Critical timing of L-arginine treatment in post-ischemic myocardial apoptosis-role of NOS isoforms.

BACKGROUND: The role of nitric oxide (NO) in apoptotic cell death has been extensively studied in recent years. However, reported results are inconsistent and often controversial, and the mechanisms underlying its diverse effects in apoptosis regulation remain unidentified. The present study attempted to determine whether in vivo administration of L-arginine, the substrate for NOS, at different time points during the course of myocardial ischemia and reperfusion may differentially regulate post-ischemic myocardial apoptosis, and if so, to investigate the mechanisms involved. METHODS AND
RESULTS: Male adult rats were subjected to 30 min of myocardial ischemia followed by 5 h of reperfusion. L-Arginine was administered as a bolus at either 10 min before (early treatment) or 3 h after reperfusion (late treatment). There was no difference in myocardial eNOS expression between any groups studied. Myocardial iNOS expression was detected at 3 h after reperfusion but not at 1 h after reperfusion. Administration of L-arginine 10 min before reperfusion markedly decreased TUNEL-positive staining cardiomyocytes, reduced myocardial caspase-3 activity, inhibited iNOS expression, and reduced myocardial nitrotyrosine content. In strict contrast, administration of L-arginine 3 h after reperfusion, a time point when iNOS was expressed, resulted in a significant increase in myocardial NO(x) content, myocardial injury, and toxic peroxynitrite formation as measured by nitrotyrosine.
CONCLUSION: Our results demonstrated for the first time that L-arginine administered at different time points during ischemia/reperfusion exerted different effects on post-ischemic myocardial injury, and suggests that stimulation of eNOS reduces nitrative stress and decreases apoptosis whereas stimulation of iNOS increases nitrative stress and enhances myocardial reperfusion injury.