Single-Channel Conductances and Models of Transport

Abstract

Kacy Cole once said that the specific membrane capacitance is the nearest thing in biology to a physical constant. I always took that to mean that the ubiquitous 1 µf/cm² membrane capacitance values show that cell membranes everywhere are made of thin sheets of low-dielectric-constant material. Since a uniform low-dielectric barrier would make ion permeation much too difficult, the large conductance increases which occur during excitation are most easily explained by the activation of sparsely distributed highly conductive ion channels. One of the advances of the last few years is that we can now measure the conductance of these elementary ionic channels. I would like to first review how the unit channel conductance can be determined from electrical noise measurement and then discuss how one can directly observe the activation of individual channels. At present, we see that ion-selective channels form a special class of interesting membrane proteins. There are various kinds of channel structure-not only the variety of gated channels which are switched on by various stimuli during excitation, but also cytolytic channels in various immune and attack systems, channels connecting adjacent epithelial cells at gap junctions, and probably others. The unit channel conductances form a new set of physical parameters for membrane transport which, like the capacitance, give us a bit of the picture of the membrane as a physical structure.