NEUTROPHIL ACCUMULATION IN THE LARGE COLON OF HORSES DURING LOW-FLOW ISCHEMIA AND REPERFUSION

Histomorphologic/morphometric evaluation, leukocyte scintigraphy, and myeloperoxidase activity were used to determine whether neutrophils ac cumulate in the large colon of horses during low-flow ischemia and rep erfusion. Twenty-four adult horses were assigned to 1 of 3 groups: gro up 1, sham-operated (n = 6); group 2, 6 hours of ischemia (n = 9); and group 3, 3 hours of ischemia and 3 hours of reperfusion (n = 9). Low- flow ischemia of the large colon was induced in horses of groups 2 and 3 by reducing colonic arterial blood flow to 20% of baseline. Radiola beled (Tc-99m) autogenous neutrophils were injected at 175 minutes, wh ich corresponded to 5 minutes prior to reperfusion in group-3 horses. Full-thickness biopsy specimens of the left ventral colon were collect ed at baseline and at 30-minute intervals for 6 hours; a portion of th e biopsy specimen was placed in formalin for histologic examination, a nd the remainder was used to measure mucosal radioactivity and myelope roxidase activity. There were no differences in baseline mucosal neutr ophil index, mucosal neutrophil numbers, submucosal venular neutrophil numbers, mucosal radioactivity, or mucosal myeloperoxidase activity a mong groups, or over time in group-1 horses. Neutrophils accumulated i n the colonic mucosa during ischemia and further increased at reperfus ion, as indicated by neutrophil index (morphology) and mucosal neutrop hil numbers (morphometry); mucosal neutrophil index was significantly (P < 0.05) greater in group-3 horses during reperfusion than at the co rresponding periods of ischemia in group-2 horses. Neutrophil numbers were significantly (P < 0.05) increased in submucosal venules at 10 mi nutes of reperfusion in group-3 horses and were significantly (P < 0.0 5) greater in group-3 than in group-2 horses during the interval from 3 to 6 hours. Mucosal radioactivity significantly (P < 0.05) increased at reperfusion in group-3 horses; there was a trend (P = 0.076) towar d greater mucosal radioactivity in group-3, compared with group-2 hors es, throughout the 3- to 6-hour interval. There were no differences in mucosal myeloperoxidase activity among or within any of the 3 groups over time. Neutrophils accumulated in the large colon of horses during low-flow ischemia and reperfusion. Neutrophil infiltration was detect ed by histologic examination and leukocyte scintigraphy, but not by me asurement of myeloperoxidase activity. The accumulation of neutrophils during ischemia and the further neutrophil infiltration during reperf usion indicate that neutrophils may contribute to reperfusion injury o f the large colon.