This is consistent with other reports and may be particularly important for the haemopoietic cells which are internally activated by VEGF. Thus, cell specific activation of VEGF synthesis may be considered one of the mechanisms involved in the hyperoxia-induced haemopoietic response. The expression of VEGF may be transcriptionally activated by HIF-1a and previous study have shown that HIF-1a, although the major transcriptional OTX015 factor involved in response to hypoxia, may also be activated by hyperoxia. In the present work, however, HIF-1a immunostaining was not correlated with anti-VEGF positivity in hepatocytes. An increase in the percentage of anti-HIF-1a stained hepatocytes was found only in rats exposed to 60% hyperoxia. An increased expression of HIF-1a in haemopoietic cells would have possibly explained the upregulation of VEGF synthesis but changes were not found between the different experimental conditions. However, it must be considered that VEGF expression may also be activated by other alternative signaling pathways and regulators, such as PGC1a, which could be investigated in the future. The expression of eNOS was also evaluated as hyperoxia effects on the NOS expression and function have been reported and NO is widely interrelated with the other factors considered. eNOS may be activated by VEGF or by stimuli such as hemodynamic shear stress and modified oxygen supply. It mediates, through NO production, vasodilatation, angiogenesis and increased vascular permeability. Differential changes in eNOS expression have previously been reported in response to hyperoxia in various tissues. For instance, in a previous study of our group, the eNOS protein level was increased in the lung of newborn rats exposed to 60% hyperoxia for the first 14 postnatal days. Conversely, eNOS protein content was decreased in the heart tissue of newborn rats exposed to 60% and 95% hyperoxia for the first 14 postnatal days and the protein levels of eNOS and iNOS have been reported not to be modified in the liver of six-week-old mice exposed to.95% O2 for 72 or 96 h. In the present study we also found different responses in hepatocytes and haemopoietic cells. In hepatocytes, eNOS expression was selectively reduced in moderate hyperoxia. In the liver tissue, the changes in eNOS and MMP9 expressions are similar and may be partially explained by previously reported finding that eNOS is essential for the efficient early induction of MMP-9 in damaged liver. In haemopoietic foci, eNOS expression was progressively increased with increased degree of hyperoxia.