Renal tubular damage/dysfunction: key to the formation of kidney stones

Supersaturation is the driving force behind crystal formation in the kidneys. It can, however, result only in the formation of crystals which can be harmlessly expelled. For stone formation, crystals must form in the kidneys and be retained there, which is indeed a rare occurrence. Crystalluria is common while stone formation is not. Only pathological changes in the kidneys including renal injury and dysfunction can accomplish crystal retention. Lethal epithelial cellular injury promotes crystal nucleation, aggregation and retention. Sub-lethal injury or dysfunctional cells may produce ineffective crystallization modulators and localized areas of supersaturation in the interstitium. The former will affect crystallization in the urine while the latter may cause precipitation in the interstitium and development of Randall’s plaques.     

Crystallization in the nephron

Recent experimental studies on the crystallization of calcium salts at different nephron levels support the theory that the initial formation of calcium concrements starts with an intratubular crystallization of calcium phosphate (CaP) and calcium oxalate (CaOx). CaP seems to be the initial crystallization product in pure CaP and mixed calcium phosphate–calcium oxalate (CaPCaOx) concrements, with the formation of CaP crystals at a nephron level above the collecting duct. Urinary macromolecules and cellular degradation products most probably promote this process. During the passage through the collecting duct, CaP might partly or completely dissolve at the lower pH encountered there. This might result in an increased concentration of calcium and hence an increased supersaturation with CaOx, which in turn can bring about a heterogeneous nucleation of CaOx on or around preformed CaP crystals or crystal aggregates. The final result will be mixed CaOxCaP or pure CaOx concrements. Pure CaOx concrements might also be the result of an initial CaOx crystallization at nephron levels above or in the collecting duct under conditions with a high urinary excretion of oxalate. Whether intratubular crystallization of calcium salts results in the formation of small harmless crystals excreted with urine or calcium stones appears to be determined by a complex process, involving kinetic factors that influence crystal growth and crystal aggregation and crystal retention. Received: 24 December 1998 / Accepted: 11 March 1999     

Effect of magnesium on calcium oxalate urolithiasis

Previous studies have shown that hypomagnesuria induced by magnesium deficient diet causes calcium oxalate crystal deposition in renal tubules of hyperoxaluric rats and administration of magnesium to these rats results in prevention of calcium oxalate crystallization in their kidneys. Based on these studies magnesium was claimed to be beneficial for calcium oxalate stone patients. However, hypomagnesuria is not a common phenomenon. To better understand the role of magnesium as an inhibitor of calcium oxalate crystallization in urine, we studied the effect of magnesium on calcium oxalate urolithiasis in rats on a regular diet and a hyperoxaluric protocol. Excess magnesium was administered to male rats on regular diet and a lithogenic protocol. Magnesium administration to hyperoxaluric rats did not result in significant changes in urinary excretion of calcium or oxalate or in calcium oxalate relative supersaturation. Urinary excretion of citrate was also not significantly altered. Some animals from both groups, those on magnesium therapy and those not on magnesium therapy had crystals deposited in their renal tubules. We conclude that excess magnesium has no significant effect on calcium oxalate urolithiasis in normomagnesuric conditions.     

Expression of inter-alpha inhibitor related proteins in kidneys and urine of hyperoxaluric rats

PURPOSE: To investigate the involvement of the inter-alpha inhibitor family of proteins in calcium oxalate stone formation we determined immunohistochemical distribution in the kidneys and excretion in the urine of these proteins in normal and hyperoxaluric rats. Various members of the family have been shown to inhibit the formation and retention of calcium oxalate crystals in the kidneys. MATERIALS AND METHODS: Hyperoxaluria was induced in male Sprague-Dawley rats by administering 0.75% ethylene glycol. The inter-alpha inhibitor family consists of inter-alpha inhibitor, pre-alpha inhibitor, the so-called heavy chains H1, H2 and H3, and the light chain bikunin. Antibodies against these molecules were used to localize various proteins in rat kidneys by immunohistochemical techniques. Urine was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis to determine the expression of various members of the inter-alpha inhibitor family. RESULTS: In normal kidneys staining for inter-alpha inhibitor and other members of the family was mostly limited to the proximal tubules and generally to their luminal contents. Eight weeks after the induction of hyperoxaluria various sections of renal tubules stained positive for inter-alpha inhibitor, bikunin and H3. Positive staining was observed in the tubular lumina as well as in the cytoplasm of epithelial cells. Crystal associated material was heavily stained. Western blot analysis recognized 7 protein bands in the urine. The urinary expression of H1, H3 and pre-alpha-inhibitor was significantly increased. CONCLUSIONS: Apparently hyperoxaluria and renal calcium oxalate crystal deposition result in the increased expression of crystallization inhibitors, such as inter-alpha-inhibitor related proteins, in the kidneys and urine. Results indicate that kidneys respond to nephrolithic challenges by producing proteins that inhibit crystal formation and retention.     

Nephrocalcinosis in animal models with and without stones

Nephrocalcinosis is the deposition of calcium salts in renal parenchyma and can be intratubular or interstitial. Animal model studies indicate that intratubular nephrocalcinosis is a result of increased urinary supersaturation. Urinary supersaturation with respect to calcium oxalate (CaOx) and calcium phosphate (CaP) are generally achieved at different locations in the renal tubules. As a result experimental induction of hyperoxaluria in animals with CaP deposits does not lead to growth of CaOx over CaP. Interstitial nephrocalcinosis has been seen in mice with lack of crystallization modulators Tamm–Horsfall protein and osteopontin. Sodium phosphate co-transporter or sodiumhydrogen exchanger regulator factor-1 null mice also produced interstitial nephrocalcinosis. Crystals plug the tubules by aggregating and attaching to the luminal cell surface. Structural features of the renal tubules also play a role in crystal retention. The crystals plugging the terminal collecting ducts when exposed to the metastable pelvic urine may promote the formation of stone.     

Calcium phosphate supersaturation regulates stone formation in genetic hypercalciuric stone-forming rats

BACKGROUND: Hypercalciuria is the most common metabolic abnormality observed in patients with nephrolithiasis. Hypercalciuria raises urine supersaturation with respect to the solid phases of calcium oxalate and calcium phosphate, leading to an enhanced probability for nucleation and growth of crystals into clinically significant stones. However, there is little direct proof that supersaturation itself regulates stone formation. Through successive inbreeding of the most hypercalciuric progeny of hypercalciuric Sprague-Dawley rats, we have established a strain of rats, each of which excrete abnormally large amounts of urinary calcium and each of which forms calcium phosphate kidney stones. We used these hypercalciuric (GHS) rats to test the hypothesis that an isolated reduction in urine supersaturation, achieved by decreasing urine phosphorus excretion, would decrease stone formation in these rats. METHODS: Thirty 44th-generation female GHS rats were randomly divided into three groups. Ten rats received a high-phosphorus diet (0.565% phosphorus), 10 a medium-phosphorus diet (0.395% phosphorus), and 10 a low-phosphorus diet (0.225% phosphorus) for a total of 18 weeks. The lowered dietary phosphorus would be expected to result in a decrease in urine phosphorus excretion and a decrease in urinary supersaturation with respect to the calcium phosphate solid phase. Every two weeks, 24-hour urine collections were obtained. All relevant ions were measured, and supersaturation with respect to calcium oxalate and calcium hydrogen phosphate were determined. At the conclusion of the experiment, each rat was killed, and the kidneys, ureters, and bladder were dissected en block and x-rayed to determine whether any stones formed. A decrease in stone formation with a reduction in urinary supersaturation would support the hypothesis that supersaturation alone can regulate stone formation. RESULTS: Decreasing the dietary phosphorus intake led to a progressive decrease in urine phosphorus excretion and an increase in urine calcium excretion, the latter presumably caused by decreased intestinal calcium phosphate binding and increased calcium absorption. With decreasing dietary phosphorus intake, there was a progressive decrease in saturation with respect to the calcium phosphate solid phase. Fifteen of the 20 kidneys from the 10 rats fed the high-phosphorus diet had radiographic evidence of kidney stone formation, whereas no kidneys from the rats fed either the medium- or low-phosphorus diet developed kidney stones. CONCLUSIONS: A decrease in urine phosphorus excretion not only led to a decrease in urine supersaturation with respect to the calcium phosphate solid phase but to an elimination of renal stone formation. The results of this study support the hypothesis that variation in supersaturation alone can regulate renal stone formation. Whether a reduction of dietary phosphorus will alter stone formation in humans with calcium phosphate nephrolithiasis remains to be determined.     

Intratubular crystallization of calcium oxalate in the presence of membrane vesicles: an in vitro study

BACKGROUND: Since urine spends only a few minutes in the renal tubules and has a low supersaturation with respect to calcium oxalate (CaOx), nucleation of CaOx crystals in the kidneys is most probably heterogeneous. We have proposed that membranes of cellular degradation products are the main substrate for crystal nucleation. The purpose of our study was to determine the site of membrane-mediated crystal nucleation within the renal tubules and the required lag time, factors that determine whether crystallization results in crystalluria or nephrolithiasis. METHODS: Nucleation of CaOx was allowed to occur in five different artificial urine solutions with ionic concentrations simulating urine in proximal tubules (PTs), descending (DLH) and ascending (ALH) limbs of the loop of Henle, distal tubules (DTs), and collecting ducts (CDs). A constant composition crystallization system was used. Experiments were run for two hours with or without the renal tubular brush border membrane (BBM) vesicles. RESULTS: The addition of BBM significantly reduced the nucleation lag time and increased the rate of crystallization. The average nucleation lag time decreased from 84.6 +/- 43.4 minutes to 24.5 +/- 19 minutes in PTs, from 143.6 +/- 29 to 70.2 +/- 53.4 minutes in DLH, from 17.6 +/- 8.6 minutes to 0.625 +/- 0.65 minutes in DTs and from 9.54 +/- 3. 03 minutes to 0.625 +/- 0.65 minutes in CDs. There was no nucleation in the ALH solution without BBM for two hours. CaOx dihydrate (COD) was common in most solutions. Calcium phosphate (CaP) also nucleated in the DLH and CD solutions. CONCLUSIONS: In the absence of membrane vesicles, there was no crystallization in any of the solutions within the time urine spends in the renal tubules. As a result, homogeneous nucleation of crystals anywhere within the nephron appears unlikely. However, BBM-supported nucleation is possible in the DTs as well as CDs. A high crystallization rate in CDs would promote rapid crystal growth and aggregation, resulting in crystal retention within the kidneys and development of nephrolithiasis.     

肾结石大鼠肾组织bikunin mRNA的表达

目的 :研究bikunin在实验性肾草酸钙结石大鼠肾组织的表达及意义。方法 :采用乙二醇和氯化铵诱导大鼠肾草酸钙结石模型形成 ,检测各组大鼠肾功能、肾组织Ca2 + 含量和草酸钙晶体沉积、尿生化指标 ,并用逆转录聚合酶链反应 (RT PCR)检测bikuninmRNA在肾组织的表达情况。结果 :模型组大鼠的血清Cr、BUN、肾Ca2 + 含量、2 4h尿Ca2 + 、草酸 (Ox)分泌量和肾组织bikuninmRNA的表达均明显高于正常组 (P <0 .0 5 )。结论 :高草酸尿和草酸钙结晶的沉积能促使大鼠肾脏通过合成更多的bikunin来抑制大鼠肾组织草酸钙晶体的形成。     

Experimentally induced hyperoxaluria in MCP-1 null mice

Experimental animal model studies suggest that calcium oxalate (CaOx) crystal deposition in the kidneys is associated with the development of oxidative stress, epithelial injury and inflammation. There is increased production of inflammatory molecules including osteopontin (OPN), monocyte chemoattractant protein-1 (MCP-1) and various subunits of inter-alpha-inhibitor such as bikunin. What does the increased production of such molecules suggest? Is it a cause or consequence of crystal deposition? We hypothesized that over-expression and increased production of MCP-1 is a result of the interaction between renal epithelial cells and CaOx crystals after their deposition in the renal tubules. We induced hyperoxaluria in MCP-1 null as well as wild type mice and examined pathological changes in their kidneys and urine. Both wild type and MCP-1 null male mice became hyperoxaluric and demonstrated CaOx crystalluria. Neither of them developed crystal deposits in their kidneys. Both showed some morphological changes in their renal proximal tubules. Significant pathological changes such as cell death and increased urinary excretion of LDH were not seen. Results suggest that at least in mice (1) Increase in oxalate and decrease in citrate excretion can lead to CaOx crystalluria but not CaOx nephrolithiasis; (2) MCP-1 does not play a role in crystal retention within the kidneys; (3) Expression of OPN and MCP-1 is not increased in the kidneys in the absence of crystal deposition; (4) Crystal deposition is necessary for significant pathological changes and movement of monocytes and macrophages into the interstitium.     

Chinese herbal medicines and their efficacy in treating renal stones

In herbal treatment of kidney stones, anti-lithics are used to “dissolve” the stones or aid their passing to guard against further retention. Diuretic action is also needed to increase the amount of fluid going through the kidneys and flush out the deposits. Previous clinical studies have shown that herbal medicines and their concoctions could be used to inhibit calcium oxalate crystallization. However, the pharmacodynamics and in-vitro effects of such medicines have not been established. Five Chinese herbal medicines were selected based on their usefulness in treating stone disease. A 96-well plate oxalate-induced turbidity in artificial urine was used to evaluate the efficacies of the different herbal medicines on calcium oxalate crystallization. The metastable limit was determined and the nucleation rate was derived from 12-min time-course measurement of turbidity at 405 nm. Phase-contrast microscopy was used to visualize the crystals. The results showed that with increasing concentrations of herbal extracts, smaller calcium oxalate crystal sizes were observed. Overall, the five herbal medicinal extracts tested were able to promote nucleation of calcium oxalate crystals while at the same time decreasing the size. This in-vitro crystallization confirms that prophylaxis of renal stones could be achieved by reducing overall supersaturation through promotion of small crystal nucleates and concomitant pharmacological diuretic action of herbal medicines. Clinical studies will provide more definitive conclusions.     

Hyperoxaluria: a gut-kidney axis?

Hyperoxaluria leads to urinary calcium oxalate (CaOx) supersaturation, resulting in the formation and retention of CaOx crystals in renal tissue. CaOx crystals may contribute to the formation of diffuse renal calcifications (nephrocalcinosis) or stones (nephrolithiasis). When the innate renal defense mechanisms are suppressed, injury and progressive inflammation caused by these CaOx crystals, together with secondary complications such as tubular obstruction, may lead to decreased renal function and in severe cases to end-stage renal failure. For decades, research on nephrocalcinosis and nephrolithiasis mainly focused on both the physicochemistry of crystal formation and the cell biology of crystal retention. Although both have been characterized quite well, the mechanisms involved in establishing urinary supersaturation in vivo are insufficiently understood, particularly with respect to oxalate. Therefore, current therapeutic strategies often fail in their compliance or effectiveness, and CaOx stone recurrence is still common. As the etiology of hyperoxaluria is diverse, a good understanding of how oxalate is absorbed and transported throughout the body, together with a better insight in the regulatory mechanisms, is crucial in the setting of future treatment strategies of this disorder. In this review, the currently known mechanisms of oxalate handling in relevant organs will be discussed in relation to the different etiologies of hyperoxaluria. Furthermore, future directions in the treatment of hyperoxaluria will be covered.     

Acidic polyanion poly(acrylic acid) prevents calcium oxalate crystal deposition

Acidic macromolecules inhibit calcium oxalate nucleation, growth, aggregation and attachment to cells in vitro. To test for such an effect in vivo we used osmotic minipumps to continuously infuse several doses of the 5.1 kDa poly(acrylic acid) (pAA(5.1)) into rats fed a diet which causes renal calcium oxalate crystal deposition. Although kidneys of rats receiving the saline control contained calcium oxalate crystals, measured by polarized light microscopy, those of animals given pAA(5.1) had significantly lower numbers of crystals in various zones of the kidney. Delivery of pAA(5.1) to urine was confirmed by measuring excretion of infused biotinylated pAA(5.1). Both the derivatized and unlabelled pAA(5.1) had the same effects on crystallization in vitro. Our study shows that acidic polymers hold promise as effective therapies for kidney stones likely through prevention of calcium oxalate crystal aggregate formation.     

Acute hyperoxaluria, renal injury and calcium oxalate urolithiasis.

Single intraperitoneal injections of three, seven, or 10 mg. of sodium oxalate per 100 gm. of rat body weight were administered to male Sprague-Dawley rats. At various times after the injection, urine samples were analyzed for oxalate, and urinary enzymes, alkaline phosphatase, leucine aminopeptidase, gamma-glutamyl transpeptidase, and N-acetyl-beta-glucosaminidase. The kidneys were processed for light microscopy and renal calcium and oxalate determination. Oxalate administration resulted in an increase in urinary oxalate and formation of calcium oxalate crystals in the kidneys. The amount and duration of urinary excretion of excess oxalate and retention of crystals in the kidneys correlated with the dose of sodium oxalate administered. At a low oxalate dose of three mg./100 gm., crystals moved rapidly down the nephron and cleared the kidneys. At higher doses crystals were retained in kidneys and at a dose of 10 mg./100 gm. were still there seven days post-injection. Crystal retention was associated with enhanced excretion of urinary enzymes indicating renal tubular epithelial injury.     

Urinary crystallization inhibitors do not prevent crystal binding

PURPOSE: Renal stone formation requires the persistent retention of crystals in the kidney. Calcium oxalate monohydrate (COM) crystal binding to Madin Darby canine kidney strain I (MDCK-I), a cell line that resembles the epithelium in the renal distal tubule/collecting duct, is developmentally regulated, while LLC-PK1 cells (American Type Tissue Collection), which are widely used as a model of the renal proximal tubule, bind crystals irrespective of their stage of epithelial development. Whereas to our knowledge the binding molecules for COM at the surface of LLC-PK1 cells are still unknown, crystals adhere to the hyaluronan (HA) rich pericellular matrix transiently expressed by mobile MDCK-I cells. In the current study we investigated whether crystal binding to either cell type is influenced by urinary substances, including glycoprotein inhibitors of crystallization MATERIALS AND METHODS: We studied crystal binding to MDCK-I cells during wound repair, to confluent LLC-PK1 cells and to HA immobilized on a solid surface using [14C] COM pretreated or not pretreated with urine from healthy male volunteers. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis were performed to assess whether the crystals became coated with urine derived proteins RESULTS: Western blot analysis demonstrated that pretreated COM crystals were covered with protein inhibitors of crystallization. However, this protein coat had no significant effect on the level of crystal binding to either cell type. In contrast, the adherence of urine treated crystals to immobilized HA was significantly reduced CONCLUSIONS: The adherence of crystals to pericellular matrixes may encompass more than their simple fixation to the polysaccharide HA. Calcium oxalate crystal retention is not prevented by coating crystals with urinary constituents such as glycoproteins and, therefore, may predominantly depend on the surface properties of the renal tubular epithelium.     

Preconditioning of the distal tubular epithelium of the human kidney precedes nephrocalcinosis

BACKGROUND: Preterm neonates and renal transplant patients frequently develop nephrocalcinosis. Experimental studies revealed that crystal retention in the distal nephron, a process that may lead to nephrocalcinosis, is limited to proliferating/regenerating tubular cells expressing hyaluronan and osteopontin at their luminal surface. Fetal and transplant kidneys contain proliferating and/or regenerating cells since nephrogenesis is not completed until 36 weeks of gestation, while ischemia and nephrotoxic immunosuppressants may lead to injury and repair in renal transplants. This prompted us to investigate the expression of hyaluronan and osteopontin and to correlate this to the appearance of tubular calcifications both in fetal/preterm and transplanted kidneys. METHODS: Sections of fetal/preterm kidneys and protocol biopsies of transplanted kidneys (12 and 24 weeks posttransplantation from the same patients) were stained for osteopontin, hyaluronan, and calcifications (von Kossa). RESULTS: Hyaluronan and osteopontin were expressed at the luminal surface of the epithelial cells lining the distal tubules of all fetal kidneys at birth and in all kidney graft protocol biopsies 12 and 24 weeks posttransplantation. In 7 out of 18 surviving (at least 4 days) preterm neonates crystal retention developed. In renal allografts a striking increase (from 2/10 to 6/10) in tubular crystal retention between 12 and 24 weeks posttransplantation was observed. In addition, crystals were selectively retained in distal renal tubules containing cells with hyaluronan and osteopontin at their luminal surface. CONCLUSION: The results of this study show that luminal expression of hyaluronan and osteopontin preceded renal distal tubular retention of crystals in preterm neonates and renal transplant patients. We propose that the presence of this crystal binding phenotype may play a general role in renal calcification processes.     

Sialic acid-containing glycoproteins on renal cells determine nucleation of calcium oxalate dihydrate crystals.

BACKGROUND: The interaction between the surfaces of renal epithelial cells and calcium oxalate dihydrate (COD), the most common crystal in human urine, was studied to identify critical determinants of kidney stone formation. METHODS: A novel technique utilizing vapor diffusion of oxalic acid was employed to nucleate COD crystals onto the apical surface of living cells. Confluent monolayers were grown in the inner 4 wells of 24-well culture plates. To identify cell surface molecules that regulate crystal nucleation, cells were pretreated with a protease (trypsin or proteinase K) to alter cell surface proteins, neuraminidase to alter cell surface sialoglycoconjugates, or buffer alone. COD crystals were nucleated on the surface of cells by diffusion of oxalic acid vapor into a calcium-containing buffer overlying the cells. Crystal face-specific nucleation was evaluated by scanning electron microscopy. RESULTS: Nucleation and growth of a COD crystal onto an untreated control cell occurred almost exclusively via its (001) face, an event rarely observed during COD crystallization. In contrast, when COD crystals were nucleated onto protease- or neuraminidase-treated cells, they did so via the (100) face of the crystal. CONCLUSIONS: Specific sialic acid-containing glycoproteins, and possibly glycolipids (sialoglycoconjugates), appear to be critical determinants of face-specific nucleation of COD crystals on the apical renal cell surface. We hypothesize that crystal retention within the nephron, and the subsequent development of a kidney stone, may result when the number or composition of these cell surface molecules is modified by genetic alterations, cell injury, or drugs in tubular fluid.     

Effect of angiotensin II receptor blockage on osteopontin expression and calcium oxalate crystal deposition in rat kidneys

Hyperoxaluria leads to calcium oxalate (CaOx) crystallization and development of tubulointerstitial lesions in the kidneys. Treatment of hyperoxaluric rats with angiotensin II (Ang II) type I receptor blocker (ARB) reduces lesion formation. Because Ang II mediates osteopontin (OPN) synthesis, which is involved in both macrophage recruitment and CaOx crystallization, it was hypothesized that ARB acts via OPN. Hyperoxaluria was induced in 10-wk-old male Sprague-Dawley rats, and they were treated with ARB candesartan. At the end of 4 wk, kidneys were examined for crystal deposits, ED-1-positive cells, and expression of OPN mRNA. PCR was used to quantify OPN, renin, and angiotensin-converting enzyme (ACE) mRNA in kidneys. RIA was used to determine renal, plasma, and urinary OPN; plasma renin; Ang II and ACE; and renal Ang II. For evaluating oxidative stress, malondialdehyde was measured. Urinary calcium, oxalate, creatinine, and albumin were also determined. Despite similar urinary calcium and oxalate levels, kidneys of hyperoxaluric rats on candesartan had fewer CaOx crystals, fewer ED-1-positive cells, reduced OPN expression, and reduced malondialdehyde than hyperoxaluric rats. Urinary albumin excretion and serum creatinine levels improved significantly on candesartan treatment. mRNA for OPN, renin, and ACE were significantly elevated in hyperoxaluric rats. OPN synthesis and production increased with hyperoxaluria but to a lesser extent in candesartan-treated hyperoxaluric rats. These results show for the first time that oxalate can activate the renal renin-angiotensin system and that oxalate-induced upregulation of OPN is in part mediated via renal renin-angiotensin system.     

Characterization of Tamm-Horsfall protein in a rat nephrolithiasis model

PURPOSE: The role of Tamm-Horsfall protein in calcium oxalate stone formation is controversial. It is unclear whether Tamm-Horsfall protein has a role in crystallization. If it does, does it act as an inhibitor or promoter of crystallization? To elucidate the nature of its involvement we characterized Tamm-Horsfall protein in a rat model of calcium oxalate nephrolithiasis by in vivo and in vitro techniques. MATERIALS AND METHODS: Calcium oxalate nephrolithiasis was induced in male Sprague-Dawley rats. The amino acid and carbohydrate composition of Tamm-Horsfall protein from normal rats and those with nephrolithiasis was determined. The Tamm-Horsfall protein gene and protein expression in the kidneys were examined by in situ hybridization and immunohistochemistry. Furthermore, the interaction of Tamm-Horsfall protein and calcium oxalate crystals was assessed by an in vitro crystal aggregation assay. RESULTS: Tamm-Horsfall protein from rats with nephrolithiasis was biochemically similar to that from normal rats. Although Tamm-Horsfall protein was associated with crystal deposits in the renal papillae of rats with nephrolithiasis, Tamm-Horsfall protein messenger RNA expression in the kidneys remained unchanged. In each group Tamm-Horsfall protein inhibited calcium oxalate crystal aggregation by 47%, indicating no change in functional capabilities. CONCLUSIONS: The results of this study indicate that urinary excretion, and the biochemical nature and functional capabilities of Tamm-Horsfall protein remain unchanged during experimental calcium oxalate nephrolithiasis. Although staining for Tamm-Horsfall protein was evident in the papillae of rats with nephrolithiasis, the site of Tamm-Horsfall protein synthesis remained cells of the thick ascending limbs of the loop of Henle.     

Calcium oxalate crystal interaction with renal tubular epithelium,mechanism of crystal adhesion and its impact on stone development

The interaction between renal epithelial cells and calcium oxalate (CaOx) crystals and/or oxalate ions plays a critical role in the formation of urinary stones. Epithelial cells respond to hyperoxaluria and the presence of CaOx crystals in the kidneys by increased enzymuria and internalization of the crystals. Crystal cell interaction results in movement of crystals from the luminal to the basolateral side between the cells and the basement membrane. Once beneath the epithelium, crystals adhere to the basement membrane and become anchored inside the kidneys. Crystals anchored to basement membrane of the peripheral collecting duct aggregate with other crystals and move through an eroding epithelium to the papillary surface, furnishing an encrustation platform or a nidus for future development of a kidney stone. Thus interaction between renal epithelial cells and CaOx crystals and/or oxalate ions is an essential element in the development of urinary stone disease.