Binding of agonists and antagonists to muscarinic receptors

Abstract

The binding of one irreversible and two reversible radioactive antagonists to muscarinic receptors in synaptosome preparations of rat cerebral cortex has been studied. The ligands all bind to the same receptor pool and directly and competitively yield self-consistent binding constants closely similar to those obtained by pharmacological methods on intact smooth muscle. The binding process for antagonists seems to be a simple mass action-determined process with a Hill slope of 1.0. The quantitative correlations strongly support the view that the receptor studied by ligand binding corresponds to the receptor studied by pharmacological methods. Inhibition of antagonist binding by most agonists shows a reduced Hill slope which also applies to direct binding studies of [3H] acetylcholine. Mechansims that might account for the behavior of agonists are discussed but do not conclusively point to any single mechanism.

Muscarinic receptor subtypes coupled to generation of different second messengers in isolated tracheal smooth muscle cells

“…As an alternative approach to define receptor subtypes, the determination of antagonist affinities can provide more accurate information than the use of agonist relative potencies (Birdsall et al, 1978). From the experiments, the effects of discriminating antagonists have been analysed at the level of functional and biochemical responses.…”

The effect of p‐chloromercuribenzoate on structure‐binding relationships of muscarinic receptors in the rat cerebral cortex

“…The preparation of a crude synaptosomal fraction (P2) from rat cerebral cortex is described elsewhere (Hulme, Birdsall, Burgen & Mehta, 1978), as is the general protocol for the centrifugation binding assay (Hulme et al, 1978;Birdsall et al, 1983) and the non-linear least squares analysis of the binding curves (Hulme et al, 1978;Birdsall, Burgen & Hulme, 1978 20 mM HEPES-Na (pH 7.0) and incubations were all carried out at 30°C on freshly prepared membranes. When necessary, acetylcholinesterase was inhibited by neostigmine (10-M) added 5 min before the binding assay.…”

“…It is clear, therefore, that the three agonist binding curves are affected in different ways, although there is a general tendency for the Hill coefficients for the binding curves to increase. The effects on the affinities of high and low affinity agonist binding sites (H and L sites;Birdsall et al, 1978) are presented in Table 1. The data could not be satisfactorily fit if it was postulated that PCMB induced an interconversion between H and L sites but the model in which only changes in affinity of the H and L sites had occurred did provide excellent fits (Figure 1).…”

The Interaction of Amine Local Anaesthetics With Muscarinic Receptors

“…One point which may be noted is that none of the amines give Hill coefficients significantly < 1 and thus show no evidence of any selectivity between the two states of the muscarinic receptor which are apparently distinguished by the binding of muscarinic agonists (Birdsall & Hulme, 1976;Ward & Young, 1977;Birdsall et al, 1978).…”