How Can We Study Ion Channels?

The ion channels in neuronal membranes are too tiny to be seen in detail, even with the electron microscope. How, then, can investigators determine their structures and modes of operation? Several techniques have been used, providing more complete knowledge about ion channels. Molecular genetic analysis of potassium and sodium channels in a variety of cells, including muscle cells and neurons, has yielded a detailed portrait of ion channel structure and function. These studies reveal that multiple genes encode the structure and function of ion channel proteins. Other studies make use of pharmacology, patch clamp techniques, or X-ray crystallography, which we will describe next.

X-Ray Crystallography

In this technique, billions of copies of a protein molecule are induced to crystallize, and X-rays are bounced off the resulting structure. The identity and location of the atoms that make up the protein can be inferred from the pattern of reflection, and through mathematical modeling the overall configuration of the protein can be reconstructed. In addition to establishing how K+ channels selectively admit K+ ions at high rates (described in the text), Rod MacKinnon and his collaborators have used this technique to make breakthrough discoveries about the gating mechanisms of voltage-gated K+ channels (Jiang et al., 2003; S. B. Long et al., 2005).

Although the exact details of the gating mechanism are far from certain, the data from MacKinnon’s lab suggest that K+ channels employ electrically charged “paddles,” located within the lipid bilayer of the cell membrane, that act as voltage sensors. Attracted to membrane charges, these paddles mechanically pop open the ion channel when the membrane potential changes appropriately (see Figure 1). Because these channels belong to a large and diverse family, it is likely that similar mechanisms will be described for other varieties of voltage-gated channels. MacKinnon’s work was recognized with the 2003 Nobel Prize in Chemistry.

Figure 1  Voltage-Gated Potassium Channels

Pharmacological Techniques

In pharmacological experiments, certain toxins are used to block specific ion channels—some affecting the outer end of the channel, and others inhibiting the inner end. By specifically blocking only some channels, these toxins provide information about those channels, as well as about the remaining, unblocked channels. Two examples that are discussed in the text are tetrodotoxin (TTX) and saxitoxin (STX), both of which selectively block voltage-gated Na+ channels. The size and structure of TTX and STX, together with those of other molecules that do or do not alter Na+ permeability, indicate the dimensions of the sodium channel.

Patch Clamp Recordings

In the patch clamp technique, a small patch of membrane is sealed by suction to the end of a micropipette, enabling investigators to record currents through single ion channels (Figure 2). Erwin Neher and Bert Sakmann were awarded the 1991 Nobel Prize in Physiology or Medicine for devising this technique. Patch clamp recordings have been made not only in nerve cells, but also in glial cells and muscle cells. The recordings show that gated channels open abruptly and remain open only briefly (see Figure 2, bottom).

Figure 2  Patch Clamp Recording from a Single Ion Channel

Opening of some channels is made more likely by changes in voltage; one example is the voltage-activated Na+ channel that is responsible for the rising phase of the nerve impulse. This type of channel responds extremely rapidly. Another major family of gated channels respond to chemical substances applied to the surface of the cell; their responses are slower than those of the voltage-gated channels. Examples of these chemically gated channels will be given a little later in this chapter, when we discuss what happens at chemical synapses.