The effect of citrate on calcium oxalate (CaOx) crystal growth was studied in a system in which series of samples containing [45Ca]calcium chloride were brought to different levels of supersaturation with various concentrations of oxalate. The crystallization was assessed by measuring the amount of isotope remaining in solution 30 min after the addition of CaOx seed crystals to samples containing citrate in concentrations corresponding to those in final urine. The experiments were carried out both in pure salt solutions and in solutions with dialysed urine. Increased concentrations of citrate resulted in a reduced crystallization of CaOx in both the presence and absence of dialysed urine, but with the lowest rate of crystallization in the samples containing urine. The increased concentration of 45Ca remaining in solution reflected a reduced crystallization, which could possibly be explained both by a reduced supersaturation and by an increased inhibition of CaOx crystal growth. The direct effects of citrate on CaOx crystal growth were assessed by calculating the ion-activity product of CaOx (APCaOx) at corresponding degrees of crystallization. The APCaOx recorded at a 30% reduction of the amount of isotope in solution increased with increasing concentrations of citrate between 1.0 and 1.5 mmol/l samples both with and without dialysed urine. These findings indicate that citrate has a weak direct inhibitory effect on CaOx crystal growth, which adds to the reduced growth rate brought about by urinary macromolecules and a decreased supersaturation.

The biochemical risk situation in calcium oxalate stone formers can be summarized by quotient 2 or 3.

Crystals of calcium phosphate (CaP) added to solutions with a composition corresponding to that at different levels of the collecting duct (CD) and with different pH were rapidly dissolved at pH 5.0, 5.25 and 5.5. Only minor or no dissolution was observed at higher pH levels. Despite this effect, CaP crystals induced nucleation or heterogeneous crystallization of CaOx up to a pH of 6.1, whereas CaP was the type of crystalline material that precipitated at higher pH. Accordingly, small crystal volumes were recorded at pH 5.5 and great volumes at pH 6.7 4 h after the addition of CaP crystals to the solutions. Dialyzed urine appeared to counteract the dissolution of CaP and to reduce the rate of secondary crystallization. The CaP induced crystallization of CaOx was confirmed by a reduction of (14)C-labeled oxalate in solution. The AP(CaOx) required for a nucleation or heterogeneous crystallization of CaOx in the presence of CaP was around 1.5 x 10(-8) (mol/l)(2). For CaP crystal formation on CaP, an AP(CaP) ((a)Ca(2+) x (a)PO(4)(3-)) of approximately 50 x 10(-14) (mol/l)(2) appeared to be necessary. The CaOx crystals formed were microscopically found in association with the CaP crystalline material and were most frequently of CaOx dihydrate type. Step-wise crystallization experiments comprising supersaturation with CaP (Step A), supersaturation with CaOx (Step B) and subsequently acidification (Step C) showed that CaOx crystal formation occurred when CaP crystals were dissolved and thereby served as a source of calcium. The ensuing formation of CaOx crystals is most likely the result from high local levels of supersaturation with CaOx caused by the increased concentration of calcium. These experimental studies give support to the hypothesis that crystallization of CaOx at lower nephron levels or in caliceal urine might be induced by dissolution of CaP formed at nephron levels above the CD, and that a low pH is prerequisite for the precipitation of CaOx. The observations accordingly provide additional evidence for the important role of calcium phosphate in the crystallization of calcium oxalate, that might occur both at the surface of Randall's plaques and intratubularly at the papillary tip.

The rate of crystal sedimentation in a suspension of calcium oxalate monohydrate (COM) crystals was determined spectrophotometrically in the presence and absence of dialysed urine and citrate. A reduced rate of crystal sedimentation after stirring was recorded in suspensions containing citrate in concentrations between 0.33 and 1.67 mmol/l. The sedimentation rate was reduced in the presence of a 0.3-3.3% concentration of dialysed urine, with increased inhibition of crystal sedimentation when the concentration of urine was increased. A comparison of the inhibition of COM crystal sedimentation in whole urine and in dialysed urine from normal subjects and stone-formers disclosed significantly higher values (P < 0.05) in the dialysed urine. The results support previous observations that physiological concentrations of citrate might efficiently inhibit the aggregation of COM crystals. Furthermore even low concentrations of both whole urine and dialysed urine are apparently very efficient inhibitors of COM crystal aggregation.

The effect of pH changes on the crystallization in solutions with an ion-composition assumed to correspond to that of urine in the distal part of the distal tubule was examined by recording the number and volume of crystals with a Coulter Multisizer and by studying the crystal morphology by scanning electron microscopy at different degrees of volume reduction. The experiments were carried out with 100 ml samples at different starting pH without and with 20% of dialyzed urine (dU).The number of crystals increased in response to volume reduction. In solutions without dU, 100 or more crystals with diameters in between 2.4 and 45 µm were observed already at a volume reduction of 40% when the initial pH was 7.28. For solutions with pH of 5.80 and 6.45, the corresponding values were 60% and 80%, respectively. In the presence of dU, an appearance of crystals was recorded at volume reductions of less than 20%. In solutions with an initial pH of 5.80 and 6.45, the crystal number was greater with dU than without; such a difference was not recorded at pH 7.28.In samples containing dU, the mean crystal volume (MCV) varied very little when the sample volume was reduced. The same was found in solutions without dU when the initial pH was 5.80 and 7.28; the MCV was greater in the samples with pH 6.45.Scanning electron microscopy of solutions reduced to 30-40% of the original volume showed that calcium phosphate had formed in solutions with a starting pH of 7.28 and 6.45. In solutions with pH 5.80, calcium oxalate crystals were observed with calcium phosphate.