Electric fields modulate bone cell function in a density‐dependent manner

The influence of an extremely low frequency (ELF) electric field stimulus (30 Hz at 6 μV/cm rms), known to promote bone formation in vivo, was evaluated for its ability to affect bone cell function in vitro. To accomplish this, we developed an apparatus for the exposure of monolayer cell systems to electric fields in a manner that provides relatively uniform electric field exposure of multiple cell samples as well as a rigorous sham exposure. We show that field exposure significantly limits the normal increase in osteoblastic cell number and enhances alkaline phosphatase activity compared to sham‐exposed samples. Moreover, these alterations are shown to occur in a cell density‐dependent manner. Samples plated at 6 × 10³ cells/cm² show no effect of field exposure. In samples plated at 30 × 10³ cells/cm², 72 h of field exposure resulted in 25% fewer cells in the exposed samples, and a doubling of alkaline phosphatase activity in those cells compared to sham exposure. Experiments using a 12 h exposure to preclude significant changes in cell number during the exposure show this density‐dependent response to be biphasic. Sparse cultures (>50 × 10³ cells/cm²) were not found to be affected by the field exposure, but increases in alkaline phosphatase activity occurred in cultures at densities of 50–200 × 10³ and 200–350 × 10³ cells/cm² and no effect on alkaline phosphatase activity was seen in confluent cell cultures of greater than 350 × 10³ cells/cm². This work suggests that the demonstrated osteogenic effect of this specific electric field stimulus on bone tissue may be initiated by an alteration of the differentiated status of the osteoblasts in the tissue rather than by stimulating cell proliferation, as has been previously suggested. As importantly, a more thorough characterization of this density‐dependent interaction with electric fields may help to identify the mechanism of transduction by which ELF electric fields interact with cells.