EFFECT OF SHORT COURSE OF 1,25-DIHYDROXYVITAMIN D3ON BIOCHEMICAL MARKERS OF BONE REMODELLING IN POSTMENOPAUSAL WOMEN

This study was designed to test the hypothesis that a short treatment course of 1,25(OH)₂D₃elicits a stimulation of osteoblast activity without any action on the osteoclast. To test this, oral daily doses of 0.5μg or 1μg of 1,25(OH)₂D₃were administered for 7 days to two groups (n=5 andn=7, respectively) of postmenopausal women with low bone mineral density. Markers of osteoblast activity, i.e. osteocalcin (BGP), total alkaline phosphatase activity (ALP) and bone alkaline phosphatase activity (BALP), and markers of osteoclast activity, i.e. hydroxylysyl-pyridinoline (Pyr), lysyl-pyridoline (D-Pyr), and galactosyl-hydroxylysine (GHyl) were measured in plasma and in fasting urinary samples, respectively, at sequential times during and after 1,25(OH)₂D₃administration. It resulted that short term 1μg 1,25(OH)₂D₃oral administration induced a significant (P<0.05) rise of BGP serum level without any associated increase ofD-Pyr and GHyl, the latter also expressed as GHyl to GGHyl ratio. Urinary Pyr increased significantly after 1μg daily doses of 1,25(OH)₂D₃. Thus, a short course of 1μg daily doses of 1,25(OH)₂D₃elicits a stimulation of osteoblast activity without any enhancement ofD-Pyr, the most specific marker of osteoclast activity. The enhancement of Pyr after 1μg daily doses of 1,25(OH)₂D₃might be due to the activation of extraosseous metabolic pathways rather than to the activation of osteoclast.