Block of Na+ ion permeation and selectivity of Ca channels

Ionic selectivity of the Ca channel is better accounted for by models that incorporate specific binding properties of the channel for the ion to be selected. Ca selectivity through ion sieving by a rigid energy barrier appears unlikely for reasons stemming from the physical properties of earthalkaline ions as compared with those of Na ion and other monovalent ions (see Tsien et al. ’). It is also inconsistent with a significant passage of monovalent ions in the absence of the preferred divalent ions. Multiple binding site models and especially those that allow for transport facilitation by ion-ion interactions*6a ppear capable of describing the basic features of ion selectivity and specific transport. Eventual insufficiencies of such models are mostly of a quantitative nature and could be removed by introducing different modes of ion permeation. This allows for the possibility that the apparently blocked channel is specifically conductive for the blocking ion. This could indeed be the case because blocking and conducting states cannot be distinguished as long as the concentration of the blocking ion is too low to produce measurable unitary currents. Ion-channel interaction leading to a Ca-specific conformation of the channels appears to be suitable to explain apparent channel block due to prolonged closures that is the dominant feature in the case of the Iva Ca channel. This does not invalidate the virtues of multiple binding site models but can provide a means of understanding the extraordinary capability of the Ca channel to select its ion.