Function of cell membranes in cerebral cortical tissue of newborn piglets after hypoxia and inhibition of nitric oxide synthase

Pathophysiology of an hypoxic–ischemic insult during the perinatal period

Hypoxia-ischemia is a leading cause of morbidity and mortality in the perinatal period with an incidence of 1/4000 live births. Biochemical events such as energy failure, membrane depolarization, brain edema, an increase of neurotransmitter release and inhibition of uptake, an increase of intracellular Ca(2+), production of oxygen-free radicals, lipid peroxidation, and a decrease of blood flow are triggered by hypoxia-ischemia and may lead to brain dysfunction and neuronal death. These abnormalities can result in mental impairments, seizures, and permanent motor deficits, such as cerebral palsy. The physical and emotional strain that is placed on the children affected and their families is enormous. The care that these individuals need is not only confined to childhood, but rather extends throughout their entire life span, so it is very important to understand the pathophysiology that follows a hypoxic-ischemic insult. This review will highlight many of the mechanisms that lead to neuronal death and include the emerging area of white matter injury as well as the role of inflammation and will provide a summary of therapeutic strategies. Hypothermia and oxygen will also be discussed as treatments that currently lack a specific target in the hypoxic/ischemic cascade.

Effect of neuronal nitric oxide synthase inhibition on CA2+/calmodulin kinase kinase and CA2+/calmodulin kinase IV activity during hypoxia in cortical nuclei of newborn piglets

The present study tests the hypothesis that cerebral tissue hypoxia results in increased Ca(2+)/calmodulin (CaM) kinase kinase activity and that the administration of nitric oxide synthase inhibitors (N-nitro-l-arginine [NNLA], or 7-nitroindazole sodium [7-NINA]) prior to the onset of hypoxia will prevent the hypoxia-induced increase in the enzyme activity. To test this hypothesis, CaM kinase kinase and CaM kinase IV activities were determined in normoxic, hypoxic, NNLA-treated hypoxic, and 7-NINA-treated hypoxic piglets. Hypoxia was induced (FiO(2)=0.05-0.08x1 h) and confirmed biochemically by tissue levels of ATP and phosphocreatine. CaM kinase kinase activity was determined in a medium containing protein kinase and phosphatase inhibitors, calmodulin, and a specifically designed CaM kinase kinase target peptide. CaM kinase IV activity was determined by (33)P-incorporation into syntide-2 in a buffer containing protein kinase and phosphatase inhibitors. Compared with normoxic animals, ATP and phosphocreatine levels were significantly lower in all hypoxic piglets whether or not pretreated with nitric oxide synthase inhibitors. There was a significant difference among CaM kinase kinase activity (pmol/mg protein/min) in normoxic (76.84+/-14.1), hypoxic (138.86+/-18.2, P<0.05 vs normoxia), NNLA-pretreated hypoxic (91.34+/-19.3; P=NS vs normoxia, P<0.05 vs hypoxia) and 7-NINA-pretreated hypoxic animals (100.12+/-23.3; P=NS vs normoxia, P<0.05 vs hypoxia). There was a significant difference among CaM kinase IV activity (pmol/mg protein/min) in normoxia (1270.80+/-126.1), hypoxia (2680.80+/-136.7; P<0.05 vs normoxia), NNLA-pretreated hypoxia (1666.00+/-154.8; P<0.05 vs normoxia, P<0.05 vs hypoxia), and 7-NINA-pretreated hypoxic (1712.9+/-231.5; P=NS vs normoxia, P<0.05 vs hypoxia). We conclude that the hypoxia-induced increase in CaM kinase kinase and CaM kinase IV activity is mediated by neuronal NOS-derived NO.

Nitric oxide-mediated expression of Bax protein and DNA fragmentation during hypoxia in neuronal nuclei from newborn piglets

The present study tests the hypothesis that nitric oxide mediates the hypoxia-induced increase in expression of Bax and in DNA fragmentation in the cerebral cortex of newborn piglets, and that administration of N-nitro-L-arginine (NNLA), a nitric oxide synthase inhibitor, will prevent a change in hypoxia-induced expression of apoptotic genes and DNA damage. Piglets were assigned to normoxic, hypoxic, or NNLA-pretreated hypoxic groups. Cerebral tissue hypoxia was documented biochemically by measuring ATP and phosphocreatine (PCr) levels. Cerebral cortical neuronal nuclei were isolated and nuclear proteins were separated electrophoretically and probed with specific antibodies against Bcl-2 or Bax proteins. Neuronal nuclear DNA from normoxic, hypoxic, and NNLA-pretreated hypoxic animals was isolated, separated by electrophoresis on 1% agarose gel and stained with ethidium bromide. Cerebral hypoxia resulted in an increase in nuclear membrane Bax protein levels from 121.33+/-47.7 optical density (OD)xmm(2) in normoxic to 273.67+/-67.3 ODxmm(2) in hypoxic group (P<0.05 vs. normoxic), but levels in NNLA-pretreated hypoxic group were 155.78+/-48.3 ODxmm(2) (P<0.05 vs. hypoxic, P=NS vs. normoxic). Similarly, cerebral hypoxia resulted in the density of DNA fragments increasing from 1530.3+/-309.8 OD/mm(2) in the normoxic group to 5383.3+/-775 OD/mm(2) in the hypoxic group (P<0.05), while levels in NNLA-pretreated hypoxic group were 3574.0+/-952 OD/mm(2) (P<0.05 compared to hypoxic and normoxic groups). The data show that NNLA-pretreatment prevents the hypoxia-induced increase in Bax expression and DNA fragmentation demonstrating that the hypoxia-induced Bax gene expression and the DNA fragmentation are NO-mediated.

Impaired early neurologic outcome in newborn piglets reoxygenated with 100% oxygen compared with room air after pneumothorax-induced asphyxia

Birth asphyxia is a serious problem worldwide, resulting in 1 million deaths and an equal number of neurologic sequelae annually. It is therefore important to develop new and better ways to treat asphyxia. In the present study we tested the effects of reoxygenation with room air or with 100% oxygen (O2) after experimental pneumothorax-induced asphyxia on the blood oxidative stress indicators, early neurologic outcome, and cerebral histopathology of newborn piglets. Twenty-six animals were studied in three experimental groups: 1) sham-operated animals (SHAM, n = 6), 2) animals reoxygenated with room air after pneumothorax (R21, n = 10), and 3) animals reoxygenated with 100% O2 after pneumothorax (R100, n = 10). In groups R21 and R100, asphyxia was induced under anesthesia with bilateral intrapleural room air insufflation. Gasping, bradyarrhythmia, arterial hypotension, hypoxemia, hypercarbia, and combined acidosis occurred 62 +/- 6 min (R21) or 65 +/- 7 min (R100; mean +/- SD) after the start of the experiments; then pneumothorax was relieved, and a 10-min reoxygenation period was started with mechanical ventilation with room air (R21) or with 100% O2 (R100). The newborn piglets then breathed room air spontaneously during the next 3 h. Blood oxidative stress indicators (oxidized and reduced glutathione, plasma Hb, and malondialdehyde concentrations) were measured at different stages of the experiments. Early neurologic outcome examinations (neurologic score of 20 indicates normal, 5 indicates brain-dead) were performed at the end of the study. The brains were next fixed, and various regions were stained for cerebral histopathology. In the SHAM group, the blood gas and acid-base status differed significantly from those measured in groups R21 and R100. In group R100, arterial PO2 was significantly higher after 5 (13.8 +/- 5.6 kPa) and 10 min (13.2 +/- 6.3 kPa) of reoxygenation than in group R21 (8.7 +/- 2.8 kPa and 9.2 +/- 3.1 kPa). The levels of all oxidative stress indicators remained unchanged in the study groups (SHAM, R21, and R100). The neurologic examination score in the SHAM group was 18 +/- 0, in group R21 it was 13.5 +/- 3.1, and in group R100 it was 9.5 +/- 4.1 (significant differences between SHAM and R21 or R100, and between R21 and R100). Cerebral histopathology revealed marked damage of similar severity in both asphyxiated groups. We conclude that the blood oxidative stress indicators and cerebral histopathology did not differ significantly after a 10-min period of reoxygenation with room air or with 100% O2 after pneumothorax-induced asphyxia, but reoxygenation with 100% O2 might impair the early neurologic outcome of newborn piglets.

Effect of hypoxia on DNA fragmentation in different brain regions of the newborn piglet

DNA fragmentation has been studied in different regions of the newborn piglet brain following different times of normobaric hypoxia (5% O(2), 95% N(2)). After 1 hr of hypoxia, fragmented DNA was observed in cerebellum, cortex, hippocampus, and striatum but not in hypothalamus. More fragmentation occurred in these areas of the brain when the animals were kept under hypoxia for times up to 8 hr 45 min. When the animals were submitted to hypoxia for two and a half hours, integrity of DNA was recovered respectively after 3 hr of exposure to the ambient atmosphere in hippocampus and striatum, but 4 hr of recovery were necessary for cerebellum and cortex. These results are discussed in terms of the consequences of neonatal hypoxia and apnea for newborn infants and economical impact for farm animals.

Effects of hypoxia-ischemia and inhibition of nitric oxide synthase on cerebral energy metabolism in newborn piglets

The present study was designed to examine the effects of inhibition of nitric oxide synthase on cerebral energy metabolism after hypoxia-ischemia in newborn piglets. Ten 1- to 3-d-old piglets received N(omega)-nitro-L-arginine (NNLA), an inhibitor of nitric oxide synthase (NNLA-hypoxia, n = 5), or normal saline (hypoxia, n = 5) 1 h before cerebral hypoxia-ischemia. After the infusion, hypoxia-ischemia was induced by bilateral occlusion of the carotid arteries and decreasing FiO2 to 0.07 and maintained for 60 min. Thereafter, animals were resuscitated and ventilated for another 3 h. Using 1H- and 31P-magnetic resonance spectroscopy, cerebral energy metabolism was measured in vivo at 15-min intervals throughout the experiment. Phosphocreatine to inorganic phosphate ratios decreased from 2.74 +/- 0.14 to 0.74 +/- 0.36 (hypoxia group) and 2.32 +/- 0.17 to 0.18 +/- 0.10 (NNLA-hypoxia group) during hypoxia-ischemia. Thereafter, phosphocreatine to inorganic phosphate ratios returned rapidly to baseline values in the hypoxia group, but remained below baseline values in the NNLA-hypoxia group. Intracellular pH decreased during hypoxia-ischemia and returned to baseline values on reperfusion in both groups. Intracellular pH values were lower in the NNLA-hypoxia group (p < 0.001, ANOVA). Lactate was not present during the baseline period. After hypoxia-ischemia, lactate to N-acetylaspartate ratios increased to 1.34 +/- 0.28 (hypoxia group) and 2.22 +/- 0.46 (NNLA-hypoxia group). Lactate had disappeared after 3 h of reperfusion in the hypoxia group, whereas lactate to N-acetylaspartate ratios were 1.37 +/- 1.37 in the NNLA-hypoxia group. ANOVA demonstrated a significant effect of NNLA on lactate to N-acetylaspartate ratios (p < 0.001). Inhibition of nitric oxide synthase by NNLA tended to compromise cerebral energy status during and after cerebral hypoxia-ischemia in newborn piglets.