Detection of extracellular ion gradients enables characterization and functional mapping of ion channels and transporters

Abstract

Conventional, voltage clamp methods have not been successful for noninvasive characterization of ion channels, studying the vast majority of slow rate and no‐current passing (electroneutral) transporters, or the direct identification of transport stoichiometry. It is for these reasons that we explored the use of electrochemical sensors for near real time measurement of extracellular ion gradients that result from ion passage through channels and transporters. Ion‐selective electrodes possess the speed and sensitivity to capture extracellular ion activity changes from single ion channels. We monitored Ca2+‐activated K+ channel activity expressed in CHO cells using a fast response K+‐selective electrode. The K+ gradients measured using this approach fit well with a diffusion‐based model, enabling noninvasive, functional characterization of the channels and mapping of their location. When ion‐selective electrodes were used in a modulation mode, termed self‐referencing, the signal to noise ratio was increased, enabling the measurement of small, relatively steady fluxes from ion transporters. Near real‐time characterization of slow rate and electroneutral transporters could be performed. In conclusion, we have developed a novel extracellular, self‐referencing microelectrode system that will be useful for the noninvasive, long‐term monitoring of ion transport in cells and epithelia. Supported by the NIH.