The properties of cholinergic muscarinic receptor sites from bovine atria reconstituted after solubilization

Abstract

We have previously shown that the digitonin-cholate solubilization of the muscarinic receptor from bovine atrial membranes disrupts the heterogeneity of carbamylcholine binding sites and their modulation by guanine nucleotides. Polyethylene glycol precipitation of the detergent extract restores these properties, while causing the formation of vesicular structures containing the receptor. Here we have studied the kinetic and pharmacological properties of [(3)H]N-methyl-scopolamine ([(3)H]NMS) binding to this reconstituted receptor. This binding is linear with the amount of protein, saturable and reversible. The apparent dissociation (K?(d)) constant determined by saturation experiments at equilibrium is 0.8 nM. By kinetic experiments an association rate constant (k(on)) of 0.28 min(?1) x nM(?1) and a dissociation rate constant (k(off)) of 0.17 min(?1) were obtained, whose ratio k(off)/k(on) = 0.62 nM is in agreement with k?(d). The potency of a range of muscarinic ligands in displacing [(3)H]NMS is atropine > scopolamine > methylatropine > oxotremorine > pilocarpine > carbamylcholine > b?thanechol. The Hill coefficients for the antagonists and partial agonists are near to 1 indicating a single class of binding sites for these ligands. In contrast, the Hill coefficients for full agonists are smaller than 1, suggesting heterogeneity of sites. Finally, it was observed that the binding of the agonists but not of the antagonists is modulated by guanine nucleotides. These results show that the binding properties of the native and the reconstituted receptors are very similar.