Background: Excess amounts of NO produced by an inducible NO synthase (iNOS) in response to cytokines may be cytotoxic and can be destructive to tissue. We investigated the role of NO in the development of myocardial damage and the effects of aminoguanidine (AG), an inhibitor of iNOS, on experimental autoimmune myocarditis in rats.
Methods and results: Autoimmune myocarditis was induced in 20 Lewis rats by injection of porcine cardiac myosin. Ten of the 20 rats were administered AG. The severity of myocarditis was evaluated by measuring the size of myocarditic lesion and serum levels of CK-MB. Serum NO levels were determined using the Cd/Cu method. Tissue specimens were immunohistochemically examined for iNOS and nitrotyrosine. Histopathological study revealed extensive myocardial destruction and massive inflammatory cell infiltration in AG-untreated rats but only focal mononuclear cell infiltration in AG-treated rats. The mean percent areas of inflammatory lesions in the untreated and treated rats were 56 +/- 13% and 3 +/- 2%, respectively (P < .001). NO levels were 102 +/- 23 and 25 +/- 9 IU/L, respectively (P < .01). CK-MB levels were 68 +/- 13 and 16 +/- 13 nmol/L, respectively (P < .01). Superoxide production as measured with an ex vivo monitoring system was also significantly decreased in the treated rats. Nitrotyrosine relating to the generation of peroxynitrite was detected through immunostaining in the inflammatory lesions of untreated rats but not in those of treated rats.
Conclusions: Excess amounts of NO produced by iNOS appear to contribute to the progression of myocardial damage in myocarditis. AG may prove to be useful in the treatment of myocarditis.