Franken_2000_Naunyn.Schmiedebergs.Arch.Pharmacol_361_107

Gallamine, alcuronium and W84 (hexane-1,6-bis[dimethyl-3'-phthalimidopropyl-ammonium bromide]) are prototype allosteric modulators of the G-protein coupled muscarinic acetylcholine receptor family, especially of the M2-subtype. In order to probe the specificity of muscarinic allosteric modulation, we checked whether these agents interact with histamine H1-receptors which have a high homology with muscarinic receptors. Binding experiments (38 mM Na2HPO4, 12 mM KH2PO4, pH 7.5) were performed with the H1-receptor antagonist [3H]mepyramine ([3H]MEP) in guinea pig cerebellar homogenates. For the sake of comparison, binding of [3H]N-methylscopolamine ([3H]NMS) at muscarinic M2-receptors was measured in porcine cardiac homogenates under identical conditions. The modulators retarded [3H]NMS dissociation (t1/2 control=1.3 min) concentration-dependently indicating their allosteric action with half-maximum effects for gallamine at EC50,discs=27 microM, for alcuronium at EC50,diss=53 nM, and for W84 at EC50,diss=170 nM. In contrast, [3H]MEP dissociation from H1-receptors (t1/2,control=2.6 min) remained unchanged up to concentrations of 1 mM of the modulators. Equilibrium binding of [3H]NMS (KD=0.46 nM, Bmax=98 fmol/mg protein) was inhibited by gallamine, elevated by alcuronium and left almost unchanged by W84, indicating negative, positive and nearly neutral cooperativity, respectively, with the radioligand. The ternary complex model of allosteric actions yielded the equilibrium dissociation constants K(A) for the binding of the allosteric modulators to free M2-receptors: K(A,gallamine)=100 nM, K(A,alcuronium)=450 nM, K(A,W84)=69 nM. In H1-receptors, more than 1,000-fold higher concentrations than in M2-receptors were required to elicit an effect on the binding of [3H]MEP (KD=1.2 nM, Bmax=205 fmol/mg protein). Half-maximal reduction was observed at 10 mM for gallamine, 1 mM for alcuronium and 92 microM for W84. In conclusion, the muscarinic modulators have little effect on the histamine H1-receptors.