Receptor ion channel

HT3 receptors belong to the ligand-gated ion channel receptor superfamily, similar to the nicotinic acetylcholine or GABAa receptors and share elec-trophysiological and structural patterns. The receptors... 

The ion channel receptors are relatively simple in functional terms because the primary response to receptor activation is generated by the ion channel which is an integral part of the protein. Therefore, no accessory proteins are needed to observe the response to nicotinic AChR activation and the full functioning of the receptor can be observed by isolating and purifying the protein biochemically and reconstituting the protein in an artificial lipid membrane. In contrast, the G-protein-coupled receptors require both G-proteins and those elements such as phospholipase-C illustrated in Fig. 3.1, in order to observe the response to receptor activation (in this case a rise in intracellular calcium concentration resulting from the action of IP3 on intracellular calcium stores). 

Transmitter Ion channel receptors G-protein-coupled receptors... 

Except for glycine, all fast transmitters act on both ion channel receptors and G-protein-coupled receptors. Within each receptor class, there may be several subtypes. 

The ion channel receptors are multi-subunit proteins which may be either homomeric (made up of multiple copies of a single type of subunit) or heteromeric (composed of more than one subunit type). These subunits come together after synthesis in the endoplasmic reticulum to form the mature receptor. Notice that stoichiometry is denoted by a subscript number. A receptor composed of two a and three /I subunits is therefore denoted as having a stoichiometry of This can cause confusion when related subunits are given sequential numbers /II, j]2, 3, etc. The convention is therefore that subunits are numbered normally while stoichiometry is indicated by subscripts so that a pentamer of a4 and j33 subunits might have a stoichiometry of a42/133. 

KINETICS AND MECHANISMS OF AGONIST ACTION (a) Ion channel receptors... 

This chapter deals mainly with information that can be obtained from equilibrium, or at least steady-state, recordings of ion-channel receptor activity. However, a great deal of information has also been obtained from kinetic studies of ion channels where the aim has been to determine values for the rate constants in a receptor mechanism. In general, only equilibrium constants can be determined from equilibrium studies. 

The simplest agonist mechanism that can be used to describe activation of the ligand-gated ion-channel receptors is that first suggested by del Castillo and Katz (1957) for activation of nAChRs at the neuromuscular junction ... 

Changes in the occupancy of the open-channel state of the receptor as a function of time (pA2R (t)) in response to a perturbation of the receptor equilibrium can be used to obtain information about the rates of channel gating and the interaction of dmgs with ion-channel receptors. The system is said to relax towards a new equilibrium. The time course of the relaxation is used to measure rates from the average behavior of many ion channels in a recording, while noise analysis uses the frequency of the moment-to-moment fluctuations in occupancy of the open-channel state at equilibrium to provide information about the rates in the receptor mechanism. 

A simple mechanism for competitive antagonism of a ligand-gated ion-channel receptor would be as follows ... 

Figure 2.5. The synapse. GPCR = guanine nucleotide-binding protein-coupled receptor, LGICR = ligand-gated ion channel receptor, SB = synaptic bouton, T = neurotransmitter, YOC = voltage-operated ion channel protein, YOCC = voltage-operated calcium channel protein, Ast = astrocyte, AA = axoaxonal synapse, ASD = axosomatic or axodendritic synapse. GPCR 1 = receptor protein, 2 = G-protein, 3 = enzyme, 4 = ion channel protein.

Muscle cell membranes of nematodes possess ion channel receptors that are opened by neurotransmitters and which are gated by selective therapeutic agents. This chapter is an introduction to the physiology and pharmacology of ligand-gated ion channels of nematode muscle. 

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