title: Variability of the calcium ion activity with pH in stone-forming and non-stone-forming urine.
abstract: In recurrent renal stone-formers (N = 20) and matched healthy adults (N = 20), the actual activity of ionized calcium (alpha Ca2+) and pH were determined in whole urine with an ion-selective electrode. No significant difference was found for the actual median activity of ionized calcium, however the actual median pH was significantly higher in stone-formers 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 collected urine by titration with HCl/NaOH. In all urines the Ca2+ activity decreased with increasing pH in a typical bifasic manner. All curves showed a characteristic "breaking point" at a similar median pH in the stone-formers and in the healthy adults (pH = 6.81 vs. pH = 6.77) (NS). However the slope of the curves in the stone-formers and healthy adults changed from a median value of delta lg alpha Ca2+/delta 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 increased binding/precipitation of Ca2+ in stone-formers than in healthy adults 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 hydrogen ion as a major factor in stone-formation. The close relationship between Ca2+ activity and pH indicates the need for simultaneous measurements 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.
authors.affiliation: Department of Clinical Chemistry, Gentofte Hospital, Hellerup, Denmark.
authors.forename: J
authors.initials: J
authors.lastname: Thode
authors.forename: R B
authors.initials: RB
authors.lastname: Holgersen
authors.forename: T
authors.initials: T
authors.lastname: Gerstenberg
mesh_terms: D000328:Adult; D000368:Aged; D002118:Calcium / Q000652:urine*; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D008875:Middle Aged; D014545:Urinary Calculi / Q000517:prevention & control / Q000652:urine*
chemical_list: D002118:Calcium