EFFECTS OF URINARY MACROMOLECULES ON THE NUCLEATION OF CALCIUM-OXALATE IN IDIOPATHIC STONE FORMERS AND HEALTHY CONTROLS

Urinary macromolecules have attracted great interest because of their possible role as both promoters and inhibitors of calcium oxalate (CaO x) crystallization and it remains unclear whether there is any differe nce, in their nucleating activity, between stone formers and controls. We selected 9 male idiopathic CaOx stone formers whose 24-h urines pr esented no evidence of common urinary stone risk factors such as hyper calciuria, hyperoxaluria, hyperuricosuria, hypocitraturia, hypomagnesi uria or low glycosaminoglycans excretion and 12 male controls (matched for age and body weight) whose 24-h urines did not differ from those of stone formers. The study of urinary CaOx nucleation was made in fre shly voided overnight urines whose biochemical composition was almost identical in the two groups. In filtered (0.22 mu m) and ultrafiltered (10 kDa) urine we performed an oxalate tolerance test to determine th e permissible increment of oxalate, the oxalate level for nucleation a nd the permissible increment of CaOx relative supersaturation (CaOx RS ). In filtered urine from stone formers the permissible increment of o xalate was lower than controls (30 +/- 10.2 vs. 46.7 +/- 9.7 mg/l, P=0 .001), the oxalate level for nucleation was lower (64.4 +/- 14.2 vs. 7 9.5 +/- 15.6 mg/l, P = 0.035) and the permissible increment of CaOx RS was also lower (9.71 +/- 2.59 vs. 13.39 +/- 3.62, P = 0.018). In ultr afiltered urine these differences disappeared because the removal of m acromolecules in stone formers significantly enhanced the oxalate-tole rance values. The difference between the change of the oxalate permiss ible increment of filtered and ultrafiltered urine allowed a distincti on to be made between stone formers and controls that was not feasible in other ways (7.6 +/- 5.3 vs. 3.3 +/- 5.9 mg/l, P < 0.0001). The stu dy suggests that, in idiopathic CaOx stone formers free from common ur inary risk factors of CaOx crystallization, there is an increased tend ency for CaOx nucleation in urine, which is mediated by macromolecular components.