CONCLUSION AND OUTLOOK (Ion Transport Across Biological Membranes)

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-/sec 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 neurotransmit-ter 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 how the 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.

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