VARIABILITY OF THE CALCIUM-ION ACTIVITY WITH PH IN STONE-FORMING AND NON-STONE-FORMING URINE

In recurrent renal stone-formers (N = 20) and matched healthy adults ( N = 20), the actual activity of ionized calcium (aCa2+) and pH were de termined in whole urine with an ion-selective electrode. No significan t difference was found for the actual median activity of ionized calci um, however the actual median pH was significantly higher in stone-for mers compared to healthy adults (pH = 5.57 vs. pH = 5.24; p < 0.005). The relationship between Ca2+ activity and pH was studied in each coll ected urine by titration with HCl/NaOH. In all urines the Ca2+ activit y decreased with increasing pH in a typical bifasic manner. All curves showed a characteristic ''breaking point' at a similar median pH in t he stone-formers and in the healthy adults (pH = 6.81 vs. pH = 6.77)(N S). However the slope of the curves in the stone-formers and healthy a dults chanced from a median value of triangle lgaCa2+/triangle pH of - 0.139 and -0.173 (NS) respectively, to a highly significant difference of -1.326 and -1.053 (p < 0.0001) between the groups, indicating incr eased binding/precipitation of Ca2+ in stone-formers than in healthy a dults supporting the theory of the lack of inhibitors in stone-formers . The strong relationship between the activity of ionized calcium and pH, combined with a higher actual pH and a higher decrease of ionized calcium with pH in stone-formers than in healthy adults, indicates hyd rogen ion as a major factor in stone-formation. The close relationship between Ca2+ activity and pH indicates the need for stimultaneous mea surements of the pH in order to interpret data for the Ca2+ activity. In order to preserve a low urinary pH, where Ca2+ is predominantly in a free ionic state, our results suggest that treatment with acidifying salts could be a logical choice in order to prevent stone-formation.