Neutrophil and platelet activity and quantification following delayed tPA therapy in a rabbit model of thromboembolic stroke

Although there is considerable interest in the role of neutrophils and platelets in acute cerebral ischemiareperfusion, there are very little data related to the effect of systemic thrombolytic therapy on these blood elements. In the present study a rabbit model was used to examine the effects of cerebral ischemia, tissue-plasminogen activator therapy, or both on neutrophil and platelet peripheral counts and activity, the latter studied by stimulated neutrophil and platelet impedance aggregation and neutrophil oxygen-free radical chemiluminescence. New Zealand white rabbits (n=25) were randomized to receive either tissue-plasminogen activator (6.3 mg/kg IV; 20% bolus, remainder as a 2-hour infusion) or vehicle (0.9% saline) 3 hours following either autologous clot embolization or sham carotid artery isolation. Thus, four groups were examined: sham (n=4), tPA only (n=4), stroke only (n=8), and stroke plus tPA (n=9). Two hours after completion of thrombolytic therapy or vehicle infusion, the experiments were terminated, that is, 7 hours following autologous clot embolization or sham instrumentation. Blood was sampled from the thoracic aorta, and neutrophil and platelet peripheral counts and activity were determined prior to embolization and 0.5, 2.0, 4.0, and 7.0 hours following autologous clot embolization. No significant difference in platelet counts, either over time or between groups, was noted. In contrast to the platelet counts, the neutrophil count significantly increased over time, rising approximately 2.5-fold from baseline in all four groups (p<0.001). No significant increase in neutrophil accumulation (myeloperoxidase assay; 10⁷ PMNs/g tissue; mean ± SEM) was noted within infarcted regions of either the stroke (1.26±0.07; n=5) or stroke plus tissue-plasminogen activator (1.26±0.09; n=5) groups when compared to either viable brain regions within the ischemic hemisphere (1.29±0.03; n=4) or in sham controls (1.36±0.35; n=4). Neutrophil activity (aggregation, oxygen-free radical release) in both groups undergoing autologous clot embolization demonstrated a trend toward higher values when compared to the two sham-operated groups. Tissue-plasminogen activator administration did not significantly affect ex vivo neutrophil activity. In contrast, platelet aggregation was significantly reduced by the administration of tPA with (p=0.001) or without (p<0.01) autologous clot embolization. Thus, in the present rabbit model platelet but not neutrophil activity is modulated by the administration of tissue-plasminogen activator, while autologous clot embolization results in a trend toward acute neutrophil activation.