Effects of microgravity on urinary osteopontin

Increased risk of renal stone formation during space flight has been linked primarily to increased calcium excretion from bone demineralization induce d by space flight. Other factors contributing to increased risk include inc reased urinary calcium oxalate supersaturation, while urinary citrate, magn esium and volume are all decreased. The aim of this study was to increase t he predictive value of stone risk profiles for crew members during space fl ight by evaluating the excretion of urinary protein inhibitors of calcium c rystallization so that more comprehensive stone risk profiles could relate mineral saturation to the concentrations of inhibitor proteins. Levels of u rinary osteopontin (uropontin) are reported in a series of 14 astronauts st udied before, during, and after space flights. During space flight, a compe nsatory increase in uropontin excretion was not observed. However, the urop ontin excretion of a majority of astronauts was increased during the period after space flight and was maximal at 2 wk after landing. The downward shi ft in the molecular size of uropontin observed in samples obtained during s pace flight was shown to result from storage at ambient temperature during flight, rather than an effect of microgravity on uropontin synthesis.