1. Channels, receptors & receptor operated channels (ROCs)

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1. Channels, receptors & receptor operated channels (ROCs)

In the element 3 lectures we shall consider how the activity ROCs and ion channels that are gated by voltage coordinate to excite first the NMJ, and subsequently the entire muscle fibre. To understand basic cell physiology, which covers ion transport, bioelectricity and the behaviour of excitable membranes, you need to understand the properties of several different types of membrane proteins. You should be comfortable with basic cell physiology including bioelectricity, the implications of the Nernst equation and Gibbs-Donnan theory. If these areas are not clear in your mind, I suggest that you revise them as you look at the material in this and subsequent sections.
This can be a difficult field to understand if you are not familiar with the meaning of some of terms used or do not appreciate that some are used interchangeably in your textbooks. To pass examinations you will need to refer correctly to the various structures, relationships and sequences in order to convince the examiner that you really do understand what it is you are trying to say.

NOTE: It is important that you realise that cell signalling, mediated by G protein-coupled receptors, including ion channel modulation, is covered in element 4 by Dr. Guy Rutter. The information presented here is only to make you aware of the distinction between ROCs and G-protein coupled receptors and between ROCs and receptor mediated modulation of ion channel function.

Important definitions

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Let’s define a few terms to start with:

ROC - (Receptor Operated Channel) a receptor protein which has an ion channel, as an integral part of its structure that is gated, when the normal ligand binds to the receptor.
Receptor - (in this context) a protein that binds a specific extracellular signalling molecule (see ligand) and initiates a response in the cell.
Ligand - a general term used to describe a molecule that binds with some specificity to a receptor protein (from Latin ligare, to bind). It is common to find that receptors draw their name(s) from the ligand(s), (eg. Acetylcholine, histamine, glutamate etc.). [see ROCs section below].
Ion channel - a transmembrane protein complex that forms a water-filled pore (or channel) through the membrane lipid bilayer through which inorganic ions can diffuse down their electrochemical gradients (see ion specificity).
Gated - a term used in ion channel literature to describe the transition from open to closed or closed to open. (Open and closed refers to the ion channel pore - when closed no ions flow). Ion channels can be gated by voltage, intracellular second messengers (eg. Ca²⁺, G proteins, cGMP ), mechanical stimuli and extracellular signalling molecules (see ROCs).
Ion specificity - Ion channels often display specificity (sometimes called selectivity). Some ion channels pass cations but not anions and others anions but not cations. Still others can discriminate between cations and are referred to as sodium, calcium or potassium channels to denote this.
Ion channel regulation - Ion channels that are not gated directly by an integral receptor (ie. Not a ROC) can have their behaviour altered as a result of receptor activation. This normally involves phosphorylation or dephosphorylation of the channel complex. This is modulation, and channels affected in this way should not be regarded as ROCs.

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Last modified: 9 Jun 1999 20:11
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