Hyperoxia arrests pulmonary development in newborn rats via disruption of endothelial tight junctions and downregulation of Cx40

This study investigated changes in vascular endothelial cell tight junction structure and the expression of the gene encoding connexin 40 (Cx40) at the early pneumonedema stage of hyperoxia-induced bronchopulmonary dysplasia (BPD) in a newborn rat model. A total of 96 newborn rats were randomly assigned to one of the following two groups, the hyperoxia group (n=48) and the control group (n=48). A hyperoxia-induced BPD model was established for the first group, while rats in the control group were maintained under normoxic conditions. Extravasation of Evans Blue (EB) was measured; the severity of lung injury was assessed; a transmission electron microscope (TEM) was used to examine the vascular endothelial cell tight junction structures, and immunohistochemical assay, western blotting and reverse transcription-polymerase chain reaction (RT-PCR) were used to evaluate the expression of Cx40 at the mRNA and protein level. Our findings showed that injuries due to BPD are progressively intensified during the time-course of exposure to hyperoxic conditions. Pulmonary vascular permeability in the hyperoxia group reached the highest level at day 5, and was significantly higher compared to the control group. TEM observations demonstrated tight junctions between endothelial cells were extremely tight. In the hyperoxia group, no marked changes in the tight junction structure were found at days 1 and 3; paracellular gaps were visible between endothelial cells at days 5 and 7. Immunohistochemical staining revealed that the Cx40 protein is mainly expressed in the vascular endothelial cells of lung tissue. Western blotting and RT-PCR assays showed a gradual decrease in Cx40 expression, depending on the exposure time to hyperoxic conditions. However, the Cx40 mRNA level reached a trough at 5 days. Overall, our study demonstrated that exposure to hyperoxia damages the tight junction structures between vascular endothelial cells and downregulates Cx40. We therefore conclude that hyperoxia may participate in the regulation of pulmonary vascular endothelial permeability.