Inhibition of Nitric Oxide Synthesis Causes Myocardial Ischemia in Endotoxemic Rats

Inhibitors of nitric oxide (NO) synthesis have been used in the treatment of septic and endotoxic shock. However, several studies question the beneficial effect of inhibiting NO production in sepsis and endotoxemia. We have investigated the effect of inhibition of NO synthesis after endotoxemia in the isolated perfused rat heart. In hearts from endotoxintreated animals, coronary flow was elevated 64% and oxygen consumption was elevated 20% compared with control hearts. NADH fluorescence imaging was used as an indicator of regional hypoperfusion. A homogeneous low-surface NADH fluorescence, indicative of adequate tissue perfusion, was observed in both control and endotoxin-treated hearts. The increase in coronary flow and oxygen consumption could only partially be prevented by pretreatment of the animals with dexamethasone. Addition of N-nitro-L-arginine (NNLA), an inhibitor of NO synthesis, to the perfusion medium eliminated differences in coronary flow and oxygen consumption between normal and endotoxin-treated hearts. However, NADH surface fluorescence images of endotoxin-treated hearts after NNLA revealed areas of high fluorescence, indicating local ischemia, whereas the control hearts remained without signs of ischemia. The ischemic areas were present at various perfusion pressures and disappeared after the infusion of L-arginine, the natural precursor of NO, or the exogenous NO donor sodium nitroprusside. Methylene blue (MB), an inhibitor of soluble guanylate cyclase, the effector enzyme of NO, also eliminated differences in coronary flow and produced similar areas of local myocardial ischemia in endotoxin-treated hearts but not in control hearts. Reducing coronary flow by direct vasoconstriction with vasopressin resulted in similar patterns of myocardial ischemia as with NNLA and MB. Our results suggest that the coronary vasodilation in the isolated rat heart after endotoxemia is caused by an increased release of NO. Coronary flow reduction with NNLA, MB, or direct vasoconstriction with vasopressin causes local areas of myocardial ischemia in endotoxin-treated hearts but not in untreated hearts. These data suggest that endotoxemia promotes myocardial ischemia in vulnerable areas of the heart after inhibition of the NO pathway or direct vasoconstriction.(Circ Res. 1995;76:418-425.)