The effects of cortisol on bone resorption and osteoclastic activity are not fully understood. Although glucocorticoid-induced osteoporosis is considered to occur as a result of inhibited bone formation, some researchers have suggested that bone resorption is stimulated
by glucocorticoid treatment. Jia et al [20] reported that the osteoclastic life span is prolonged by the direct action of glucocorticoids on osteoclasts, resulting in the activation of bone resorption and a reduction in bone density. Kuroki et al [17] also observed increased
bone resorption markers in humans treated with 40 mg of prednisolone daily. However, Gram et al reported a significant decrease in bone resorption markers in healthy men treated with 10 mg prednisolone twice daily. Here, we also observed a significant decrease in both TRAP5b and HYP. The reason for this discrepancy is unclear. Further studies are necessary to elucidate the direct or indirect effects of cortisol or exogenous glucocorticoids on osteoclasts.
The present bone metabolic marker data indicate
that the periparturient cortisol state reproduced via 12-h
ACTH infusion inhibited bone resorption and formation
in cows depleted of gonadal steroid hormones.
However, TRAP5b and BALP peaked on the day of
calving in our previous study [21]. Therefore, these
results suggest that a number of factors are involved in
the regulation of bone metabolism and that some of
these alleviate the inhibitory effect of cortisol on bone
metabolism. Notably, the influence of calcitropic hormones
such as parathyroid hormone and calcitriol on
bone metabolism was not evaluated in the present
study. Thus, further studies are necessary to assess the
influence of other endocrine factors on bone metabolism
and ACTH stimulation.