AVOIDING CaHPO4 KIDNEY STONES DURING OLIGURIA

CaHPO 4 kidney stones grow when the (Ca) X HPO 4 2− ion product exceeds its solubility‐product constant (K SP ). For example, when the U FR (1.2 ml/min) declines 4‐fold to its nadir (0.3 ml/min), this ion product should rise 16 ‐fold if there were no change in the excretions of calcium and phosphate. Our objective was to determine how much the CaHPO 4 ion product would rise when the U FR fell after 16‐h of water deprivation. We studied 16 healthy subjects who did not eat or drink for 16‐hr beginning at 20:00‐h (0‐h). Completeness of urine collections was ensured by creatinine excretion rates. The U FR at 0‐h & 16‐h of water deprivation were 0.7±0.1 and 0.3±0.05 ml/min, respectively (p<0.01). The urine Ca (2.6±0.5 & 1.2±0.3 mM), phosphate (39±4 & 22±3 mM), and citrate (3.4±1.3 & 2.2±0.4 mM) concentrations fell over time. In addition, the Na+K excretion rate fell significantly (155±20 & 88±14 μmol/min) when the U FR reached its nadir; however there was no change in the U osm (1059±62 & 1067±29 mOsm/L). We conclude that instead of an expected > 2‐fold increase in the concentrations of calcium and phosphate when the U FR fell, these ion concentrations decreased by > 50 % suggesting marked stimulation of Ca and phosphate reabsorption while oliguric; there was, however, no significant change in the ECF volume. Since the fall in calcium exceed that of citrate, the ion product for CaHPO 4 decreased during oliguria. Thus the reabsorption of Ca and phosphate was enhanced while the avidity for Na reabsorption was enhanced. Hence oliguria in normal subjects is not a risk factor for precipitation of CaHPO 4 overnight.