Receptors With Intrinsic Ion Channel

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Chapter: Essential pharmacology : Pharmacodynamics Mechanism Of Drug Action; Receptor Pharmacology

These cell surface receptors, also called ligand gated ion channels, enclose ion selective channels (for Na+, K+, Ca2+ or Cl¯) within their molecules. Agonist binding opens the channel...



These cell surface receptors, also called ligand gated ion channels, enclose ion selective channels (for Na+, K+, Ca2+ or Cl¯) within their molecules. Agonist binding opens the channel (Fig. 4.4) and causes depolarization/hyperpolarization/ changes in cytosolic ionic composition, depending on the ion that flows through. The nicotinic cholinergic, GABAA, glycine (inhibitory), excitatory AA (kainate, NMDA or NmethylDaspartate, quisqualate) and 5HT3 receptors fall in this category.


The receptor is usually a pentameric protein; all subunits, in addition to large intra and extracellular segments, generally have four membrane spanning domains in each of which the AA chain traverses the width of the membrane six times. The subunits are thought to be arranged round the channel like a rosette and the α subunits usually bear the agonist binding sites.


Certain receptoroperated (or ligandgated) ion channels also have secondary ligands which bind to an allosteric site and modulate the gating of the channel by the primary ligand, e.g. the benzodiazepine receptor modulates GABAA gated Cl¯channel.


Thus, in these receptors, agonists directly operate ion channels, without the intervention of any coupling protein or second messenger. The onset and offset of responses through this class of receptors is the fastest (in milliseconds).


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