calcium oxalate crystal aggregation induced by urate in human urine in vitro.

1. Increasing the concentration of dissolved urate promotes calcium oxalate crystallization in urine from which Tamm-Horsfall mucoprotein, an inhibitor of calcium oxalate crystal aggregation, has almost completely been removed. This study aimed to determine whether the effect of urate could be reduced or abolished by a physiological concentration of Tamm-Horsfall mucoprotein. This was approached in two ways. 2. The effect of Tamm-Horsfall mucoprotein on calcium oxalate crystallization induced by urate was tested in ultrafiltered (10 kDa) urine samples from 10 healthy men. Tamm-Horsfall mucoprotein (35 mg/l) was added to half of each specimen, the urate concentration was increased by the addition of sodium urate solution and crystallization was induced by a standard load of oxalate. The remainder of each urine specimen was used as a control; these specimens were treated with an identical amount of urate solution, but contained no Tamm-Horsfall mucoprotein. Tamm-Horsfall mucoprotein had no effect on the urinary metastable limit or on the deposition of calcium oxalate, but significantly reduced the size of the particles precipitated. 3. The effect of increasing the urate concentration in the presence of Tamm-Horsfall mucoprotein was tested. Tamm-Horsfall mucoprotein (35 mg/l) was added to 10 ultrafiltered urine samples as before, the samples were divided, and the concentration of urate was increased in half of each specimen. Compared with the control to which no urate was added, urate significantly reduced the amount of oxalate required to induce spontaneous calcium oxalate nucleation and increased the median volume and the particle size of the material deposited.(ABSTRACT TRUNCATED AT 250 WORDS)

Limited Risk of Kidney Stone Formation During Long-Term Calcium Citrate Supplementation in Nonstone Forming Subjects

The physiological and physicochemical effects of long-term calcium citrate supplementation (25mmol, calcium per day) were assessed in 7 normal premenopausal women. Calcium citrate increased urinary calcium from 3.27 ± 0.42mmol, per day (standard deviation) before treatment to 5.16 ± 0.75mmol, per day after 1-month of treatment (p <0.0125). After 3 months of treatment urinary calcium decreased from the 1-month value to 4.54 ± 0.67mmol, per day (p <0.0125) but remained higher than the pretreatment value (p <0.0125). Fractional intestinal calcium absorption and serum 1,25-dihydroxyvitamin D levels decreased marginally at 1 month of calcium citrate therapy, from 0.457 ± 0.092 to 0.374 ± 0.035 (p <0.05) and from 103 ± 7 to 77 ± 14 pmol./l. (p <0.05), respectively. After 3 months of treatment fractional intestinal calcium absorption decreased further to 0.341 ± 0.061 (p <0.0125 compared to pretreatment), whereas serum 1,25-dihydroxyvitamin D remained unchanged at 82 ± 14pmol./l.Calcium citrate treatment decreased urinary phosphorus levels significantly from 18.9 ± 3.3 to 15.0 ± 2.5mmol, per day (p <0.0125) and 14.0 ± 2.5mmol, per day (p <0.05) at 1 and 3 months, respectively. Mean urinary oxalate decreased by 15 to 20% and urinary citrate increased marginally during treatment. Urinary saturation of calcium oxalate and brushite did not change during calcium citrate therapy, except at 1 month when the saturation of calcium oxalate increased marginally. The inhibitory activity of urine against spontaneous nucleation of calcium oxalate and brushite (formation product) did not change during treatment.In conclusion, long-term calcium citrate supplementation in normal subjects does not increase the propensity for crystallization of calcium salts in the urine. This protective effect is probably due to the attenuated increase in urinary calcium excretion (from a decrease in fractional intestinal calcium absorption), a decrease in urinary phosphorus and an increase in urinary citrate.

Heterogeneous Nucleation of Calcium Oxalate on Native Oxide Surfaces

ABSTRACTThe aqueous deposition of calcium oxalate onto colloidal oxides has been studied as a model system for understanding heterogeneous nucleation processes of importance in biomimetic synthesis of ceramic thin films. Calcium oxalate nucleation has been monitored by measuring induction times for nucleation using Constant Composition techniques and by measuring nucleation densities on extended oxide surfaces using an atomic force microscope. Results show that the dependence of calcium oxalate nucleation on solution supersaturation fits the functional form predicted by classical nucleation theories. Anionic surfaces appear to promote nucleation better than cationic surfaces, lowering the effective energy barrier to heterogeneous nucleation.

Glycosaminoglycans and Oxalocalcic Urolithiasis

A very simple analytical procedure with dimethylmethylene blue as photometric reagent was applied for the evaluation of glycosaminoglycan (GAG) urinary excretions and concentrations in both sexes, calcium oxalate stone formers, and a control group. The GAG concentrations varied significantly in stone formers (males and females) and in the control group; moreover, in some individual cases, a deficit of urinary GAGs was clearly detected. Apart from important inhibitory effects of GAGs on heterogenous calcium oxalate nucleation, the low urinary GAG content could be the cause of a pathological epithelium, favoring stone formation. The dimethylmethylene blue method is recommended as a quick screening procedure to determine a GAG deficit.

Heterogeneous nucleation of calcium oxalate crystals in the presence of membrane vesicles

Membrane-assisted crystallization of calcium oxalate was studied in vitro, using constant composition methodology. Rat renal tubular brush border membrane vesicles were incubated in supersaturated solution of calcium oxalate. Calcium and oxalate depletion started much earlier in the presence of the vesicles than in their absence; within 8, 32, or 258 min of the incubation of vesicles in calcium oxalate solutions of relative supersaturation of 12, 10 or 6 respectively. Thin plate-like crystals with jagged edges formed in association with the membrane vesicles. Since crystal nucleation in the presence of membrane vesicles started within 8 min at a relative supersaturation as low as 12, it will start significantly earlier in the urine of stone formers which is known to have higher relative supersaturation with respect to calcium oxalate. These results demonstrate that cellular membranes can efficiently induce nucleation of calcium oxalate crystals from a metastable solution in an vitro system. Similar membrane induced heterogeneous nucleation of calcium oxalate in vivo within the renal tubules is a distinct possibility.

Urinary macromolecules are promoters of calcium oxalate nucleation in human urine: Turbidimetric studies

Calcium oxalate crystallization was induced in the filtered, ultrafiltered (10 kDa) and retentate fractions of 24-h urine specimens obtained from 15 male controls and 10 male stone formers, by administration of an aqueous sodium oxalate challenge to each test solution. Crystallization rates were followed by monitoring of the increase in turbidity in these fractions as a function of time. A laboratory nephelometer, previously calibrated against a Coulter counter, was used for this purpose. In addition, to facilitate interpretation of turbidity data, a Malvern particle size analyzer was used to determine crystal sizes and numbers in control urines. Crystallization rates, crystal numbers and crystal sizes were generally lower in ultrafiltered fractions than in filtered or retentate fractions, indicating that urinary macromolecules are promoters of calcium oxalate nucleation. Data for stone formers suggest that the urinary macromolecules in this group may be more potent nucleation promoters than those in controls.