It should be mentioned that examples of two types of ion
channels have been given. (1) So-called slow channels involve
second messengers. The examples given here are the
light-activated channels that are opened by cGMP and the
transmembrane membrane ion transport that requires the
hydrolysis of ATP. (2) Fast activated channels. For activation,
these just require the binding of a ligand to the channel
protein or a change in transmembrane voltage. The
examples given are the neurotransmitter-activated channels
and the voltage-activated K+ channel.
The development of techniques using crystallography,
NMR, electron diffraction, and molecular biology to produce
specific proteins in large amounts to determine the
structure of transmembrane channels formed by proteins is
a very active field. It is expected that an increasing number
of high-resolution transmembrane structures will be
forthcoming in the next few years. These structures, together
with kinetic measurements, are expected to give
detailed information about the mechanism by which inorganic
ions are transported across the cell membrane.
The single-channel current-recording technique is ideally
suited for studying channels that open because of the concentration
gradient of inorganic ions and the resulting
voltage changes. The change in transmembrane voltage
determines whether or not a signal is propagated. Rapid
chemical kinetic techniques with a 100-µsec time resolution,
and suitable for investigations of ligand-gated
ion channels on cell surfaces, are also now available.
They are expected to provide additional information about
ligand-gated ion channels and their mechanism of action.
The ability to determine the effect of neurotransmitter
concentration on the rate of transmembrane ion flux
and, therefore, the change in transmembrane voltage is
expected to provide important insight into how cells perceive,
store, and transmit information. It is also expected
to indicate howthe receptor mechanism is changed by diseases
of the nervous system and by the hundreds of drugs
that affect the mechanism of these proteins. This information
is expected to be essential in devising strategies for
curing mental diseases and overcoming drug addiction.