Single-channel properties and gating of Na+ and K+ channels in the squid giant axon

Abstract

This chapter describes patch clamp recordings made from the inner membrane surface of the cut-open squid giant axon, allowing examination of macroscopic ionic currents, gating currents, and single channel fluctuations. The cut-open axon method that allows the access of patch pipettes from the interior of the axon is described. The high density of channels on the squid axon surface makes it ideal for recording gating currents with a large signal-to-noise ratio, and these measurements have provided insights into the possible molecular mechanisms underlying the opening and closing of the ionic channels. To constrain the number of possible models of the gating process it is necessary to gather information on macroscopic ionic and gating currents, and measurements of single-channel fluctuations. In addition, these three types of measurement must be done in the same biological preparation, because the characteristics of ionic channels differ in different species and tissues. The chapter also talks about the properties of the Na+ channel in the squid giant axon, as seen with single-channel recording. These recordings provide direct information on the events occurring close to the open state(s); therefore this type of experiment, combined with the results on macroscopic currents and gating currents, should constrain the models of channel gating. With the cut-open axon technique it is possible to measure not only aspects of the opening and closing of single channels, but also gating currents and macroscopic currents in patches with high channel density. The characteristics of potassium channels of the squid axon are also briefly summarized at the end.