The role of platelet-activating factor (PAF) in mediating the colonic damage that develops after large-colon torsion was studied in 14 ponies. Morphologic changes in areas of the ascending colon and selected abdominal and thoracic viscera after 1 hour of large-colon torsion and 3 to 5 hours of reperfusion were determined, as well as the protective effects of systemic administration of a specific PAF antagonist (WEB 2086). Ponies were selected then allocated at random and in equal numbers to 2 groups that received 1 of 2 treatments prior to induction of large-colon torsion: group 1 - control (saline solution), and group 2 WEB 2086 (3 mg/kg of body weight loading dose and 3 mg/kg/h for the remainder of the study). In each pony, full-thickness tissue specimens from the gastrointestinal tract-cecum, pelvic flexure, left and right ventral colon, and right dorsal colon - heart, left lung, liver, left adrenal gland, spleen, and right kidney were collected and histologically evaluated. Edema, mucosal necrosis, and neutrophil infiltration in colonic sections were graded from 0 (normal) to 3 (most severe changes). Sections of liver and lung from 3 ponies in each group, and colon from 1 pony in each group, also were examined by transmission electron microscopy to determine the presence of ultrastructural alterations. Ischemia and reperfusion induced marked changes in all sections of colon in all ponies: moderate to severe submucosal edema, moderate necrosis of the superficial epithelium and lamina propria, and necrosis of the mucosal crypt epithelium. Extra- vascular neutrophil accumulation was evident in all sections of colon and cecum, but not in other tissues. Ultrastructural lesions were not present in hepato- cytes or pneumocytes, or in the endothelial cells of liver, lung, and colon. Bacteria were observed by electron microscopy in 5% of hepatic sinusoids. Administration of a specific PAF antagonist, WEB 2086, failed to reduce severity of the observed lesions, indicating that it was not cytoprotective at the dosage used in this model of ischemia-reperfusion injury.

Effects of low-flow ischemia and reperfusion of the large colon on systemic and colonic hemodynamic and metabolic variables were determined in horses. Twenty-four adult horses were randomly allocated to 3 groups: sham-operated (n = 6), 6 hours of ischemia (n = 9), and 3 hours of ischemia and 3 hours of reperfusion (n = 9). Low-flow ischemia was induced in groups 2 and 3 by reducing colonic arterial blood flow to 20% of baseline. Heart rate, arterial blood pressures, cardiac index, pulmonary artery pressure, right atrial pressure, and colonic blood flow were monitored. Arterial, mixed-venous, and colonic venous blood gas and oximetry analyses; PCV; and blood lactate and pyruvate and plasma total protein concentrations were measured. Data were recorded, and blood samples were collected at baseline and at 30-minute intervals for 6 hours; additionally, data were collected at 185, 190, and 195 minutes (corresponding to 5, 10, and 15 minutes of reperfusion in group-3 horses). There were no differences among groups at baseline or across time for any systemic hemodynamic or metabolic variable. Colonic blood flow did not change across time in group-1 horses. Colonic blood flow significantly (P < 0.05) decreased to 20% of baseline at induction of ischemia in horses of groups 2 and 3 and remained significantly decreased throughout the ischemic period in horses of groups 2 (6 hours) and 3 (3 hours). Colonic blood flow significantly (P < 0.05) increased above baseline by 5 minutes of reperfusion in group-3 horses. Colonic oxygen delivery and oxygen consumption, and colonic venous pH, PO2, percentage saturation of hemoglobin, and oxygen content were significantly (P < 0.05) decreased within 30 minutes after induction of ischemia in horses of groups 2 and 3; colonic venous PCO2, colonic oxygen extraction ratio, and lactate and pyruvate concentrations were significantly (P < 0.05) increased by 30 minutes of ischemia. These alterations continued throughout ischemia, but within 5 minutes of reperfusion in group-3 horses, these variables either returned to baseline (pH, PCO2, lactate, pyruvate), significantly (P < 0.05) increased above baseline (PO2, oxygen content, % saturation of hemoglobin), or significantly (P < 0.05) decreased below baseline (colonic oxygen extraction ratio). Colonic oxygen consumption remained decreased during reperfusion in group-3 horses. Colonic mucosal ischemia-reperfusion injury observed in this model of ischemia was associated with local colonic hemodynamic and metabolic alterations in the presence of systemic hemodynamic and metabolic stability. Reactive hyperemia was observed at restoration of colonic blood flow in group-3 horses and persisted during reperfusion. Colonic venous metabolic alterations were corrected at reperfusion, indicating adaptation of the colon to the return of blood flow and oxygen delivery with resultant decrease in anaerobic metabolism. The early alterations in these variables may simply represent a washout of metabolic by-products.

To assess right colic artery blood flow and relevance of xanthine dehydrogenase/xanthine oxidase after experimentally induced strangulation obstruction and reperfusion of the colon, 5 ponies were subjected to 2.5 hours of complete ischemia of the left dorsal and ventral colons, allowed to recover from surgery, and monitored during a 48-hour reperfusion period. Five ponies were subjected to sham surgery and served as controls. All ponies had a Doppler ultrasound blood flow monitor implanted on the right colic artery near the pelvic flexure 10 to 14 days prior to the ischemic period. Colic artery blood flow was monitored prior to, during, and for 4 hours after surgery. Blood samples from the right colic artery and vein distal to the obstruction site were collected during surgery (prior to ischemia, after 1 and 2 hours of ischemia, and after 10 and 60 minutes of reperfusion) for determination of arterial and venous blood gas tensions and electrolytes. Prior to surgery, blood selenium and plasma vitamin E (alpha-tocopherol) concentrations and blood glutathione peroxidase (GPX) activity were determined to assess the status of endogenous antioxidants. Combined xanthine dehydrogenase (XDH) plus xanthine oxidase (XO) activity, and XO activity alone (nanomoles per minute per gram of tissue) were determined, using a dual-spectrophotometric technique. Xanthine dehydrogenase and oxidase activities were determined prior to ischemia, after 1 and 2 hours of ischemia, and at 1 and 48 hours after reperfusion. Median blood flow in the experimental and control groups (156 ml/min and 110 ml/min, respectively) was not statistically different before surgery, and was significantly (P < 0.02) lower in the experimental (4 ml/min) vs the control group (72.5 ml/min) during the ischemic period. Experimental ponies had significantly (P < 0.03) lower right colic artery blood flow during the 4 hours immediately after recovery from anesthesia. Significant difference was not observed in right colonic venous bicarbonate concentration between groups at any time. Median right colonic venous P(CO2), pH, and standard base excess were different (P < 0.001) between groups during the ischemic period only. Median venous oxygen saturation and median venous P(O2) were significantly (P < 0.001) lower in the experimental ponies at the end of 2 hours of ischemia, but were significantly (P < 0.05) increased during the reperfusion phase. Median venous potassium concentration was significantly (P < 0.01) higher in experimental ponies during the ischemic and reperfusion phases. Vitamin E and GPX values were within normal limits for all ponies. Median selenium concentration was < 15 microgram/dl; however, there were no significant differences between control and experimental ponies. Only 3 of 10 ponies had measurable XHH/XO activity at the beginning of the experiment. Enzyme activity was detected in 1 additional pony during the ischemic period. However, in all 4 ponies in which XDH/XO activity was detected, enzyme activity was low (10 to 36 nmol/min/g). On the basis of macroscopic and histologic examination of the large colon, evidence of reperfusion injury was not found in 4 of the 5 experimental ponies. The only pony with gross evidence of reperfusion injury did not have detectable XO activity. Results of the study indicate that hypoperfusion of the colon during the postischemic period may be a factor in deterioration of the colon observed clinically in equids with surgical correction of large-colon volvulus. Additionally, if reperfusion injury develops in the large colon, it probably is not mediated through the xanthine oxidase enzyme system: the activity of this enzyme in the large colon, when present, is negligible.

Effects of low-flow ischemia and reperfusion of the large colon on mucosal architecture were determined in horses. Twenty-four adult horses were randomly allocated to 3 groups: sham-operated (n = 6), 6 hours of ischemia (n = 9), and 3 hours of ischemia and 3 hours of reperfusion (n = 9). Low-flow ischemia was induced in horses of groups 2 and 3 by reducing colonic arterial blood flow to 20% of baseline values. Systemic hemodynamic and metabolic variables were maintained constant and in a normal physiologic range. Full-thickness biopsy specimens were obtained from the left ventral colon for histomorphologic and morphometric examination at baseline and at 30-minute intervals for 6 hours; additional biopsy specimens were collected at 185, 190, and 195 minutes (corresponding to 5-, 10-, and 15-minute periods of reperfusion in group-3 horses). There were no differences among groups at baseline or across time in group-1 horses for any of the histopathologic variables. There were significant (P < 0.05) increases in percentage of surface mucosal disruption, estimated and measured percentage depth of mucosal loss, mucosal hemorrhage, mucosal edema, and cellular debris index during 0 hour to 3 hours, compared with baseline, and from 3 hours to 6 hours, compared with 3 hours in horses of groups 2 and 3. Estimated percentage depth of mucosal loss and cellular debris index were significantly (P < 0.05) greater in group-3 horses, compared with group-2 horses during the interval from 3 to 6 hours. There were trends toward greater percentage of surface mucosal disruption and mucosal edema during the early phase of reperfusion (3 to 4 hours) and greater mucosal hemorrhage, measured percentage depth of mucosal loss, and mucosal interstitial-to-crypt ratio during the late phase (4 to 6 hours) of reperfusion in group-3 horses vs group-2 horses. Reestablishment of colonic arterial blood flow after low-flow ischemia caused greater mucosal injury than did a comparable period of continued ischemia. Thus, reperfusion injury was detected in the large colon of horses after low-flow arterial ischemia. The serial mucosal alterations that developed in the colon were comparable in horses of groups 2 and 3; however, reperfusion exacerbated colonic mucosal injury.

Histomorphologic/morphometric evaluation, leukocyte scintigraphy, and myeloperoxidase activity were used to determine whether neutrophils accumulate in the large colon of horses during low-flow ischemia and reperfusion. Twenty-four adult horses were assigned to 1 of 3 groups: group 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. Radiolabeled (99mTc) autogenous neutrophils were injected at 175 minutes, which corresponded to 5 minutes prior to reperfusion in group-3 horses. Full-thickness biopsy specimens of the left ventral colon were collected at baseline and at 30-minute intervals for 6 hours; a portion of the biopsy specimen was placed in formalin for histologic examination, and the remainder was used to measure mucosal radioactivity and myeloperoxidase activity. There were no differences in baseline mucosal neutrophil index, mucosal neutrophil numbers, submucosal venular neutrophil numbers, mucosal radioactivity, or mucosal myeloperoxidase activity among groups, or over time in group-1 horses. Neutrophils accumulated in the colonic mucosa during ischemia and further increased at reperfusion, as indicated by neutrophil index (morphology) and mucosal neutrophil numbers (morphometry); mucosal neutrophil index was significantly (P < 0.05) greater in group-3 horses during reperfusion than at the corresponding periods of ischemia in group-2 horses. Neutrophil numbers were significantly (P < 0.05) increased in submucosal venules at 10 minutes of reperfusion in group-3 horses and were significantly (P < 0.05) 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) toward greater mucosal radioactivity in group-3, compared with group-2 horses, 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 detected by histologic examination and leukocyte scintigraphy, but not by measurement of myeloperoxidase activity. The accumulation of neutrophils during ischemia and the further neutrophil infiltration during reperfusion indicate that neutrophils may contribute to reperfusion injury of the large colon.