SUMMARY The possible role of leukocytes in the cerebral microcirculation following ischaemia was assessed in the gerbil. The no-reflow phenomenon seen after 30 minutes of severe bilateral hemispheric ischaemia during hypotensive reperfusion was compared in control animals and in a group made leukopenic by pretreatment with cyclophosphamide. Neither the incidence nor the severity of the no-reflow phenomenon differed between the two groups. The evidence from this study casts doubt on the hypothesis that leukocyte plugging plays a major role in the cerebral microcirculation's response to ischaemia.There is increasing interest in the rheological properties of leukocytes.' The peripheral blood leukocyte count has been shown to be a predictor of myocardial infarction, and stroke,26 and leukocyte counts correlate with the size of myocardial infarcts.7 A poor prognosis after myocardial infarction,89 and stroke'" is associated with a high leukocyte count. In experimental models of myocardial infarction, reducing the leukocyte count reduces infarct size." The role of leukocytes in the cerebral circulation has received less attention,'2 but cellular plugging has been suggested as a contributory factor in the genesis of the "no-reflow" phenomenon.'3 This is characterised by patchy poor perfusion after arrest of the cerebral circulation, if reperfusion is initially at an untreated low perfusion pressure. ' We sought to investigate the role ofleukocytes in the cerebral circulation by studying the effect of lowering the peripheral blood leukocyte count on the development of the no-reflow phenomenon in the gerbil brain. MethodsThe no-reflow phenomenon was produced in the gerbil by 30 minutes of severe bilateral hemispheric ischaemia followed by reperfusion without blood pressure support. Adult gerbils of either sex (60-80 g) were anaesthetised with intraperitoneal pentobarbitone 60 mg/kg (Sagatal May and Baker Ltd). Through a midline cervical incision a tracheostomy was performed and both common carotid arteries were isolated with 5-0 silk sutures. A cannula was placed in the left femoral artery for blood sampling and continuous blood pressure monitoring. The left femoral vein was cannulated for injection of India ink. Arterial blood (0-1 ml) was removed for measurement of leukocyte count and haematocrit and in some of the animals a platelet count.'5 Scoville-Lewis aneurysm clips were then placed on both carotid arteries for 30 minutes. Ten minutes after their removal, an intravenous injection of 1 ml of an isotonic filtered suspension of India ink was given over the course of 30 seconds. (Pulmonary filtration further removes larger particles and the cerebral microcirculation is thereby fully perfused.) This technique also ensures that the intravascular marker circulates at the prevailing arterial blood pressure. After a further minute of reperfusion with India ink the animal was decapitated and the brain removed and fixed in 10% neutral formal saline. Coronal (1 mm) sections were dehydrated in alcohol and cleared in oil of W...

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