Studies on the role of calcium phosphate in the process of calcium oxalate crystal formation

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 ¹⁴C-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 × 10⁻⁸ (mol/l)². For CaP crystal formation on CaP, an AP_CaP (^aCa²⁺ × ^aPO₄ ³⁻) of approximately 50 × 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.