Brain-stem death induces up-regulation of the endothelin axis and increased levels of matrix

John F Fraser, Fiona Kermeen, Alison Sutherland, John Dunning

Critical Care Research Group, Prince Charles Hospital, Brisbane, QLD

Aim

The molecular mechanisms of ischaemia–reperfusion injury after lung transplantation remain unclear. Alveolar macrophages are implicated in the development of acute lung injury. Endothelin (ET)-1 has been shown to be raised after lung transplantation and in acute inflammatory processes. Matrix metalloproteinases (MMPs) are known to cause acute lung injury and, in some situations, are up-regulated by the increase in ET-1. The link between ET-1, MMP-2 and MMP-9 in the alveolar macrophage has not been described in lung transplant recipients or donors.

Methods

Fourteen Wistar–Kyoto rats were anaesthetised and ventilated, and a catheter was positioned in the subdural space. This was inflated inducing brain-stem death (BSD) in eight rats, and left non-inflated in the other six (control) rats. After 4 hours of ventilation, lung specimens were labelled immunohistochemically to demonstrate ET-1, ET-A and ET-B, as well as MMP-2 and MMP-9. CD68 staining was used to characterise alveolar macrophages.

Results

The ratio of alveolar macrophages to polymorphonuclear neutrophils was significantly greater in the BSD group than in the control group (mean ± SD, 9.07 ± 4.13 v 3.09 ± 0.59; P=0.002). ET-1, ET-A and ET-B levels were elevated in the BSD group (27.57 ± 5.26 v 7.01 ± 1.75, 36.1 ± 4.57 v 17.73 ± 2.56,and 54.98 ±7.07 v 19.75 ±3.73 cells per high power field, respectively; P<0.0001). MMP-2 and MMP-9 expression in the experimental group was more than double that in the control group (14.88 ±3.42 v 30.68 ±3.38 and 14.15 ±2.18 v 37 ±3.67 cells per high power field, respectively; P<0.0001).

Conclusions

In a murine model, BSD was associated with up-regulation of the pulmonary endothelin axis and significant rises in c.5oncentrations of the gelatinases MMP-2 and MMP-9. The ratio of alveolar macrophages to infiltrating neutrophils was significantly increased after 4 hours of BSD compared with the control group. Alveolar macrophages expressed significantly higher levels of ET-1, MMP-2 and MMP-9 in BSD compared with the control. Endothelin blockade in BSD donors may reduce the risk of ischaemia–reperfusion injury and diminish the degree of tissue degradation by MMPs.