CARDIAC PRESERVATION IS ENHANCED IN A HETEROTOPIC RAT TRANSPLANT MODEL BY SUPPLEMENTING THE NITRIC-OXIDE PATHWAY

Nitric oxide (NO) is a novel biologic messenger with diverse effects b ut its role in organ transplantation remains poorly understood. Using a porphyrinic microsensor, the first direct measurements of coronary v ascular and endocardial NO production were made. NO was measured direc tly in the effluent of preserved, heterotopically transplanted rat hea rts stimulated with L-arginine and bradykinin; NO concentrations fell from 2.1 +/- 0.4 mu M for freshly explanted hearts to 0.7 +/- 0.2 and 0.2 +/- 0.08 mu M for hearts preserved for 19 and 38 h, respectively. NO levels were increased by SOD, suggesting a role for superoxide-medi ated destruction of NO. Consistent with these data, addition of the NO donor nitroglycerin (NTG) to a balanced salt preservation solution en hanced graft survival in a time- and dose-dependent manner, with 92% o f hearts supplemented with NTG surviving 12 h of preservation versus o nly 17% in its absence. NTG similarly enhanced preservation of hearts stored in University of Wisconsin solution, the clinical standard for preservation. Other stimulators of the NO pathway, including nitroprus side, L-arginine, or 8-bromoguanosine 3',5'monophosphate, also enhance d graft survival, whereas the competitive NO synthase antagonist N-G-m onomethyl-L-arginine was associated with poor preservation. Likely mec hanisms whereby supplementation of the NO pathway enhanced preservatio n included increased blood flow to the reperfused graft and decreased graft leukostasis. NO was also measured in endothelial cells subjected to hypoxia/reoxygenation and detected based on its ability to inhibit thrombin-mediated platelet aggregation and serotonin release. NO beca me undetectable in endothelial cells exposed to hypoxia followed by re oxygenation and was restored to normoxic levels on addition of SOD. Th ese studies suggest that the NO pathway fails during preservation/tran splantation because of formation of oxygen free radicals during reperf usion, which quench available NO. Augmentation of NO/cGMP-dependent me chanisms enhances vascular function after ischemia and reperfusion and provides a new strategy for transplantation of vascular organs.