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.     

The use of a test for the differential diagnosis of hypercalciuria.

28 renal stone formers (18 men and 10 women) with idiopathic hypercalciuria (IH) and 27 controls have been subjected to a test proposed for the diagnosis of absorptive, resorptive and renal hypercalciurias. Fasting serum calcium concentration, urinary calcium and cyclic AMP excretion were measured after overnight fasting and an oral load of calcium. Absorptive hypercalciuria was demonstrated in 14 patients. High fasting urinary calcium first suggested resorptive or renal hypercalciurias in 5 other patients, but since fasting urinary calcium was normalized following cellulose phosphate therapy, absorptive hypercalciuria was more likely. Renal hypercalciuria was a possibility in 1 single case. Both fasting and post-load urinary calcium were normal in 7 men and 1 woman. The test did not appear as useful as expected since it was of no diagnostic value in about 30% of the cases and erroneously suggested resorptive or renal hypercalciuria in about 15% of the cases. On the other hand it indicated that absorptive IH is common and renal IH exceptional.     

New insights into the pathogenesis of idiopathic hypercalciuria

Idiopathic hypercalciuria (IH) is the most common metabolic abnormality in patients with calcium kidney stones. It is characterized by normocalcemia, absence of diseases that cause increased urine calcium, and calcium excretion that is greater than 250 mg/d in women and 300 mg/d in men. Subjects with IH have a generalized increase in calcium turnover, which includes increased gut calcium absorption, decreased renal calcium reabsorption, and a tendency to lose calcium from bone. Despite the increase in intestinal calcium absorption, a negative calcium balance is seen commonly in balance studies, especially on a low-calcium diet. The mediator of decreased renal calcium reabsorption is not clear; it is not associated with either an increase in filtered load of calcium or altered parathyroid hormone levels. There is an increased incidence of hypercalciuria in first-degree relatives of those with IH, but IH appears to be a complex polygenic trait with a large contribution from diet to expression of increased calcium excretion. Increased tissue vitamin D response may be responsible for the manifestations of IH in at least some patients.     

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.     

Claudins and the Kidney

Claudins are tight-junction membrane proteins that function as both pores and barriers in the paracellular pathway in epithelial cells. In the kidney, claudins determine the permeability and selectivity of different nephron segments along the renal tubule. In the proximal tubule, claudins have a role in the bulk reabsorption of salt and water. In the thick ascending limb, claudins are important for the reabsorption of calcium and magnesium and are tightly regulated by the calcium-sensing receptor. In the distal nephron, claudins need to form cation barriers and chloride pores to facilitate electrogenic sodium reabsorption and potassium and acid secretion. Aldosterone and the with-no-lysine (WNK) proteins likely regulate claudins to fine-tune distal nephron salt transport. Genetic mutations in claudin-16 and -19 cause familial hypomagnesemic hypercalciuria with nephrocalcinosis, whereas polymorphisms in claudin-14 are associated with kidney stone risk. It is likely that additional roles for claudins in the pathogenesis of other types of kidney diseases have yet to be uncovered.     

Pathophysiological studies in hypercalciuric and normocalciuric stone formers

This study has been carried out in order to investigate the effects of a standard oral calcium load in 18 hypercalciuric and 18 normocalciuric recurrent stone formers. Following oral loading there was not a significant difference in the two groups studied, as far as serum calcium increment was concerned; however, urinary calcium excretion was significantly greater in hypercalciuric stone formers. The results obtained, although they do not exclude the presence of intestinal calcium hyperabsorption, seem to demonstrate the existence of a diminution of renal tubular calcium reabsorption in patients with idiopathic hypercalciuria.     

Diet, vitamin D and vertebral mineral density in hypercalciuric calcium stone formers

To elucidate the pathophysiology of dietary calcium independent hypercalciuria, 42 calcium stone formers (Ca SF) were selected because they had on free diet a calciuria greater than 0.1 mmol/kg/day. For four days they were put on a diet restricted in calcium (Ca RD) by exclusion of the dairy products. They collected 24 hour urines on free diet and on day 4 of Ca RD as well as the two-hour fasting urines on the morning of the day 5 and the four-hour urines passed after an oral calcium load of 1 g, for measurement of creatinine, Ca, PO4, urea and total hydroxyprolinuria (THP). On day 5 fasting plasma concentrations of Ca, PO4, intact PTH, Gla protein, calcidiol and calcitriol were measured. The patients were firstly classified into dietary hypercalciuria (DH, 18 patients) and dietary calcium-independent hypercalciuria (IH, 24 patients) on the basis of the disappearance or not of hypercalciuria on Ca RD. Then the patients with IH were subclassified into absorptive hypercalciuria (AH) because of normal fasting calciuria (8 patients) and into fasting hypercalciuria (16 patients). Fasting hypercalciuric patients were subsequently divided according to the PTH levels into renal hypercalciuria (RH, 1 patient) with elevated fasting PTH becoming normal after the Ca load and undetermined hypercalciuria (UH, 15 patients) with normal PTH levels. Furthermore, their vertebral mineral density (VMD) was measured by quantitative computerized tomography which was normal in DH (91 +/- 6% of the normal mean for age and sex) but was decreased in IH to 69 +/- 4%. No difference in VMD was observed between AH and UH. Urinary excretions of urea, phosphate and THP was higher in IH than in DH and comparable in AH and UH. Sodium excretion Ca RD was the same in all groups and subgroups as well as the plasma parameters. Plasma calcitriol was increased in IH and DH comparatively to normal in spite of normal plasma calcidiol. Calciuria increase after oral calcium load, an index of Ca absorption, was higher in IH than in controls and comparable in IH and DH as well as in the three subgroups of IH. From these data and correlation studies in IH it is concluded: (1.) VMD is decreased in Ca stone formers with IH but not in those with DH, making the distinction of these two groups of hypercalciuria patients clinically relevant.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inherited hypercalciuric syndromes: Dent's disease (CLC-5) and familial hypomagnesemia with hypercalciuria (paracellin-1)

Dent's disease and familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) are inherited diseases in which hypercalciuria, nephrocalcinosis, and renal failure are prominent features. Dent's disease resembles a Fanconi syndrome, with impaired reabsorption in the proximal tubule; FHHNC, with urinary loss of magnesium and calcium, is associated with impaired cation transport in the thick ascending limb of Henle's loop. Gene mapping in families and positional cloning led in both cases to identification of the responsible gene. Dent's disease is associated with mutations that disrupt function of a voltage-gated chloride channel, CLC-5, expressed in subapical endosomes of the proximal tubule and in other nephron segments. Impaired function of this channel disturbs reabsorption of filtered proteins, as well as other transport functions of the proximal tubule, and leads, apparently indirectly, to hypercalciuria and renal failure. FHHNC results from mutations in paracellin-1, a tight-junction protein that appears to be important in conducting or regulating paracellular cation transport. Impaired function of paracellin-1 leads specifically to urinary losses of magnesium and calcium, but because transcellular transport is intact these patients do not have hypokalemia or salt wasting. Identification of both genes represent triumphs of a genetic approach to solving problems of pathophysiology.     

Low calcium diet in idiopathic urolithiasis: a risk factor for osteopenia as great as in primary hyperparathyroidism

The bone mineral content of the radius was measured in 32 male renal stone formers, 18 of them presenting with idiopathic urolithiasis and 14 with primary hyperparathyroidism, a disease known to disturb bone metabolism. The idiopathic stone formers had been on regular treatment with a low calcium diet. The bone mineral content of the radius was reduced to a similar level in both groups of patients. The data suggest that idiopathic stone formers on a low calcium diet are at risk of osteopenia; the factors which could lead to a negative calcium balance included uncompensated renal hypercalciuria, hypophosphataemia and exaggerated serum levels of 1,25-dihydroxyvitamin D. To treat idiopathic hypercalciuria, thiazide diuretics, which reduce the renal excretion of calcium and have been shown to be beneficial for bone, seem safer than a low calcium diet.     

Resorptive and absorptive hypercalciuria. Therapy with sodium cellulose phosphate or thiazides.

Patients with recurrent stone disease and hypercalciuria were cleared up according to Nordin's schedule. In cases of absorptive hypercalciuria, an ion exchanger operating in the intestine, sodium cellulose phosphate (SCP), is applied under strict control of oxalate, calcium and magnesium excretion as well as ionized calcium in serum. Under treatment with SCP (27 patients), we found a reduction in the renal excretion of calcium and magnesium, and, as a side effect, a significant augmentation of the renal oxalate excretion. In cases of resorptive or resorptive/absorptive hypercalciuria, except in patients with primary HPT, 23 patients were mediated by thiazides (Esidrix). This drug effects a marked decrease of urinary calcium based on a higher rate of reabsorption of calcium in the distal tubule. No severe side effects especially primary HPT were observed.     

Urolithiasis associated with hypercalciuria

Fifty male patients with urolithiasis (UL), associated with idiopathic hypercalciuria (IH), were studied in comparison to a group of 18 male normocalcemic patients with inactive calcium stone disease of unknown etiology. In the group of IH-UL, in addition to hypercalciuria, statistically significant hyperphosphaturia with decreased tubular reabsorption of phosphate and hyperuricemia were observed; there was a tendency to hypophosphatemia although non-significant. In 36% of the IH-UL patients the first episode of renal colic appeared at age 40 to 50. Thirty-eight per cent of the IH-UL patients had recurrent stone formation. Twenty per cent of the IH-UL patients had a family history of urolithiasis. Forty-six per cent of all stones contained oxalate in addition to calcium, and 25% of the stones contained oxalate and phosphate.     

The relationship of 3' vitamin D receptor haplotypes to urinary supersaturation of calcium oxalate salts and to age at onset and familial prevalence of nephrolithiasis.

BACKGROUND: Idiopathic hypercalciuria (IHc) and idiopathic hypocitraturia are frequently associated with calcium nephrolithiasis. We investigated the relationship of vitamin D receptor (VDR) polymorphisms (BsmI, TaqI and FokI) to urinary supersaturation of calcium oxalate salts in recurrent calcium oxalate stone formers with IHc and the clinical relevance of this relationship. METHODS: The study included 110 Caucasian stone formers with IHc and 127 unrelated healthy controls without history of nephrolithiasis. Age at onset of nephrolithiasis, familial history score (FHS) and the ion activity product of calcium oxalate salts in urine (AP(CaOx)) were tabulated. BsmI, TaqI and FokI VDR polymorphisms were evaluated in all participants. RESULTS: Patients and controls were classified as homozygous (bbTT and BBtt) or heterozygous in relation to BsmI and TaqI polymorphisms. Compared with BBtt patients, bbTT homozygous stone formers showed lower citrate excretion (1.91+/-0.89 vs 3.46+/-1.39 mmol/24 h, P = 0.004) and higher AP(CaOx) (2.02+/-0.51 vs 1.53+/-0.53, P = 0.006). Among controls, there were similar differences in citrate excretion and AP(CaOx) between the two groups, but they were not statistically significant. Compared with BBtt, bbTT patients showed lower mean age at onset of nephrolithiasis (29.7+/-12.1 vs 38.1+/-12.7 years, P = 0.008) and higher values of FHS (2.45+/-1.9 vs 0.83+/-0.7, P = 0.006). Similar results were obtained for individual BsmI and TaqI alleles. The analysis of FokI alleles was not informative. CONCLUSIONS: Recurrent calcium oxalate stone formers with IHc and the bT VDR haplotype have more aggressive kidney stone diseases as indicated by a higher familial incidence and lower mean age at onset. This clinical severity is associated with the higher urinary supersaturation of calcium oxalate salts and abnormalities of renal citrate handling.     

Aromatase deficiency causes altered expression of molecules critical for calcium reabsorption in the kidneys of female mice *.

Kidney stones increase after menopause, suggesting a role for estrogen deficiency. ArKO mice have hypercalciuria and lower levels of calcium transport proteins, whereas levels of the klotho protein are elevated. Thus, estrogen deficiency is sufficient to cause altered renal calcium handling. INTRODUCTION: The incidence of renal stones increases in women after menopause, implicating a possible role for estrogen deficiency. We used the aromatase deficient (ArKO) mouse, a model of estrogen deficiency, to test the hypothesis that estrogen deficiency would increase urinary calcium excretion and alter the expression of molecular regulators of renal calcium reabsorption. MATERIALS AND METHODS: Adult female wildtype (WT), ArKO, and estradiol-treated ArKO mice (n = 5-12/group) were used to measure urinary calcium in the fed and fasting states, relative expression level of some genes involved in calcium reabsorption in the distal convoluted tubule by real-time PCR, and protein expression by Western blotting or immunohistochemistry. Plasma membrane calcium ATPase (PMCA) activity was measured in kidney membrane preparations. ANOVA was used to test for differences between groups followed by posthoc analysis with Dunnett's test. RESULTS: Compared with WT, urinary Ca:Cr ratios were elevated in ArKO mice, renal mRNA levels of transient receptor potential cation channel vallinoid subfamily member 5 (TRPV5), TRPV6, calbindin-D28k, the Na+/Ca+ exchanger (NCX1), and the PMCA1b were significantly decreased, and klotho mRNA and protein levels were elevated. Estradiol treatment of ArKO mice normalized urinary calcium excretion, renal mRNA levels of TRPV5, calbindin-D(28k), PMCA1b, and klotho, as well as protein levels of calbindin-D28k and Klotho. ArKO mice treated with estradiol had significantly greater PMCA activity than either untreated ArKO mice or WT mice. CONCLUSIONS: Estrogen deficiency caused by aromatase inactivation is sufficient for renal calcium loss. Changes in estradiol levels are associated with coordinated changes in expression of many proteins involved in distal tubule calcium reabsorption. Estradiol seems to act at the genomic level by increasing or decreasing (klotho) protein expression and nongenomically by increasing PMCA activity. PMCA, not NCX1, is likely responsible for extruding calcium in response to in vivo estradiol hormonal challenge. These data provide potential mechanisms for regulation of renal calcium handling in response to changes in serum estrogen levels.     

Dietary factors and medullary sponge kidneys as causes of the so-called idiopathic renal leak of calcium

Out of 89 stone formers with idiopathic hypercalciuria, 51 remained hypercalciuric on a low calcium diet over 5 days: a renal leak of calcium could thus have been suspected in them. Dietary factors such as high sodium or high animal protein intake, and metabolic factors such as obesity with or without hyperinsulinemia, which all might account for the hypercalciuria of these patients, have been evaluated. This evaluation revealed conditions known to be associated with hypercalciuria in 37 of these 51 patients: 15 had hypercalciuria related to a high sodium intake, 7 had severe hyperuricosuria (greater than 1 g/24 h) reflecting a high animal protein intake, 20 were obese (greater than 120% of ideal weight) with (7 cases) or without (13 cases) concomitant high fasting plasma level of insulin (greater than 18 microU/ml). A careful retrospective analysis of the intravenous pyelograms disclosed medullary sponge kidneys in 8 cases which had remained undiagnosed so far. One of them was studied histologically. Only 14 out of 51 patients had an otherwise unexplained hypercalciuria on a low calcium diet. It is concluded that dietary causes appear to play a key role in 'idiopathic' hypercalciuria, that the incidence of a primary renal leak of calcium among idiopathic stone formers is much smaller than initially thought, and that this condition can hide unrecognized medullary sponge kidneys.     

Urine risk factors in children with calcium kidney stones and their siblings

Calcium nephrolithiasis in children is increasing in prevalence and tends to be recurrent. Although children have a lower incidence of nephrolithiasis than adults, its etiology in children is less well understood; hence, treatments targeted for adults may not be optimal in children. To better understand metabolic abnormalities in stone-forming children, we compared chemical measurements and the crystallization properties of 24-h urine collections from 129 stone formers matched to 105 non-stone-forming siblings and 183 normal, healthy children with no family history of stones, all aged 6 to 17 years. The principal risk factor for calcium stone formation was hypercalciuria. Stone formers have strikingly higher calcium excretion along with high supersaturation for calcium oxalate and calcium phosphate, and a reduced distance between the upper limit of metastability and supersaturation for calcium phosphate, indicating increased risk of calcium phosphate crystallization. Other differences in urine chemistry that exist between adult stone formers and normal individuals such as hyperoxaluria, hypocitraturia, abnormal urine pH, and low urine volume were not found in these children. Hence, hypercalciuria and a reduction in the gap between calcium phosphate upper limit of metastability and supersaturation are crucial determinants of stone risk. This highlights the importance of managing hypercalciuria in children with calcium stones.     

BMP2及Msx2在特发性高钙尿肾结石患者肾乳头组织表达的研究

目的：研究骨形态发生蛋白2（BMP2）及成骨样细胞转录因子Msx2在特发性高钙尿（IH）肾结石患者肾乳头组织中表达以及探讨其在IH患者结石形成中作用机制。方法：筛选特发性高钙尿肾结石患者8例（IH组）,排除各种已知可能影响血清钙或者尿钙的继发疾病;选择同期因肾肿瘤或非结石所致的无功能肾需行肾切除术的患者8例（NC组）。分别取16例患者肾乳头组织若干,各标本应用实时荧光定量PCR检测BMP2和Msx2mRNA的表达,并应用Westernblot测定两组蛋白质表达水平。结果：IH组BMP2的mRNA表达量为（1．491±0．121）,而NC组BMP2的tuRNA为（1．032±0．034）,两组间表达量差异有统计学意义（P〈0．05）;而1H组与NC组Msx2的mRNA表达量分别为（1．432±0．091）和（1．015±0．017）,两组数据差异有统计学意义（P〈0．05）。Westernblotting检测BMP2蛋白提示NC组和IH组蛋白质表达量分别为（1．475±0．042）和（1．681±0．153）,两组数据差异有统计学意义（P〈0．05）;测定Msx2蛋白水平表达显示NC组为（1．531±0．134）,而IH组（1．603±0．156）,两者差异无统计学意义（P〉0．05）。结论：特发性高钙尿（IH）肾结石患者肾乳头BMP2和Msx2mRNA表达增强为间质异位钙化特征,BMP2信号通路在特发性高钙尿结石患者Randall钙斑形成中具有一定作用。     

Critical role of oxalate restriction in association with calcium restriction to decrease the probability of being a stone former: insufficient effect in idiopathic hypercalciuria

The probability of being a stone former (PSF) was calculated in 3 groups of idiopathic calcium stone formers [with normocalciuria (NC), dietary hypercalciuria (DH) and idiopathic hypercalciuria (IH)] in 4 conditions: while on a free diet; on a calcium- and oxalate-restricted diet during 4 days; after an oxalate load, while on a 1.5-gram calcium diet, and after an oxalate load while on a calcium-restricted diet. Combined calcium and oxalate restriction significantly decreased PSF only in NC and DH whereas the decrease was not significant in IH because of a concomitant significant increase in oxalate excretion. Increase of PSF with the oxalate load was significantly greater during a calcium-restricted diet than during the 1.5-gram calcium diet in all groups of patients (4, 6 and 12 times greater in NC, DH and IH, respectively). These data show the critical role of oxalate restriction when calcium is restricted in order to decrease the PSF. This combined restriction is however not sufficient in idiopathic hypercalciuric patients to decrease their PSF.     

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.     

Osteoporosis and urolithiasis

Several studies have indicated that up to 60% of idiopathic calcium stone formers present hypercalciuria. Many authors have described reduced bone mineral density (BMD) in stoneformers with hypercalciuria, but osteopenia has also been found in normocalciuric patients. Moreover, Jaeger's group found that bone mass was reduced in all patients with calcium stone disease, independently of hypercalciuria. Many factors may contribute to the pathogenesis of osteopenia in stone formers. A predominant role has been given to the low-calcium diet that is still prescribed in nephrolithiasis. Also slight metabolic acidosis, which is frequently present in stone formers eating a diet rich in animal protein, can contribute to bone loss. Finally, some authors described a pathogenetic role for cytokines, prostaglandins and vitamin D receptor gene polymorphisms.     

Prevalence of hyperoxaluria in idiopathic calcium oxalate kidney stone disease

Urinary excretion of oxalate, calcium and urate has been investigated in 88 patients affected by idiopathic calcium oxalate stone disease and in 20 normal subjects. Of these ions, only oxalate was found significantly higher in stone formers. Defining hyperoxaluria as urinary oxalate excretion greater than 2 SD above normal, 50% of stone-forming people were found to be hyperoxaluric. When stone formers were classified in normo- and hyperoxaluric, the prevalence of hypercalciuria, hyperuricuria, family history of stone disease and recurrencies in stone formation was the same in both groups. It is concluded that hyperoxaluria is a frequent finding in finding in idiopathic calcium oxalate renal stone disease.