The Role of Nitric Oxide Production during Cardiac Allograft Rejection

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The role of nitric oxide (NO) production in acute allograft rejection is not known. In normal human vasculature, nitric oxide synthase is constitutively expressed within endothelial cells (cNOS), and small quantities of NO produced by this pathway mediate vasorelaxation via a cyclic GMP-dependent mechanism. During cardiac allograft rejection, NO may also be produced by the inducible isoform of nitric oxide synthase (iNOS), present in endothelial cells, smooth muscle cells and cardiac myocytes. Unlike cNOS activity, which is an integral part of cardiovascular homeostasis, iNOS activity is a component of the inflammatory response and may have both beneficial and deleterious effects during rejection. Early reports suggested that acute cardiac allograft rejection was associated with an increase in nitric oxide production. The general aim of this study was to document the way in which nitric oxide production is affected by allograft rejection, and to determine the significance of these changes. Enhanced nitric oxide synthesis may have a graft protective effect by inhibiting clonal expansion of immune cells targeting alloantigen. However, immunosuppressive agents have been shown to block NO production, raising the possibility that NO may modulate graft function and/or survival. The following hypotheses were tested; that acute cardiac allograft rejection is associated with a rise in NO production and an increase in iNOS expression, that conventional immunosuppressive agents reduce NO production by limiting iNOS expression, that an increase in substrate availability for nitric oxide synthase would increase NO production and decrease graft survival, and that L-NMMA, a competitive inhibitor of both constitutive and inducible isoforms would prolong graft survival.