ABSTRACT:The optimal oxygen concentration for newborn resuscitation is still discussed. Oxygen administration during reoxygenation may induce short-and long-term pathologic changes via oxidative stress and has been associated to later childhood cancer. The aim was to study changes in oxidative stress-associated markers in liver and lung tissue of newborn pigs after acute hypoxia followed by reoxygenation for 30 min with 21, 40, or 100% oxygen compared with room air or to ventilation with 100% oxygen without preceding hypoxia. Nine hours after resuscitation, we found a dose-dependent increase in the matrix metalloproteinase gelatinase activity in liver tissue related to percentage oxygen supply by resuscitation (100% versus 21%; p ϭ 0.002) pointing at more extensive tissue damage. Receiving 100% oxygen for 30 min without preceding hypoxia decreased the expression of VEGFR2 and TGFBR3 mRNA in liver tissue, but not in lung tissue. MMP-, VEGF-, and TGF-superfamily are vital for the development, growth, and functional integrity of most tissues and our data rise concern about both short-and longterm consequences of even a brief hyperoxic exposure. O ver the last two decades, the traditionally recommended use of 100% oxygen for newborn resuscitation has been extensively challenged. Meta-analyses have shown room air to be as efficient as 100% oxygen and that 100% oxygen increases both mortality and morbidity (1-3). In addition, an association has been found between newborn resuscitation with 100% oxygen and later childhood cancer (4,5) pointing at the outmost necessity to search for the underlying cause.Perinatal asphyxia may induce hypoxia-reoxygenation (HR) injury to various organs including liver and lungs, which have important roles in host defense after both organic and inorganic environmental insults (6). Liver-tissue responses to potentially damaging neonatal hypoxia and reoxygenation are of particular interest because of the unique and remarkable capacity of this organ to regenerate after injury via DNA replication and mitosis (7). The lungs are in a direct and continuous communication with the outside environment and are exposed directly to the highest partial pressure of inspired O 2. The time course of reactive oxygen species (ROS) generation during hypoxia and reperfusion is not clearly understood (8), but experimental evidence indicates a worsening of the hypoxic injury by reoxygenation (9,10). The underlying mechanisms of hypoxia induced liver or lung injury, and the nature of the detrimental biofactors set in motion these circumstances, are still poorly understood. Among the molecules whose expression and activity are modulated in tissue damage and repair are the matrix metalloproteinases (MMPs). MMPs belong to a family of zinc-containing endopeptidases best known for their roles in the physiologic and pathologic remodeling of the extracellular matrix (ECM) during angiogenesis, embryogenesis, wound healing, tumor metastasis, and various cardiovascular and inflammatory diseases (11). MMPs are important in...