Decreased nitric oxide levels stimulate osteoclastogenesis and bone resorption both in vitro and in vivo on the chick chorioallantoic membrane in association with neoangiogenesis

High nitric oxide (NO) levels inhibit osteoclast (OC)-mediated bone resorpt ion in vivo and in vitro, and nitrate donors protect against estrogen-defic ient bone loss in postmenopausal women. Conversely, decreased NO production potentiates OC bone resorption in vitro and is associated with in vivo bon e loss in rats and humans. Previously, we reported that bone sections from rats administered aminoguanidine (AG), a selective inhibitor of NO producti on via inducible NO synthase, exhibited both increased OC resorptive activi ty as well as greater numbers of OC, Here, we investigated further whether AG promoted osteoclastogenesis, in addition to stimulating mature OC functi on, using a modified in vivo chick chorioallantoic membrane (CAM) system an d an in vitro chick bone marrow OC-like cell developmental model. AG, focal ly administered in small agarose plugs placed directly adjacent to a bone c hip implanted on the CAM, dose-dependently elicited neoangiogenesis while s timulating the number, size, and bone pit resorptive activity of individual OC ectopically formed in vivo. In addition to enhancing OC precursor recru itment via neoangiogenesis, AG also exerted other vascular-independent effe cts on osteoclastogenesis. Thus, AG promoted the in vitro fusion and format ion from bone marrow precursor cells of larger OC-Pike cells that contained more nuclei per cell and exhibited multiple OC differentiation markers. AG stimulated development was inversely correlated with declining medium nitr ite levels. In contrast, three different NO donors each dose-dependently in hibited in vitro OC-like cell development while raising medium nitrite leve ls. Therefore, NO sensitively regulates OC-mediated bone resorption through affecting OC recruitment (angiogenesis), formation (fusion and differentia tion), and bone resorptive activity in vitro and in vivo. Possibly, the sti mulation of neoangiogenesis and OC-mediated bone remodeling via AG or other pro-angiogenic agents may find clinical applications in reconstructive sur gery, fracture repair, or the treatment of avascular necrosis.