Potassium current in the squid giant axon.

The squid giant axon was the first preparation to be investigated with the voltage clamp technique over 30 years ago by Cole (1949) and Hodgkin et al. (1952). During the intervening years it has continued to serve as a useful preparation for the development of other new techniques such as internal perfusion (Baker et al., 1962), gating current measurements (Armstrong and Bezanilla, 1974), and patch clamp measurements (Conti and Neher, 1980). It also has served as a useful comparative preparation for investigations of sodium and potassium currents in other excitable membrane preparations. This article has focused on the activation kinetics and the instantaneous current-voltage relation of the potassium component. The squid axon is well suited for studies of IK, because it appears to have only a single type of potassium channel, and the leakage current is relatively small under ideal conditions. The IK component is activated in a sigmoidal manner following membrane depolarization. It deactivates with a single exponential time constant following return of the membrane potential to the holding level, although the deactivation time constant varies with changes in the external potassium concentration. There has not, as yet, appeared a self-consistent model which describes all of these results. The current-voltage relation is a nonlinear function of driving force, which is approximately described by the Goldman-Hodgkin-Katz equation, although a model of the IV based on single file diffusion of ions through a channel is more in tune with the modern view of the ion permeation process (Hodgkin and Keynes, 1955; Hille and Schwarz, 1978; Clay and Shlesinger, 1977, 1983, 1984). Further progress in this area will probably be achieved both by the traditional techniques and by the patch clamp technique. The traditional method is well suited for studying tail current kinetics and the slow inactivation process. The patch clamp technique is well suited for studying the distribution of channels in the membrane and the kinetics of channel gating in steady state conditions.