Age-related differences of neutrophil activation in a skeletal muscle ischemia-reperfusion model

Free tissue transfers and replantation of amputated limbs are better tolerated by young adolescents than mature adults. The authors hypothesized that this observation may be, in part, because of an attenuated ischemia-reperfusion (IR) injury in younger patients. Because neutrophils have been identified as a critical cell line responsible for IR injury, the authors investigated the effects of animal age on the degree of neutrophil activation in a rat model. Activation was evaluated by monitoring expression of integrin surface markers (mean fluorescence intensity [MFI] of CD11b) and oxidative burst potential (MFI of dihydrorhodamine [DHR] oxidation) by flow cytometry in neutrophils analyzed after 4 hours of ischemia and 1, 4, and 16 hours of reperfusion in a gracilis muscle flap model in mature adult and young adolescent rats. Neutrophil activation was also evaluated in control sham-operated animals, which underwent elevation of gracilis muscle flaps without exposure to an ischemic insult. Muscle edema, determined by wet-to-dry muscle weight ratio, and muscle viability, determined by nitro blue tetrazolium (NBT) staining, were completed for gracilis muscles exposed to ischemia after 24 hours of reperfusion for each of the groups. Integrin expression, assessed by MFI of CD11b, was increased significantly in ischemic muscles of mature adult rats at 4 hours of reperfusion (71.10 +/- 3.53 MFI vs. 54.88 +/- 12.73 MFI, p = 0.025). Neutrophil oxidative potential, assessed by MFI of DHR oxidation, was increased significantly in ischemic muscles of mature adult rats compared with young adolescent rats at 1 hour of reperfusion (78.10 +/- 9.53 MFI vs. 51.78 +/- 16.91 MFI, p = 0.035) and 4 hours of reperfusion (83.69 +/- 15.29 MFI vs. 46.55 +/- 8.09 MFI, p = 0.005). Increased edema formation was observed in the ischemic muscles of mature adult rats when compared with young adolescent rats (1.25 +/- 0.04 vs. 1.12 +/- 0.05, p = 0.031) after 24 hours of reperfusion. A trend toward decreased muscle viability was observed in the mature adult rats when compared with young adolescent rats (23.7 +/- 3.1% NBT staining vs. 32.3 +/- 13.7% NBT staining, p = 0.189) after 24 hours of reperfusion. The authors present evidence of an attenuated IR injury in young adolescent animals when compared with mature adult rats. These findings emphasize the importance that studies involving IR injury should be performed with consideration of animal age.