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

Abstract

Objective: To simulate the crystallization process that might occur in the nephron segment from the distal part of the distal tubule (DTd) to the end of the collecting duct (CD), we evaporated salt solutions with an ion composition assumed to correspond to that in the DTd.

Material and methods: 100-mL samples of DTd solutions with and without dialysed urine (dU) and with hydroxyapatite (HAP) seed crystals were evaporated to a final volume of 10-20 mL at different starting pH levels. After evaporation, the number and volume of crystals were recorded in a Coulter Multisizer and the crystal morphology examined by scanning electron microscopy and x-ray crystallography.

Results and conclusions: Volume reduction of solutions with a pH of 6.5 resulted in a crystallization of calcium phosphate (CaP) and calcium oxalate (CaOx). An increased fraction of CaOx in the precipitate was observed when the pH was decreased. In solutions with an initial pH of 5.5, 6.0 and 6.5, the number and volume of crystals were smaller in samples analysed after 60 min than in those analysed immediately after the evaporation. This might reflect dissolution of crystals, as expected, most pronounced in the most acid solutions, but apparently counteracted by dU. The results obtained in these experiments suggest that CaP formed high in the nephron might induce heterogeneous nucleation of CaOx when subjected to the lower pH levels and increased supersaturation with CaOx encountered in the CD. With an alkaline pH in the CD, a pronounced crystallization of CaP can be anticipated.