Signal Transduction in Electrically Stimulated Bone Cells

Background: Electrical stimulation is used to treat nonunions and to augment spinal fusions. We studied the biochemical pathways that are activated in signal transduction when various types of electrical stimulation are applied to bone cells. Methods: Cultured MC3T3-E1 bone cells were exposed to capacitive coupling, inductive coupling, or combined electromagnetic fields at appropriate field strengths for thirty minutes and for two, six, and twenty-four hours. The DNA content of each dish was determined. Other cultures of MC3T3-E1 bone cells were exposed to capacitive coupling, inductive coupling, or combined electromagnetic fields for two hours in the presence of various inhibitors of signal transduction, with or without electrical stimulation, and the DNA content of each dish was determined. Results: All three signals produced a significant increase in DNA content per dish compared with that in the controls at all time-points (p < 0.05), but only exposure to capacitive coupling resulted in a significant, ever-increasing DNA production at each time-period beyond thirty minutes. The use of specific metabolic inhibitors indicated that, with capacitive coupling, signal transduction was by means of influx of Ca2+ through voltage-gated calcium channels leading to an increase in cytosolic Ca2+ (blocked by verapamil), cytoskeletal calmodulin (blocked by W-7), and prostaglandin E2 (blocked by indomethacin). With inductive coupling and combined electromagnetic fields, signal transduction was by means of intracellular release of Ca2+ leading to an increase in cytosolic Ca2+ (blocked by TMB-8) and an increase in activated cytoskeletal calmodulin (blocked by W-7). Conclusions: The initial events in signal transduction were found to be different when capacitive coupling was compared with inductive coupling and with combined electromagnetic fields; the initial event with capacitive coupling is Ca2+ ion translocation through cell-membrane voltage-gated calcium channels, whereas the initial event with inductive coupling and with combined electromagnetic fields is the release of Ca2+ from intracellular stores. The final pathway, however, is the same for all three signals—that is, there is an increase in cytosolic Ca2+ and an increase in activated cytoskeletal calmodulin. Clinical Relevance: Electrical stimulation in various forms is currently being used to treat fracture nonunions and to augment spinal fusions. Understanding the mechanisms of how bone cells respond to electrical signals—that is, understanding signal transduction and the metabolic pathways utilized in electrically induced osteogenesis—will allow optimization of the effects of the various bone-growth-stimulation signals.