Human chagasic IgGs bind to cardiac muscarinic receptors and impair L-type Ca²⁺ currents

Objectives: Antibodies against cardiac G protein-coupled receptors have been reported in sera from chronic chagasic patients (CChP) and other non-parasitic cardiomyopathies, but the effects and underlying mechanism of interaction between these antibodies and heart cells are not fully established. To address this point, binding of antibodies purified from sera of CChP patients and normal blood donors (NBD) to cardiac muscarinic acetylcholine receptors (mAChR) and their effect on L-type Ca²⁺ currents were examined. Methods and Results: Saturation [³H]NMS binding experiments with porcine atrial membranes showed that B_max in the presence of CChP-immunoglobulin G (IgG) decreased from 280.2±16.08 fmol/mg (control) to 91.00±5.98 fmol/mg, with no apparent change in K_D, while NBD-IgG did not significantly alter these parameters. At the single channel level, CChP-IgG decreased both the fast and slow mean open times and P_o (from 0.074±0.023 to 0.025±0.007) without changes in single channel conductance. I/V plots of isoproterenol-stimulated whole-cell L-type Ca²⁺ currents (I_Ca) from rabbit ventricular cardiomyocytes showed a significant reduction in peak I_Ca during perfusion with CChP-IgG (at 0 mV: from 10.61±2.97 to 8.45±2.54 pA/pF). NBD-IgGs had no effect on I_Ca. A CChP-IgG purified against a peptide corresponding to the second extracellular loop of the M₂ receptor also impaired L-type Ca²⁺ currents. All effects of CChP-IgG were blocked by atropine. Conclusions: Our results show that antibodies from CChP bind to mAChR in a non-competitive manner and are able to activate the receptor in an agonist-like form resulting in L-type Ca²⁺ current inhibition.