Thermal mechanisms of millimeter wave stimulation of excitable cells

Interactions between millimeter waves (MMWs) and biological systems have received increasing attention due to the growing use of MMW radiation in technologies ranging from experimental medical devices to telecommunications and airport security. Studies have shown that MMW exposure alters cellular function, especially in neurons and muscles. However, the biophysical mechanisms underlying such effects are still poorly understood. Due to the high aqueous absorbance of MMW, thermal mechanisms are likely. However, nonthermal mechanisms based on resonance effects have also been postulated. We studied MMW stimulation in a simplified preparation comprising Xenopus laevis oocytes expressing proteins that underlie membrane excitability. Using electrophysiological recordings simultaneously with 60 GHz stimulation, we observed changes in the kinetics and activity levels of voltage-gated potassium and sodium channels and a sodium-potassium pump that are consistent with a thermal mechanism. Furthermore, we showed that MMW stimulation significantly increased the action potential firing rate in oocytes coexpressing voltage-gated sodium and potassium channels, as predicted by thermal terms in the Hodgkin-Huxley model of neurons. Our results suggest that MMW stimulation produces significant thermally mediated effects on excitable cells via basic thermodynamic mechanisms that must be taken into account in the study and use of MMW radiation in biological systems.