STUDIES OF HYPOXEMIC REOXYGENATION INJURY - WITHOUT AORTIC CLAMPING .5. ROLE OF THE L-ARGININE-NITRIC OXIDE PATHWAY - THE NITRIC-OXIDE PARADOX/

This study tests the hypothesis that nitric oxide, which is endothelia l-derived relaxing factor, produces reoxygenation injury via the L-arg inine-nitric oxide pathway in hypoxemic immature hearts when they are placed on cardiopulmonary bypass. Twenty 3-week-old piglets undergoing 2 hours of hypoxemia (oxygen tension about 25 mm Hg) on a ventilator were reoxygenated by initiating cardiopulmonary bypass (oxygen tension about 400 mm Hg). Five animals were not treated, whereas the pump cir cuit was primed with the nitric oxide-synthase inhibitor N-G-nitro-L-a rginine methyl ester (L-NAME, 4 mg/kg) in five piglets. L-Arginine, th e substrate for nitric oxide, was administered in a fivefold excess (2 0 mg/kg), together with L-NAME in five piglets (L-NAME and L-arginine) , and given alone in five other piglets (L-arginine). Five normoxemic, instrumented piglets served as a control group, and five others under went 30 minutes of cardiopulmonary bypass without preceding hypoxemia. Left ventricular contractility was determined as end-systolic elastan ce by pressure-dimension loops. Myocardial conjugated dienes were meas ured as a marker of lipid peroxidation, and the antioxidant reserve ca pacity (malondialdehyde production in tissue incubated with t-butylhyd roperoxide) was measured. Nitric oxide level was determined in coronar y sinus plasma as its spontaneous oxidation product, nitrite. Cardiopu lmonary bypass per se did not alter left ventricular contractility, ca use lipid peroxidation, or lower antioxidant capacity. Reoxygenation w ithout treatment depressed cardiac contractility (end-systolic elastan ce 38% +/- 12% of control), raised nitric oxide (127% above hypoxemic values), increased conjugated dienes (1.3 +/- 0.2 vs 0.7 +/- 0.1, con trol), and reduced antioxidant reserve capacity (910 +/- 59 vs 471 +/ - 30, control). Inhibition of nitric oxide production by L-NAME impro ved end-systolic elastance to 84% +/- 12%,* limited conjugated diene elution (0.8 +/- 0.1 vs 1.3 +/- 0.2, no treatment*), and improved ant ioxidant reserve capacity (679 +/- 69 vs 910 +/- 59, no treatment*). Conversely, L-arginine counteracted these beneficial effects of L-NAME , because left ventricular function recovered only 24% +/- 6%, conjug ated dienes were 1.2 +/- 0.1, and antioxidant reserve capacity was 82 6 +/- 70. L-Arginine alone caused the same deleterious biochemical ch anges as L-NAME/L-arginine and resulted in 60% mortality. The close re lationship between postbypass left ventricular dysfunction (percent en d-systolic elastance) and myocardial conjugated diene production (r = 0.752) provides in vivo evidence that lipid peroxidation contributes t o myocardial dysfunction after reoxygenation. These results suggest th e L-arginine-nitric oxide pathway, which is normally of physiologic be nefit by producing endothelial-derived relaxing factor, is involved in the pathogenesis of myocardial reoxygenation injury (p < 0.05 vs con trol; *p < 0.05 vs no treatment, analysis of variance).