QUANTITATIVE STUDIES OF HUMAN URINARY-EXCRETION OF UROPONTIN

Uropontin is the urinary form of osteopontin, an aspartic acid-rich ph osphorylated glycoprotein. Uropontin has been previously shown to be a potent inhibitor of the nucleation, growth and aggregation of calcium oxalate crystals and the binding of these crystals to renal epithelia l cells. Quantitative data defining the excretion of this protein are necessary to determine its role in urinary stone formation. In the pre sent studies, we determined uropontin excretion rates of normal humans . Urine samples were obtained under conditions of known dietary intake from young adult human volunteers with no history, radiographic or la boratory evidence of renal disease. Urinary concentrations of uroponti n were measured by a sensitive ELISA employing an affinity purified po lyclonal antiserum to uropontin. Thirteen normal subjects ingested a c onstant diet providing 1 gram of calcium, 1 gram of phosphorus, 150 mE q of sodium and 1 gram of protein per kilogram of body wt per day duri ng an eight day study period. The relationship of urinary volume to ur opontin excretion was assessed by varying fluid intake on the last fou r days of the study to change the mean urine volume/24 hr by > 500 mi. Urine collected in six hour aliquots for eight days was analyzed for uropontin by ELISA, and for calcium, and creatinine. Daily uropontin e xcretion of 13 individual Subjects was 3805 +/- 1805 mu g/24 hr (mean +/- 1 so). The mean urinary levels (1.9 mu g/ml) detected in the prese nt study are sufficient for inhibition of crystallization; our previou s studies have demonstrated that the nucleation, growth and aggregatio n of calcium oxalate crystals and their binding to renal cells in vitr o are inhibited by this concentration of purified uropontin. In contra st to the regular pattern of diurnal variation of calcium excretion se en in most subjects, uropontin excretion showed no regularity of diurn al variation and was not directly related to either calcium or creatin ine excretion or changes in urinary volume. However, uropontin concent ration varied inversely with urine volume (P less than or equal to 0.0 01), so that the highest uropontin concentrations occurred when urine volume was the lowest. We conclude that the physiologic characteristic of an inverse relationship of uropontin concentration to urine volume favors protection from urinary crystallization of calcium oxalate by uropontin. Our quantitative definition of urinary uropontin excretion of normal adults provides the basis for the evaluation of uropontin ex cretion by individuals who have formed urinary stones.