Ischemia shifts the anticoaugulant/procoagulant balance of the endothelium in favor of activation of coagulation. We studied whether cheek pouch microcirculation of leukopenic hamsters was protected by tissue plasminogen activator (tPA) (50 microg/100 g body wt) against ischemia-reperfusion injury. Adherent leukocytes, total perfused capillary length (PCL), permeability increase, and arteriolar and venular red blood cell (RBC) velocity were investigated by fluorescence microscopy. Measurements were made at control, 30 or 60 min of ischemia, and at 30 or 60 min of reperfusion. Hamsters were made leukopenic by treatment with cyclophosphamide (20 mg/100 g body wt ip, 4 days before the experiment), which decreased circulating leukocyte count by 85-90%. Leukopenic hamsters undergoing 30 min of ischemia followed by 30 min of reperfusion showed no significant decrease in PCL or increased permeability. Leukopenic hamsters undergoing 60 min of ischemia followed by 60 min of reperfusion presented a significant decrease in microvascular perfusion where PCL was 28 +/- 7% of baseline, low-flow conditions, and increased permeability. In leukopenic hamsters treated with tPA there was complete protection of capillary perfusion with no significant changes in permeability or arteriolar and venular RBC velocity. In conclusion, thrombus formation may be an additional and independent factor that with leukocyte-mediated mechanisms determines ischemia-reperfusion injury.