Gating of a muscle K+ channel and its dependence on the permeating ion species

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Abstract
In excitable cells, ions permeate the cell membrane through ionic channels, some of which open and close in response to changes in the potential differences across the membrane. This opening and closing (or gating) process seems largely independent of the permeating ion. The gating of the resting K permeability of frog skeletal muscle actually depends on the species of ion which carries the current across the membrane. The K permeability investigated allows K+ to move in across the membrane more easily than out. This property is known as inward or anomalous rectification and is shared by cell membranes of skeletal muscle, egg and certain other cells. In both egg cells and skeletal muscle fibers, the group IIIB metal ion TI+, which can replace K+ in several other systems in experimental conditions, permeates the inward rectifier. TI+ is more permeant than K+. When TI+ carries current inwards across the membrane, the inward rectifier inactivates over a brief period when the membrane is hyperpolarized. When K+ carries current, the permeability increases with time under hyperpolarization.