[Effect of hypokinesia on vitamin D metabolism in rats]

The effect of short- (7 days) and long-term (28 days) hypokinesia on 25-oxycholecalciferol metabolism was investigated in rats that were fed diets containing adequate amounts of vitamin D, calcium and phosphorus. Eighteen hours before sacrifice the animals were injected 3H-25(OH)D3. 3H-metabolites of D3 formed in vivo were separated by high-performance liquid chromatography, and their concentrations in the serum, kidneys, intestinal mucosa and bones were measured. Long-term hypokinesia decreased the content of 3H-1,25(OH)2D3 and increased that of 3H-24,25(OH)2D3 in the serum and kidneys. The exposure diminished the content of both 3H-1,25(OH)2D3 and 3H-24,25(OH)2D3 in the intestinal mucosa and bones and increased that of nonmetabolized 3H-25(OH)D3. The data obtained are indicative of a lower synthesis of 1,25(OH)2D3 and a higher synthesis of 24,25(OH)2D3 in the kidneys, as well as of a reduced binding of 24,25(OH)2D3 in the intestinal mucosa and bones in hypokinetic rats. Possible causes of variations in the biosynthesis of vitamin D active metabolites and their contribution to the disorders of calcium metabolism and bone density during hypokinesia are discussed.