Molecular aspects of neuronal voltage-dependent K+ channels.

K+ channels are probably the oldest channel types since they are present in virtually all eukaryotic cells from yeast to the mammalian neuron. In the neuron, K+ channels terminate the action potential, repolarize the membrane and set its resting potential (Hille, 1984; Rudy, 1988). Due to this multitude of functions, K+ channels are a highly diverse class of channels. A rough classification discriminates the ATP-dependent K+ channels, the Ca2+-dependent K+ channels, the Na+-activated K+ channels and the voltage-dependent K+ channels. Voltage-dependent K+ channels are further subdivided on the basis of their gating characteristics into: (a) so-called A channels which activate and inactivate quickly upon membrane depolarization; (b) delayed rectifier channels which activate slowly and do not or only slowly inactivate and (c) the inward rectifier channels which open upon membrane hyperpolarization. Each of these groups has several members and the delineations between the groups are flexible.