Effect of the NADPH oxidase inhibitor apocynin on ischemia-reperfusion lung injury

Apocynin (4-hydroxy-3-methoxy-acetophenone) inhibits NADPH oxidase in activated polymorpho-nuclear (PMN) leukocytes, preventing the generation of reactive oxygen species. To determine if apocynin attenuates ischemia-reperfusion lung injury, we examined the effects of apocynin (0.03, 0.3, and 3 mM) in isolated in situ sheep lungs. In diluent-treated lungs, reperfusion with blood (180 min) after 30 min of ischemia (ventilation 28% O₂, 5% CO₂) caused leukocyte sequestration in the lung and increased vascular permeability [reflection coefficient for albumin (σ(alb)) 0.47 ± 0.10, filtration coefficient (K(f)) 0.14 ± 0.03 g · min^-1 · mmHg^-1 · 100 g^-1] compared with nonreperfused lungs (σ(alb) 0.77 ± 0.03, K(f) 0.03 ± 0.01 g · min^-1 · mmHg^-1 · 100 g^-1; P < 0.05). Apocynin attenuated the increased protein permeability at 0.3 and 3 mM (σ(alb)) 0.69 ± 0.05 and 0.91 ± 0.03, respectively, P < 0.05); K(f) was decreased by 3 mM apocynin (0.05 ± 0.01 g · min^-1 · mmHg¹ · 100 g^-1, P < 0.05). Diphenyleneiodonium (DPI, 5 μM), a structurally unrelated inhibitor of NADPH oxidase, worsened injury (K(f) 0.32 ± 0.07 g · min^-1 · mmHg^-1 · 100 g^-1, P < 0.05). Neither apocynin nor DPI affected leukocyte sequestration. Apocynin and DPI inhibited whole blood chemiluminescence and isolated PMN leukocyte-induced resazurin reduction, confirming NADPH oxidase inhibition. Apocynin inhibited pulmonary artery hypertension and perfusate concentrations of cyclooxygenase metabolites, including thromboxane B₂. The cyclooxygenase inhibitor indomethacin had no effect on the increased vascular permeability, suggesting that cyclooxygenase inhibition was not the explanation for the apocynin results. Apocynin prevented ischemia-reperfusion lung injury, but the mechanism of protection remains unclear.