Ileal oxalate absorption and urinary oxalate excretion are enhanced in Slc26a6 null mice

Intestinal oxalate transport, mediated by anion exchange proteins, is important to oxalate homeostasis and consequently to calcium oxalate stone diseases. To assess the contribution of the putative anion transporter (PAT)1 (Slc26a6) to transepithelial oxalate transport, we compared the unidirectional and net fluxes of oxalate across isolated, short-circuited segments of the distal ileum of wild-type (WT) mice and Slc26a6 null mice [knockout (KO)]. Additionally, urinary oxalate excretion was measured in both groups. In WT mouse ileum, there was a small net secretion of oxalate (J(net)(Ox) = -5.0 +/-5.0 pmol.cm(-2).h(-1)), whereas in KO mice J(net)(Ox) was significantly absorptive (75 +/- 10 pmol.cm(-2)h.h(-1)), which was the result of a smaller serosal-to-mucosal oxalate flux (J(sm)(Ox)) and a larger mucosal-to-serosal oxalate flux (J(ms)(Ox)). Mucosal DIDS (200 microM) reduced J(sm)(Ox) in WT mice, leading to reversal of the direction of net oxalate transport from secretion to absorption (J(net)(Ox) = 15.0 +/- 5.0 pmol.cm(-2).h(-1)) , but DIDS had no significant effect on KO ileum. In WT mice in the absence of mucosal Cl(-), there were small increases in J(ms)(Ox) and decreases in J(sm)(Ox) that led to a small net oxalate absorption. In KO mice, J(net)(Ox) was 1.5-fold greater in the absence of mucosal Cl(-), due solely to an increase in J(ms)(Ox). Urinary oxalate excretion was about fourfold greater in KO mice compared with WT littermates. We conclude that PAT1 is DIDS sensitive and mediates a significant fraction of oxalate efflux across the apical membrane in exchange for Cl(-); as such, PAT1 represents a major apical membrane pathway mediating J(sm)(Ox).     

Cholinergic signaling inhibits oxalate transport by human intestinal T84 cells

Urolithiasis remains a very common disease in Western countries. Seventy to eighty percent of kidney stones are composed of calcium oxalate, and minor changes in urinary oxalate affect stone risk. Intestinal oxalate secretion mediated by anion exchanger SLC26A6 plays a major constitutive role in limiting net absorption of ingested oxalate, thereby preventing hyperoxaluria and calcium oxalate urolithiasis. Using the relatively selective PKC-δ inhibitor rottlerin, we had previously found that PKC-δ activation inhibits Slc26a6 activity in mouse duodenal tissue. To identify a model system to study physiologic agonists upstream of PKC-δ, we characterized the human intestinal cell line T84. Knockdown studies demonstrated that endogenous SLC26A6 mediates most of the oxalate transport by T84 cells. Cholinergic stimulation with carbachol modulates intestinal ion transport through signaling pathways including PKC activation. We therefore examined whether carbachol affects oxalate transport in T84 cells. We found that carbachol significantly inhibited oxalate transport by T84 cells, an effect blocked by rottlerin. Carbachol also led to significant translocation of PKC-δ from the cytosol to the membrane of T84 cells. Using pharmacological inhibitors, we observed that carbachol inhibits oxalate transport through the M(3) muscarinic receptor and phospholipase C. Utilizing the Src inhibitor PP2 and phosphorylation studies, we found that the observed regulation downstream of PKC-δ is partially mediated by c-Src. Biotinylation studies revealed that carbachol inhibits oxalate transport by reducing SLC26A6 surface expression. We conclude that carbachol negatively regulates oxalate transport by reducing SLC26A6 surface expression in T84 cells through signaling pathways including the M(3) muscarinic receptor, phospholipase C, PKC-δ, and c-Src.     

Nucleation of calcium oxalate crystals on an imprinted polymer surface from pure aqueous solution and urine

Calcium oxalate (CaOx) is the most common component of human kidney stones. Heterogeneous nucleation is regarded as the key mechanism in this process. In this study, we have used an imprinted 6-methacrylamidohexanoic acid/divinylbenzene co-polymer as a biomimetic surface to nucleate CaOx crystal formation. The polymer was imprinted with either calcium oxalate monohydrate (COM) or dihydrate (COD) template crystals. These were washed out of the polymer, which was then immersed in various test solutions. The test solutions were an aqueous solution of calcium chloride and sodium oxalate, artificial urine and a sample of real urine. Crystals that formed on the polymer surface were characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy, atomic absorption spectroscopy and scanning electron microscopy. Results showed that in the aqueous solution the COM-imprinted polymer induced the nucleation of COM. The COD-imprinted polymer induced only trace amounts of COD crystallization, together with larger quantities of COM. In artificial and real urines, COM also specifically precipitated on the COM-imprinted surface. The results show that, at least to some extent, the imprinted polymers direct formation of their morphologically matched crystals. In the case of COD, however, it appears that either rapid hydrate transformation of COD to COM occurs, or the more stable COM polymorph is directly co-precipitated by the polymer. Our results support the hypothesis that heterogeneous nucleation plays a key role in CaOx stone formation and that the imprinted polymer model could provide an additional and superior diagnostic tool for stone researchers to assess stone-risk in urine.Abbreviations COD calcium oxalate dihydrate - COM calcium oxalate monohydrate - COT calcium oxalate trihydrate - dvb divinylbenzene - 6-maaha 6-methylacrylamidohexanoic acid     

Oxalate, calcium and ash intake and excretion balances in fat sand rats (Psammomys obesus) feeding on two different diets

Fat sand rats Psammomys obesus feed exclusively on plants of the family Chenopodiaceae, which contain high concentrations of chloride salts (NaCl, KCl) and oxalate salts. Ingestion of large quantities of oxalate is challenging for mammals because oxalate chelates Ca(2+) cations, reducing Ca(2+) availability. Oxalate is a metabolic end-point in mammalian metabolism, however it can be broken-down by intestinal bacteria. We predicted that in fat sand rats microbial breakdown of oxalate will be substantial due to the high dietary load. In addition, since a high concentration of soluble chloride salts increases the solubility of calcium oxalate in solution, we examined whether a change in the intake of chloride salts affects microbial oxalate breakdown and calcium excretion in fat sand rats. We measured oxalate, calcium and other inorganic matter (ash) intake and excretion in fat sand rats feeding on two different diets: saltbush (Atriplex halimus), their natural diet, and goose-foot (Chenopodium album), a non-native chenopod on which fat sand rats will readily feed and that has a similar oxalate content to saltbush but only 2/3 of the ash content. In animals feeding on both diets, 65-80% of the oxalate ingested did not appear in urine or faeces. In animals consuming the more saline saltbush, significantly more oxalate was apparently degraded (p<0.001), while significantly less oxalate was excreted in urine (p<0.01) and in faeces (p<0.05). We propose, therefore, that fat sand rats rely on symbiotic bacteria to remove a large portion of the oxalates ingested with their diet, and that the high dietary salt intake may play a beneficial role in their oxalate and calcium metabolism.     

TNF-alpha down-regulates the Na+-K+ ATPase and the Na+-K+-2Cl-cotransporter in the rat colon via PGE2

TNF-alpha is believed to play a pivotal role in the pathogenesis of inflammatory bowel diseases which have diarrhea as one of their symptoms. This work studies the effect of the cytokine on electrolyte and water movements in the rat distal colon using an intestinal perfusion technique and attempts to determine its underlying mechanism of action. TNF-alpha inhibited net water and chloride absorption, down-regulated in both surface and crypt colonocytes the Na+-K+-2Cl- cotransporter, and reduced the protein expression and activity of the Na+-K+ ATPase. Indomethacin up-regulated the pump and the cotransporter in surface cells but not in crypt cells, and in its presence, TNF-alpha could not exert its effect, suggesting an involvement of PGE2 in the cytokine action. The effect of TNF-alpha on the pump and symporter was studied also in cultured Caco-2 cells in isolation of the effect of other cells and tissues, to test whether the cytokine acts directly on intestinal cells. In these cells, TNF-alpha and PGE2 had a similar effect on the pump expression and activity as that observed in crypt cells but were without any effect on the Na+-K+-2Cl- cotransporter. It was concluded that the effect of the cytokine on colonocytes is mediated via PGE2. By inhibiting the Na+-K+ ATPase, it reduces the Na+ gradient needed for NaCl absorption, and by down-regulating the expression of the Na+-K+-2Cl- symporter, it reduces basolateral Cl- entry and luminal Cl- secretion. The inhibitory effect on absorption is more significant than the inhibitory effect on secretion resulting in a decrease in net electrolyte uptake and consequently in more water retention in the lumen.     

Studies on the absorption of sodium and chloride from the rumen of sheep

The net absorption of Na and Cl from the temporarily isolated rumen of sheep was studied using an artificial ruminal fluid with different Na and K and constant Cl concentrations. The net absorption of Na and Cl was linearly correlated. The net absorption of Cl was abolished and a small net secretion was observed when no sodium was in the artificial rumen fluid. The net absorption of Na was significantly reduced under chloride free conditions. It is concluded that the active transport of Na and Cl is coupled. The mechanism of an Na-Cl cotransport can not be deduced from these studies.     

Intestinal maturation: The effect of glucocorticoids on in vivo net magnesium and calcium transport in the rat

We investigated with an $\text{in vivo}$ single pass perfusion technique, the effect of glucocorticoids on net magnesium and calcium absorption from the small and large intestine of suckling and adolescent rats. In control rats, rates of net magnesium and calcium absorption were several fold greater in both small and large intestinal segments of suckling rats compared to corresponding rates in segments of adolescent rats. Methylprednisolone 30 mg/kg body weight daily for three days, suppressed significantly net magnesium and calcium absorption from the small and large intestinal segments of suckling rats only. Methylprednisolone had no effect on either net magnesium or calcium absorption in adolescent rats. The mechanism(s) responsible for the observed decrease in net magnesium and calcium absorption in the suckling period by glucocorticoids are discussed.     

Mucilage Cells, Calcium Oxalate Crystals and Soluble Calcium in Opuntia ficus-indica

The subcellular location of soluble calcium in parenchymatousand mucilage cells of Opuntia ficus-indica was determined histochemically.Soluble calcium was observed in crystal chambers containingcalcium oxalate on the membrane of the vesicles. Calcium wasalso present in the plasmalemma, in plasmodesmata, in cell walls,in mitochondria and in the vacuoles. Especially marked was thepresence of soluble calcium in vesicles free or fused with theplasmalemma. Little free calcium was observed in other cellcompartments. In the calcium economy of tissues the location of soluble calciumand the transport of calcium to and from mucilage cells to parenchymatouscells and calcium oxalate idioblasts will play a role. Chelationof calcium by mucilage or oxalate, which depends on pH, ionicstrength, etc., will be important in this respect. Opuntia ficus-indica, calcium oxalate, mucilage cells, transport of calcium     

Increased Calcium Absorption in Nephrolithiasis Explained by Uptake Studies in Ileal Brush Border Membrane Vesicles

We previously showed that recurrent calcium renal stone formers have enhanced urinary excretions of calcium and oxalate resulting from malabsorption of citrate. In the present investigation, the mechanism of the citrate-induced increased calcium uptake was studied using guinea pig ileal brush border membrane vesicles. In this model, calcium is absorbed in a concentration dependent, single mechanism uptake with a K_m of 275 ± 30 umol/liter (SD) and a V_max of 4.0 ± 0.5 nmol/min · mg protein. Under conditions of maximal calcium uptake, both citrate and phosphate inhibited calcium absorption into brush border membrane vesicles (BBMVs). In contrast, when phosphate and citrate were added together, calcium absorption normalized. Citrate inhibition of calcium absorption appeared to be due to free citrate ions, and phosphate ions overcame this inhibition. Phosphate inhibition was mostly due to decreased concentrations of ionized calcium and partly to precipitation of insoluble calcium phosphate. These studies confirm that the effects of citrate in humans in enhancing calcium absorption occur in the lumen of the gut and are not related to further biochemical conversions of citrate by the gut cells, to effects of citrate on calcium-related hormones, or to the renal handling of calcium. Also, the effects of citrate on increasing calcium absorption should be increased or attenuated in patients who malabsorb citrate, and this explains the increased urinary calcium and oxalate excretions reported for recurrent calcium stone formers.     

An analysis of physiological states responsible for antero-posterior disintegration in Planaria dorotocephala

Summary Planaria were treated with equi-molal solutions of ammonium, potassium, sodium, magnesium, and calcium chlorides, made up in distilled water and the rates of cytolysis compared with cytolysis in distilled water. Potassium and ammonium accelerate cytolysis; some protection is afforded by sodium; still more by magnesium, and complete protection by calcium in the concentrations employed.In distilled water solutions of calcium chloride no cytolysis occurs in concentrations from M/500 to M/40,000; cytolysis is distinctly delayed in M/100,000. The protective action of M/1,000,000 is detectable.Potassium oxalate accelerates disintegration in hypotonic solutions.One per cent ethyl alcohol in distilled water causes cytolysis more rapidly than does distilled water alone, but in M/500 molal calcium chloride the alcohol solution is much less effective.Ringer's solution minus calcium affords no protection against death due to absence of calcium and death due to potassium oxalate but completely protects against cytolysis. Death in Ringer's solution minus calcium and in Ringer's solution with potassium oxalate occurs first in the anterior region and describes an antero-posterior gradient.Cytolysis in distilled water, in potassium oxalate solutions, in alcohol solutions, and in hypotonic calcium solutions of extreme dilution is initiated in the anterior end and describes an antero-posterior gradient within a zooid.Earlier work of the writer on the disintegrative action of lipoid solvents, heat, KNC, hyper- and hypotonic solutions is discussed. It is concluded that inPlanaria dorotocephala the antero-posterior gradient in cytolytic disintegration represents an antero-posterior differential in sensitivity to disturbance of the calcium-lipoid-water relation in the organism.     

Mechanism of action of ryanodine on cardiac sarcoplasmic reticulum

Ryanodine was found to initially inhibit calcium uptake by cardiac sarcoplasmic reticulum. This initial depression was followed by a later marked stimulation of calcium uptake. These effects were noted when calcium uptake was measured in the presence or absence of oxalate. The requirement for preincubation with ryanodine was highly dependent on ryanodine concentration and temperature. The mechanism of action of ryanodine clearly was not an effect on oxalate entry or calcium oxalate precipitation because the effects were also observed in the absence of oxalate. Ryanodine also had no effect on passive calcium efflux from actively loaded vesicles. Because ryanodine had no effect on Ca2+-ATPase activity under defined conditions of an ATP-regenerating system and no calcium gradient, we suggest ryanodine does not change the stoichiometry of the pump. Our results are consistent with the hypothesis that ryanodine closes a calcium channel in a subpopulation of the vesicles.     

On the formation of calcium(II) taurocholate aggregate species in aqueous solution

Abstract

The influence of the presence of calcium(II) ions in solutions containing sodium and taurocholate ions at 25°C and in 0.5 mol dm^?3 N(CH₃)₄Cl as the constant ionic medium was studied. The composition and existence range of aggregates formed by taurocholate sodium and calcium(II) were investigated by means of two different procedures. First, the increasing calcium oxalate solubility due to the presence of taurocholate ions was studied as a function of the taurocholate, sodium and hydrogen ions. The free concentration of sodium and hydrogen ions was determined in solutions equilibrated with solid calcium(II) oxalate. After filtration, the concentration of calcium(II) (by atomic absorption spectro-photometry) and that of oxalate were also determined. In the second approach, electromotive force measurements carried out in solutions containing taurocholate, sodium and calcium(II) ions provided hydrogen and sodium ions free concentrations. The results from both procedures could be explained by assuming the presence of aggregates of different composition with the participation of sodium, calcium(II) and taurocholate ions, depending on the concentration of the reagents. No protonated species were present in appreciable concentrations. All the species found have even anion aggregation numbers. A strong analogy with the composition of sodium taurocholate and glycocholate is observed, while a comparison with sodium deoxycholate, glycodeoxycholate and taurodeoxycholate shows wide differences.     

Heavy metal stress reduces the deposition of calcium oxalate crystals in leaves of Phaseolus vulgaris

Calcium oxalate (CaOx) crystals in plants may serve as a sink for the absorption of excess calcium, and they could play an important role in heavy metal detoxification. In this study, the effect of heavy metals and different calcium concentrations on the growth of calcium oxalate crystals in leaves of Phaseolus vulgaris was investigated. Different analytical techniques were used to determine the influence of exogenous lead and zinc on CaOx deposition and to detect a presence of these metals in CaOx crystals. We found a positive correlation between the calcium concentration in the nutrient medium and the production of calcium oxalate crystals in leaves of hydroponically grown plants. On the other hand, addition of the heavy metals to the nutrient medium decreased the number of crystals. Energy dispersive X-ray spectrometry did not detect the inclusion of heavy metals inside the CaOx crystals. Our investigation suggests that CaOx crystals do not play a major role in heavy metal detoxification in P. vulgaris but do play an important role in bulk calcium regulation.     

Engineering calcium oxalate crystal formation in Arabidopsis

Many plants accumulate crystals of calcium oxalate. Just how these crystals form remains unknown. To gain insight into the mechanisms regulating calcium oxalate crystal formation, a crystal engineering approach was initiated utilizing the non-crystal-accumulating plant, Arabidopsis. The success of this approach hinged on the ability to transform Arabidopsis genetically into a calcium oxalate crystal-accumulating plant. To accomplish this transformation, two oxalic acid biosynthetic genes, obcA and obcB, from the oxalate-secreting phytopathogen, Burkholderia glumae were inserted into the Arabidopsis genome. The co-expression of these two bacterial genes in Arabidopsis conferred the ability not only to produce a measurable amount of oxalate but also to form crystals of calcium oxalate. Biochemical and cellular studies of crystal accumulation in Arabidopsis revealed features that are similar to those observed in the cells of crystal-forming plants. Thus, it appears that at least some of the basic components that comprise the calcium oxalate crystal formation machinery are conserved even in non-crystal-accumulating plants.     

Effects of antidiuretic hormone on cellular conductive pathways in mouse medullary thick ascending limbs of Henle: I. ADH increases transcellular conductance pathways

Summary This paper reports experiments designed to assess the relations between net salt absorption and transcellular routes for ion conductance in single mouse medullary thick ascending limbs of Henle microperfusedin vitro. The experimental data indicate that ADH significantly increased the transepithelial electrical conductance, and that this conductance increase could be rationalized in terms of transcellular conductance changes. A minimal estimate (G _c ^min ) of the transcellular conductance, estimated from Ba⁺⁺ blockade of apical membrane K⁺ channels, indicated thatG _c ^min was approximately 30–40% of the measured transepithelial conductance. In apical membranes, K⁺ was the major conductive species; and ADH increased the magnitude of a Ba⁺⁺-sensitive K⁺ conductance under conditions where net Cl^– absorption was nearly abolished. In basolateral membranes, ADH increased the magnitude of a Cl^– conductance; this ADH-dependent increase in basal Cl^– conductance depended on a simultaneous hormone-dependent increase in the rate of net Cl^– absorption. Cl^– removal from luminal solutions had no detectable effect onG _e , and net Cl^– absorption was reduced at luminal K⁺ concentrations less than 5mm; thus apical Cl^– entry may have been a Na⁺,K⁺,2Cl^– cotransport process having a negligible conductance. The net rate of K⁺ secretion was approximately 10% of the net rate of Cl^– absorption, while the chemical rate of net Cl^– absorption was virtually equal to the equivalent short-circuit current. Thus net Cl^– absorption was rheogenic; and approximately half of net Na⁺ absorption could be rationalized in terms of dissipative flux through the paracellular pathway. These findings, coupled with the observation that K⁺ was the principal conductive species in apical plasma membranes, support the view that the majority of K⁺ efflux from cell to lumen through the Ba⁺⁺-sensitive apical K⁺ conductance pathway was recycled into cells by Na⁺,K⁺,2Cl^– cotransport.     

Enhancement of rat intestinal calcium absorption by vanadate.

Vanadate alters intestinal transport and may have a role in regulating cell function. To determine whether it influences calcium absorption, we tested the effects of acute and chronic vanadate administration on calcium absorption using single-pass perfusion of jejunal and ileal segments of the in vivo rat intestine. Acute vanadate administration increased the lumen-to-mucosa and net fluxes of calcium in both the jejunum and ileum. The increase was largely due to an enhancement of the saturable fluxes of calcium and was observed at 10(-4) M concentration of vanadate, but not at higher or lower concentrations of the oxyanion, except at the highest concentration used, 10(-2) M, where calcium absorption was inhibited. Chronic vanadate administration caused, on the other hand, no changes in calcium absorption. We have demonstrated previously that rat intestinal (Na+ + K+)-ATPase is inhibited by vanadate, an effect that could raise cell sodium and increase the efflux of sodium across the brush border membrane. The results suggest that the vanadate enhancement of calcium absorption may be related to an increased entry of calcium into the mucosa, possibly as a result of an augmented exchange through the Na+/Ca+ antiport system. Alternatively, vanadate may influence access to a calcium channel in the mucosal membrane of the intestinal epithelium, leading to the observed increase in absorption.     

Effect of D ala2 metenkephalinamide on feline jejunal and ileal water and electrolyte transport

The aim of this investigation was to compare the effect of an opioid, D ala2 metenkephalinamide (DAMA), on net jejunal and ileal water and electrolyte fluxes using the gut perfusion technique in the anesthetized cat. Intestinal transport was measured during intravenous infusion of serial doses of 2, 6, and 18 micrograms.kg-1.h-1 of DAMA in 6 cats. Each cat was its own control during an intravenous infusion of 150 mmol/l NaCl preceding the first dose of peptide and following the last dose of DAMA. Both jejunal and ileal segments were isolated by inflated balloons and were studied at the same time. Fifteen ml of an iso-osmolar test solution with hypo-osmolar ion contents and complementary mannitol were administered in the upstream tube and collected 1 h later in the downstream tube. In the jejunum, water secretion was dose-dependently reversed to an absorption from a control value of +0.5 +/- 0.4 to -0.83 +/- 0.5 ml.h-1.10 cm-1; in the ileum, water absorption was increased from -0.5 +/- 0.3 to -1.5 +/- 0.2 ml.h-1.10 cm-1. The net absorption of all electrolytes, ie sodium, chloride, bicarbonate, potassium and calcium also increased during peptide administration. However, a qualitative difference in the ion transport was observed between the jejunum and the ileum.     

The Occurrence, Type and Location of Calcium Oxalate Crystals in the Leaves of Fourteen Species of Araceae

The occurrence, type and location of calcium oxalate crystalsin the leaves of 14 species belonging to the family Araceaewere studied by light microscopy. The Pizzolato test and theRubeanic acid-silver nitrate test, used to chemically identifyand locate the crystals in cross sections of laminae, showedthe presence of four types of crystals: druses, raphides, prismaticsand crystal sand. Styloids were not observed in any of the species.Crystals identified as calcium oxalate were observed in eachtissue layer of the leaf blade, druses occurring more frequentlyin the palisade mesophyll layers, raphides more often in thespongy mesophyll. Prismatics were sparse, occurring in the mesophyllof only two species. Specialized spindle-shaped crystal idioblasts,located in the spongy mesophyll in all cases, were observedin seven of the 14 aroids. Crystal sand and variations in crystalforms were most frequently observed to be calcium compoundsother than calcium oxalate. Crystals, calcium oxalate, idioblasts, Araceae     

The effect of calcium oxalate crystallization kinetics on the kinetics of calcium uptake and calcium ATPase activity of sarcoplasmic reticulum vesicles

The ionophore A23187 is a potent inhibitor of oxalate supported calcium uptake if added before uptake is initiated by ATP and is a much weaker inhibitor of uptake once uptake has been initiated. This observation is shown to be due to a failure of oxalate to capture the transported calcium at the beginning of uptake because the rate of calcium oxalate crystallization is initially slow, thereby allowing the ionophore to release the accumulated calcium. This hypothesis is supported by the observation that calcium oxalate crystallization shows a lag phase which is absent when calcium oxalate seeds are in the reaction system. Once calcium uptake has progressed, calcium oxalate seeds are present in the sarcoplasmic reticulum and calcium oxalate crystallization proceeds sufficiently rapidly that the ionophore cannot compete successfully for calcium. That A23187 and oxalate compete for intravesicular ionic calcium is shown by the stimulation which each produces in ATPase activity and by the dependence of ionophore activity on oxalate concentration.The failure of calcium oxalate crystallization to reach equilibrium during the early phase of calcium uptake caused us to examine whether at any time during calcium uptake, crystallization reaches equilibrium. Skeletal sarcoplasmic reticulum accumulated calcium at such a high rate that oxalate, in concentrations up to 20mM, was unable to clamp intravesicular calcium at equilibrium values. The lower rate of calcium accumulation by cardiac sarcoplasmic reticulum and/or perhaps its greater permeability to oxalate apparently allows intravesicular calcium to be clamped by oxalate.