Mlx. Fu et al., AGONIST-LIKE ACTIVITY OF ANTIPEPTIDE ANTIBODIES DIRECTED AGAINST AN AUTOIMMUNE EPITOPE ON THE HEART MUSCARINIC ACETYLCHOLINE-RECEPTOR, Receptors & channels, 2(2), 1994, pp. 121-130
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.