MRI predicts area of increased plasminogen activation in permanent focal cerebral ischemia
Background: Increased proteolytic activity after ischemic stroke may promote brain edema and secondary hemorrhage. Increased plasminogen activation has previously been detected in infarcted regions after focal cerebral ischemia. In our study, we investigated whether areas of increased plasminogen activation can be related to and therefore later be predicted by intravital magnetic resonance imaging. Methods: Permanent occlusion of the middle cerebral artery (MCA) in male Fisher rats was achieved by electrocoagulation after subtemporal craniectomy. After 48 hours the extent of the ischemic lesion was determined by T2-weighted in vivo MRI. The animals were sacrificed after 72 (n=4) and 168 (n=4) hours. Areas of plasminogen activation were then detected in 10 μm brain slices by in situ zymography using an overlay containing plasminogen and the plasmin substrate casein. The area of the MRI lesion was compared to the area of increased plasminogen activation in respective brain sections using an advanced videoimaging system. Results: MRI at 48 hours consistently revealed extensive lesions affecting most of the basal ganglia and the cortex (84.9 ± 8.6 % and 86.6 ± 10.1 %, respectively). After 72 hours of permanent ischemia, plasminogen activation in respective sections measured 67.5 ± 14.6 % and 80.5 ± 6.8 % of total basal ganglia and cortex area. After 168 hours respective values were 81.8 ± 7.3 % and 88.1 ± 2.7 %. There was no significant difference in the extent of plasminogen activation between the two time intervals. Comparing individual animals, MRI lesion and plasminogen activation showed close overlapping. This colocalisation was significantly higher than expected if areas were randomely distributed (p<0.005). Discussion: In vivo MRI at day two of permanent focal ischemia predicts the extent of increased plasminogen activity. MRI may therefore reveal areas at risk for plasminogen/plasmin mediated reperfusion injury including edema and secondary hemorrhage.