Reconstitution of solubilized atrial cholinergic muscarinic receptors in liposomes

The reconstitution of solubilized bovine atrial cholinergic muscarinic receptor into liposomes made of exogenous lipids has been achieved by polyethyleneglycol precipitation. Of the different lipid mixtures used, soybean lecithins were shown to be the best on the basis of receptor recovery. The receptor reconsituted into soybean lecithins liposomes exhibited ligand binding properties very similar to those of the native receptor. The dissociation constant of ³H]-N-methyl-scopolamine (³H]NMS) was 0.46 and 0.30 nM as determined by equilibrium and kinetics experiments respectively. The potency of a range of muscarinic ligands in displacing ³H]NMS binding was atropine > methyl-atropine > scopolamine > pirenzepine oxotremorine > gallamine > carbamylcholine > pilocarpine bethanechol. The Hill slopes of the displacement curves were near 1 for the antagonists and smaller than 1 for the agonists and for gallamine. The agonist binding may be modulated by guanine nucleotides. These results indicate that soybean lecithins fulfill the lipid requirements for the reconstitution of the atrial muscarinic receptor.