Ultrastructural evaluation of the effects of vitamin D and uremia on bone in the rat.

Pharmacologic doses of vitamin D given to uremic rats significantly elevated serum calcium levels and produced dramatic ultrastructural changes in osteoblasts and osteocytes compared to placebo-treated uremic rats. Osteocytes in uremic rats given vitamin D were interpreted to have increased metabolic activity, with greater amounts of rough endoplasmic reticulum, and they contained finely granular mineral in pericellular sheaths. Osteoblasts appeared to have reduced activity with lesser amounts of rough endoplasmic reticulum, increased ribosomes and amorphous mineral frequently deposited along plasma membranes compared to uremic rats given placebo treatment. Osteoclasts were seen more frequently in uremic rats given vitamin D. Bone cell response in sham-operated rats administered vitamin D was distinctly different. Osteocytes and osteoblasts appeared to have greater metabolic activity compared to vitamin D-treated uremic rats. Osteocytes in sham-operated rats had increased amounts of rough endoplasmic reticulum and lysosomes. Osteoblasts were larger, with moderate amounts of rough endoplasmic reticulum, and had no mineral deposits along the plasma membranes. Osteoclasts were more numerous in sham-operated rats administered vitamin D. The altered response of bone cells in uremic rats given vitamin D was postulated to be due to the elevated serum phosphorus and greater release of calcitonin from thyroid C cells.