Substrate deformation levels associated with routine physical activity areless stimulatory to bone cells relative to loading-induced oscillatory fluid flow

Although it is well accepted that bone tissue metabolism is regulated by ex ternal mechani cal loads, it remains unclear to what load-induced physical signals bone cells respond. In this study, a novel computer-controlled stre tch device and parallel plate flow chamber were employed to investigate cyt osolic calcium (Ca-i(2+)) mobilization in response to a range of dynamic su bstrate strain levels (0.1-10 percent, 1 Hz) and oscillating puidJIow (2 N/ m(2), 1 Hz). In addition, we quantified the effect of dynamic substrate str ain and oscillating fluid flow on the expression of mRNA for the bone matri x protein osteopontin (OPN). Our data demonstrate that continuum strain lev els observed for routine physical activities (<0.5 percent) do not induce C a-i(2+) responses in osteoblastic cells in vitro. However, there was a sign ificant increase in the number of responding cells at larger strain levels. Moreover, we found no change in osteopontin mRNA level in response to 0.5 percent strain at 1 Hz. In contrast, oscillating fluid flow predicted to oc cur in the lacunar-canalicular system due to routine physical activities (2 N/m2, 1 Hz) caused significant increases in both Ca-i(2+) and OPN mRNA. Th ese data suggest that, relative to fluid flow, substrate deformation may pl ay less of a role in bone cell mechanotransduction associated with bone ada ptation to routine loads. [S0148-0731 (00)01204-8].