The Ionic selectivity of large protein ion channels

catalaEls canals ionics de gran amplada i de conductancia alta exerceixen una funcio molt important en el cicle de vida de les cel·lules. Permeten l’intercanvi de soluts neutres i carregats a traves de la membrana cel·lular i regulen l’aportacio de nutrients i la sortida de deixalles. Per a realitzar aquesta funcio, els canals han de discriminar entre diferents especies ioniques. Els canals mesoscopics permeten el transport multiionic passiu i generalment presenten una selectivitat moderada respecte a ions positius i negatius. Aqui revisem les metodologies mes usades per a estimar quantitativament la selectivitat ionica. Entre elles, la mesura del potencial electric necessari per a anul·lar el corrent electric a traves del canal en presencia d’un gradient de concentracio, cosa que es coneix com a potencial de corrent zero. Assenyalem els factors clau que cal tenir en compte per a una interpretacio fisica correcta d’aquests experiments d’elec- trofisiologia. EnglishLarge, 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 elec- trolyte 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.