It is known that pharmacological or toxic doses of
vitamin D induce
bone resorption both in vivo and in vitro, whereas physiological doses of the
vitamin have a protective effect on bone in vivo. To investigate the discrepancies of the dose-dependent effect of
vitamin D on
bone resorption, we examined the in vivo effect of
1,25-dihydroxyvitamin D(3) [
1,25(OH)(2)D(3)] on the expression of the
receptor activator of nuclear factor-kappaB (
NF-kappaB)
ligand (RANKL) and
osteoprotegerin (OPG) mRNAs in bone of thyroparathyroidectomized (TPTX) rats infused with or without
parathyroid hormone (PTH). Continuous infusion of 50 ng/h of PTH greatly increased the expression of RANKL
mRNA in bone of TPTX rats. Expression of OPG
mRNA was not altered by PTH infusion. When graded doses of
1,25(OH)(2)D(3) was daily administered orally for 14 days to normocalcemic TPTX rats constantly infused with PTH, 0.01 and 0.1 microg/kg of
1,25(OH)(2)D(3) inhibited the PTH-induced RANKL
mRNA expression, but 0.5 microg/kg of the
vitamin did not inhibit it. Regulator of
G protein signaling-2 (RGS-2) gene expression was suppressed by
1,25(OH)(2)D(3) dose-dependently, but
PTH/PTHrP receptor mRNA expression was not altered. Bone morphometric analyses revealed that
1,25(OH)(2)D(3) suppressed PTH-induced osteoclast number in vivo. These results suggest that pharmacological or toxic doses of
1,25(OH)(2)D(3) stimulate
bone resorption by inducing RANKL, but a certain range of physiological doses of the
vitamin inhibit PTH-induced
bone resorption, the latter mechanism appeared to be mediated, at least in part, by the suppression of the
PTH/PTHrP receptor-mediated signaling.