Mitochondrial Ultrastructural Alterations and Declined M(2) Receptor Density Were Involved in Cardiac Dysfunction in Rats after Long Term Treatment with Autoantibodies against M(2) Muscarinic Receptor

BACKGROUND: Previous studies showed that autoantibodies (M(2)-AA) against the second extracellular loop of M(2) muscarinic receptor (M(2)AChR-el2) from dilated cardiomyopathy (DCM) serum could induce DCM-like morphological changes in mice hearts. However, the effects of M(2)-AA on the cardiac function during the process of DCM and the potential mechanisms are not fully known. The present study was designed to dynamically observe the cardiac function, mitochondrial changes, and M(2) receptor binding characteristics in rats long-term stimulated with M(2)-AA in vivo. METHODS: M(2)-AA-positive model was established by actively immunizing healthy male Wistar rats with synthetic M(2)AChR-el2 peptide for 18 months. Meanwhile, vehicle group rats were administrated with physiological saline. The change of mitochondrial membrane potential (ΔΨm) was detected by radionuclide imaging. The ultrastructure of mitochondria was observed under electron microscopy. The M(2) receptor binding characteristics were determined by radioactive ligand binding assay. RESULTS: After immunization for 12 months, compared with vehicle group, M(2)AChR-el2-immunized rats showed decreased myocardial contractility and cardiac diastolic function evidenced by declined maximal rate of rise of ventricular pressure and increased left ventricular end-diastolic pressure, respectively. Additionally, mitochondrial swelling and vacuolation were observed. At 18 months, M(2)AChR-el2-immunized rats manifested significant decreased cardiac systolic and diastolic function and pathological changes such as enlargement of right ventricular cavity and wall thinning; and the mitochondrial damage was aggravated. Furthermore, the M(2) receptor maximum binding capacity (B(max)) of the M(2)AChR-el2-immunized rats significantly decreased, while the M(2) receptor dissociation constant (K(d)) was increased. CONCLUSIONS: Our study suggested that long-term stimulation with M(2)-AA leaded to the ventricular dilatation and gradual deterioration of cardiac dysfunction. Mitochondrial damage and the down-regulation of M(2) receptor density and affinity may be involved in the process.