Zoledronic acid and bone cellular respiration

Phosphorescence O₂ analyzer was used to measure calvarial bone cellular respiration (cellular mitochondrial O₂ consumption) in Taylor Outbred mice in the presence and absence of zoledronic acid. This potent bisphosphonate inhibits osteoclast-mediated calcium resorption, and its effects on bone respiration have not been previously investigated. The change of O₂ concentration with time was measured in closed vials containing phosphate-buffered saline (PBS), 5 mM glucose and 5-25 mg calvarial bone fragments, and it was complex for t = 0-30 h. Cyanide (specific inhibitor of cytochrome oxidase) halted O₂ consumption, confirming the oxidation occurred in the respiratory chain. Initial rate of respiration was estimated from the zero-order plots d[O₂]/dt for t = 0-4 h. For untreated specimens, the rate (mean ± SD) was 2.0 ± 1.2 µM O₂ h^-1 mg^-1 (n = 6). This value was 7-10 times lower than that of other murine organs, but similar to that reported for rat and Guinea pig calvaria (averaging, 2.7 nmol O₂ h^-1 mg^-1). The corresponding rate in the presence of 10-100 µM zoledronic acid was 2.7 ± 0.7 µM O₂ h^-1 mg^-1 (n = 11), p = 0.216. The first-order plots ln ([O₂] _t  ÷ [O₂] _t=0) versus time for t = 0-30 h were also used to compare treated and untreated specimens. The rate (h^-1 mg^-1 10³) for specimens incubated in PBS without glucose was 1.3 ± 0.6 (n = 3, p = 0.007), in PBS + glucose it was 10.7 ± 6.9 (n = 10), in PBS + glucose + 10 µM zoledronic acid it was 12.1 ± 6.7 (n = 10, p = 0.579), in PBS + glucose + 20 µM zoledronic acid it was 12.9 ± 3.3 (n = 9, p = 0.356), and in PBS + glucose + 100 µM zoledronic acid it was 13.7 ± 7.7 (n = 9, p = 0.447). Thus, exposure to high-doses of zoledronic acid over several hours imposed a statistically insignificant increase in calvarial bone cellular respiration.