The modes of binding of ligands to cardiac muscarinic receptors.

Ionizable groups on the cardiac M2 muscarinic receptor which regulate the binding of ligands have been examined by studying the pH dependence of the ligand affinity constants. The presence of three titratable residues (approximate pK values, 5.4, 6.8 and 7.5) whose protonation modulates antagonist binding has been demonstrated. Cardioselective antagonists are selectively affected by the protonation state of the pK 6.8 residue, whereas the binding of antagonists having differing selectivities is more strongly affected by protonation of the pK 5.4 residue on cardiac receptors. Methoctramine is capable of binding to both the pK 5.4 and 6.8 residues simultaneously. Protonation of the residue of highest pK produces a conformational change at the receptor which can affect both agonist and antagonist binding. It is now possible to demonstrate differences both in the way ligands bind to a given receptor subtype and in the way a given ligand binds to different subtypes.