Where in the Tissues of Danio rerio Is More H₂O₂ Produced During Acute Hypoxia?

The lack of oxygen (O₂) causes changes in the cell functioning. Modeling hypoxic conditions in vitro is challenging given that different cell types exhibit different sensitivities to tissue O₂ levels. We present an effective in vivo platform for assessing various tissue and organ parameters in Danio rerio larvae under acute hypoxic conditions. Our system allows simultaneous positioning of multiple individuals within a chamber where O₂ level in the water can be precisely and promptly regulated, all while conducting microscopy. We applied this approach in combination with a genetically encoded pH-biosensor SypHer3s and a highly H₂O₂-sensitive HyPer7 biosensor. Hypoxia causes H₂O₂ production in areas of brain, heart, and skeletal muscles, exclusively in the mitochondrial matrix; it is noteworthy that H₂O₂ does not penetrate into the cytosol and is neutralized in the matrix upon reoxygenation. Hypoxia causes pronounced tissue acidosis, expressed by a decrease in pH by 0.4-0.6 units everywhere. Using imaging photoplethysmography, we measured in D. rerio larvae real-time heart rate decrease under conditions of hypoxia and subsequent reoxygenation. Our observations in this experimental system lead to the hypothesis that mitochondria are the only source of H₂O₂ in cells of D. rerio under hypoxia. Antioxid. Redox Signal. 42, 292-300.