Identification of drugs competing with d-tubocurarine for an allosteric site on cardiac muscarinic receptors
Waelbroeck, M.
Molecular Pharmacology 46(4): 685-692
1994
ISSN/ISBN: 0026-895X PMID: 7969047 Document Number: 428867
d-Tubocurarine behaved as a weak allosteric inhibitor of N-(3H) methylscopolamine binding to cardiac M-2 muscarinic receptors. In a low ionic strength buffer devoid of bivalent ions, d-tubocurarine recognized cardiac M-2 receptors in the micromolar concentration range and decreased their affinity for N-(3H)methylscopolamine by at most 4-fold. To identify the compounds that preferentially recognize this accessory site (as opposed to the classical muscarinic binding site), we measured the inhibition by different drugs of N-(3H)methylscopolamine binding, in the absence or presence of d-tubocurarine. The effect of gallamine was competitively inhibited by d-tubocurarine; both drugs compete for the same accessory site on muscarinic receptors. The effects of dexetimide, levetimide, 4-diphenylacetoxy-N-ethylpiperidine ethobromide, AF-DX 116, and telenzepine on N-(3H) methylscopolamine binding were not affected or were barely affected by d-tubocurarine; these compounds preferentially recognize another binding site (probably the muscarinic binding site). The dose-effect curves for pentamethylene-bis(4-diphenylacetoxymethylpiperidine) bromide and methoctramine were shifted, but at most 1 0-fold, by d-tubocurarine. It is likely that (in this low ionic strength incubation buffer) methoctramine and pentamethylene-bis(4-diphenylacetoxymethylpiperidine) bromide had comparable affinities for the muscarinic site and the accessory site. d-Tubocurarine competitively inhibited their binding to the accessory site and allosterically inhibited their binding to the muscarinic site. This resulted in a large decrease (40-60-fold) of their overall affinity for muscarinic receptors.