The influence of pH on urinary ionized [Ca2+]: differences between urinary tract stone formers and normal subjects

To assess the role of pH in determining the solubility of Ca2+ in urine from normal subjects and stone formers and to determine the conditions which cause the formation of crystalline products.

Urine, collected from normal subjects and urinary tract stone-formers, was used in vitro. The concentration of ionized calcium, [Ca2+], and pH were measured with ion-selective electrodes and the formation of crystalline products measured as the change in turbidity of the specimen. The composition of the crystalline products was analysed by infrared spectroscopy.

The [Ca2+] decreased with increasing pH and showed a biphasic relationship with a critical pH (pHn) at which Ca-containing precipitates appeared. Compared with normal subjects, the mean (SD) value of pHn was significantly lower in urine from stone-formers, at 7.67 (0.62) and 7.21 (0.54), respectively, and nearer to their voided pH of 5.92 (0.70) and 6.26 (0.71), respectively. The lower pHn in stone-formers could not be explained by altered concentrations or binding properties of Ca(2+)-binding ligands. pHn increased after diluting the urine and the value of pHn was strongly dependent on the [Ca2+] at a standardized pH.

Crystalline precipitates appear in urine at a critical pH which is closer to the voided pH in stone-formers than in normal subjects and might explain the greater propensity of this group to form stones. The value of pHn is critically dependent on the urinary [Ca2+] and manoeuvres which reduced its concentration would reduce the tendency to form stones.