Cardiac fibrogenesis following infarction in mice with deletion of inducible nitric oxide synthase

Background: Studies have shown that the absence of inducible nitric oxide synthase (iONOS) improves cardiac function and survival after myocardial infarction (MI). The responsible mechanisms, however, remain uncertain. Cardiac iNOS is significantly increased after MI, which is colocalized with fibrous tissue formation. Herein, we tested our hypothesis that iNOS is involved in the development of cardiac fibrosis. Methods: Wild-type and iNOS-knockout mice were subjected to MI by left coronary artery ligation. At week 1, 2, 3, and 4 post-MI, we addressed cardiac expression of profibrogenic mediator, growth of collagen-producing cells, collagen synthesis, and degradation. Results: In the infarcted myocardium of wild-type and iNOS-knockout mice, transforming growth factor (TGF)-beta 1 expression was significantly increased, particularly in the early stage; myofibroblasts appeared and became abundant for over 4 weeks; matrix metalloproteinase-1 expression was low, whereas tissue inhibitor of matrix metalloproteinase-1 was significantly elevated; typed collagen mRNA was significantly increased and collagen was continuously accumulated. In the noninfarcted myocardium, TGF-beta 1 and typed collagen mRNA levels as well as collagen volume were also elevated, but less evident than infarcted myocardium. However, there was no significant difference in cardiac TGF-beta 1 expression, myofibroblast population, collagen synthesis/degradation, and collagen volume between wild-type and iNOS-knockout mice with MI. Conclusion: The current study suggests that iNOS-induced nitric oxide production may not mediate cardiac fibrosis after MI. Thus, other mechanisms are involved in nitrosative stress-induced cardiac dysfunction after MI.