Ion Channel Pore Sizing in Patch-Clamp Experiments

The physical dimensions of an ion channel pore can be estimated in patch-clamp experiments using various charged and noncharged molecular probes. Measuring the permeability to organic ions of different size and shape yields an estimate of the size of the narrowest part of the selectivity filter. Open-channel blockers represent another frequently used tool to estimate the size of an ion channel pore near the binding pocket. The special case of permeable blockage yields the size of the narrowest portion of the ion-transporting pathway. Noncharged molecular probes may also affect ionic currents through channels either by blocking them or by decreasing the effective ionic mobility within the pore. Size-dependent suppression of single-channel amplitudes by neutral polymers, such as polyethylene glycols, can be interpreted in terms of molecular partitioning between the bulk solution and pore interior. This allows us to gauge the size of channel vestibules based on standard patch-clamp data.