Crystallization during volume reduction of solutions with an ion-composition corresponding to that in the distal tubuli.

The effect of macromolecules on the crystallization in solutions with an ion-composition and a pH corresponding to that of urine in the distal part of the distal tubuli was examined by recording the number and volume of crystals in a Coulter Multisizer and by studying the crystal morphology with scanning electron microscopy at different degrees of evaporation. The experiments were carried out with 100 ml samples of salt solutions with and without different concentrations of dialysed urine (dU) from normal subjects. Addition of dU resulted in a greater number of crystals and a reduction in the mean crystal volume (MCV). Under the experimental conditions, the maximal effect of the macromolecules appeared to be accomplished in solutions with an initial dU concentration of 10%. The precipitate was strongly suggestive of calcium phosphate (CaP) as shown by scanning electron microscopy and Raman spectroscopy. This conclusion was further supported by the ion-activity products of calcium oxalate (CaOx) and different CaP salts in those samples in which crystal formation was recorded. The obtained results give support to the view that macromolecules might exert a promotive effect on the nucleation of CaP. The macromolecules also appear to counteract the development of large CaP crystals, but whether this is due to an inhibition of crystal growth, an inhibition of crystal aggregation or both could not be concluded from these experiments. The way in which CaP crystals initially form in the nephron might be of importance for the subsequent crystallization of CaOx and the formation of CaOx containing stones.