Stone formation in genetic hypercalciuric rats

Risk of Symptomatic Kidney Stones During and After Pregnancy

RATIONALE & OBJECTIVE

There are several well-known anatomical and physiological changes during pregnancy that could contribute to kidney stone formation, but evidence that they increase the risk of kidney stones during pregnancy is lacking. We determined whether there was an increased risk of a first-time symptomatic kidney stone during and after pregnancy.

STUDY DESIGN

A population-based matched case-control study.

SETTING & PARTICIPANTS

945 female first-time symptomatic kidney stone formers aged 15-45 years and 1,890 age-matched female controls in Olmsted County, MN, from 1984-2012. The index date was the date of onset of a symptomatic kidney stone for both the case and her matched controls.

EXPOSURE

The primary exposure was pregnancy with assessment for variation in risk across different time intervals before, during, and after pregnancy. Medical records were manually reviewed to determine the conception and delivery dates for pregnancies.

OUTCOME

Medical record-validated first-time symptomatic kidney stone.

ANALYTICAL APPROACH

Conditional and unconditional multivariable logistic regression analysis.

RESULTS

Compared with nonpregnant women, the odds of a symptomatic kidney stone forming in women was similar in the first trimester (OR, 0.92; P=0.8), began to increase during the second trimester (OR, 2.00; P=0.007), further increased during the third trimester (OR, 2.69; P=0.001), peaked at 0 to 3 months after delivery (OR, 3.53; P<0.001), and returned to baseline by 1year after delivery. These associations persisted after adjustment for age and race or for diabetes mellitus, hypertension, and obesity. These results did not significantly differ by age, race, time period, or number of prior pregnancies. Having a prior pregnancy (delivery date>1year ago) was also associated with a first-time symptomatic kidney stone (OR, 1.27; P=0.01).

LIMITATIONS

Observational study design in a predominantly White population. The exact timing of stone formation cannot be determined.

CONCLUSIONS

Pregnancy increases the risk of a first-time symptomatic kidney stone. This risk peaks close to delivery and then improves by 1 year after delivery, though a modest risk of a kidney stone still exists beyond 1 year after delivery.

Increased Osteoclast and Decreased Osteoblast Activity Causes Reduced Bone Mineral Density and Quality in Genetic Hypercalciuric Stone-Forming Rats

To study human idiopathic hypercalciuria (IH), we developed an animal model, genetic hypercalciuric stone-forming (GHS) rats, whose pathophysiology parallels that in IH. All GHS rats form kidney stones and have decreased BMD and bone quality compared with the founder Sprague-Dawley (SD) rats. To understand the bone defect, we characterized osteoclast and osteoblast activity in the GHS compared with SD rats. Bone marrow cells were isolated from femurs of GHS and SD rats and cultured to optimize differentiation into osteoclasts or osteoblasts. Osteoclasts were stained for TRAcP (tartrate resistant acid phosphatase), cultured to assess resorptive activity, and analyzed for specific gene expression. Marrow stromal cells or primary neonatal calvarial cells were differentiated to osteoblasts, and osteoblastic gene expression as well as mineralization was analyzed. There was increased osteoclastogenesis and increased resorption pit formation in GHS compared with SD cultures. Osteoclasts had increased expression of cathepsin K, Tracp, and MMP9 in cells from GHS compared with SD rats. Osteoblastic gene expression and mineralization was significantly decreased. Thus, alterations in baseline activity of both osteoclasts and osteoblasts in GHS rats, led to decreased BMD and bone quality, perhaps because of their known increase in vitamin D receptors. Better understanding of the role of GHS bone cells in decreased BMD and quality may provide new strategies to mitigate the low BMD and increased fracture risk found in patients with IH. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Vitamin D and Kidney Stones

This review explores the relationship between vitamin D supplementation and lithogenesis. A causal relationship has been assumed despite myriad studies demonstrating that therapeutic doses of vitamin D do not increase lithogenic risk. Select stone formers may be at increased risk for recurrence with vitamin D supplementation, possibly from CYP24A1 gene mutations. Additionally, the evidence for who is vitamin D deficient, and the benefits of supplementation in those not at risk for rickets, is sparse. Concerns may be avoidable as vitamin D screening appears unnecessary in most patients, and superior pharmacology is available which increases bone density, while decreasing stone formation.

Pseudoxanthoma Elasticum, Kidney Stones and Pyrophosphate: From a Rare Disease to Urolithiasis and Vascular Calcifications

Pseudoxanthoma elasticum is a rare disease mainly due to ABCC6 gene mutations and characterized by ectopic biomineralization and fragmentation of elastic fibers resulting in skin, cardiovascular and retinal calcifications. It has been recently described that pyrophosphate (a calcification inhibitor) deficiency could be the main cause of ectopic calcifications in this disease and in other genetic disorders associated to mutations of ENPP1 or CD73. Patients affected by Pseudoxanthoma Elasticum seem also prone to develop kidney stones originating from papillary calcifications named Randall’s plaque, and to a lesser extent may be affected by nephrocalcinosis. In this narrative review, we summarize some recent discoveries relative to the pathophysiology of this mendelian disease responsible for both cardiovascular and renal papillary calcifications, and we discuss the potential implications of pyrophosphate deficiency as a promoter of vascular calcifications in kidney stone formers and in patients affected by chronic kidney disease.

Chlorthalidone Is Superior to Potassium Citrate in Reducing Calcium Phosphate Stones and Increasing Bone Quality in Hypercalciuric Stone-Forming Rats

BACKGROUND

The pathophysiology of genetic hypercalciuric stone-forming rats parallels that of human idiopathic hypercalciuria. In this model, all animals form calcium phosphate stones. We previously found that chlorthalidone, but not potassium citrate, decreased stone formation in these rats.

METHODS

To test whether chlorthalidone and potassium citrate combined would reduce calcium phosphate stone formation more than either medication alone, four groups of rats were fed a fixed amount of a normal calcium and phosphorus diet, supplemented with potassium chloride (as control), potassium citrate, chlorthalidone (with potassium chloride to equalize potassium intake), or potassium citrate plus chlorthalidone. We measured urine every 6 weeks and assessed stone formation and bone quality at 18 weeks.

RESULTS

Potassium citrate reduced urine calcium compared with controls, chlorthalidone reduced it further, and potassium citrate plus chlorthalidone reduced it even more. Chlorthalidone increased urine citrate and potassium citrate increased it even more; the combination did not increase it further. Potassium citrate, alone or with chlorthalidone, increased urine calcium phosphate supersaturation, but chlorthalidone did not. All control rats formed stones. Potassium citrate did not alter stone formation. No stones formed with chlorthalidone, and rats given potassium citrate plus chlorthalidone had some stones but fewer than controls. Rats given chlorthalidone with or without potassium citrate had higher bone mineral density and better mechanical properties than controls, whereas those given potassium citrate did not.

CONCLUSIONS

In genetic hypercalciuric stone-forming rats, chlorthalidone is superior to potassium citrate alone or combined with chlorthalidone in reducing calcium phosphate stone formation and improving bone quality.

Hydroxycitrate: a potential new therapy for calcium urolithiasis

Alkali supplements are used to treat calcium kidney stones owing to their ability to increase urine citrate excretion which lowers stone risk by inhibiting crystallization and complexing calcium. However, alkali increases urine pH, which may reduce effectiveness for patients with calcium phosphate stones and alkaline urine. Hydroxycitrate is a structural analog of citrate, widely available as an over-the-counter supplement for weight reduction. In vitro studies show hydroxycitrate has the capacity to complex calcium equivalent to that of citrate and that it is an effective inhibitor of calcium oxalate monohydrate crystallization. In fact, hydroxycitrate was shown to dissolve calcium oxalate crystals in supersaturated solution in vitro. Hydroxycitrate is not known to be metabolized by humans, so it would not be expected to alter urine pH, as opposed to citrate therapy. Preliminary studies have shown orally ingested hydroxycitrate is excreted in urine, making it an excellent candidate as a stone therapeutic. In this article, we detail the crystal inhibition activity of hydroxycitrate, review the current knowledge of hydroxycitrate use in humans, and identify gaps in knowledge that require appropriate research studies before hydroxycitrate can be recommended as a therapy for kidney stones.

Low Sodium Diet Decreases Stone Formation in Genetic Hypercalciuric Stone-Forming Rats

BACKGROUND

Urine (u) calcium (Ca) excretion is directly dependent on dietary sodium (Na) intake leading to the recommendation for Na restriction in hypercalciuric kidney stone formers. However, there is no direct evidence that limiting Na intake will reduce recurrent stone formation.

MATERIALS AND METHODS

We used genetic hypercalciuric stone-forming (GHS) rats, which universally form Ca phosphate (P) kidney stones, fed either a low Na (LNa, 0.05%) or normal Na (NNa, 0.4%) Na diet (D) for 18 weeks. Urine was collected at 6-week intervals. Radiographic analysis for stone formation and bone analyses were done at the conclusion of the study.

RESULTS

Mean uCa was lower with LNaD than NNaD as was uP and LNaD decreased mean uNa and uChloride. There were no differences in urine supersaturation (SS) with respect to calcium phosphate (CaP) or Ca oxalate (CaOx). However, stone formation was markedly decreased with LNaD by radiographic analysis. The LNaD group had significantly lower femoral anterior-posterior diameter and volumetric bone mineral density (vBMD), but no change in vertebral trabecular vBMD. There were no differences in the bone formation rate or osteoclastic bone resorption between groups. The LNaD group had significantly lower femoral stiffness; however, the ultimate load and energy to fail was not different.

CONCLUSION

Thus, a low Na diet reduced uCa and stone formation in GHS rats, even though SS with respect to CaP and CaOx was unchanged and effects on bone were modest. These data, if confirmed in humans, support dietary Na restriction to prevent recurrent Ca nephrolithiasis.

Vitamin D, Hypercalciuria and Kidney Stones

The estimated lifetime risk of nephrolithiasis is growing nowadays, and the formation of kidney stones is frequently promoted by hypercalciuria. Vitamin D, and especially its active metabolite calcitriol, increase digestive calcium absorption—as urinary calcium excretion is directly correlated with digestive calcium absorption, vitamin D metabolites could theoretically increase calciuria and promote urinary stone formation. Nevertheless, there was, until recently, low evidence that 25-hydroxyvitamin D serum levels would be correlated with kidney stone formation, even if high calcitriol concentrations are frequently observed in hypercalciuric stone formers. Low 25-hydroxyvitamin D serum levels have been associated with a broad spectrum of diseases, leading to a huge increase in vitamin D prescription in the general population. In parallel, an increased frequency of kidney stone episodes has been observed in prospective studies evaluating vitamin D alone or in association with calcium supplements, and epidemiological studies have identified an association between high 25-hydroxyvitamin D serum levels and kidney stone formation in some groups of patients. Moreover, urinary calcium excretion has been shown to increase in response to vitamin D supplements, at least in some groups of kidney stone formers. It seems likely that predisposed individuals may develop hypercalciuria and kidney stones in response to vitamin D supplements.

Animal models of urinary stone disease

The etiology of stone disease remains unknown despite the major technological advances in the treatment of urinary calculi. Clinically, urologists have relied on 24-h urine collections for the last 30-40 years to help direct medical therapy in hopes of reducing stone recurrence; yet little progress has been made in preventing stone disease. As such, there is an urgent need to develop reliable animal models to study the pathogenesis of stone formation and to assess novel interventions. A variety of vertebrate and invertebrate models have been used to help understand stone pathogenesis. Genetic knockout and exogenous induction models are described. Surrogates for an endpoint of stone formation have been urinary crystals on histologic examination and/or urinalyses. Other models are able to actually develop true stones. It is through these animal models that real breakthroughs in the management of urinary stone disease will become a reality.

Modeling hypercalciuria in the genetic hypercalciuric stone-forming rat

PURPOSE OF REVIEW

In this review, we discuss how the genetic hypercalciuric stone-forming (GHS) rats, which closely model idiopathic hypercalciuria and stone formation in humans, provide insights into the pathophysiology and consequences of clinical hypercalciuria.

RECENT FINDINGS

Hypercalciuria in the GHS rats is due to a systemic dysregulation of calcium transport, as manifest by increased intestinal calcium absorption, increased bone resorption and decreased renal tubule calcium reabsorption. Increased levels of vitamin D receptor in intestine, bone and kidney appear to mediate these changes. The excess receptors are biologically active and increase tissue sensitivity to exogenous vitamin D. Bones of GHS rats have decreased bone mineral density (BMD) as compared with Sprague-Dawley rats, and exogenous 1,25(OH)2D3 exacerbates the loss of BMD. Thiazide diuretics improve the BMD in GHS rats.

SUMMARY

Studying GHS rats allows direct investigation of the effects of alterations in diet and utilization of pharmacologic therapy on hypercalciuria, urine supersaturation, stone formation and bone quality in ways that are not possible in humans.

Effect of Potassium Citrate on Calcium Phosphate Stones in a Model of Hypercalciuria

Potassium citrate is prescribed to decrease stone recurrence in patients with calcium nephrolithiasis. Citrate binds intestinal and urine calcium and increases urine pH. Citrate, metabolized to bicarbonate, should decrease calcium excretion by reducing bone resorption and increasing renal calcium reabsorption. However, citrate binding to intestinal calcium may increase absorption and renal excretion of both phosphate and oxalate. Thus, the effect of potassium citrate on urine calcium oxalate and calcium phosphate supersaturation and stone formation is complex and difficult to predict. To study the effects of potassium citrate on urine supersaturation and stone formation, we utilized 95th-generation inbred genetic hypercalciuric stone-forming rats. Rats were fed a fixed amount of a normal calcium (1.2%) diet supplemented with potassium citrate or potassium chloride (each 4 mmol/d) for 18 weeks. Urine was collected at 6, 12, and 18 weeks. At 18 weeks, stone formation was visualized by radiography. Urine citrate, phosphate, oxalate, and pH levels were higher and urine calcium level was lower in rats fed potassium citrate. Furthermore, calcium oxalate and calcium phosphate supersaturation were higher with potassium citrate; however, uric acid supersaturation was lower. Both groups had similar numbers of exclusively calcium phosphate stones. Thus, potassium citrate effectively raises urine citrate levels and lowers urine calcium levels; however, the increases in urine pH, oxalate, and phosphate levels lead to increased calcium oxalate and calcium phosphate supersaturation. Potassium citrate induces complex changes in urine chemistries and resultant supersaturation, which may not be beneficial in preventing calcium phosphate stone formation.

1,25(OH)₂D₃ induces a mineralization defect and loss of bone mineral density in genetic hypercalciuric stone-forming rats

Genetic hypercalciuric stone-forming (GHS) rats, bred to maximize urine (u) calcium (Ca) excretion, demonstrate increased intestinal Ca absorption, increased bone Ca resorption, and reduced renal Ca reabsorption, all leading to elevated uCa compared to the parental Sprague-Dawley (SD) rats. GHS rats have increased numbers of vitamin D receptors (VDRs) at each site, with normal levels of 1,25(OH)₂D₃ (1,25D), suggesting their VDR is undersaturated with 1,25D. We have shown that 1,25D induces a greater increase in uCa in GHS than SD rats. To examine the effect of the increased VDR on the osseous response to 1,25D, we fed GHS and SD rats an ample Ca diet and injected either 1,25D [low dose (LD) 12.5 or high dose (HD) 25 ng/100 g body weight/day] or vehicle (veh) daily for 16 days. Femoral areal bone mineral density (aBMD, by DEXA) was decreased in GHS+LD and GHS+HD relative to GHS+veh, while there was no effect on SD. Vertebral aBMD was lower in GHS compared to SD and further decreased in GHS+HD. Both femoral and L6 vertebral volumetric BMD (by μCT) were lower in GHS and further reduced by HD. Histomorphometry indicated a decreased osteoclast number in GHS+HD compared to GHS+veh or SD+HD. In tibiae, GHS+HD trabecular thickness and number increased, with a 12-fold increase in osteoid volume but only a threefold increase in bone volume. Bone formation rate was decreased in GHS+HD relative to GHS+veh, confirming the mineralization defect. The loss of BMD and the mineralization defect in GHS rats contribute to increased hypercalciuria; if these effects persist, they would result in decreased bone strength, making these bones more fracture-prone. The enhanced effect of 1,25D in GHS rats indicates that the increased VDRs are biologically active.

Nephrolithiasis in free-ranging North American river otter (Lontra canadensis) in North Carolina, USA

The North American river otter (Lontra canadensis) serves as an indicator species for environmental monitoring, is prized as a valuable furbearer, and is a popular display animal in zoologic collections. Nephrolithiasis has been reported as a frequent problem in other free-ranging and captive otter species but is rarely reported in North American river otters. In this study, we compared the prevalence of nephrolithiasis diagnosed using routine gross pathologic examination techniques with the use of computed tomography (CT) of excised kidneys. We also evaluated whether otter nephroliths could be accurately classified by their CT densities, and we examined the renal tissue uric acid concentrations in free-ranging otters in North Carolina, USA. Kidneys were collected from carcasses of legally trapped, free-ranging animals. Nephroliths were observed in 16.2% of the individuals (n = 229). Associations were found between age and nephrolith status and between capture location and nephrolith status (P = 0.026 and < 0.001, respectively). Computed tomography Hounsfield unit density measurements were not useful in determining nephrolith chemical composition in this study. Renal tissue uric acid concentrations were similar across genders, age groups, and stone status. The chemical composition of the nephroliths was determined by scanning electron microscopy-energy dispersive X-ray spectroscopy to be calcium phosphate in the carbonate form.

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

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

The relation between bone and stone formation

Hypercalciuria is the most common metabolic abnormality found in patients with calcium-containing kidney stones. Patients with hypercalciuria often excrete more calcium than they absorb, indicating a net loss of total-body calcium. The source of this additional urinary calcium is almost certainly the skeleton, the largest repository of calcium in the body. Hypercalciuric stone formers exhibit decreased bone mineral density (BMD), which is correlated with the increase in urine calcium excretion. The decreased BMD also correlates with an increase in markers of bone turnover as well as increased fractures. In humans, it is difficult to determine the cause of the decreased BMD in hypercalciuric stone formers. To study the effect of hypercalciuria on bone, we utilized our genetic hypercalciuric stone-forming (GHS) rats, which were developed through successive inbreeding of the most hypercalciuric Sprague-Dawley rats. GHS rats excrete significantly more urinary calcium than similarly fed controls, and all the GHS rats form kidney stones while control rats do not. The hypercalciuria is due to a systemic dysregulation of calcium homeostasis, with increased intestinal calcium absorption, enhanced bone mineral resorption, and decreased renal tubule calcium reabsorption associated with an increase in vitamin D receptors in all these target tissues. We recently found that GHS rats fed an ample calcium diet have reduced BMD and that their bones are more fracture-prone, indicating an intrinsic disorder of bone not secondary to diet. Using this model, we should better understand the pathogenesis of hypercalciuria and stone formation in humans to ultimately improve the bone health of patients with kidney stones.

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

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

Increased biological response to 1,25(OH)(2)D(3) in genetic hypercalciuric stone-forming rats

Genetic hypercalciuric stone-forming (GHS) rats, bred to maximize urine (U) calcium (Ca) excretion, have increased intestinal Ca absorption and bone Ca resorption and reduced renal Ca reabsorption, leading to increased UCa compared with the Sprague-Dawley (SD) rats. GHS rats have increased vitamin D receptors (VDR) at each of these sites, with normal levels of 1,25(OH)(2)D(3) (1,25D), indicating that their VDR is undersaturated with 1,25D. We tested the hypothesis that 1,25D would induce a greater increase in UCa in GHS rats by feeding both strains ample Ca and injecting 1,25D (25 ng · 100 g body wt(-1) · day(-1)) or vehicle for 16 days. With 1,25D, UCa in SD increased from 1.7 ± 0.3 mg/day to 24.4 ± 1.2 (Δ = 22.4 ± 1.5) and increased more in GHS from 10.5 ± 0.7 to 41.9 ± 0.7 (Δ = 29.8 ± 1.8; P = 0.003). To determine the mechanism of the greater increase in UCa in GHS rats, we measured kidney RNA expression of components of renal Ca transport. Expression of transient receptor potential vanilloid (TRPV)5 and calbindin D(28K) were increased similarly in SD + 1,25D and GHS + 1,25D. The Na(+)/Ca(2+) exchanger (NCX1) was increased in GHS + 1,25D. Klotho was decreased in SD + 1,25D and GHS + 1,25D. TRPV6 was increased in SD + 1,25D and increased further in GHS + 1,25D. Claudin 14, 16, and 19, Na/K/2Cl transporter (NKCC2), and secretory K channel (ROMK) did not differ between SD + 1,25D and GHS + 1,25D. Increased UCa with 1,25D in GHS exceeded that of SD, indicating that the increased VDR in GHS induces a greater biological response. This increase in UCa, which must come from the intestine and/or bone, must exceed any effect of 1,25D on TRPV6 or NCX1-mediated renal Ca reabsorption.

Sex modifies genetic effects on residual variance in urinary calcium excretion in rat (Rattus norvegicus)

Conventional genetics assumes common variance among alleles or genetic groups. However, evidence from vertebrate and invertebrate models suggests that residual genotypic variance may itself be under partial genetic control. Such a phenomenon would have great significance: high-variability alleles might confound the detection of "classically" acting genes or scatter predicted evolutionary outcomes among unpredicted trajectories. Of the few works on this phenomenon, many implicate sex in some aspect of its control. We found that female genetic hypercalciuric stone-forming (GHS) rats (Rattus norvegicus) had higher coefficients of variation (CVs) for urinary calcium (CV = 0.14) than GHS males (CV = 0.06), and the reverse in normocalciuric Wistar-Kyoto rats (WKY) (CV(♂) = 0.14; CV(♀) = 0.09), suggesting sex-by-genotype interaction on residual variance. We therefore investigated the effect of sex on absolute-transformed residuals in urinary calcium in an F(2) GHS × WKY mapping cohort. Absolute residuals were associated with genotype at two microsatellites, D3Rat46 (RNO3, 33.9 Mb) and D4Mgh1 (RNO4, 84.8 MB) at Bonferroni thresholds across the entire cohort, and with the microsatellites D3Rat46, D9Mgh2 (RNO9, 84.4 Mb), and D12Rat25 (RNO12, 40.4 Mb) in females (P < 0.05) but not males. In GHS chromosome 1 congenic lines bred onto a WKY genomic background, we found that congenic males had significantly (P < 0.0001) higher CVs for urinary calcium (CV = 0.25) than females (CV = 0.15), supporting the hypothesis of the inheritance of sex-by-genotype interaction on this effect. Our findings suggest that genetic effects on residual variance are sex linked; heritable, sex-specific residuals might have great potential implications for evolution, adaptation, and genetic analysis.

Clinical review. Kidney stones 2012: pathogenesis, diagnosis, and management

CONTEXT

The pathogenetic mechanisms of kidney stone formation are complex and involve both metabolic and environmental risk factors. Over the past decade, major advances have been made in the understanding of the pathogenesis, diagnosis, and treatment of kidney stone disease.

EVIDENCE ACQUISITION AND SYNTHESIS

Both original and review articles were found via PubMed search reporting on pathophysiology, diagnosis, and management of kidney stones. These resources were integrated with the authors' knowledge of the field.

CONCLUSION

Nephrolithiasis remains a major economic and health burden worldwide. Nephrolithiasis is considered a systemic disorder associated with chronic kidney disease, bone loss and fractures, increased risk of coronary artery disease, hypertension, type 2 diabetes mellitus, and the metabolic syndrome. Further understanding of the pathophysiological link between nephrolithiasis and these systemic disorders is necessary for the development of new therapeutic options.

Chlorthalidone improves vertebral bone quality in genetic hypercalciuric stone-forming rats

We have bred a strain of rats to maximize urine (u) calcium (Ca) excretion and model hypercalciuric nephrolithiasis. These genetic hypercalciuric stone-forming (GHS) rats excrete more uCa than control Sprague-Dawley rats, uniformly form kidney stones, and similar to patients, demonstrate lower bone mineral density. Clinically, thiazide diuretics reduce uCa and prevent stone formation; however, whether they benefit bone is not clear. We used GHS rats to test the hypothesis that the thiazide diuretic chlorthalidone (CTD) would have a favorable effect on bone density and quality. Twenty GHS rats received a fixed amount of a 1.2% Ca diet, and half also were fed CTD (4 to 5 mg/kg/d). Rats fed CTD had a marked reduction in uCa. The axial and appendicular skeletons were studied. An increase in trabecular mineralization was observed with CTD compared with controls. CTD also improved the architecture of trabecular bone. Using micro-computed tomography (µCT), trabecular bone volume (BV/TV), trabecular thickness, and trabecular number were increased with CTD. A significant increase in trabecular thickness with CTD was confirmed by static histomorphometry. CTD also improved the connectivity of trabecular bone. Significant improvements in vertebral strength and stiffness were measured by vertebral compression. Conversely, a slight loss of bending strength was detected in the femoral diaphysis with CTD. Thus results obtained in hypercalciuric rats suggest that CTD can favorably influence vertebral fracture risk. CTD did not alter formation parameters, suggesting that the improved vertebral bone strength was due to decreased bone resorption and retention of bone structure.

Elevated vitamin D receptor levels in genetic hypercalciuric stone-forming rats are associated with downregulation of Snail

Patients with idiopathic hypercalciuria (IH) and genetic hypercalciuric stone-forming (GHS) rats, an animal model of IH, are both characterized by normal serum Ca, hypercalciuria, Ca nephrolithiasis, reduced renal Ca reabsorption, and increased bone resorption. Serum 1,25-dihydroxyvitamin D [1,25(OH)(2)D] levels are elevated or normal in IH and are normal in GHS rats. In GHS rats, vitamin D receptor (VDR) protein levels are elevated in intestinal, kidney, and bone cells, and in IH, peripheral blood monocyte VDR levels are high. The high VDR is thought to amplify the target-tissue actions of normal circulating 1,25(OH)(2)D levels to increase Ca transport. The aim of this study was to elucidate the molecular mechanisms whereby Snail may contribute to the high VDR levels in GHS rats. In the study, Snail gene expression and protein levels were lower in GHS rat tissues and inversely correlated with VDR gene expression and protein levels in intestine and kidney cells. In human kidney and colon cell lines, ChIP assays revealed endogenous Snail binding close to specific E-box sequences within the human VDR promoter region, whereas only one E-box specifically bound Snail in the rat promoter. Snail binding to rat VDR promoter E-box regions was reduced in GHS compared with normal control intestine and was accompanied by hyperacetylation of histone H(3). These results provide evidence that elevated VDR in GHS rats likely occurs because of derepression resulting from reduced Snail binding to the VDR promoter and hyperacetylation of histone H(3).

Genetic hypercalciuric stone-forming rats have a primary decrease in BMD and strength

Kidney stone patients often have a decrease in BMD. It is unclear if reduced BMD is caused by a primary disorder of bone or dietary factors. To study the independent effects of hypercalciuria on bone, we used genetic hypercalciuric stone-forming (GHS) rats. GHS and control (Ctl) rats were fed a low Ca (0.02% Ca, LCD) or a high Ca (1.2% Ca, HCD) diet for 6 wk in metabolic cages. All comparisons are to Ctl rats. Urine Ca was greater in the GHS rats on both diets. GHS fed HCD had reduced cortical (humerus) and trabecular (L(1)-L(5) vertebrae) BMD, whereas GHS rats fed LCD had a reduction in BMD similar to Ctl. GHS rats fed HCD had a decrease in trabecular volume and thickness, whereas LCD led to a approximately 20-fold increase in both osteoid surface and volume. GHS rats fed HCD had no change in vertebral strength (failure stress), ductibility (failure strain), stiffness (modulus), or toughness, whereas in the humerus, there was reduced ductibility and toughness and an increase in modulus, indicating that the defect in mechanical properties is mainly manifested in cortical, rather than trabecular, bone. GHS rat cortical bone is more mineralized than trabecular bone and LCD led to a decrease in the mineralization profile. Thus, the GHS rats, fed an ample Ca diet, have reduced BMD with reduced trabecular volume, mineralized volume, and thickness, and their bones are more brittle and fracture prone, indicating that GHS rats have an intrinsic disorder of bone that is not secondary to diet.

Strain differences in urinary factors that promote calcium oxalate crystal formation in the kidneys of ethylene glycol-treated rats

Ethylene glycol (EG)-induced hyperoxaluria is the most commonly employed experimental regimen as an animal model of calcium oxalate (CaOx) stone formation. The variant sensitivity to CaOx among different rat strains has not been fully explored, although the Wistar rat is known to accumulate more CaOx in kidney tissue after low-dose EG exposure than in the Fischer 344 (F344) rats. Supersaturation of CaOx in tubular fluid contributes to the amount of CaOx crystal formation in the kidney. We hypothesized that the urinary supersaturation of CaOx in Wistar rats is higher than that of F344 rats, thereby allowing for greater CaOx crystal deposition in the Wistar rat. Age-matched male Wistar and F344 rats were treated with 0.75% EG or drinking water for 8 wk. Twenty-four-hour urine was collected at 0, 2, 4, 6, and 8 wk for analysis of key electrolytes to calculate the CaOx supersaturation. Plasma oxalate level was also measured. Our data confirmed the different sensitivity to renal toxicity from EG between the two rat strains (Wistar > F344). After EG treatment, the plasma oxalate level and urine oxalate excretion were markedly greater in the Wistar rats than in the F344 rats, while urine calcium was slightly decreased in Wistars. Thus, the CaOx supersaturation in urine of Wistar rats was higher, which led to a greater crystal deposition in kidney in Wistar rats. These studies suggest that during EG treatment, changes in urine electrolytes and in CaOx supersaturation occur to a greater extent in the Wistar rat, in agreement with its greater sensitivity to EG toxicity.

Thiosulfate reduces calcium phosphate nephrolithiasis

An uncontrolled trial reported that sodium thiosulfate reduces formation of calcium kidney stones in humans, but this has not been established in a controlled human study or animal model. Using the genetic hypercalciuric rat, an animal model of calcium phosphate stone formation, we studied the effect of sodium thiosulfate on urine chemistries and stone formation. We fed genetic hypercalciuric rats normal food with or without sodium thiosulfate for 18 wk and measured urine chemistries, supersaturation, and the upper limit of metastability of urine. Eleven of 12 untreated rats formed stones compared with only three of 12 thiosulfate-treated rats (P < 0.002). Urine calcium and phosphorus were higher and urine citrate and volume were lower in the thiosulfate-treated rats, changes that would increase calcium phosphate supersaturation. Thiosulfate treatment lowered urine pH, which would lower calcium phosphate supersaturation. Overall, there were no statistically significant differences in calcium phosphate supersaturation or upper limit of metastability between thiosulfate-treated and control rats. In vitro, thiosulfate only minimally affected ionized calcium, suggesting a mechanism of action other than calcium chelation. In summary, sodium thiosulfate reduces calcium phosphate stone formation in the genetic hypercalciuric rat. Controlled trials testing the efficacy and safety of sodium thiosulfate for recurrent kidney stones in humans are needed.

Pharmacology of stone disease

Kidney stone disease remains a major health and economic burden on the nation. It has been increasingly recognized that nephrolithiasis can be both a chronic or systemic illness. There have been major limitations in the development of new drugs for the prevention and management of this disease, largely due to our lack of understanding of the complex pathophysiologic mechanisms involving the interaction of three major target organs: the kidney, bone, and intestine. We also do not yet understand the molecular genetic basis of this polygenic disorder. These limitations are coupled with the incorrect perception that kidney stone disease is solely an acute illness, and the lack of reliable tests to assess outcome measures. All of these factors combined have diminished the willingness of the pharmaceutical industry to engage in the development of novel drugs.

Effect of bolus and divided feeding on urine ions and supersaturation in genetic hypercalciuric stone-forming rats

Because urine ion excretion varies throughout the day, clinicians monitor 24 h urine samples to measure ion excretion and supersaturation in kidney stone patients. However, these results are averages and may not reflect maximal supersaturation which drives stone formation. We measured ion excretion and saturation in genetic hypercalciuric stone-forming rats on both a normal or low calcium diet over 0-3, 3-6 and 6-24 h using two feeding protocols, where the daily food allotment was fed either as a bolus or divided into three portions. With a normal calcium diet, urine calcium, oxalate, volume, and calcium oxalate supersaturation were significantly greater on the bolus compared to the divided feeds in the prandial and postprandial periods. Bolus eaters also excreted more calcium and oxalate and had increased volume over 24 h. Maximal calcium oxalate supersaturation was greater during the initial time periods than during the entire 24 h, regardless of the feeding schedule. With the low calcium diet, the effect of bolus feeding was reduced. Thus, urine ion excretion and supersaturation vary with the type of feeding. If these results are confirmed in man, it suggests that eating as a bolus may result in greater prandial and postprandial calcium oxalate supersaturation. This may increase growth on Randall's plaques and promote stone disease.

Idiopathic hypercalciuria

Hypercalcuria is the most common metabolic disorder found in patients with nephrolithiasis. As the prevalence of kidney stones rises in industrialized nations, understanding the pathogenesis and treatment of hypercalciuria becomes increasingly important. Idiopathic hypercalciuria (IH), defined as an excess urine calcium excretion without an apparent underlying etiology, is the most frequent cause of hypercalciuria and will be the focus of this paper. Calcium homeostasis is tightly controlled and slight disturbances in transport at the level of the intestine, bone, and/or kidney can lead to excessive urine calcium excretion and promote stone formation. IH is a systemic disorder with dysregulation of calcium transport at a combination of these calcium regulatory sites. The goal of treatment is to prevent stone formation and relies on a combination of dietary and pharmaceutical interventions. Dietary management includes increasing fluid intake, salt restriction, animal protein restriction, and maintaining a normal calcium intake. Thiazide diuretics have proven effective in preventing calcium stone formation by reducing the urinary excretion of calcium. It is important to note that while decreasing urinary calcium excretion is important the clinician should focus primarily on reducing the supersaturation of calcium oxalate as this determines the true tendency for stone formation.

Genetic hypercalciuric stone-forming rats

PURPOSE OF REVIEW

We will describe the pathophysiology of hypercalciuria and the mechanism of the resultant stone formation in a rat model and draw parallels to human hypercalciuria and stone formation.

RECENT FINDINGS

Through inbreeding we have established a strain of rats that excrete 8-10 times more urinary calcium than control rats. These genetic hypercalciuric rats absorb more dietary calcium at lower 1,25-dihydroxyvitamin D3 levels. Elevated urinary calcium excretion on a low-calcium diet indicated a defect in renal calcium reabsorption and/or an increase in bone resorption. Bone from hypercalciuric rats released more calcium when exposed to 1,25-dihydroxyvitamin D3. Bisphosphonate significantly reduced urinary calcium excretion in rats fed a low-calcium diet. Clearance studies showed a primary defect in renal calcium reabsorption. The intestine, bone and kidneys of the hypercalciuric rats had increased numbers of vitamin D receptors. When hydroxyproline is added to their diet they form calcium oxalate stones, the most common stone type in humans. Increased numbers of vitamin D receptors may cause hypercalciuria in these rats and humans.

SUMMARY

Understanding the mechanism of hypercalciuria and stone formation in this animal model will help clinicians devise effective treatment strategies for preventing recurrent stone formation in humans.

Vitamin D and calcium receptors: links to hypercalciuria

PURPOSE OF REVIEW

In idiopathic hypercalciuria, patients have increased intestinal Ca absorption and decreased renal Ca reabsorption, with either elevated or normal serum levels of 1,25-dihydroxyvitamin D. As 1,25-dihydroxyvitamin D exerts its biologic effects through interactions with the vitamin D receptor, we examine the actions of this receptor and 1,25-dihydroxyvitamin D in animals with genetic hypercalciuria.

RECENT FINDINGS

In genetic hypercalciuric stone-forming rats intestinal calcium transport is increased and renal calcium reabsorption is reduced, yet serum 1,25-dihydroxyvitamin D levels are normal. Elevated intestinal and kidney vitamin D receptors suggest that increased tissue 1,25-dihydroxyvitamin D-vitamin D receptor complexes enhance 1,25-dihydroxyvitamin D actions on intestine and kidney, and vitamin D-dependent over-expression of renal calcium-sensing receptor alone can decrease tubule calcium reabsorption. In TRPV5-knockout mice, ablation of the renal calcium-influx channel decreases tubular calcium reabsorption, and secondary elevations in 1,25-dihydroxyvitamin D increase intestinal calcium transport.

SUMMARY

1,25-Dihydroxyvitamin D or vitamin D receptor may change intestinal and renal epithelial calcium transport simultaneously or calcium-transport changes across renal epithelia may be primary with a vitamin D-mediated secondary increase in intestinal transport. The extent of homology between the animal models and human idiopathic hypercalciuria remains to be determined.

Effect of cinacalcet on urine calcium excretion and supersaturation in genetic hypercalciuric stone-forming rats

Idiopathic hypercalciuria is the most common metabolic abnormality in patients with nephrolithiasis. Through successive inbreeding, we have developed a strain of rats whose urine calcium (UCa) excretion is approximately 8-10-fold greater than that of control rats and who spontaneously form kidney stones. We have termed these rats genetic hypercalciuric stone-forming (GHS) rats. The physiology of the hypercalciuria in the GHS rats closely parallels that of man. We have recently shown that the GHS rat kidneys have an increased number of receptors for calcium (CaR) compared to Sprague-Dawley rats, the strain of rats originally bred to develop the GHS rats. Calcimimetics, such as cinacalcet (Cin), increase the sensitivity of the CaR to Ca. The effects of Cin on UCa are complex and difficult to predict. We tested the hypothesis that Cin would alter urinary (U) Ca and supersaturation with respect to calcium hydrogen phosphate (CaHPO(4)) and calcium oxalate (CaOx). GHS or control rats were fed a normal Ca diet (0.6% Ca) for 28 days with Cin (30 mg/kg/24 h) added to the diet of half of each group for the last 14 days. The protocol was then repeated while the rats were fed a low Ca (0.02% Ca) diet. We found that Cin led to a marked reduction in circulating parathyroid hormone and a modest reduction in serum Ca. Cin did not alter UCa when the GHS rats were fed the normal Ca diet but lowered UCa when they were fed the low Ca diet. However, Cin did not alter U supersaturation with respect to either CaOx or CaHPO(4) on either diet. If these findings in GHS rats can be confirmed in man, it suggests that Cin would not be an effective agent in the treatment of human idiopathic hypercalciuria and resultant stone formation.

Isolation and confirmation of a calcium excretion quantitative trait locus on chromosome 1 in genetic hypercalciuric stone-forming congenic rats

Hypercalciuria is the most common risk factor for kidney stones and has a substantial genetic component. The genetic hypercalciuric stone-forming (GHS) rat model displays complex changes in physiology involving intestine, bone, and kidney and overexpression of the vitamin D receptor, thereby reproducing the human phenotype of idiopathic hypercalciuria. Through quantitative trait locus (QTL) mapping of rats that were bred from GHS female rats and normocalciuric Wistar Kyoto (WKY) male rats, loci that are linked to hypercalciuria and account for a 6 to eight-fold phenotypic difference between the GHS and WKY progenitors were mapped. GHS x WKY rats were backcrossed to breed for congenic rats with the chromosome 1 QTL HC1 on a normocalciuric WKY background. Ten generations of backcrosses produced N10F1 rats, which were intercrossed to produce rats that were homozygous for GHS loci in the HC1 region between markers D1Mit2 and D1Mit32. On a high-calcium diet (1.2% calcium), significantly different levels of calcium excretion were found between male congenic (1.67 +/- 0.71 mg/24 h) and male WKY control rats (0.78 +/- 0.19 mg/24 h) and between female congenic (3.11 +/- 0.90 mg/24 h) and female WKY controls (2.11 +/- 0.50 mg/24 h); the congenics preserve the calcium excretion phenotype of the GHS parent strain. Microarray expression analyses of the congenic rats, compared with WKY rats, showed that of the top 100 most changed genes, twice as many as were statistically expected mapped to chromosome 1. Of these, there is a clear bias in gene expression change for genes in the region of the HC1. Of >1100 gene groups analyzed, one third of the 50 most differentially expressed gene groups have direct or secondary action on calcium metabolism or transport. This is the first QTL for hypercalciuria to be isolated in a congenic animal.

Regulation of renal calcium receptor gene expression by 1,25-dihydroxyvitamin D3 in genetic hypercalciuric stone-forming rats

Hypercalciuria in inbred genetic hypercalciuric stone-forming (GHS) rats is due, in part, to a decrease in renal tubule Ca reabsorption. Activation of the renal Ca receptor (CaR) may decrease renal tubule Ca reabsorption and cause hypercalciuria through suppression of Ca-sensitive potassium channel activity. Because the rat renal CaR gene is regulated by extracellular calcium and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and GHS rats have increased renal vitamin D receptor content, the current study was undertaken to determine the level of CaR gene expression in GHS rat kidney and whether CaR gene expression is regulated by 1,25(OH)2D3. Male GHS and normal control (NC) rats were fed a Ca-sufficient diet (0.6% Ca). Western blotting revealed a four-fold increase in CaR protein in GHS rat renal tissue, and 1,25(OH)2D3 administration increased renal CaR in both GHS and NC rats. Northern blot analysis of extracts of renal cortical tissue from GHS and NC rats revealed a major 7-kb transcript of CaR and a more modest 4-kb transcript, both of which were readily detectable. Both Northern blotting and real-time reverse transcription-PCR revealed increased basal CaR mRNA expression levels in GHS rat kidney. 1,25(OH)2D3 administration increased renal CaR mRNA levels 2.0- and 3.3-fold in GHS and NC rats, respectively. Despite the greater incremental increase by 1,25(OH)2D3 in NC rats, CaR mRNA levels remained higher in GHS rat kidney, and the elevation was more sustained. 1,25(OH)2D3 increased CaR mRNA through both elevated CaR gene expression and prolonged tissue half-life. These results demonstrate that GHS rats have high levels of CaR gene expression and CaR protein that may contribute to the hypercalciuria and calcium nephrolithiasis.

Mechanism and function of high vitamin D receptor levels in genetic hypercalciuric stone-forming rats

The functional status and mechanism of increased VDR in GHS rats were investigated. Basal VDR and calbindins were increased in GHS rats. 1,25(OH)(2)D(3) increased VDR and calbindins in controls but not GHS rats. VDR half-life was prolonged in GHS rats. This study supports the mechanism and functional status of elevated VDR in GHS rats.

INTRODUCTION

Genetic hypercalciuric stone-forming (GHS) rats form calcium kidney stones from hypercalciuria arising from increased intestinal calcium absorption and bone resorption and decreased renal calcium reabsorption. Normal serum 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels and increased vitamin D receptor (VDR) protein suggest that high rates of expression of vitamin D-responsive genes may mediate the hypercalciuria. The mechanism of elevated VDR and state of receptor function are not known.

MATERIALS AND METHODS

GHS and non-stone-forming control (NC) male rats (mean, 249 g), fed a normal calcium diet, were injected intraperitoneally with 1,25(OH)2D3 (30 ng/100 g BW) or vehicle 24 h before cycloheximide (6 mg/100 g, IP) and were killed 0-8 h afterward. Duodenal VDR was measured by ELISA and Western blot, and duodenal and kidney calbindins (9 and 28 kDa) were measured by Western blots.

RESULTS AND CONCLUSIONS

Duodenal VDR protein by Western blot was increased 2-fold in GHS versus NC rats (633 +/- 62 versus 388 +/- 48 fmol/mg protein, n = 4, p < 0.02), and 1,25(OH)2D3 increased VDR and calbindins (9 and 28 kDa) further in NC but not GHS rats. Duodenal VDR half-life was prolonged in GHS rats (2.59 +/- 0.2 versus 1.81 +/- 0.2 h, p < 0.001). 1,25(OH)2D3 prolonged duodenal VDR half-life in NC rats to that of untreated GHS rats (2.59 +/- 0.2 versus 2.83 +/- 0.3 h, not significant). This study supports the hypothesis that prolongation of VDR half-life increases VDR tissue levels and mediates increased VDR-regulated genes that result in hypercalciuria through actions on vitamin D-regulated calcium transport in intestine, bone, and kidney.

Thiazides Reduce Brushite, but not Calcium Oxalate, Supersaturation, and Stone Formation in Genetic Hypercalciuric Stone–Forming Rats

Over 59 generations, a strain of rats has been inbred to maximize urine calcium excretion. The rats now excrete eight to 10 times as much calcium as controls. These rats uniformly form calcium phosphate (apatite) kidney stones and have been termed genetic hypercalciuric stone-forming (GHS) rats. The addition of a common amino acid and oxalate precursor, hydroxyproline, to the diet of the GHS rats leads to formation of calcium oxalate (CaOx) kidney stones. Hydroxyproline-supplemented GHS rats were used to test the hypothesis that the thiazide diuretic chlorthalidone would decrease urine calcium excretion, supersaturation, and perhaps stone formation. All GHS rats received a fixed amount of a standard 1.2% calcium diet with 5% trans-4-hydroxy-l-proline (hydroxyproline) so that the rats would exclusively form CaOx stones. Half of the rats had chlorthalidone (Thz; 4 to 5 mg/kg per d) added to their diets. Urine was collected weekly, and at the conclusion of the study, the kidneys, ureters, and bladders were radiographed for the presence of stones. Compared with control, the addition of Thz led to a significant reduction of urine calcium and phosphorus excretion, whereas urine oxalate excretion increased. Supersaturation with respect to the calcium hydrogen phosphate fell, whereas supersaturation with respect to CaOx was unchanged. Rats that were fed Thz had fewer stones. As calcium phosphate seems to be the preferred initial solid phase in patients with CaOx kidney stones, the reduction in supersaturation with respect to the calcium phosphate solid phase may be the mechanism by which thiazides reduce CaOx stone formation.

Calcium oxalate crystal localization and osteopontin immunostaining in genetic hypercalciuric stone-forming rats

BACKGROUND

The inbred genetic hypercalciuric stone-forming (GHS) rats develop calcium phosphate (apatite) stones when fed a normal 1.2% calcium diet. The addition of 1% hydroxyproline to this diet does not alter the type of stone formed, while rats fed this diet with 3% hydroxyproline form mixed apatite and calcium oxalate stones and those with 5% hydroxyproline added form only calcium oxalate stones. The present study was designed to determine the localization of stone formation and if this solid phase resulted in pathologic changes to the kidneys.

METHODS

GHS rats were fed 15 g of the standard diet or the diet supplemented with 1%, 3%, or 5% hydroxyproline for 18 weeks. A separate group of Sprague-Dawley rats (the parental strain of the GHS rats), fed the standard diet for a similar duration, served as an additional control. At 18 weeks, all kidneys were perfusion-fixed for structural analysis, detection of crystal deposits using the Yasue silver substitution method, and osteopontin immunostaining.

RESULTS

There were no crystal deposits found in the kidneys of Sprague-Dawley rats. Crystal deposits were found in the kidneys of all GHS rats and this Yasue-stained material was detected only in the urinary space. No crystal deposits were noted within the cortical or medullary segments of the nephron and there was no evidence for tubular damage in any group. The only pathologic changes occurred in 3% and 5% hydroxyproline groups with the 5% group showing the most severe changes. In these rats, which form only calcium oxalate stones, focal sites along the urothelial lining of the papilla and fornix of the urinary space demonstrated a proliferative response characterized by increased density of urothelial cells that surrounded the crystal deposits. At the fornix, some crystals were lodged within the interstitium, deep to the proliferative urothelium. There was increased osteopontin immunostaining in the proliferating urothelium.

CONCLUSION

Thus in the GHS rat, the initial stone formation occurred solely in the urinary space. Tubular damage was not observed with either apatite or calcium oxalate stones. The apatite stones do not appear to cause any pathological change while those rats forming calcium oxalate stones have a proliferative response of the urothelium, with increased osteopontin immunostaining, around the crystal deposits in the fornix.

Endoscopic renal papillary biopsies: a tissue retrieval technique for histological studies in patients with nephrolithiasis

PURPOSE

The mechanisms behind calcium nephrolithiasis remain unclear. Previous research has relied on animal models or cell lines, yielding limited insight into the pathophysiology of human calcium stone disease. To determine changes occurring in the human kidney during active stone disease we used an endoscopic renal papillary biopsy protocol in calcium stone formers undergoing percutaneous nephrolithotomy.

MATERIALS AND METHODS

Following stone burden clearance via percutaneous nephrolithotomy 15 idiopathic calcium oxalate and 4 ileal bypass stone formers underwent flexible and rigid nephroscopy. Biopsies from select papillae in the peripheral and interpolar regions were obtained with 5Fr flexible cup biopsy forceps. A papilla adjacent to the accessed calix was biopsied with 10Fr cup biopsy forceps. Cortical biopsies along the access tract were also obtained with the 10Fr forceps.

RESULTS

All patients had successful biopsy completion. No complications were attributable to the biopsy process and no blood transfusions were required. Of the 19 patients 12 were contacted for followup at a mean of 21.7 +/- 9.0 months with none experiencing adverse sequelae such as bleeding or significant pain. A total of 14 patients had followup serum creatinine available showing that the difference in mean preoperative and postoperative values was not clinically significant (1.00 +/- 0.27 and 1.11 +/- 0.27 mg/dl, respectively). The quality of biopsied tissue permitted accurate immunohistochemical staining of crystal deposits and mineral analysis.

CONCLUSIONS

Endoscopic papillary biopsies were performed safely in a small patient population. Tissue obtained using this protocol can be used for detailed histological and analytical studies, which may lead to significant advances in our understanding of stone formation mechanisms.

Quantitative trait loci for hypercalciuria in a rat model of kidney stone disease

Hypercalciuria is the most common risk factor for kidney stones and has a recognized familial component. The genetic hypercalciuric stone-forming (GHS) rat is an animal model that closely resembles human idiopathic hypercalciuria, with excessive intestinal calcium absorption, increased bone resorption, and impaired renal calcium reabsorption; overexpression of the vitamin D receptor (VDR) in target tissues; and calcium nephrolithiasis. For identifying genetic loci that contribute to hypercalciuria in the GHS rat, an F2 generation of 156 rats bred from GHS female rats and normocalciuric WKY male rats was studied. The calcium excretion was six- to eightfold higher in the GHS female than in the WKY male progenitors. Selective genotyping of those F2 rats with the highest 30% and lowest 30% rates of calcium excretion was performed, scoring 98 markers with a mean interval of 23 cM across all 20 autosomes and the X chromosome. With the use of strict criteria for significance, significant linkage was found between hypercalciuria and a region of chromosome 1 at D1Rat169 (LOD, 2.91). Suggestive linkage to regions of chromosomes 4, 7, 10, and 14 was found. The proportion of phenotypic variance contributed by the region on chromosome 1, with appropriate adjustments, was estimated to be 7%. Candidate genes encoding the VDR and the calcium-sensing receptor were localized to regions on rat chromosomes 7 and 11, respectively, but the suggestive quantitative trait locus on chromosome 7 was not in the region of the VDR gene locus. Identification of genes that contribute to hypercalciuria in this animal model should prove valuable in understanding idiopathic hypercalciuria and kidney stone disease in humans.

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.

Effects of an aqueous extract from Phyllantus niruri on calcium oxalate crystallization in vitro

Phyllanthus niruri is a plant used in Brazilian folk medicine for the treatment of urolithiasis. It was previously observed that P. niruri shows no toxicity, potentially increases calculus voiding by stone forming patients and inhibits the endocytosis of calcium oxalate (CaOx) crystals by MDCK cells. In addition, in a rat model of urolithiasis it reduced calculus growth. In the present study, we evaluated the effect of an aqueous extract of P. niruri on CaOx crystallization in vitro. CaOx precipitation was induced by the addition of 0.1 M sodium oxalate to unfiltered urine samples from Wistar rats (n=14) and normal humans (n=18) in the presence or absence of P. niruri extract (0.25 mg/ml of urine). The presence of CaOx crystals was evaluated immediately and 24 h later. In vitro crystallization of human urine produced typical mono- and dihydrated CaOx crystals, but only a few typical CaOx crystals were found in rat urine. The presence of P. niruri extract did not inhibit CaOx precipitation and even more crystals were obtained, although they were significantly smaller than those in the control urine. Crystal aggregation observed 24 h after crystallization was also inhibited by P. niruri extract. The results showed an inhibitory effect of P. niruri extract on CaOx crystal growth and aggregation in human urine, suggesting that it may interfere with the early stages of stone formation and may represent an alternative form of treatment and/or prevention of urolithiasis

Calcium oxalate stone formation in genetic hypercalciuric stone-forming rats

BACKGROUND

Over 54 generations, we have successfully bred a strain of rats that maximizes urinary calcium excretion. The rats now consistently excrete 8 to 10 times as much calcium as controls, uniformly form poorly crystalline calcium phosphate kidney stones, and are termed genetic hypercalciuric stone-forming (GHS) rats. These rats were used to test the hypothesis that increasing urinary oxalate excretion would not only increase the supersaturation with respect to the calcium oxalate solid phase, but also would increase the ratio of calcium oxalate-to-calcium phosphate supersaturation and result in calcium oxalate stone formation.

METHODS

To increase urine oxalate excretion an oxalate precursor, hydroxyproline, was added to the diet of male GHS rats. The GHS rats were fed a standard 1.2% calcium diet alone or with 1%, 3% or 5% trans-4-hydroxy-l-proline (hydroxyproline).

RESULTS

The addition of 1% hydroxyproline to the diet of GHS rats led to an increase in urinary oxalate excretion, which did not increase further with the provision of additional hydroxyproline. The addition of 1% and 3% hydroxyproline did not alter calcium excretion while the provision of 5% hydroxyproline led to a decrease in urine calcium excretion. The addition of 1% hydroxyproline led to an increase in urinary calcium oxalate supersaturation, which did not further increase with additional hydroxyproline. The addition of 1% and 3% hydroxyproline did not alter urinary supersaturation with respect to calcium hydrogen phosphate while the addition of 5% hydroxyproline tended to lower this supersaturation. Compared to rats fed the control and the 3% hydroxyproline diet the addition of 5% hydroxyproline increased the ratio of calcium oxalate supersaturation to calcium phosphate supersaturation. Virtually all rats formed stones. In the control and 1% hydroxyproline group, all of the stones were composed of calcium and phosphate (apatite), in the 3% hydroxyproline group the stones were a mixture of apatite and calcium oxalate, while in the 5% hydroxyproline group all of the stones were calcium oxalate.

CONCLUSIONS

The provision of additional dietary hydroxyproline to GHS rats increases urinary oxalate excretion, calcium oxalate supersaturation and the ratio of calcium oxalate-to-calcium phosphate supersaturation, resulting in the formation of calcium oxalate kidney stones. Thus, with the addition of a common amino acid, the GHS rats now not only model the most common metabolic abnormality found in patients with nephrolithiasis, hypercalciuria, but form the most common type of kidney stone, calcium oxalate.

Effect of acidosis on urine supersaturation and stone formation in genetic hypercalciuric stone-forming rats

BACKGROUND

We have successively inbred over 45 generations a strain of rats to maximize urine calcium excretion. The rats now consistently excrete 8 to 10 times as much calcium as controls and uniformly form poorly crystalline calcium phosphate kidney stones. In humans with calcium nephrolithiasis, consumption of a diet high in acid precursors is often cited as a risk factor for the development of calcium-based kidney stones; however, the effect of this diet on urinary supersaturation with respect to the common solid phases found in kidney stones has not been determined.

METHODS

To determine the effect of the addition of an acid precursor on urine ion excretion, supersaturation, and stone formation, we fed these genetic hypercalciuric stone-forming (GHS) rats 13 g/day of a 1.2% calcium diet with 0.0, 0.5, 1.0, or 1.5% NH4Cl in the drinking water for 14 weeks (N = 8 for each). Urine was collected and analyzed every two weeks.

RESULTS

As expected, the addition of dietary NH4Cl led to a progressive fall in urine pH and urine citrate, while urine ammonium increased. Urine calcium and phosphorus increased, while urine oxalate fell. Increasing dietary NH4Cl led to a fall in supersaturation with respect to CaHPO4 (brushite) and CaOx and a rise in supersaturation with respect to uric acid. In spite of differences in supersaturation, most rats in each group formed stones that contained calcium phosphate and not calcium oxalate.

CONCLUSIONS

Thus, while the provision of additional dietary acids alters urinary ion excretion and lowers supersaturation with respect to CaHPO4 and CaOx, it does not change the character or rate of stone formation in the GHS rats.