Endothelial nitric oxide synthase mediates protective effects of hypoxic preconditioning in lungs

To elucidate the protective mechanism of whole-body hypoxic preconditioning (WHPC) on pulmonary ischemia-reperfusion injury focussing on nitric oxide synthases (NOS), mice were placed in a hypoxic chamber (FIO2 = 0.1) for 4 h followed by 12 h of normoxia. Then, pulmonary ischemia for 1 h followed by 5 h of reperfusion was performed by clamping the left hilum in vivo (I/R). WHPC protected WT mice from pulmonary leukocyte infiltration as assessed by myeloperoxidase (MPO) activity, associated with a mild further increase in endothelial permeability (Evans Blue extravasation). When all NOS isoforms were inhibited during WHPC by L-NAME, mortality and MPO activity after I/R markedly increased. To determine the responsible NOS isoform, quantitative RT-PCR was performed for eNOS and iNOS mRNA, showing that only eNOS was upregulated in response to WHPC. While eNOS total protein expression remained unchanged, the amount of phosphorylated eNOS also increased. The WHPC/IR experiments were then repeated with eNOS knockout mice. Here, we found that the protective effect of WHPC on pulmonary leukocyte sequestration was abrogated, and endothelial leakage was further exacerbated. We conclude that WHPC limits neutrophil sequestration via an eNOS-dependent mechanism, and that eNOS helps preserve endothelial permeability during hypoxia and I/R.