THE INFLUENCE OF HYDROXYAPATITE SEED ON THE CRYSTALLIZATION INDUCED BY VOLUME REDUCTION OF SOLUTIONS WITH AN ION COMPOSITION CORRESPONDING TO THAT IN THE DISTAL TUBULE AT DIFFERENT PH LEVELS

Objective: To simulate the crystallization process that might occur in the nephron segment from the distal part of the distal tubule (DTd) t o the end of the collecting duct (CD), we evaporated salt solutions wi th an ion composition assumed to correspond to that in the DTd. Materi al and Methods: 100-mL samples of DTd solutions with and without dialy sed urine (dU) and with hydroxyapatite (HAP) seed crystals were evapor ated to a final volume of 10-20 mL at different starting pH levels. Af ter evaporation, the number and volume of crystals were recorded in a Coulter Multisizer and the crystal morphology examined by scanning ele ctron microscopy and x-ray crystallography. Results and Conclusions: V olume reduction of solutions with a pH of 6.5 resulted in a crystalliz ation of calcium phosphate (CaP) and calcium oxalate (CaOx). An increa sed fraction of CaOx in the precipitate was observed when the pH was d ecreased. In solutions with an initial pH of 5.5, 6.0 and 6.5, the num ber and volume of crystals were smaller in samples analysed after 60 m in than in those analysed immediately after the evaporation. This migh t reflect dissolution of crystals, as expected, most pronounced in the most acid solutions, but apparently counteracted by dU. The results o btained in these experiments suggest that CaP formed high in the nephr on might induce heterogeneous nucleation of CaOx when subjected to the lower pH levels and increased supersaturation with CaOx encountered i n the CD. With an alkaline pH in the CD, a pronounced crystallization of CaP can be anticipated.