Effects of MgSO4 Administration on Cerebral cortical Nuclear DNA During Acute Hypoxia in the Newborn Piglet • 180

Abstract

Previous studies have shown that brain tissue hypoxia increases lipid peroxidation in the brain cell membranes. It has also been shown that Mg2+ blocks the activation of the cerebral NMDA receptors during hypoxia. The present study tests the hypothesis that Mg2+ inhibits the hypoxia-induced perioxidative changes in the nuclear membrane in the cerebral cortex, and that Mg2+ inhibits the fragmentation of cerebral cortical nuclear DNA in the newborn piglet during hypoxia. Three groups of anesthetized, ventilated piglets (n=6) were studied: normoxic controls, untreated hypoxic and Mg2+-treated hypoxic piglets. Cerebral hypoxia was induced by lowering the FiO2 (0.05-0.07) to achieve a PaO2 of 16-21 mmHg for 60 min, and documented by decreased tissue ATP and phosphocreatine (Pcr) levels. Prior to hypoxia the Mg2+-treated group received MgSO4 600 mg/kg over 30 min followed by 300 mg/kg infused during 60 min of hypoxia. Cerebral cortical nuclei were isolated. Nuclear membrane lipids were extracted and conjugated dienes and fluorescent compounds, indices of membrane lipid peroxidation, were determined. DNA was isolated from cerebral cortical nuclei. DNA samples were separated by gel electrophoresis on 1% agarose and the bands of DNA:ethydium bromide complex were analyzed. The level of conjugated dienes and fluorescent compounds in the non-treated hypoxic nuclear membranes increased from 0 in normoxic to 0.753μmoles/g protein, and from 6.7 to 14.0 mg quinine sulfate/g protein respectively. In the Mg2+-treated hypoxic group the levels were 0.09μmoles/g protein and 7.7 mg quinine sulfate/g protein respectively. In the normoxic nuclei there was only one DNA band whereas in the hypoxic nuclei there were several bands ranging from 100 to 2000 basepairs. In Mg2+-treated hypoxic nuclei only one DNA band was observed. As previously demonstrated there was an increased peroxidation of neuronal nuclear membranes during hypoxia, decreased in the Mg2+-treated hypoxic neuronal nuclear membranes. The data also show that there is fragmentation of nuclear DNA, a hallmark of programmed cell death, in non-treated hypoxia. The DNA fragmentation was not seen in Mg2+ treated hypoxia. We speculate that the free radical species generation during hypoxia-induced peroxidation of the nuclear membrane may cause randomized fragmentation of genomic DNA and lead to programmed cell death, and that this changes can be inhibited by the administration of MgSO4.