Release of nitric oxide, but not prostaglandin E2, by bone cells depends on fluid flow frequency.

Loading frequency is an important parameter for the stimulation of bone formation in vivo. It is still unclear how the information of external loading characteristics is conveyed to osteoblasts and osteoclasts. Osteocytes are thought to detect mechanical loads by sensing fluid flow through the lacuno-canalicular network within bone and to translate this information into chemical signals. The signaling molecules nitric oxide (NO) and prostaglandin E2 (PGE2) are known to play important roles in the adaptive response of bone to mechanical loads. We have investigated the effects of fluid flow frequency on the production of PGE2 and NO in bone cells in vitro. Pulsatile fluid flow with different frequencies stimulated the release of NO by MC3T3-E1 osteoblasts in a dose-dependent manner. In contrast, PGE2 production was enhanced consistently by all fluid flow regimes, independent of flow frequency. This implies that the NO response may play a role in mediating the differential effects of the various loading patterns on bone.

Nitric oxide production by bone cells is fluid shear stress rate dependent.

Shear stress due to mechanical loading-induced flow of interstitial fluid through the lacuno-canalicular network is a likely signal for bone cell adaptive responses. Moreover, the rate (determined by frequency and magnitude) of mechanical loading determines the amount of bone formation. Whether the bone cells' response to fluid shear stress is rate dependent is unknown. Here we investigated whether bone cell activation by fluid shear stress is rate dependent. MC3T3-E1 osteoblastic cells were subjected for 15 min to fluid shear stress of varying frequencies and amplitudes, resulting in peak fluid shear stress rates ranging from 0 to 39.6 Pa-Hz. Nitric oxide production, a parameter for bone cell activation, was found to be linearly dependent on the fluid shear stress rate; the slope was steepest at 5 min (0.11 Pa-Hz(-1)) and decreased to 0.03 Pa-Hz(-1) at 15 min. We conclude that the fluid shear stress rate is an important parameter for bone cell activation.