22-oxacalcitriol suppresses 25-hydroxycholecalciferol-1 alpha-hydroxylase in rat kidney

22-oxacalcitriol can decrease the serum level of 1,25-dihydroxycholecalciferol by increasing its metabolic clearance rate and decreasing its production rate. To determine whether 22-oxacalcitriol suppressed the renal 25-hydroxycholecalciferol-1 alpha- hydroxylase we treated rats with 200 ng of 22-oxacalcitriol daily for 1 week. Enzyme activity was measured in vitro by measurement of production of 1,25-dihydroxycholecalciferol by renal slices incubated with 25-hydroxycholecalciferol. 22-oxacalcitriol significantly decreased the 25-hydroxycholecalciferol-1 alpha-hydroxylase activity from 797 +/- 208 pg of 1,25-dihydroxycholecalciferol/g of kidney/h in the control group to 257 +/- 150 pg of 1,25-dihydroxycholecalciferol/g of kidney/h (p less than 0.05). This is the first demonstration of suppression of the renal 25-hydroxycholecalciferol-1 alpha-hydroxylase by the vitamin D3 analog, 22-oxacalcitriol.