Skeletal response to dietary calcium in a rat model simulating weightlessness.

Unweighting the hindlimbs of a rat by tail suspension leads to a decrease in bone in the unweighted hindlimbs, but not in the normally weighted forelimbs. We evaluated whether increments in dietary calcium could prevent this. Growing rats were fed diets ranging in calcium content from 0.1% to 2.4%. After the rats were suspended for two weeks, we found no differences between suspended and control animals fed the same diet with respect to calcium transport or serum levels of calcium, phosphorus, 1,25-dihydroxyvitamin D, and parathyroid hormone. In both groups, increasing dietary calcium reduced active intestinal calcium transport and serum 1,25-dihydroxyvitamin D levels. The calcium content of the tibia and lumbar vertebra (but not the humerus) was reduced in suspended rats compared to control rats fed the same diet. However, increasing dietary calcium increased the calcium content of all bones in both suspended and control animals. The bone formation rate at the tibiofibular junction (measured by double-label tetracycline) was reduced in the suspended animals compared to controls and was not altered by dietary calcium. However, the marrow area of the tibia, an indication of bone resorption, did not differ between suspended and control animals and was equally reduced in both groups when dietary calcium was increased. Our data suggest that the deleterious effects of skeletal unweighting on bone formation cannot be explained by changes in the calciotropic hormones and are not reversed by increments in dietary calcium. However, increasing dietary calcium can increase bone calcium, even in unweighted limbs, by decreasing bone resorption.