Evidence for a role of PDZ domain-containing proteins to mediate hypophosphatemia in calcium stone formers

Background

Hypophosphatemia (HYP) is common among calcium stone formers (SFs) and in rare cases is associated with mutations in sodium-phosphate cotransporters or in Na+/H+ exchanger regulatory factor 1 (NHERF1), but the majority of cases are unexplained. We hypothesized that reduced sodium-phosphate cotransporter activity mediated via NHERF1 or a similar PDZ domain-containing protein, causes HYP. If so, other transport activities controlled by NHERF1, such as NHE3 and URAT1, might be reduced in HYP.

Methods

To test this idea, we analyzed two large but separate sets of 24-h urine samples and paired serums of 2700 SFs from the University of Chicago and 11 073 SFs from Litholink, a national laboratory. Patients were divided into quintiles based on serum phosphate.

Results

Males were more common in the lowest phosphate tiles in both datasets. Phosphate excretion did not vary across the quintiles, excluding diet as a cause of HYP. Tubule maximum (Tm) phosphate per unit glomerular filtration rate decreased and fractional excretion increased with decreasing phosphate quintiles, indicating reduced tubule phosphate reabsorption was responsible for HYP. Urine pH and serum chloride increased with decreasing serum phosphate, suggesting a coordinate change in NHE3 activity. Serum uric acid and Tm uric acid decreased significantly with decreasing serum phosphate, while uric acid excretion did not vary. Conclusion. HYP in SFs results from decreased tubule phosphate reabsorption and, being associated with related changes in other proximal tubule transporters, may arise from alterations in or signaling to PDZ-containing proteins.

Evidence for durable kidney stone prevention over several decades

OBJECTIVE

To analyse three outcomes of stone prevention strategies in one clinic devoted to that activity since 1969, i.e. stone recurrence rates, stone-related procedures and 24-h urinary stone risk factor, to assess whether such treatment can be maintained over long periods.

PATIENTS AND METHODS

We selected 2509 patients with at least one laboratory follow-up after initial clinical and laboratory evaluation. We divided them into five time cohorts of 5, 10, 15, 20 and >20 years of follow-up. Rates of new stones and stone-related procedures, and 24-h urinary stone risk factors were compared between the cohorts using analysis of variance and general linear modelling.

RESULTS

Stone rates and rates of stone-related procedures declined in all five cohorts, as did 24-h urinary stone risk indices. We found no diminution of treatment effects for any of these three over time.

CONCLUSION

Those patients who remained under active care had significant reductions in stone recurrence and rates of stone-related urological procedures for up to >20 years. However, only a small fraction of patients who entered the clinic remained for such long periods. Urine testing substantiates impressive and sustained reductions in supersaturation, the principle driving force for stone formation. Overall, for those patients willing to remain in continuous treatment for periods of up to three decades, medical stone prevention appears to be effective in maintaining low recurrence and procedure rates.

Evidence for increased postprandial distal nephron calcium delivery in hypercalciuric stone-forming patients

A main mechanism of idiopathic hypercalciuria (IH) in calcium stone-forming patients (IHSF) is postprandial reduction of renal tubule calcium reabsorption that cannot be explained by selective reduction of serum parathyroid hormone levels; the nephron site(s) responsible are not as yet defined. Using fourteen 1-h measurements of the clearances of sodium, calcium, and endogenous lithium during a three-meal day in the University of Chicago General Clinical Research Center, we found reduced postprandial proximal tubule reabsorption of sodium and calcium in IHSF vs. normal subjects. The increased distal sodium delivery is matched by increased distal reabsorption so that urine sodium excretions do not differ, but distal calcium reabsorption does not increase enough to match increased calcium delivery, so hypercalciuria results. In fact, urine calcium excretion and overall renal fractional calcium reabsorption both are high in IHSF vs. normal when adjusted for distal calcium delivery, strongly suggesting a distal as well as proximal reduction of calcium reabsorption. The combination of reduced proximal tubule and distal nephron calcium reabsorption in IHSF is a new finding and indicates that IH involves a complex, presumably genetic, variation of nephron function. The increased calcium delivery into the later nephron may play a role in stone formation via deposition of papillary interstitial apatite plaque.

Urine pH in renal calcium stone formers who do and do not increase stone phosphate content with time

BACKGROUND

Calcium phosphate (CaP) renal stones appear to be increasing in prevalence, and are caused by high urine CaP supersaturation, which arises from genetic hypercalciuria and high urine pH. Renal damage from stones or procedures, or treatments for stone could raise urine pH; alternatively pH may be intrinsically high in some people who are thereby predisposed to CaP stones.

METHODS

To distinguish these alternatives we sequenced changes in urine pH and stone CaP content asking which occurs first in patients whose stones showed progressive increase in CaP over time. From 4767 patients we found 62 in whom we could document transformation from calcium oxalate (CaOx) to CaP stones, and 134 CaOx controls who did not transform. Laboratory and clinical finding were contrasted between these groups.

RESULTS

Even when patients were forming relatively pure CaOx stones, those destined to increase stone CaP had higher urine pH than controls who never did so. Their higher pH was present before and during treatments to prevent new stone formation. Shock wave lithotripsy was strongly associated with increasing stone CaP but urine pH bore no relationship to number of procedures.

CONCLUSION

We conclude that high pH may not be acquired as a result of stones or their treatments but may precede transformation from CaOx to CaP stones and arise from diet or possibly heredity.

Evidence that postprandial reduction of renal calcium reabsorption mediates hypercalciuria of patients with calcium nephrolithiasis

Idiopathic hypercalciuria (IH) is common among calcium stone formers (IHSF). The increased urinary calcium arises from increased intestinal absorption of calcium, but it is unclear whether increased filtered load or decreased renal tubular reabsorption of calcium is the main mechanism for the increased renal excretion. To explore this question, 10 IHSF and 7 normal subjects (N) were studied for 1 day. Urine and blood samples were collected at 30- to 60-min intervals while subjects were fasting and after they ate three meals providing known amounts of calcium, phosphorus, sodium, protein, and calories. Fasting and fed, ultrafiltrable calcium levels, and filtered load of calcium did not differ between N and IHSF. Urine calcium rose with meals, and fractional reabsorption fell in all subjects, but the change was significantly higher in IHSF. The changes in calcium excretion were independent of sodium excretion. Serum parathyroid hormone levels did not differ between N and IHSF, and they could not account for the greater fall in calcium reabsorption in IHSF. Serum magnesium and phosphorus levels in IHSF were below N throughout the day, and tubule phosphate reabsorption was lower in IHSF than N after meals. The primary mechanism by which kidneys ferry absorbed calcium into the urine after meals is via reduced tubule calcium reabsorption, and IHSF differ from N in the magnitude of the response. Parathyroid hormone is not likely to be a sufficient explanation for this difference.

Renal Function in Patients With Nephrolithiasis

PURPOSE

We describe kidney function, as measured by creatinine clearance in stone formers, and classified by type of stone formed and systemic etiologies of stone formation.

MATERIALS AND METHODS

The mean of 3 pretreatment 24-hour creatinine clearance measurements in each of 1,856 stone formers and creatinine clearance in 153 normal individuals were used. Clearance was adjusted for patient sex, age and body weight using general linear modeling.

RESULTS

As a group, all stone formers had decreased clearance adjusted for age, sex and body weight compared to that in normal individuals. Although clearance was particularly low in cystine and struvite stone formers, they were below normal in even common CaOx stone formers.

CONCLUSIONS

As a rule, patients with kidney stones do not have normal kidney function. In clinical management all efforts must be made to minimize renal injury, balancing the risks of obstruction from stones against those of urological procedures.

Reduced renal function and benefits of treatment in cystinuria vs other forms of nephrolithiasis

A significant study from the USA compares cystine stone formers and routine stone formers; the former group had a higher requirement for therapeutic procedures, but this was less if they took chelating agents, although remaining higher than in the latter group. Other interesting findings are also presented.

OBJECTIVE

First, to compare two types of stone formers (SF), those with cystinuria and those without, for effects of treatments for stones, as cystinuria leads to recurrent stones that are difficult to fragment with shock-wave lithotripsy, and there is disagreement about the efficacy of current treatments. Second, to compare these two groups with respect to blood pressure (BP) and renal function, as cystine stones may be associated with more morbidity than are routine stones.

PATIENTS AND METHODS

Fifty-two cystinuric patients (cystine SF) entering our programme since 1970 were compared with 3215 SF without cystinuria (routine SF), of whom 114 had a single functioning kidney (routine SF + nephrectomy). All patients had three 24-h urine and blood samples taken to determine the risk of stones before their first clinic visit; these studies were repeated after therapy was initiated, and at regular intervals to monitor therapy. Cystine was measured in the urine samples of the cystine SF. All stone-related procedures were recorded, and BP measured at clinic visits. Creatinine clearances (CCr) were calculated from each set of serum and urine values. Cystine supersaturation (SS) was directly measured in 16 urine samples collected before treatment and 13 afterward.

RESULTS

Patients were treated with increased fluid intake, potassium alkali and chelating agents such as alpha-mercapto-propionyl-glycine, as needed. The mean (sd) CCr, corrected for age and gender, was significantly lower at entry in cystine SF than in routine SF, at 91 (6) vs 160 (1) L/day, respectively (P < 0.001), and remained so at the last CCr. Neither systolic nor diastolic BP, similarly corrected, differed between the groups, but cystine SF had significantly more procedures, corrected for time at risk, before treatment than did routine SF, at 4.0 (0.4) vs 1.86 (0.06), respectively (P < 0.001); time-adjusted procedures decreased significantly in both groups during treatment, but remained higher in cystine SF, at 0.88 (0.14) vs 0.23 (0.02), respectively, (P < 0.001). Urine volume and pH were significantly higher in cystine SF than in routine SF, both before and during treatment. Cystine SS decreased during treatment, consistent with the increase in urine volume and decline in procedure rates during treatment.

CONCLUSION

Cystine SF have significantly higher procedure rates than routine SF, but procedure rates decline during therapy, although they remain higher than in routine SF. The lower CCr in cystinurics suggests that treatment to prevent stone recurrence and the need for procedures is particularly important, and emphasizes the need for a close follow-up. Use of cystine SS measurements may allow closer monitoring of the effect of treatment on the risk of stone recurrence.

Crystal-associated nephropathy in patients with brushite nephrolithiasis

BACKGROUND

We have biopsied the renal cortex and papillae of patients who form brushite renal stones asking if this unusual stone type is associated with specific tissue changes. We contrasted these with biopsies of 15 calcium oxalate stone formers, three stone formers with intestinal bypass, and four normal subjects.

METHODS

We studied all ten brushite stone formers treated with percutaneous nephrolithotomy (PNL) during the past 3 years using digital video imaging of renal papillae, and obtained cortical and papillary biopsies. Biopsies were analyzed by light and electron microscopy, microinfrared spectroscopy, and electron diffraction.

RESULTS

Apatite crystals plugged scattered terminal collecting ducts whose cells were injured or dead, and surrounding interstitium inflamed and fibrotic. White papillary deposits of interstitial apatite particles, so called Randall's plaque, were also present. Glomerular changes and cortical tubular atrophy and interstitial fibrosis were moderate to severe.

CONCLUSION

Brushite stone formers combine the interstitial plaque of calcium oxalate stone formers with the collecting duct apatite plugs found in stone formers with intestinal bypass. Collecting duct injury and interstitial fibrosis are severe. Prominent cortical fibrosis, tubule atrophy, and glomerular pathology seem secondary to the collecting duct plugging. We believe crystallization obstructs and destroys terminal collecting duct segments thereby damaging nephrons, perhaps via intranephronal obstruction, and producing a hitherto unrecognized renal disease.

Nephrolithiasis and nephrocalcinosis in rats with small bowel resection

Intestinal resection (IR) may lead to hyperoxaluria and nephrolithiasis. A rat model of IR was developed, in which kidney stones form. We describe the urine chemistries and histopathologic features. Rats underwent resection of 40-45 cm of distal ileum (n=16) or sham resection (SR) (n=8), and were then fed a 1% Na oxalate, 0.02% Ca diet. After 1 week on the diet, 24 h urine samples were obtained for stone chemistries. At 4-7 months after surgery, kidneys were examined grossly and by light microscopy. The extent and location of crystallization was assessed by polarized light. Histochemistry and infrared spectroscopy were used to determine crystal composition. IR rats had higher urine oxalate excretion (P<0.01) and concentration (P<0.001) than SR rats, and lower urine citrate excretion; only IR rats formed kidney stones (12/15 surviving rats). Tissue calcification was found only in kidneys from IR rats, located in the cortex (83% of kidneys), medulla (73%) and papillary tip (47%). Crystals, composed of CaOx, apatite, and calcium carbonate, filled collecting duct lumens, and were associated with tubular obstruction, and interstitial inflammation. Crystals in the papillary interstitium incited inflammation with tubular destruction and development of progressive papillary erosion. This new rat model of nephrolithiasis and nephrocalcinosis resembles the pattern of urinary abnormalities and tissue calcification that may be seen in humans with small bowel resection. The model allows further studies of the mechanisms of renal crystal formation, and possible therapeutic interventions.

Stone formation is proportional to papillary surface coverage by Randall's plaque

PURPOSE

Randall's plaques are common in calcium oxalate (CaOx) stone formers (SF). Plaque coverage correlates directly with urine calcium excretion and inversely with urine volume. We hypothesize that plaque coverage should increase proportionally with increasing stone number. We measured plaque areas in idiopathic CaOx stone formers and nonstone formers (NSF), and identified significant relationships with quantified stone histories.

MATERIALS AND METHODS

A total of 13 SFs and 4 control NSFs underwent nephroscopic papillary mapping with representative still images and MPEG (Moving Pictures Experts Group) movies used to identify plaque and papillary borders. Stone histories were obtained through patient interviews, and from medical records and radiographs. The relationship of plaque coverage to clinical stone events was assessed by general multivariate linear modeling. Log transformation normalized the distribution of percent plaque coverage and stone number.

RESULTS

Plaque surface area in SFs differed significantly from that in NSFs (p <0.0001). The duration of stone disease and the log transformed percent plaque coverage correlated significantly with the number of stones (0.677 and 0.620, p = 0.003 and 0.008, respectively). On multivariate analysis and correcting for the duration of stone disease total percent plaque coverage correlated significantly with the number of stones (R = 0.496, p = 0.05). Disease duration and plaque coverage did not correlate significantly (p = 0.257).

CONCLUSIONS

Percent plaque coverage directly correlates with the number of stones formed even when corrected for the duration of stone disease. However, plaque coverage does not correlate with the duration of stone disease. These results support the hypothesis that the pathogenesis of CaOx stones begins with Randall's plaques.

Clinical implications of abundant calcium phosphate in routinely analyzed kidney stones

BACKGROUND

To better portray the clinical phenotype of kidney stone patients with high calcium phosphate (CaP) stone abundance, we present here clinical and laboratory findings of large numbers of stone formers (SF) with stone CaP ranging from 0% to 100%. Our purpose was to inform clinicians and highlight areas that seem to deserve further research.

METHODS

We calculated average percent CaP (CaP%) in all stones of 1201 patients, and classified them into CaOx (N= 1011) or CaP (N= 190). Sex differences, stone formation rates, urine stone risk factors, extracorporeal shock wave lithotripsy (ESWL) treatments, and relapse during treatment were quantified in relation to stone CaP content.

RESULTS

CaP% has risen for three decades, especially among women. ESWL rates adjusted for numbers of stones and duration of stone disease were higher in CaP SF (0.6 vs. 1.86 and 0.73 vs. 1.82, CaOx vs. CaP, men and women, respectively, P < 0.001), and especially when stones contained brushite (2.90 vs. 1.02 and 3.11 vs. 1.35, brushite vs. not, males and females, respectively, P < 0.001). Urine pH and CaP supersaturation rose in proportion to CaP% in a dose response manner. Relapse rates of CaP and CaOx SF did not differ, and both did well with medical prevention.

CONCLUSION

Stone CaP% has risen for three decades. CaP SF, particularly with brushite stones, receive more ESWL treatments than CaOx SF, not explained by stone number or duration of stone disease. Urine supersaturations explain the high CaP%. High CaP% does not hamper medical stone prevention.

Association of urinary pH with body weight in nephrolithiasis

BACKGROUND

The prevalence of kidney stone disease in the United States is progressively increasing, paralleling the growing rate of obesity. Uric acid nephrolithiasis, a condition associated with a low urinary pH, has been linked to obesity and insulin resistance. Based on these observations, we hypothesized that urinary pH may be inversely associated to body weight in nephrolithiasis.

METHODS

Data were retrieved from 4883 patients with nephrolithiasis who underwent ambulatory evaluation at two established stone clinics in Dallas and Chicago. The patients collected 24-hour urine samples on an outpatient basis, while avoiding any drug that could alter urinary pH. Patients were divided in increasing sextiles of body weight, and urinary pH was adjusted for urinary creatinine and for age.

RESULTS

Urinary pH had a strong, graded inverse association with body weight. Urinary creatinine and age were both found to be significant covariates of urinary pH, while gender was not a significant independent variable after adjustment for urinary creatinine. Mean 24-hour urinary pH, adjusted for age and urinary creatinine, were 6.09, 6.04, 6.01, 5.99, 5.97, and 5.91 for sextiles of body weight in increasing order from Dallas (P for linear trend <0.0001), and 6.18, 6.10, 6.04, 6.02, 5.97, and 5.88 for the sextiles from Chicago (P for linear trend <0.0001).

CONCLUSION

We conclude that urinary pH is inversely related to body weight among patients with stones. The results confirm the previously proposed scheme that obesity may sometimes cause uric acid nephrolithiasis by producing excessively acid urine due to insulin resistance.

Association of urinary pH with body weight in nephrolithiasis

BACKGROUND

The prevalence of kidney stone disease in the United States is progressively increasing, paralleling the growing rate of obesity. Uric acid nephrolithiasis, a condition associated with a low urinary pH, has been linked to obesity and insulin resistance. Based on these observations, we hypothesized that urinary pH may be inversely associated to body weight in nephrolithiasis.

METHODS

Data were retrieved from 4883 patients with nephrolithiasis who underwent ambulatory evaluation at two established stone clinics in Dallas and Chicago. The patients collected 24-hour urine samples on an outpatient basis, while avoiding any drug that could alter urinary pH. Patients were divided in increasing sextiles of body weight, and urinary pH was adjusted for urinary creatinine and for age.

RESULTS

Urinary pH had a strong, graded inverse association with body weight. Urinary creatinine and age were both found to be significant covariates of urinary pH, while gender was not a significant independent variable after adjustment for urinary creatinine. Mean 24-hour urinary pH, adjusted for age and urinary creatinine, were 6.09, 6.04, 6.01, 5.99, 5.97, and 5.91 for sextiles of body weight in increasing order from Dallas (P for linear trend <0.0001), and 6.18, 6.10, 6.04, 6.02, 5.97, and 5.88 for the sextiles from Chicago (P for linear trend <0.0001).

CONCLUSION

We conclude that urinary pH is inversely related to body weight among patients with stones. The results confirm the previously proposed scheme that obesity may sometimes cause uric acid nephrolithiasis by producing excessively acid urine due to insulin resistance.

Urine calcium and volume predict coverage of renal papilla by Randall's plaque

BACKGROUND

Renal papillary plaques are common in calcium stone formers. We hypothesized that plaque should increase directly with urine calcium excretion, and inversely with urine volume. To test this, we measured papillary plaque areas in both idiopathic calcium oxalate stone formers and nonstone formers and examined 24-hour urine data to identify significant correlations.

METHODS

Fourteen stone formers and four nonstone forming controls underwent papillary mapping with flexible nephroscopy. For each papillum, representative still images and moving pictures expert group (MPEG) movies were used to identify plaque extent and papillary borders. The mean fractional plaque coverage for each polar region (upper, inter, lower) and per papillum was calculated. The relationship of the plaque coverage data to urine measurements was assessed with general multivariate linear modeling.

RESULTS

Mean polar fractional plaque coverage was higher in the calcium oxalate stone formers (7.4% vs. 0.5%, P= 0.012) as was mean fractional plaque per papillum (7.6% vs. 0.6%, P= 0.011). When correlating mean polar plaque coverage to urine data, urine volume and calcium excretion were the only measurements with independent relationships to plaque (P= 0.002, adjusted multiple R2= 0.521), with higher calcium and lower volume increasing coverage. The same relationships hold for mean plaque per papillum, except that urine pH also becomes an independent factor (P= 0.001, adjusted multiple R2= 0.606).

CONCLUSION

Utilizing advanced digital video and endoscopic equipment, we have achieved the most accurate estimation of papillary plaque coverage to date. Our findings support the idea that urine volume and calcium are the main correlates of plaque coverage.

Outcome Of Metabolic Evaluation and Medical Treatment for Calcium Nephrolithiasis in a Private Urological Practice

PURPOSE

We determine if medications that have been proven effective for kidney stone prevention in prospective controlled trials can reduce kidney stone recurrence in a private practice of urology better than life-style advice, including hydration.

MATERIALS AND METHODS

Between July 1, 1995 and December 31, 1996, 203 patients with stones received care from 1 private practice. Physicians chose to evaluate these patients metabolically based on clinical judgement. Thiazide, potassium citrate and allopurinol are recognized in this study as active treatments, and drug treatment intervals were calculated. Relapse and recurrent stones were counted as those stones manifesting after the initial index event. A stone was called a relapse stone if its date was included in the active treatment interval.

RESULTS

The association between use of active therapy and ordering of metabolic evaluation was highly significant. Using survival tables, we separately considered all patients and only those who had formed more than 1 stone. For both populations active treatment reduced stone recurrence significantly more than diet advice and hydration.

CONCLUSIONS

Medications validated in trials and guided by metabolic evaluation lower stone recurrence when used in a private practice setting as they do in trials.

Causes and consequences of kidney loss in patients with nephrolithiasis

BACKGROUND

It is unknown whether stone formers may safely donate a kidney. Nephrectomy could accelerate stone formation, or loss of filtration with age. We contrast, here, the course of stone patients with two versus one kidney.

METHODS

One hundred fifteen patients with a single functioning kidney were compared with 3151 patients with two kidneys. Cause of kidney loss was determined, along with stone types, rates of stone formation, urine stone risk factors, and creatinine clearance.

RESULTS

Women were 49.6% of the patients with kidney loss, compared to 33.6% of ordinary stone formers. Obstruction, stone burden, and infection were the most common reasons for kidney loss. We found an increased number of struvite and calcium phosphate stones among single kidney patients. Before and during treatment, single kidney patients had fewer stones than ordinary stone formers. Creatinine clearance was lower in the single kidney patients; rate of loss of kidney function with age was higher among single kidney males versus two kidney males if all patients are considered. Among males >age 45 years, the difference disappears. Females with one and two kidneys lost function with age at equivalent rates. Compared with nonstone formers, male stone formers lose kidney function with age at an accelerated rate.

CONCLUSION

Nephrectomy does not worsen stone disease. It may increase loss of renal function among younger males. The pattern of renal function loss with age differs between stone formers and nonstone formers.

Thiazide does not affect urine oxalate excretion

PURPOSE

We assessed the effects of thiazide treatment on urine oxalate excretion in patients with kidney stones.

MATERIALS AND METHODS

Of 537 patients (231 women), 209 (81 women) received thiazide for stone prevention. Urine oxalate was measured in 3, 24-hour urines before treatment, and in 1, 24-hour urine after 6 to 12 weeks of treatment. This was an observational cohort and treatment was not randomized.

RESULTS

Urine oxalate increased in general. There was no difference in increase with or without thiazide, whether one considered simple t test comparisons or used ANOVA with pretreatment oxalate excretion as a covariate.

CONCLUSIONS

Thiazide administration exerts no measurable effect on urine oxalate excretion that can be detected in clinical practice.

Gender differences in seasonal variation of urine stone risk factors

PURPOSE

We determined whether men and women differ in urine stone risk factors during the 4 seasons of the year.

MATERIALS AND METHODS

Measurements from 28,498, 24-hour urines from stone forming patients prior to treatment were analyzed to determine whether monthly variation was significant and whether the sexes differed using ANOVA. Locations of supersaturation maxima were determined.

RESULTS

The 2 sexes showed modest sodium depletion in summer with a corresponding decrease in urine calcium but men showed a remarkable decrease in urine volume, causing high calcium oxalate supersaturation. Women had maximum calcium oxalate supersaturation in early winter because of decreasing urine volume and increasing urine calcium excretion. Urine pH was reduced in the 2 sexes during summer but the decrease was far more marked in men, who had a uric acid supersaturation spike.

PURPOSE

Overall the sexes differ markedly in the timing of stone risk. Men show a dual summer calcium oxalate and uric acid high risk, while women show a high early winter calcium oxalate high risk.

Randall's plaque of patients with nephrolithiasis begins in basement membranes of thin loops of Henle

Our purpose here is to test the hypothesis that Randall's plaques, calcium phosphate deposits in kidneys of patients with calcium renal stones, arise in unique anatomical regions of the kidney, their formation conditioned by specific stone-forming pathophysiologies. To test this hypothesis, we performed intraoperative biopsies of plaques in kidneys of idiopathic-calcium-stone formers and patients with stones due to obesity-related bypass procedures and obtained papillary specimens from non-stone formers after nephrectomy. Plaque originates in the basement membranes of the thin loops of Henle and spreads from there through the interstitium to beneath the urothelium. Patients who have undergone bypass surgery do not produce such plaque but instead form intratubular hydroxyapatite crystals in collecting ducts. Non-stone formers also do not form plaque. Plaque is specific to certain kinds of stone-forming patients and is initiated specifically in thin-limb basement membranes by mechanisms that remain to be elucidated.

Changes in urine volume accomplished by physicians treating nephrolithiasis

PURPOSE

We quantified the changes in urine volume and sodium accomplished in various practice settings and the consequent effects on calcium oxalate supersaturation.

MATERIALS AND METHODS

We determined comprehensive urine stone risk factors in 2,877 patients treated in 14 practices, including a university referral center and private sector practices. Changes in urine volume and stone risk factors were measured.

RESULTS

In a wide range of practice settings the volume increase was about 0.3 l. daily. Urine sodium excretion increased with volume for unexplained reasons, as did urine calcium excretion. As expected, thiazide lowered calcium excretion but the effect progressively decreased as urine volume increased. Therefore, urine calcium and calcium oxalate supersaturation changes were the result of opposing forces. The net effect was a decrease partly offset by sodium and calcium excretion increases.

CONCLUSIONS

Urine volume increments are modest in practice and they are modestly offset by increases in urine sodium due to increased sodium intake. Clinicians should strive for higher volume increases than are currently achieved and be vigilant concerning what seems to be a strong tendency toward a higher sodium intake with more fluids.

Bone mineral density and urine calcium excretion among subjects with and without nephrolithiasis

BACKGROUND

Bone mineral density (BMD) is reduced among patients with idiopathic hypercalciuria (IH) and nephrolithiasis. To disentangle effects of diet, stone formation, and physiology upon BMD, we studied vertebral and femoral neck BMD among relatives of hypercalciuric stone formers, and contrasted those with to those without stones.

METHODS

Among 59 subjects from 11 families, vertebral and femoral neck BMD, diet calcium intake, urine excretions of calcium, sodium, ammonium, titratable acid, sulfate, urea nitrogen, and serum levels of calcitriol and markers of bone turnover were studied.

RESULTS

Stone formers (SF) consumed less calcium than non-stone formers (NSF). Spine and femoral neck BMD z-scores varied inversely with urine calcium loss and urine ammonium excretion among SF but not NSF. No correlations of BMD z-score were found for bone markers, calcitriol, or any of the other measurements.

CONCLUSION

SF consumed less calcium, presumably to prevent more stones, and displayed a bone mineral responsiveness to calcium loss and ammonium excretion not present among NSF, who ate more calcium. Lowered calcium consumption in IH, perhaps in response to stone formation, alters bone responses in a direction that can predispose to mineral loss and eventual fracture.

Urine stone risk factors in nephrolithiasis patients with and without bowel disease

BACKGROUND

The prevalence of nephrolithiasis among patients with bowel disease is higher than in the general population. We examined urine stone risk factors and clinical characteristics of these patients, contrasted with a large group of stone forming patients without systemic disease.

METHODS

A total of 180 patients with bowel disease were compared with a group of 2048 nephrolithiasis patients with calcium or uric acid stones and without systemic diseases. Bowel diseases included inflammatory bowel disease with and without bowel resections, bowel resections from cancer or trauma, and bypass procedures for obesity or hypercholesterolemia. Urine stone risk factors, stone rates, stone compositions, and creatinine clearance were measured.

RESULTS

Compared to ordinary stone forming patients, bowel patients formed stones higher in rate of recurrence and in uric acid content. Uric acid content was highest when colon surgery had occurred. Urine volumes were low among all bowel patients except those with a bypass. Average creatinine clearance values were low among all bowel patients. Urine oxalate excretion was modestly elevated after small bowel resection, but very high with bypass. Supersaturations were increased mainly by low urine volume and-for uric acid-low pH. Patients with no surgery were indistinguishable from routine stone formers.

CONCLUSIONS

Low urine volume and pH are the main stone-forming abnormalities in bowel disease patients. Hyperoxaluria is extreme after bypass, but only modest after small bowel surgery. In the absence of surgery, bowel disease patients with stones cannot be distinguished from common stone formers by comprehensive stone risk measurements.

Reduced crystallization inhibition by urine from women with nephrolithiasis

BACKGROUND

Human urine is known to inhibit growth, aggregation, nucleation, and cell adhesion of calcium oxalate monohydrate (COM) crystals, the main solid phase of human kidney stones. This study tested the hypothesis that low levels of inhibition are present in women with calcium oxalate stones and, therefore, could promote stone production.

METHODS

In 17 stone-forming women and 17 normal women matched in age within five years, inhibition by dialyzed urine proteins of COM growth and aggregation was examined, as well as whole urine upper limits of metastability (ULM) for COM and calcium phosphate (CaP) in relation to the corresponding supersaturation (SS).

RESULTS

Compared to normals, patient urine showed a reduced ULM in relation to SS. In contrast to men, there was no difference in growth inhibition.

CONCLUSIONS

Reduced CaP and CaOx ULM values in relation to SS are a characteristic of female stone formers. This defect could promote stones by facilitating crystal nucleation. Abnormal inhibition may well be a very important cause of human nephrolithiasis.

Urinary stone risk factors in the siblings of patients with calcium renal stones

PURPOSE

We determined which, if any, urinary stone risk factors accurately discriminate stone forming and nonstone forming siblings of patients with calcium renal stones.

MATERIALS AND METHODS

A total of 252 siblings of stone formers provided 2, 24-hour urine samples, which were sent overnight and analyzed at a central laboratory. Standard stone risk factors were measured and the supersaturation of calcium oxalate, calcium phosphate and uric acid was calculated.

RESULTS

Discriminant functions were derived for each gender by multivariate analysis. In stone forming sisters higher urinary calcium and pH discriminated with a success rate of 70%. In stone forming brothers higher urinary calcium, lower urinary potassium and older age discriminated with a success rate of 79%.

CONCLUSIONS

Select urinary measurements as well as age classify siblings into those with and without stones with fair accuracy. Calcium excretion and urinary pH in females, and calcium excretion, urinary potassium and age in males are feasible identifiers of stone forming siblings. To determine whether these measurements can be used to predict new stone onset may require years of observation of our current cohort.

A single 24-hour urine collection is inadequate for the medical evaluation of nephrolithiasis

PURPOSE

We determined the adequacy of a single 24-hour urine sample for evaluating patients for medical renal stone prevention.

MATERIALS AND METHODS

A total of 459 patients from a private urology practice specializing in the treatment of urolithiasis and 683 from a university stone research clinic provided 2 and 3, 24-hour urine samples, respectively. We used samples 1 and 2 from private practice patients, and 1 and 3 from university clinic patients for analysis, and compared each to the others by correlation coefficients and calculation of the mean difference plus or minus standard deviation (SD) of the difference. Urine risk factors were measured by standard methods.

RESULTS

Although the correlation of urine values 1 and 2 was excellent for all stone risk factors, SD values for the differences were large enough that within 1 SD on either side of 0, which included 68.8% of cases, by chance urine 1 would depart from urine 2 by clinically important amounts. These departures would be more than sufficient to misdiagnose common metabolic disorders.

CONCLUSIONS

A single 24-hour sample is not sufficient for evaluating patients before metabolic treatment for stone prevention because misdiagnosis is common, leading to inappropriate treatment.

Patient adherence to long-term medical treatment of kidney stones

PURPOSE

We determine patient adherence to and quality of outcome of medical kidney stone treatment during a 30-year duration at a single university based referral clinic. We also analyze time trends in adherence and timing of followup measurements, and supersaturation reduction during treatment.

MATERIALS AND METHODS

Data on all patients who entered the University of Chicago Kidney Stone Prevention Program from 1970 to 2000 were analyzed. Fractions of new patients who had any followup and those remaining in followup at increasing intervals were analyzed. Timing of followup was measured. Changes in adherence during the 3 decades were also analyzed, as was reduction in supersaturation in regard to calcium oxalate, calcium phosphate and uric acid.

RESULTS

A total of 70% to 80% of patients were retained at each successive followup cycle with 2 physicians, and a clinical protocol that always required 6-week followup with 24-hour urine collection and a yearly one thereafter for stone risk factors. Retention decreased during the last 5 years of the 1990s. Supersaturation reduction was present by the first followup and remained constant or improved with time. Timing of followup measurements was in accord with our protocol.

CONCLUSIONS

At best, one can retain only 70% to 80% of patients in a followup program at each interval, and achieve supersaturation reductions that are constant and significant during the long term. Timing of followup measurements can be close to that of the protocol in use.

Proportional reduction of urine supersaturation during nephrolithiasis treatment

PURPOSE

During metabolic stone therapy, urine supersaturation decreases in proportion to pretreatment levels. We gauge the quantitative contribution of regression to the mean for reducing urine supersaturation from high pretreatment to lower values during therapy.

MATERIALS AND METHODS

The 24-hour urine supersaturations for calcium oxalate, calcium phosphate and uric acid were measured on 2 pretreatment and at least 1 treatment 24-hour collection for each of the 2,667 patients in 2 networks and at a university based specialty clinic. Changes in supersaturation between the first and second pretreatment collections were an estimate of random change and compared to therapeutic changes.

RESULTS

Supersaturations decreased between the first and second pretreatment collections, proportional to the supersaturation in the first collection. However, the magnitude of this effect was minor compared to therapeutic changes. Also, mean change between pretreatment collections was 0, whereas mean change with therapy was greater than 0 for all 3 supersaturations.

CONCLUSIONS

Although regression to the mean can be detected, it cannot be responsible for the decrease in urine supersaturation with therapy or the fact that the decrease is proportional to pretreatment mean supersaturation. The mechanisms responsible for proportional reduction remain to be clarified.

Solid phase assay of urine cystine supersaturation in the presence of cystine binding drugs

PURPOSE

We report a new kind of assay system for urine cystine supersaturation that is accurate in the presence of cystine binding thiol drugs. We measured the molar ratio of cystine dissolved per mole of drug.

MATERIALS AND METHODS

Measured amounts of cystine crystals were incubated in buffer or urine for 48 hours with stirring. The solid phase remaining was pelleted by centrifugation, extracted into a high pH buffer and measured. D-penicillamine, tiopronin and captopril were added to determine their effect on solid phase dissolution.

RESULTS

Total cystine calculated from urine and solid measurements closely matched the amounts of cystine weighed in, meaning that the assay system successfully recovered the total cystine from the 2 phases. Each drug dissolved solid cystine in a specific and fixed proportion to its molar concentration in the range of 0.2 to 0.4 mM. dissolution per mM. of drug. Solution measurements were not a reliable gauge to the actual amounts of cystine dissolved.

CONCLUSIONS

Changes in solid phase cystine accurately reflect buffer or urine supersaturation when thiol drugs are present. The solid phase assay is a technically straightforward and reliable way of assessing cystine movement into and out of urine that avoids complexity of measurement and distortions of assay systems by drugs. This assay enables one to assess the level of drug effect and the need for a change in dosing.

Differences in urine volume and supersaturation in 2 physician networks

PURPOSE

We determined whether a network of practices devoted to a broad range of urological care would achieve a decrease in metabolic stone risk comparable to that achieved by a network of similar practices that emphasized kidney stone management as a distinct specialized interest, provided that each was given equivalent access to high level urine testing and software support.

MATERIALS AND METHODS

Pretreatment and treatment 24-hour urine samples were obtained from patients in a large network of practices related by the shared use of lithotripsy facilities and instruments (group 2) and a contrasting network of practices that emphasize stone treatment over other concerns (group 1). All known urine risk factors, including supersaturation, were measured and calculated.

RESULTS

Treatment supersaturation values in group 2 exceeded those in group 1. The reason was unpredicted and unexplained but highly consistent lower urine volume in group 2 patients that was present before and persisted during treatment. Group 2 physicians mostly achieved changes in urine volume and stone risk factors equivalent to those of group 1 physicians but began with higher supersaturation due to lower urine volume.

CONCLUSIONS

A network of physicians not specialized for stone care may achieve a decreased risk equivalent to that of more specialized physicians. Initial patient characteristics may vary significantly in the groups for reasons that are unknown to date, greatly affecting treatment outcome.

Protein fluorescence measurements within electrospray droplets

The conformation of cytochrome c molecules within electrospray droplets is investigated by monitoring the laser induced fluorescence of its single tryptophan residue (Trp-59). By increasing the alcohol concentration of the electrosprayed solutions, protein denaturation is induced, giving rise to significant changes in the intensity of the detected fluorescence. Comparison with analogous denaturation experiments in solution provides information about the relative protein conformations and differences between the bulk-solution and droplet environments. Both electrospray-plume and bulk-solution fluorescence measurements using low methanol concentration solutions indicate the presence of folded protein structures. At high methanol content, fluorescence measurements are consistent with the presence of partly denatured or unfolded conformations. At intermediate methanol content, differences are observed between the extent of denaturation in solution and that within the droplets, suggesting electrosprayed proteins have more compact structures than those detected in bulk measurements using solutions of similar composition. This infers that some fraction of the proteins within the droplets have refolded relative to their bulk-solution conformation. Protein denaturation experiments using the low vapor pressure solvent 1-propanol indicate that differences between the droplet and solution measurements are not due to solvent evaporation effects. It is suggested that different droplet conformations are more likely the result of protein diffusion to the droplet surface and effects of the droplet/air interface. To our knowledge, these are the first reported measurements of protein fluorescence within electrospray droplets.

Diffraction of trapped (CsI)(n)Cs+: the appearance of bulk structure

We report an investigation of (CsI)(n)Cs (+) cluster structures (n = 30-39) studied using the recently developed technique of trapped ion electron diffraction. Contributions to diffraction from both rock salt (NaCl) and cesium chloride lattice (CsCl) derived isomeric structures are observed at size n = 32. This size can form a closed shell rhombic dodecahedron corresponding to the CsI bulk structure. All other sizes, n not equal32, are dominated by the NaCl structure. Density functional calculations and molecular dynamic simulations identify the presence of a stable CsCl lattice derived structure isomer which is consistent with these results.

Clinical use of cystine supersaturation measurements

PURPOSE

We measured the concentration and solubility of cystine in urine from patients with cystinuria or calcium stones and from normal subjects to determine whether urine cystine supersaturation can be calculated from a standard nomogram of solubility versus pH or needs to be measured directly. We also evaluated whether increasing pH of the 24-hour collection recovered enough crystallized cystine to increase cystine supersaturation.

MATERIALS AND METHODS

Cystine concentration, pH and usual stone risk factors were measured on 50 ml. aliquots of 24-hour collections from 24 patients with cystinuria, 22 calcium stone formers and 15 normal subjects. After 48 hours of incubation with sodium bicarbonate, a second aliquot was taken from the 24-hour collection for cystine concentration. The original urine at its ambient pH was incubated with an excess of cystine crystals for 24, 48, 72 or 96 hours at 37C to determine solubility and kinetics of equilibration.

RESULTS

Cystine solubility varied so widely at any pH range that no predictive nomogram could be relied on for calculating supersaturation. Addition of sodium bicarbonate to the 24-hour urine significantly increased cystine concentration. Urine from stone formers had higher cystine solubility than urine from normal subjects.

CONCLUSIONS

Clinical management of cystinuria can be improved by direct measurement of cystine solubility because it varies widely at any given pH. Increasing 24-hour collection pH with sodium bicarbonate additionally improves accuracy of supersaturation measurement by recovering crystallized cystine.

Safety of losartan in hypertensive patients with thiazide-induced hyperuricemia

BACKGROUND

Losartan, an angiotensin II receptor antagonist, has been shown to decrease serum uric acid and to increase urinary excretion of uric acid.

METHODS

To determine if this effect can increase the risk of acute urate nephropathy, 63 hypertensive patients with thiazide-induced asymptomatic hyperuricemia (serum uric acid 7.0 to 12.0 mg/dl) were randomized double-blind to losartan 50 mg every day (q.d.), losartan 50 mg plus hydrochlorothiazide (HCTZ) 50 mg q.d., HCTZ 50 mg q.d., or placebo for three weeks. To potentiate the risk of crystal formation, patients received a 2 g/kg protein diet one day prior to each clinic visit on days 0 (baseline), 1, 7, and 21.

RESULTS

Adverse events typically associated with acute urate nephropathy, for example, flank pain, hematuria, or increased blood urea nitrogen/creatinine, were not reported. Uric acid excretion and urine pH increased four and six hours after losartan on day 1 compared with day 0. Dihydrogen urate, the primary risk factor for crystal formation, decreased at four and six hours on day 1 compared with day 0 associated with the concurrent rise in urine pH. Day 7 and 21 changes, compared with day 0, in uric acid excretion rate, urine pH, and dihydrogen urate with losartan were comparable to day 1 results but were not statistically significant. Serum uric acid was significantly reduced after 21 days of therapy with losartan.

CONCLUSION

Losartan decreased serum uric acid and increased uric acid excretion without increasing urinary dihydrogen urate, the primary risk factor for acute urate nephropathy, during 21 days of dosing in hypertensive patients with thiazide-induced hyperuricemia.

Reduced crystallization inhibition by urine from men with nephrolithiasis

BACKGROUND

Human urine is known to inhibit growth, aggregation, nucleation, and cell adhesion of calcium oxalate monohydrate (COM) crystals, the main solid phase of human kidney stones. This study tests the hypothesis that low levels of inhibition are present in men with calcium oxalate stones and could therefore promote stone production.

METHODS

In 17 stone-forming men and 17 normal men that were matched in age to within five years, we studied the inhibition by dialyzed urine proteins of COM growth, aggregation, and binding to cultured BSC-1 renal cells, as well as whole urine upper limits of metastability (ULM) for COM and calcium phosphate (CaP) in relationship to the corresponding supersaturation (SS).

RESULTS

Compared with normals, patient urine showed reduced COM growth inhibition and reduced ULM in relationship to SS. When individual defects were considered, 15 of the 17 patients were abnormal in one or more inhibition measurements. ULM and growth inhibition defects frequently coexisted.

CONCLUSIONS

Reduced COM growth and CaP and CaOx ULM values in relationship to SS are a characteristic of male stone formers. Both defects could promote stones by facilitating crystal nucleation and growth. Abnormal inhibition may be a very important cause of human nephrolithiasis.

Divergence between stone composition and urine supersaturation: clinical and laboratory implications

PURPOSE

In general high urine supersaturation with respect to calcium oxalate, calcium phosphate or uric acid is associated with that phase in stones. We explore the exceptions when supersaturation is high and a corresponding solid phase is absent (type 1), and when the solid phase is present but supersaturation is absent or low (type 2).

MATERIALS AND METHODS

Urine supersaturation values for calcium oxalate, calcium phosphate and uric acid, and other accepted stone risk factors were measured in 538 patients at a research clinic and 178 at stone prevention sites in a network served by a single laboratory.

RESULTS

Of the patients 14% lacked high supersaturation for the main stone constituent (type 2 structural divergence) because of high urine volume and low calcium excretion, perhaps from changes in diet and fluid intake prompted by stones. Higher calcium excretion and low urine volume caused type 1 divergences, which posed no clinical concern.

CONCLUSIONS

Type 1 divergence appears to represent a condition of low urine volume which raises supersaturation in general. Almost all of these patients are calcium oxalate stone formers with the expected high supersaturation with calcium oxalate as well as high uric acid and calcium phosphate supersaturations without either phase in stones. Type 2 divergence appears to represent an increase in urine volume and decrease in urine calcium excretion between stone formation and urine testing.

Medical reduction of stone risk in a network of treatment centers compared to a research clinic

PURPOSE

We determined whether a network of 7 comprehensive kidney stone treatment centers supported by specialized stone management software and laboratory resources could achieve reductions in urine supersaturation comparable to those in a single research clinic devoted to metabolic stone prevention.

MATERIALS AND METHODS

Supersaturation values for calcium oxalate, calcium phosphate and uric acid in 24-hour urine samples were calculated from a set of kidney stone risk factor measurements made at a central laboratory site for the network and research laboratory for the clinic. Individual results and group outcomes were presented to each center in time sequential table graphics. The decrease in supersaturation with treatment was compared in the network and clinic using analysis of variance.

RESULTS

Supersaturation was effectively reduced in the network and clinic, and the reduction was proportional to the initial supersaturation value and increase in urine volume. The clinic achieved a greater supersaturation reduction, higher fraction of patient followup and greater increase in urine volume but the treatment effects in the network were, nevertheless, substantial and significant.

CONCLUSIONS

Given proper software and laboratory support, a network of treatment centers can rival but not quite match results in a dedicated metabolic stone research and prevention clinic. Therefore, large scale stone prevention in a network system appears feasible and effective.

Renal stone disease in older adults

The pathophysiology of stone disorder in older adults, as compared to their younger counterparts, has not been thoroughly investigated. This article examines the differences in serum and urine chemistries between groups that are younger and older than 60 years of age. The principal finding is that stone formation occurs at lower urinary supersaturations in older patients, suggesting that other unexplored factors are significant contributors. The authors then review the possible effect of age on the morbidity of stone disease and the implications of stone disease for the development and management of osteoporosis.

Metabolic urinary correlates of calcium oxalate dihydrate in renal stones

PURPOSE

We determined what metabolic features of the 24-hour urine predict calcium oxalate dihydrate in kidney stones. Prior studies have suggested that low urine magnesium, high urine calcium, high calcium-to-oxalate ratio and high urine supersaturation with respect to calcium oxalate monohydrate predict calcium oxalate dihydrate.

MATERIALS AND METHODS

Stone analyses and results from 2, 24-hour pretreatment urine collections from 96 patients with nephrolithiasis were drawn from 3 kidney stone prevention centers. Standard stone risk measurements were made on the urine, including supersaturation for calcium oxalate monohydrate, brushite and uric acid.

RESULTS

The main differences in metabolic urine findings were between patients with no calcium oxalate dihydrate and those with any calcium oxalate dihydrate in stones. Percent calcium oxalate dihydrate itself did not correlate with urine findings. Patients with no calcium oxalate dihydrate in stones showed a biphasic pattern of urine calcium oxalate monohydrate supersaturation, about half had values below almost any found among patients with calcium oxalate dihydrate in stones (less than 7) and the rest overlapped with the calcium oxalate dihydrate group. Except for higher calcium oxalate monohydrate supersaturation, patients with calcium oxalate dihydrate in stones had higher urine calcium excretion and lower urine citrate concentrations, even after calcium oxalate monohydrate supersaturation was considered.

CONCLUSIONS

Patients with low calcium oxalate monohydrate supersaturation (less than 7) are unlikely to have calcium oxalate dihydrate in renal stones. However, many patients with no calcium oxalate dihydrate have higher calcium oxalate monohydrate supersaturation values, and so prediction of calcium oxalate dihydrate or its absence from urine findings is imperfect. Urine magnesium and the calcium-to-oxalate ratio are unrelated to calcium oxalate dihydrate.

Contribution of human uropontin to inhibition of calcium oxalate crystallization

Uropontin (UP) is known to inhibit the growth and nucleation of calcium oxalate monohydrate (COM) crystals, and it also impedes attachment of calcium oxalate crystals to cultured renal epithelial cells. However, its role in normal defense against renal crystallization, and in pathogenesis of nephrolithiasis is unclear. In this study we determined the effect of UP on aggregation of COM crystals as well as the inhibitory activity of UP on COM crystal growth and nucleation in a series of normal subjects, in order to assess the potential of UP as an important urinary inhibitor. The mean urinary excretion of UP measured by ELISA was 185 +/- 12 nmol/24 hr (mean +/- SEM) with a mean urine UP concentration of 131 +/- 13 nM. Uropontin isolated by immunoaffinity chromatography was a very potent inhibitor of COM crystal aggregation, with a mean UP concentration of 28 +/- 4 nM required for a 50% reduction in aggregation. The kDa for COM crystal growth inhibition determined from Langmuir type isotherms was 21 +/- 3 nM and the concentration required for 50% reduction in COM crystal growth rate was 16 +/- 2 nM. Inhibition of secondary nucleation was measured at a single concentration of 200 nM, which reduced the nucleation rate to 42 +/- 3% of control. Using a theoretical model of growth and aggregation inhibition at varying urine flow rates, we showed that inhibitory activity of UP would be significant for all subjects over a wide range of urine flow rates. Overall, UP is a potent inhibitor of COM aggregation as well as growth and nucleation. The urinary concentration of UP is in the range in which its contribution to inhibition of growth and aggregation are likely to be substantial. Thus, UP appears to be an important natural defense against renal crystallizations and nephrolithiasis.

Dependence of upper limit of metastability on supersaturation in nephrolithiasis

Formation of renal stones requires supersaturation (SS) high enough to induce crystallization; such a SS is referred to as the upper limit of metastability (ULM). The ULM for calcium oxalate (CaOx) or calcium phosphate can be measured by adding oxalate or calcium to urine, respectively, and noting the point at which overt crystallization occurs as evidenced by clouding. In principle, the urine should be more prone to form stone crystals as its SS approaches the ULM, and the SS ULM distance has been used as an index of stone forming potential. In addition, one would expect the ULM and initial SS to be unrelated, as the starting urine SS has no apparent link to the amount of calcium or oxalate that urine can dissolve without leading to crystal formation. However, in rats, we have found a surprising correlation between ULM and SS, such that ULM appears to rise with initial SS, for CaOx, and, to a lesser extent, for brushite (Br), a typical calcium phosphate initial phase. In this study, we measured CaOx and Br ULM, and SS, in urine of 50 patients and 11 normal people, to determine if ULM and SS were correlated, as in rats, and to explore the relationship between SS and ULM. We found the same dependence of ULM on SS as in rats, for both CaOx and Br, and found no differences between patients and normal people with respect to this dependency. However, for Br, patients showed a lower ULM than normals, but the same initial SS, meaning that patients were closer to their crystal formation threshold than normals. Treatments for stones had no apparent effect on the SS-ULM dependency. We conclude that in humans, as in rats, ULM is related to initial SS, and that this relationship is the same in patients as in normals for CaOx, but shifted in a stone forming direction for Br among patients. The ULM-SS interaction is unaffected by contemporary conventional stone treatments, and is more marked for CaOx than Br. The mechanisms of the dependence are unknown. The smaller difference between ULM and initial SS for Br in patients than normal supports prior evidence suggesting a defect in stone patients that could lead to calcium phosphate crystallization, subsequent nucleation of CaOx, and stone disease.

Relationship between supersaturation and crystal inhibition in hypercalciuric rats

Calcium oxalate (CaOx) and calcium phosphate (CaP) crystals do not precipitate in large amounts in normal urine despite considerable supersaturation (SS), partly because urine inhibits crystal nucleation, aggregation, and growth. In normal rats and rats bred for hypercalciuria (GHS), we varied SS by varying calcium intake to test the hypothesis that increased SS might deplete inhibitors and reduce inhibition of crystal formation. In normal rats when compared to a low calcium diet (0.02% Ca), a high calcium diet (1.2% Ca) raised the SS of CaOx from 0.8 to 8.2. The high calcium diet also raised the upper limit of metastability (ULM) of CaOx (the SS at which crystals form in urine) from 11.8 to 36. In GHS rats, diet change altered CaOx SS from 1.5 to 12, and ULM from 17 to 50 (all differences, P < 0.001). Because ULM rose with SS, the increased SS had little potential to increase CaOx stone risk. For CaP, however, SS rose from 0.6 to 2.4 and 1.1 to 8 in normal and GHS rats (P < 0.001 for both), respectively, whereas ULM for CaP did not increase significantly (8 vs. 7 and 7 vs. 11; P = NS, both changes). Therefore, CaP SS rose close to the ULM, posing a high stone risk. The stones formed by these rats are composed of CaP. Increasing CaOx SS by diet raises ULM for CaOx thereby offsetting the risk of CaOx stones in rats.

Correspondence between stone composition and urine supersaturation in nephrolithiasis

Supersaturation (SS) with respect to calcium oxalate monohydrate (COM), brushite (Br) and uric acid (UA), obtained in three 24-hour pretreatment urine samples from patients with stone disease were compared to the mineral composition of stones passed by the same patients to determine whether sparse urine SS measurements accurately reflect the long-term average SS values in the kidney and final urine. Among males and females elevation of SS above same sex normals corresponded to composition. As well, treatments that reduced stone rates also reduced these SS values. The degree of calcium phosphate (CaP) admixture was accurately matched by shifting magnitudes of COM and Br SS. As well, increasing CaP content was associated with falling urine citrate and rising urine pH, suggesting renal tubular acidosis. We conclude that sparse urine SS measurements accurately track stone admixtures, and are a reliable index of average renal and urine SS.