Stochastic resonance of ELF-EMF in voltage-gated channels: the case of the cardiac I_Ks potassium channel

We applied a periodic magnetic field of frequency 16 Hz and amplitude 16 nT to a human I_Ks channel, expressed in a Xenopus oocyte and varied the membrane depolarization between -100 mV and +100 mV. We observed a maximal increase of about 9% in the potassium current at membrane depolarization between 0 mV and 8 mV (see Figure 3). A similar measurement of the potassium current in the KCNQ1 channel, expressed in an oocyte, gave a maximal increase of 16% at the same applied magnetic field and membrane depolarization between -14 mV and -7 mV (see Figure 4). We attribute this resonant behavior to stochastic resonance between the thermal activations of the configuration of interacting ions in the I_Ks channel over a low potential barrier inside the closed state of the channel and the periodic electromotive force induced across the membrane by the periodic magnetic field. The partial synchronization of the random jumps with the periodic force changes the relative times spent on either side of the barrier, thereby changing the open probability of the spontaneously gating open channel. This, in turn, changes the conductance of the channel at the particular depolarization level and frequency and is expressed in the Hodgkin-Huxley equations as a bump at the given voltage in the conductance-voltage relation. We integrate the modified Hodgkin-Huxley equations for the current into the Luo-Rudy model of a Guinea pig ventricular cardiac myocyte and obtain increased conductance during the plateau of the action potential in the cell. This shortens both the action potential and the cytosolic calcium concentration spike durations, lowers its amplitude, increases cytosolic sodium, and lowers cytosolic potassium concentrations. The shortening of the ventricular calcium signal shortens the QT period of the ECG.