Effects of immunosuppressive therapy on expression of inducible nitric oxide synthase (iNOS) during cardiac allograft rejection.

Recent studies have indicated a role for nitric oxide (NO) in alloimmune responses and in allograft rejection. iNOS mRNA, protein and enzyme activity are induced in myocardium during cardiac allograft rejection. NO produced by iNOS is negatively inotropic and has the potential to be cytotoxic to cardiac myocytes. To investigate whether immunosuppressive agents would alter the expression of iNOS during cardiac allograft rejection, hearts from Wistar-Furth rats were transplanted into the abdomen of Lewis recipients. At day 5 allografts from treated and untreated animals were removed for pathological and biochemical examination. At day 5 the untreated allografts exhibited histological evidence of marked rejection (edema, infiltration with macrophages and lymphocytes, necrosis of cardiac muscle fibers). Abundant iNOS mRNA was apparent in Northern blots and iNOS enzyme activity was increased in ventricular homogenates and in cardiac myocytes purified from the untreated rejecting allografts. Incubation of isolated purified cardiac myocytes from normal rats for 24 h with cytokines known to be present during allograft rejection (IL-1 beta, TNF-alpha and IFN-gamma) was also associated with increased iNOS mRNA and enzyme activity. When Wistar-Furth to Lewis allografts were treated from time of transplantation with FK 506, cyclosporine A, dexamethasone or a combination of all three drugs, histological evidence of rejection and the levels of iNOS mRNA and enzyme activity in ventricular homogenates were reduced significantly below those observed in the untreated allografts. The data in a rat model indicate that immunosuppressive drugs reduce myocardial iNOS mRNA and enzyme activity in rejecting cardiac allografts. The results are consistent with the hypothesis that the alloimmune response and cytokine release are involved in the expression of iNOS during cardiac transplantation rejection.