The Ionic selectivity of large protein ion channels

Abstract

Large, highly conductive ion channels have a major functional role in the cell life cycle. They allow the exchange of charged and neutral solutes across the cell membrane envelope and regulate the influx of nutrients and the extrusion of waste products. To perform this function, channels must discriminate between different ionic species. Mesoscopic channels allow multiionic, passive transport and are usually moderately selective toward positive or negative ions. Here we review one of the most common approaches used for the quantitative estimation of channel selectivity: the measurement of the potential needed to get zero current across a channel in the presence of an electrolyte concentration gradient, also known as Reversal Potential. We highlight several key points that need to be addressed for a correct physical interpretation of these experiments in electrophysiology.