Vitamin D metabolism in nephrotic rats.

Vitamin D metabolites were measured in sera of normal, vitamin D deficient and nonazotemic nephrotic rats. The concentrations of all metabolites were reduced in nephrotic and vitamin D deficient animals although 1,25-dihydroxyvitamin D values remained relatively normal in the nephrotic group. Twenty-four hours after the intravenous injection of tritiated 25-hydroxycholecalciferol, approximately 34% of the injected radioactivity appeared in the urine of the nephrotic animals compared with 0.4% in the controls. In extracts from nephrotic sera subjected to high performance liquid chromatography, the percentage of radioactive counts comigrating with 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol was significantly increased. The various metabolites were present in urine in approximately the same ratios as in serum. Dynamic histomorphometry of tibial metaphyses showed no abnormality. Urinary losses of vitamin D metabolites constitute the major cause for low serum values in nephrotic rats. The apparent synthetic rates are not impaired.