BackgroundExperimental studies in vitro suggest that neutrophils can modulate platelet function and vasomotor responses. In the present study, the interactions among neutrophils, platelets, and arterial responses to injury in vivo were assessed.Methods and ResultsThe acute thrombotic and vasomotor responses of porcine carotid arteries to balloon injury in vivo were evaluated in three groups of animals: neutropenic pigs treated (n= 11) or not treated (n= 12) with aspirin and healthy untreated control pigs (n= 15). Neutropenia was achieved by treatment with cyclophosphamide (50 mg/kg, 4 days before the experiment), which decreased circulating leukocyte count by 92% and almost abolished neutrophil aggregation toN-formyl-methionyl- leucyl-phenylalanine without affecting blood platelet count, hematocrit, hemoglobin concentration, or whole blood platelet aggregation to ADP.51Cr platelet deposition on deeply injured and uninjured arterial segments was not statistically influenced by neutrophil depletion, whereas the angio-graphic vasoconstrictive response at the site of endothelial injury distally was significantly reduced by 41% from 46.3±2.9% in the control group to 27.2±4.1% in the neutropenic group (P<.05). Aspirin treatment in combination with neutropenia produced a 50% reduction in whole blood platelet aggregation, resulted in a significant inhibition of platelet deposition to deeply injured arteries, and decreased vasocon-striction by 66% to 15.6±3.0% (P<.05 versus control and neutropenic).ConclusionsNeutrophils can influence the vasoconstrictive response at the site of endothelial injury in vivo. In addition to platelets, neutrophil interaction with the injured vessel wall may be implicated in the pathophysiological response to arterial injury in vivo.