Effect of dietary calcium on the response of bone to 1,25 (OH)2D3.

The purpose of this investigation was to evaluate, by static and dynamic histomorphometry, the comparative influence of dietary calcium on the effect of 1,25(OH)2D3 on bone. Young adult female rats were fed a diet containing 0.3% phosphorus and 0.05%, 0.5%, or 1.0% calcium. Rats in each group received placebo, or 27 ng or 135 ng of 1,25(OH)2D3, intraperitoneally, for 10 days. A dramatic osteosclerosis and hyperosteoidosis characterized by an increase in metaphyseal hard tissue percentage and percentage metaphyseal osteoid, a decrease in osteoclasts per millimeter of trabecular surface perimeter and longitudinal growth, and hypertrophy of osteoblasts developed in the proximal tibial metaphysis of rats fed 1.0% calcium diet and given 135 ng of 1,25(OH)2D3 compared with placebo-treated rats fed the same diets. Similar histomorphometric changes, with the exception of increased metaphyseal hard tissue percentage, occurred in rats fed 0.5% dietary calcium and treated with 135 ng of 1,25(OH)2D3. Metaphyseal hard tissue percentage, percentage metaphyseal osteoid, and osteoclasts per millimeter of trabecular surface perimeter were not affected by treatment with 135 ng of 1,25(OH)2D3 in rats fed a 0.05% calcium diet. Serum calcium was significantly elevated in all 1,25(OH)2D3-treated rats. Dietary calcium appears to modulate the effects of 1,25(OH)2D3 on bone. Only diets containing adequate dietary calcium permit exogenous pharmacologic levels of 1,25(OH)2D3 to produce a positive skeletal balance either by decreasing osteoclasis and/or increasing bone matrix synthesis.