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Microgravity and Bone Cell Mechanosensitivity- Bone Cell Activation by Vibration Stress.  R.G. Bacabac1, T.H. Smit2, J.J.W.A. van Loon1,3, J. Klein-Nulend1, 1Dept Oral Cell Biology, ACTA-Vrije Universiteit, Amsterdam, NL, 2Dept Physics and Medical Technology, VUMC-Vrije Universiteit, 3DESC, Vrije Universiteit.

   Bone loss in space is likely resulting from an exceptional form of disuse. It is possible that the mechanosensitivity of bone cells is directly modulated by microgravity. Mechanical loading is likely transferred to the bone cells by strain-induced fluid flow through the lacuno-canalicular system. We have shown earlier that bone cell activation by fluid shear stress (upto 9 Hz), is linearly rate dependent. Using animal models, other studies showed that low magnitude (<10) high-frequency (10-100Hz) mechanical loading stimulated bone growth and inhibited disuse osteoporosis in vivo. Thus, high frequencies seem to be stimulatory to bone cells. To test bone cell mechanosensitivity in space, our in vitro model will use fluid flow to induce fluid shear stress on bone cells. However, vibration stress during launch, which might have high frequencies, might affect bone cell response to stress. Thus, we studied the nitric oxide (NO) production of MC3T3-E1 osteoblast-like cells in response to vibration stress at a wide frequency range (5Hz-100Hz), at different amplitudes. We found that NO production in rapid response (after 5min) to vibration stress linearly correlated with the applied peak acceleration rate (p<0.027). NO production did not linearly correlate with the peak velocity. Also, accumulative NO  was unchanged after 120 min of post-incubation without vibration stress. NO production is an essential step for mechanical loading-induced bone formation as observed in rats in vivo.  Hence, NO production in response to vibration stress is a meaningful parameter for measuring bone cell activation. Our results imply that the joint effect of the loading frequency and amplitude might exhibit similar properties for different types of stresses on bone cells. (Funded by SRON: MG-055)