CALCITONIN RECEPTOR DOWN-REGULATION RELATES TO CALCITONIN RESISTANCE IN MATURE MOUSE OSTEOCLASTS

We have reported that short calcitonin (CT) treatment of mature mouse osteoclast-like cells (OCLs) in culture induced prolonged down-regulat ion of the CT receptor (CTR) and desensitization to CT rechallenge, at the level of adenylate cyclase activity: In this study, we have exten ded those studies to examine the bone resorbing activity of OCLs pretr eated with CT. OCLs, which formed on gelled type I collagen, were pret reated with salmon CT (sCT) (10(-9) M, 1 h) and 24 h later were replat ed onto plastic dishes or dentine slices after removal from the gel by collagenase digestion. The number and population of either mononuclea r or multinuclear OCLs that adhere to either surface was not affected by sCT pretreatment. It was found that OCLs pretreated with sCT regain ed reduced but significant bone resorbing capacity, which was quantita ted as the surface area resorbed by OCLs on dentine slices. However, c ompared with control, the number of resorption pits produced by sCT-pr etreated OCLs was slightly reduced, and the total pit area was decreas ed by approximately 40-50%. The distribution of individual pit sizes w as altered by sCT-pretreatment so that the number of larger pits was p redominantly reduced, suggesting that short sCT treatment may produce a long lasting decrease in osteoclast mobility. sCT was able to inhibi t bone resorptive activity of CT-pretreated OCLs (ED(50): similar to 1 0(-13)-10(-12) M). Importantly, the ED(50) of sCT inhibition of bone r esorption in sCT-pretreated OCLs was approximately 100-fold greater th an for control, indicating resistance of the OCLs to CT rechallenge. C onsistent with these results, treatment of OCLs with sCT greatly decre ased the expression of CTR messenger RNA, whereas no significant effec t was observed on the tartrate-resistant acid phosphatase messenger RN A expression, a marker of resorptive capacity of osteoclasts. These re sults indicate, therefore, that an important component of escape of os teoclastic resorption from CT inhibition is CT resistance of mature os teoclasts, which regain bone resorbing function.