NITRIC-OXIDE CARBON-MONOXIDE - A MOLECULAR SWITCH FOR MYOCARDIAL PRESERVATION DURING ISCHEMIA

Background In heart, NO is produced from L-arginine catalyzed by NO sy nthase, and CO is formed during the conversion of bilirubin from heme by the action of heme oxygenase. NO, which exerts its biological actio ns through cGMP and heme, has recently been implicated in myocardial p rotection during ischemia and reperfusion. We hypothesized that the in tracellular signaling by NO may be modulated by heme oxygenase. Method s and Results To test this hypothesis, isolated rat hearts were perfus ed for 10 minutes with one of the following: (1) buffer alone; (2) 3 m mol/L L-arginine, a precursor for NO; (3) 650 mu mol/L zinc protoporph yrin, a heme oxygenase inhibitor; (4) 3 mmol/L L-arginine plus 650 mu mol/L zinc protoporphyrin; (5) 15 mu mol/L methylene blue, a cGMP inhi bitor; or (6) 3 mmol/L L-arginine plus 15 mu mol/L methylene blue. Hea rts were then made ischemic for 30 minutes, followed by 30 minutes of reperfusion. L-Arginine afforded significant myocardial protection, as evidenced by increased developed pressure (DP) (53.3 +/- 4.3 versus 3 5.2 +/- 1.8 for control), dP/dt(max) (2405 +/- 125 versus 1758 +/- 117 for control), aortic flow (23.0 +/- 1.5 versus 9.4 +/- 1.6 for contro l), and coronary flow (CF) (23.0 +/- 0.8 versus 19.0 +/- 1.6 for contr ol) at the end of reperfusion. Protoporphyrin tended to reduce these v alues compared with L-arginine alone (DP, 27.5 +/- 1.4; dP/dt(max), 14 00 +/- 78; CF, 17 +/- 0.5), suggesting a contribution of heme oxygenas e in addition to NO for myocardial preservation. Increased mRNAs for t he heme oxygenase were noticed in the ischemic reperfused myocardium. Contents of cGMP, the second messenger for NO signaling, increased in the L-arginine group (1.6 +/- 0.1 versus 1.1 +/- 0.1 for control) and were reduced by protoporphyrin. cGMP was completely inhibited by methy lene blue, which also retarded postischemic myocardial functional reco very. Malonaldehyde formation, a presumptive marker for free radical g eneration, was decreased in the L-arginine group (0.053 +/- 0.003) com pared with control (0.089 +/- 0.005) but was increased in the protopor phyrin group (0.09 +/- 0.003) compared with the L-arginine group. In v itro studies demonstrated that NO was able to reduce the reactive oxyg en species produced by myoglobin, especially oxoferrylmyoglobin, which either are present in heart or are formed in high concentrations duri ng the reperfusion of ischemic myocardium. Conclusions The results sug gest that NO contributes to myocardial preservation by both cGMP-depen dent and cGMP-independent mechanisms, the former being modulated by CO signaling and the latter by virtue of its antioxidant action.