Effects of endogenous and exogenous calcitonin on inflammation-mediated osteopenia in the rat

Inflammation-mediated osteopenia (IMO) in the rat is characterized by loss of bone mass within 3 weeks after induction of nonspecific inflammation (s.c. talcum injections) in growing rats. Histologically, this shows as marked inhibition of osteoblasts 3 days after the initiation of IMO. The role of calcitonin (CT) was investigated in the present study. A reversible increase of serum CT levels was found after intraperitoneal calcium challenge in rats on day 4 after induction of IMO, which was thought to be a result from calcium efflux from bone. No difference in stimulated serum CT levels between the rats with and without IMO was seen on any other day during 4 weeks after initiation of IMO. Bone loss after IMO was more pronounced in normocalcemic and euthyroid rats with deficiency of endogenous CT (thyroidectomy with parathyroid gland reimplanted) (-12.9%) compared with sham operated controls with IMO (-3.25%). Daily subcutaneous injections of 100 mIU salmon CT in rats with and without IMO did not prevent the development of bone loss. This might have been due to the growing state of rats of this age group. Our results support the hypothesis that endogenous CT physiologically has a bone protective role. They furthermore are consistent with the view that endogenous CT itself is not pathogenetically involved in the development of osteoporosis.