Suppression of a differentiation response in MC-3T3-E1 osteoblast-like cells by sustained, low-level, 30 Hz...

Extremely low-frequency (ELF) magnetic fields have been reported to be capable of influencing both tissue remodeling and cell phenotypic expression in culture. However, whether the cells or tissues respond directly to the magnetic flux or to the electric field induced by the time-changing magnetic flux remains a controversial topic. To address this question, we developed an osteoblast cell assay based on the activity of alkaline phosphatase, an enzyme whose activity is up-regulated during the differentiation of bone cells. MC-3T3-E1 cells plated at a confluent density were allowed to proceed through the differentiation process for 3 days, after which they were exposed to a 30 Hz, 1.8-mT r.m.s. magnetic field inducing a spatially varying electric field with a maximum intensity of 0.9 mV/m r.m.s. In situ assays of alkaline phosphatase activity at 4, 8, 16 and 64 h demonstrated a progressive inhibition of enzyme activity, the pattern of which maps to the intensity of the induced electric field (R(2) = 0.5, P<0.001). We interpret these results to indicate that cells are capable of responding to ELF induced electric fields at intensities below 1 mV/m, and that the principal effect on cells is an inhibition of differentiation.