Solvent Substitution as a Probe of Gating Processes in Voltage-Dependent Ion Channels

Abstract

Although it is widely accepted that ion transport across excitable membranes occurs via voltage-dependent ion-selective channels, and much has been learned about the microscopic events which occur prior to, during, and following activation of such channels, many important questions remain unresolved. For example, does the transient conductivity of channels arise from a relatively small alteration in the conformation of a preexisting structure (a “gate”), or is it due to a field-dependent activation of channel precursors, which must then interact locally to form a conducting pathway? Because the transmembrane electric field is the independent variable that controls permeability, activation of channels is likely to be triggered by some intramembrane charge movement, but what is the molecular origin of this displacement current and what relationship does it have to the creation of a conducting channel? Once opened, what is the nature of the pathway an ion sees in traversing the membrane? Does physiological inactivation of the Na⁺ channel depend on the existence of another specialized gate-like structure, or on the interaction of a blocking particle with the conducting channel? Is inactivation perhaps just a consequence of the relative stability of one of several intermediate, nonconducting protein conformations, so that the term “inactivation” becomes a misnomer to the extent that it is taken to imply the existence of a separate kinetic process? In an attempt to answer such questions investigators have drugged, irradiated, and digested axons, have grossly altered internal and external electrolyte compositions, and have subjected axons to increasingly more complex voltage-clamp protocols.