Conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 in renal slices from the rat.

Isolated renal cortical slices were used to study the conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and 24,25-dihydroxyvitamin D3 [24,25](OH2)D3] by the rat kidney. Production of 1,25-(OH)2D3 and 24,25-(OH)2D3 was linear with time (30-90 min) and tissue weight (40-250 mg). Production of 1,25-(OH)2D3 was greatest (134 +/- 17 pg/mg tissue.h) in animals fed a low calcium, vitamin D-deficient diet. The greatest 24,25-(OH)2D3 production (106 +/- 17 pg/mg tissue.h) was seen in animals fed a high calcium, vitamin D-replete diet, 1,25-(OH)2D3 production was reduced to 23% of maximum by the addition of 1.2% calcium or 0.8% strontium to the vitamin D-deficient, low calcium diet. Production of 1,25-(OH)2D3 and 24,25-(OH)2D3 was greatly reduced in renal cortical slices that had been heated before incubation. Slices of renal medulla produced only small amounts of 1,25-(OH)2D3 compared to slices of renal cortex. These studies provide direct evidence for the production of 1,25-(OH)2D3 and 24,25-(OH)2D3 by the mammalian renal cortex. They also demonstrate that this production may be modulated by dietary calcium, strontium, and vitamin D.