Vitamin D insufficiency reduces the protective effect of bisphosphonate on ovariectomy-induced bone loss in rats

The present study was carried out to obtain an experimental model of vitamin D (vit D) insufficiency and established osteopenia (experiment 1) to then investigate whether vit D status, i.e. normal or insufficient, interferes with bone mass recovery resulting from bisphosphonate therapy (experiment 2). Rats (n = 40) underwent OVX (n = 32) or a sham operation (n = 8). The first 15 days post-surgery, all groups were kept under fluorescent tube lighting and fed a diet containing 200 IU% vit D (+D). They were then assigned during an additional 45 days to receive either +D or a diet lacking vit D (-D) and kept under 12 h light/dark cycles using fluorescent or red lighting. Serum 25HOD was significantly lower in -D rats (P < 0.0001). The type of lighting did not induce differences in 25OHD, calcium (sCa), phosphorus (sP), bone alkaline phosphatase (b-AL), CTX, bone density or histology. No osteoid was observed in undecalcified bone sections. Experiment 2 (105 days): rats were fed either +D or -D according to experiment 1 and were treated with either placebo or 16 mug olpadronate (OPD)/100 g rat/week during the last 45 days. Whereas 25HOD was significantly lower (P < 0.0001) in -D/OPD than in +D/OPD rats, no significant differences in sCa, sP, b-AL or CTX were observed. OPD prevented the loss of lumbar spine (LS) and proximal tibia (PT) BMD and the decrease in bone volume (BV/TV) (P < 0.05) and in the number of trabeculae observed in untreated rats. However, +D/OPD animals presented significantly higher values of LS BMD, PT BMD and BV/TV than -D/OPD rats (P < 0.05). No osteoid was observed in undecalcified sections of bone. In summary, this is the first experimental study to provide evidence that differences in vit D status may affect the anticatabolic response to bisphosphonate treatment. However, the molecular mechanism through which vit D insufficiency reduces the effect of the aminobisphosphonate remains to be defined.