LOCALIZATION OF A FUNCTIONAL AUTOIMMUNE EPITOPE ON THE MUSCARINIC ACETYLCHOLINE RECEPTOR-2 IN PATIENTS WITH IDIOPATHIC DILATED CARDIOMYOPATHY

A peptide corresponding to the sequence 169-193 of the second extracel lular loop of the human muscarinic acetylcholine receptor-2 was used a s an antigen to screen sera from patients with idiopathic dilated card iomyopathy (DCM, n = 36) and healthy blood donors (HBD, n = 40). The s era from 14 patients with DCM (38.8%) and 3 HBD (7.5%) recognized the muscarinic receptor peptide at dilutions varying from 1:20 to 1:160 in ELISA. A highly significant correlation (P = 0.006) was found between the presence of antimuscarinic receptor-2 autoantibodies and anti-bet a-adrenoceptor-1 autoantibodies in the patients' sera. Affinity-purifi ed autoantibodies from positive sera of patients with DCM recognized o n the electrotransferred protein of rat ventricular membrane a major b and of about 80 kD. Incubation of autoantibodies with membrane resulte d not only in a decrease in the maximal binding sites (B(max)) but als o in an increase in K(d) of radioligand binding in a concentration-dep endent manner. This suggests a mixed-type of inhibition. Moreover, pre incubation with atropine abolished the inhibitory effect of autoantibo dies on the receptor binding whereas carbachol appeared to have no eff ect on the activity of the autoantibodies. These data define a subgrou p of patients with idiopathic DCM who have in their sera functionally active autoantibodies against muscarinic receptor-2.