Polyanionic Inhibition Versus Supersaturation in Male and Female Recurrent Calcium-Stone Formers
Calcium-oxalate stones form because of an imbalance between the forces of inhibition and supersaturation. The main inhibitory activity is a function of a group of urinary polyanions (for example, glycosaminoglycans) which act by binding to the surface of calcium-oxalate crystals and increasing their net negative surface charge, that is the zeta potential, Urine saturation is determined by urinary volume and pH and the 24-h urine excretions of calcium, oxalate, and, possibly, uric acid. In order to determine abnormalities in the functional availability of the polyanionic inhibitors and in saturation risk factors, and to assess possible differences between males and females, the following individuals collected a 24-h urine specimen while on their normal diet and activity. Sixty-one male recurrent calcium-oxalate stone formers (MRSF), 25 male control subjects (MCS), 19 female recurrent calcium-oxalate stone formers (FRSF), and 13 female control subjects (FCS). Three ml of urine were combined with 27 ml of a supersaturated solution of calcium oxalate and the resulting zeta potential of the crystals was measured using a Zeta Meter. Each urine specimen was also analyzed for volume, pH, calcium, oxalate, uric acid, and creatinine. The mean zeta potential measurement using the urine of the MRSF was −16.5±0.4(±SEM) mV compared with −20.3±0.8 mV for the MCS (p<.0.001). The MRSF excreted 7.0±0.3 mmol of calcium per 24 h compared with 5.6±0.5 mmol by the MCS (p<0.05). Other risk factor comparisons were not significantly different. The mean zeta potential measurement using the urine of FRSF was −17.3±0.8 mV compared with −20.2±1.0 mV for the FCS (p<0.05). The FRSF excreted more calcium (5.8±1.3 vs 2.7±0.4 mmol per 24 h, p<0.0001) and oxalate (0.40±0.05 vs 0.20±0.02 mmol/24 h, p<0.005) than did the FCS. Other risk factor comparisons were not significantly different. The MCS had significantly greater 24-h excretions of calcium, uric acid, and oxalate than FCS, but MRSF had only significantly greater uric acid excretions compared with FRSF. Other inter-sex risk-factor comparisons were not significantly different. In conclusion, both MCS and FCS have a decreased risk of stone formation due to greater polyanionic inhibitory activity and a lower supersaturation. There was no difference between males and females in terms of inhibitory activity, but saturation was more of a problem in the male groups.