Medical Physiology, 3rd Edition

Receptors That are Ion Channels

Ligand-gated ion channels transduce a chemical signal into an electrical signal

The property that defines the ligand-gated ion channel class of multisubunit membrane-spanning receptors is that the signaling molecule itself controls the opening and closing of an ion channel by binding to a site on the receptor. Thus, these receptors are also called ionotropic receptors to distinguish them from the metabotropic receptors, which act via “metabolic” pathways. One superfamily of ligand-gated channels includes the ionotropic receptors for ACh, serotonin, gamma-aminobutyric acid (GABA), and glycine. Most structural and functional information for ionotropic receptors comes from the nicotinic ACh receptor (AChR) present in skeletal muscle (see Fig. 8-7). The nicotinic AChR is a cation channel that consists of four membrane-spanning subunits, α, β, γ or ε, and δ, in a stoichiometry of 2 : 1 : 1 : 1. This receptor is called nicotinic because the nicotine contained in tobacco can activate or open the channel and thereby alter Vm. Note that the nicotinic AChR is very different from the muscarinic AChR discussed below, which is not a ligand-gated channel. Additional examples of ligand-gated channels are the inositol 1,4,5-trisphosphate (IP3) receptor and the Ca2+-release channel (also known as the ryanodine receptor). Both receptors are tetrameric Ca2+ channels located in the membranes of intracellular organelles.