Repeated immobilization stress reduces rat vertebral bone growth and osteocalcin.

We previously showed that psychological stressors alter plasma levels of osteocalcin (pOC), a bone-specific mineral binding protein, in ways that differ with the type of stressor. To determine effects of chronic stress, we examined vertebrae, pOC, and corticosterone levels from conscious rats subjected to foot-restraint immobilization (Immo) daily for 1-42 times. After 40-42 Immo, basal pOC was decreased by 25% compared with unstressed rats, and the subsequent rise in pOC during Immo was blunted. Corticosterone was elevated 10-fold during Immo. Immo for seven times did not change vertebral OC concentration, but caused a slight decrease in calcium and phosphorous concentrations in younger rats. Rats Immo for 42 times exhibited reduced body weight, vertebral weight, and vertebral OC concentration but no significant differences in vertebral mineral concentrations. Body fat content was visibly decreased. We do not know the source of or the stimulus for the initial rise in pOC. We conclude that both decreased growth and bone OC concentration are due to repeatedly elevated stress hormones.