AJP - Endocrinology and Metabolism, Vol 256, Issue 5 E686-E691, Copyright © 1989 by American Physiological Society
Vitamin D kinetics in vivo: effect of 1,25-dihydroxyvitamin D administration
B. P. Halloran and M. E. Castro
Veterans Administration Medical Center, San Francisco, California.
Administration of 1,25-dihydroxyvitamin D [1,25(OH)2D] can increase the
metabolic clearance rate (MCR) of 25-hydroxyvitamin D [25(OH)D]. To
determine whether administration of 1,25(OH)2D can also influence the
metabolic clearance rates (MCR) of 1,25(OH)2D and 24,25-dihydroxyvitamin D
24,25(OH)2D, we measured metabolic clearance of 1,25(OH)2D, 24,25(OH)2D,
and 25(OH)D in rats in which the serum concentration of 1,25(OH)2D was
increased by continuous infusion. Infusion of 1,25(OH)2D (12 days at 75
pmol/day) increased serum 1,25(OH)2D from 128 +/- 11 to 244 +/- 14 pg/ml (P
less than 0.005) and increased MCR from 169 +/- 13 to 210 +/- 9
microliters.min-1.kg-1 or 24% (P less than 0.025). Increasing serum
1,25(OH)2D to 330-360 pg/ml increased MCR 72%. Infusion of 1,25(OH)2D
decreased serum 24,25(OH)2D from 3.5 +/- 0.5 to 2.4 +/- 0.3 ng/ml (P less
than 0.05), increased MCR from 25 +/- 2 to 48 +/- 6 microliters.min-1.kg-1
(P less than 0.0025), and increased the production rate (PR) from 70 +/- 11
to 124 +/- 26 pg.min-1.kg-1 (P less than 0.05). Infusion of 1,25(OH)2D
decreased serum 25(OH)D from 13.0 +/- 0.5 to 8.0 +/- 0.5 ng/ml (P less than
0.005) and increased MCR from 45 +/- 1 to 75 +/- 7 microliters.min-1.kg-1
(P less than 0.001) but had no effect on PR. The data indicate that
increasing serum 1,25(OH)2D by chronic administration can increase the MCR
of 1,25(OH)2D and suggest that 1,25(OH)2D can feedback regulate its serum
concentration by regulating its MCR. The data also suggest that 1,25(OH)2D
administration can increase the MCRs of 24,25(OH)2D and 25(OH)D.
