Monitoring Ion Channel Charge Displacements using Radio Frequencies

Abstract

Here we introduce a new technique to examine voltage-dependent ion-channel biophysics using radio frequency (RF) interrogating electric fields. The approach exposes the cell membrane to an RF electric field and measures vibrational electric current evoked by the RF field. Xenopus Oocytes transfected to express Shaker-B IR ion channels were used as the experimental model. A 500 kHz RF signal was applied to the membrane using extracellular bipolar metal electrodes, and RF charge displacement measurements were made during traditional two-electrode whole-cell voltage clamp. Voltage clamp was used to depolarize the oocytes and measure whole-cell K+ current at several transmembrane potentials. The RF interrogation signal was superimposed on top of the comparatively slow (DC) voltage clamp command signal. Results show that the measured RF membrane current was a function of DC membrane potential. The RF current was separated into conduction and displacement components to examine the voltage-dependent RF conductance, GRF, and capacitance, CRF. Remarkably, the RF capacitance, CRF, had a voltage sensitivity and half-activation voltage that correlated with the Shaker-B IR channel DC conductance measured using whole-cell voltage clamp. These data are consistent with the hypothesis that electrostatic interactions between the channel protein and K+ in the pore constrain the mobility of K+ and lead to changes in RF capacitance with membrane depolarization. The approach might offer a means to examine electrostatic interactions associated with ion channel function or to estimate voltage dependence of channel activation using extracellular RF signals. [supported by NIH R01DC04928, NSF IGERT DGE-9987616]