Affinity-purified autoantibodies and anti-peptide antibodies directed against the second extracellular loop of the beta 1-adrenoceptor increase the beating rate of cultured cardiomyocytes just like the beta-adrenergic agonist isoprenaline. Their positive chronotropic action is blocked by beta-adrenergic antagonists. Affinity-purified autoantibodies and anti-peptide antibodies directed against the muscarinic cholinergic M2 receptor exert in these myocytes, like the muscarinic cholinergic agent carbachol, a negative chronotropic effect that is antagonized by atropine. In contrast to the agonism of isoprenaline and carbachol, the described agonistic effects of the antibodies are not subject to desensitization.
