Shoichi Imai

Excessive NO produced by an inducible NO synthase (iNOS) has been implicated in many types of immune-associated disorders of the cardiovascular system, but it remains to be determined whether NO plays a role in myocarditis. Thus, the significance of iNOS expression in the development of experimental autoimmune myocarditis (EAM), an animal model of human giant cell myocarditis, was investigated. Lewis rats were immunized with cardiac myosin and were killed 7, 14, 21, 28, and 49 days after immunization. The development of severe myocarditis was observed on days 14, 21, and 28 in association with significant deterioration of hemodynamics determined by cardiac catheterization, which peaked on day 21. In parallel with histological severity of myocarditis and deterioration of cardiac performance, iNOS activity in the heart measured by [¹⁴C]l-citrulline formation was markedly increased on days 14, 21, and 28. The expression of iNOS was confirmed by immunoblotting and was localized to the infiltrating inflammatory cells found in the vicinity of necrotic myocytes by immunohistochemical analysis. Aminoguanidine, a selective inhibitor of iNOS, significantly decreased the iNOS activity (1.04±0.37 compared with 29.1±8.62 pmol·min⁻¹·mg protein⁻¹in untreated myosin-immunized rats,P<.01) and effectively attenuated histopathological changes of EAM on day 21. Hemodynamic parameters were also improved from 64±3 to 89±3 mm Hg for mean blood pressure, from 80±2 to 113±4 mm Hg for left ventricular systolic pressure, from 7.8±0.3 to 3.2±0.3 mm Hg for left ventricular end-diastolic pressure, from 2867±137 to 4180±102 mm Hg/s for +dP/dt, and from 2717±132 to 4180±184 mm Hg/s for −dP/dt (P<.01). The values after aminoguanidine treatment were not significantly different from the control values. These results suggest an important role for NO in mediating pathophysiological changes in myocarditis of autoimmune origin.