AGONIST-LIKE ACTIVITY OF ANTIPEPTIDE ANTIBODIES DIRECTED AGAINST AN AUTOIMMUNE EPITOPE ON THE HEART MUSCARINIC ACETYLCHOLINE-RECEPTOR

A synthetic peptide corresponding to amino acids 169-193 of the second extracellular loop of the M2 human muscarinic receptor was used to ra ise antibodies in rabbits. Affinity purified antibodies specifically r ecognized a major band with a molecular weight of about 80 KDa on the electrotransferred membrane proteins of both rat ventricles and chines e hamster ovary cells expressing recombinant muscarinic receptors. Inc ubation of these antibodies with rat myocardial membranes resulted not only in a decrease in the maximal binding capacity, but also in a dec rease in receptor antagonist affinity. These antibodies could also mim ic the effects of agonist stimulation as demonstrated by inhibition of isoproterenol-stimulated cAMP accumulation and by a negative chronotr opic effect on cultured cardiomyocytes. These results suggest that the second extracellular loop of the M2 muscarinic receptor is an immunol ogically and functionally important domain with properties comparable to those found for autoantibodies against the same domain in idiopathi c dilated cardiomyopathy. It strengthens the hypothesis that the secon d extracellular loop of the members of the superfamily of G-protein co upled membrane receptors could be the main immunogenic region responsi ble for a pathogenic autoimmune response.