Changes in serum concentrations of calcium and its regulating hormones during tail suspension in rats.

To study the effects of mechanical unloading on systemic calcium homeostasis, we determined the changes in serum concentration of calcium, 1,25-dihydroxyvitamin D3 and parathyroid hormone (PTH) during tail-suspension experiments in rats. The serum concentration of ionized calcium significantly increased during the 14 days of the suspension, reflecting increased bone resorption in the hind limbs. This hypercalcemic condition should cause suppression in PTH secretion. Indeed, serum PTH levels decreased on Day 3 of the suspension. This decrease was associated with lower serum levels of 1,25-dihydroxyvitamin [correction of dihyroxyvitamin] D3 probably due to a decrease in the activity of 1 alpha-hydroxylase in the kidneys resulting from a decrease in PTH secretion. Since it is known that 1,25-dihydroxyvitamin D3 stimulates osteoblastic function, it is suggested that endocrine responses evoked by tail suspension aggravate disuse atrophy of the hind limbs.