Conditions of disuse in
bed rest patients, as well as microgravity experienced by astronauts are accompanied by reduced mechanical loading, reduced
calcium absorption, and lower serum levels of
1,25(OH)2D3 (1,25-D), the active metabolite of
vitamin D, all contributing to bone loss. To determine whether 1,25-D or a less calcemic analog,
Seocalcitol or
EB1089 (1 alpha,25-dihydroxy-22,24-diene-24,26,27-trihomovitamin D3) can alleviate bone loss in a rat hindlimb unloading model of disuse
osteopenia, mature male rats originally on a
vitamin D replete diet containing 1.01%
calcium were transferred to a
vitamin D-deficient diet containing 0.48%
calcium and then tail suspended and treated for 28 days with vehicle, 0.05 microg/kg 1,25-D, or 0.05 microg/kg
EB1089. The
vitamin D-deficient diet caused a substantial decrease in bone mineral density (-8%), which may be compounded by hindlimb unloading (-10%). Exogenous 1,25-D not only prevented the bone loss but also increased the bone mineral density to greater than the baseline level (+7%).
EB1089 was less effective in preventing bone loss. Analysis of site and cell-specific effects of 1,25-D and
EB1089 revealed that 1,25-D was more active than
EB1089 in the intestine, the site of
calcium absorption, and in inducing osteoclastogenesis and
bone resorption whereas
EB1089 was more effective in inducing osteoblast differentiation. These studies suggest that elevating circulating 1,25-D levels presumably increasing
calcium absorption can counteract bone loss induced by disuse or microgravity with its associated reductions in circulating 1,25-D and decreased
calcium absorption.