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they vaguely addressed whether hyperoxic expo-sure in the neonatal period directly impaired nephrogenesis because the end points were assessed in adult rats. Popescu et al.26) reported that P5 rat pups exposed to hyperoxia showed a decreased glomerular size during postnatal nephrogenesis (80% oxygen from P3 to P10), whereas the P10 rat pups showed no change in glomerular count. Hyperoxia exposure in mice during postnatal nephrogenesis (65% oxygen from P0 to P7) did not cause overt adverse effects on renal development27), but the glomeruli were enlarged in early adulthood. Nakagawa et al.20) also reported that hyperoxia exposure suppresses glomerular development. Regarding the association between oxidative stress and immature glomeruli, the proportion of imma-ture glomeruli increases in preterm birth28). In our study, high oxygen levels significantly increased the number of immature glomeruli.With regard to the relationship between oxida-tive stress and hydrogen, hydrogen selectively scavenges harmful ROS, such as hydroxyl radical and peroxynitrite; thus, nucleic acid oxidation and lipid peroxidation are mitigated, and the cells or tissues are protected against oxidant stress and apoptotic injury23). Moreover, hydrogen increases the activity of antioxidant enzymes, including superoxide dismutase and catalase, and inhibits TNF-α and interleukin (IL)-6, thereby suppressing inflammation29). In addition to directly neutralizing highly reactive oxidants, hydrogen indirectly reduces oxidative stress by regulating the expression of various genes. In the study of Nakashima et al., hydrogen decreased apoptosis and nephrotoxicity in rat models of cisplatin nephrotoxicity30). More-over, by reducing oxidative stress and suppressing the activation of inflammatory signaling pathways and cytokine production, HW prevented chronic allograft nephropathy in a kidney transplantation model; thus, the allograft function and overall survival improved31). Therefore, HW can decrease oxidative stress and prevent damages in the kidneys.Our study revealed that through hyperoxia exposure, the number of immature glomeruli can be significantly increased, but HW can significantly suppress such increase. The abovementioned studies suggest that molecular hydrogen suppresses active oxygen and is related to the suppression of renal development disorder. This study is the first to report that molecular hydrogen suppresses the production of immature glomeruli caused by high oxygen load.In the RT-PCR analysis of kidney tissue, the levels of α-SMA, TGF-β, and TNF-α were signifi-cantly higher in the O2-PW group than in the RA-PW group. In contrast, they were significantly suppressed in the O2-HW group compared with those in the O2-PW group.TGF-β induces fibrosis by promoting α-SMA expression and enhancing collagen fibronectin secretion. It also causes glomerular capillaries in rat renal development32). Our study suggested that oxygen administration impaired the formation of glomerular capillaries and was suppressed by molecular hydrogen administration.Angiotensin II elevation resulting from renal damage and oxidative stress promotes TNF-α production33). The intravenous administration of TNF-α causes damage to glomerular endothelial cells and the glomerular endothelial surface layer34). Our study suggested that TNF-α increased by oxidative stress damaged the glomerular endothe-lium but was suppressed by molecular hydrogen administration.Most hydrophilic antioxidants cannot penetrate biomembranes and remain on the membrane surface, whereas molecular hydrogen administra-tion can be distributed rapidly into lipids and cytosol and has no cytotoxicity, even at high concentrations35). A study reported that inhalation of molecular hydrogen gas did not affect physiolog-ical parameters such as pH and blood electrolytes and had no adverse effects36). In addition, oral HW intake is a useful route for molecular hydrogen administration because it is portable, safe, and does not alter the taste, smell, or pH of foods, drinks and drugs. In our study, no adverse events were observed in rats.Our findings demonstrate the protective effect of HW intake on renal impairment. Therefore, HW may delay dialysis or kidney transplantation in children with renal impairment.In conclusion, hyperoxia exposure during nephro-genesis causes renal impairment, whereas HW reduces oxidative stress and suppresses renal impairment.239