  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.
