Summary: We studied whether administration of nitric oxide (NO) donors reduces the ischemic damage resulting from middle cerebral artery (MCA) occlusion in sponta neously hypertensive rats (SHRs). In halothane anesthetized and ventilated SHRs, the MCA was oc cluded. CBF was monitored using a laser-Doppler flow meter. Three to five minutes after MCA occlusion, the NO donors sodium nitroprusside (SNP; 3 mg/kg/h) or 3-morpholino-sydnonimine (SIN 1; 1.5-6 mg/kg/h) were administered into the carotid artery for 60 min. As a con trol, the effect of papaverine (3.6 mg/kg/h), a vasodilator that acts independently of NO, was also studied. The hypotension evoked by these agents was counteracted by intravenous infusion of phenylephrine. At the end of the infusion, rats were allowed to recover. Stroke size was determined 24 h later in thionin-stained sections. In sham occluded rats, SNP (n = 5), SIN 1 (n = 5), and papav erine (n = 5) produced comparable increases in CBF (p > 0.05 from vehicle). After MCA occlusion, SNP (n = 5) and SIN 1 (n = 5), but not papaverine (n = 5), enhanced Nitric oxide (NO) is a ubiquitous free radical gas involved in a wide variety of biological processes (Moncada, 1992). In the systemic circulation, NO, produced in endothelial cells, regulates vascular re sistance and platelet aggregation (Moncada, 1992). In the immune system, NO is responsible for the
217the recovery of CBF (p < 0.05 from vehicle) and reduced the size of the infarct by 28 ± 12 and 32 ± 7%, respec tively (mean ± SD; p < 0.05 from vehicle). To determine whether NO donors could act by inhibiting platelet aggre gation, we studied the effect of SNP on collagen-induced platelet aggregation. Intracarotid administration of SNP (3 mg/kg/h for 60 min) did not affect platelet aggregation to collagen, suggesting that the protective effect of NO donors was not due to inhibition of platelet function. We conclude that NO donors increase CBF to the ischemic territory and reduce the tissue damage resulting from fo cal ischemia. The protective effect may result from an increase in CBF to the ischemic territory, probably the ischemic penumbra. These findings suggest that NO do nors may represent a new therapeutic strategy for the management of acute stroke.