Brain cell membrane Na+,K(+)-ATPase activity following severe hypoxic injury in the newborn piglet

This study tests the hypothesis that severe brain hypoxia causes decreased Na+,K(+)-ATPase activity, resulting in permanent alterations in the neuronal cell membranes. Seventeen anesthetized piglets (normoxic control (NC), no recovery after hypoxia (Group 1), 6 h normoxic recovery (Group 2), and 48 h normoxic recovery (Group 3)) were studied. Hypoxia was induced by lowering the FiO2 to maintain PCr/Pi ratio at 25% of baseline for 1 h as monitored by 31P-NMR spectroscopy. PCr/Pi returned to 57% of baseline by 6 h and was normal by 48 h. At termination, cortical tissue Na+,K(+)-ATPase activity was determined. Na+,K(+)-ATPase activity was measured in cortical membrane preparations by determining the rate of ATP hydrolysis. NC membranes had Na+,K(+)-ATPase activity of 58.3 +/- 1.3 microM Pi/mg protein/h (mean +/- S.E.M.). Na+,K(+)-ATPase activity was reduced in Groups 1, 2, and 3 (45.8 +/- 1.3, 47.4 +/- 3.6, 48.7 +/- 2.9 microM Pi/mg protein/h) (P < 0.05 compared to NC). There was no difference in enzyme activity among Groups 1, 2, or 3. The data show that in spite of recovery of neuronal oxidative phosphorylation (PCr/Pi) by 48 h, there is a permanent decrease in Na+,K(+)-ATPase activity in cells that have undergone severe hypoxic injury. The persistent decrease in Na+,K(+)-ATPase activity indicates ongoing cell injury following severe cerebral hypoxia, and that recovery of oxidative phosphorylation as indicated by PCr/Pi values cannot be used as an index of recovery of cell function.

Effect of cerebral hypoxia on NMDA receptor binding characteristics after treatment with 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) in newborn piglets

Previous studies have shown that hypoxia modifies the NMDA receptor/ion channel complex in cortical brain cell membranes of newborn piglets. The present study tests the hypothesis that blockade of the glutamate recognition site of the NMDA receptor with the competitive antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) prevents modification of the receptor during hypoxia. Twenty seven anesthetized, ventilated newborn piglets were randomized into four groups: 7 normoxic (Nx), 6 CPP-treated normoxic (CPP-Nx), 8 hypoxic (Hx) and 6 CPP-treated hypoxic (CPP-Hx). Treatment groups received CPP 2 mg/kg i.v. The CPP-Hx group received CPP 30 min prior to hypoxia, which was induced by lowering the FiO2 to 5-7% for 1 h. Physiologic data showed no change in heart rate, blood pressure, arterial blood gas values, glucose or lactate following CPP administration. During hypoxia there was a significant decrease in PaO2, pH and an increase in lactate compared to baseline values. The CPP-Hx group had significantly higher lactate levels than the Hx group during hypoxia. P2 membrane fractions were prepared and thoroughly washed. Characteristics of the NMDA receptor ion channel were determined by [3H]MK-801 binding assays and characteristics of the glutamate recognition site by specific NMDA-displaceable [3H]glutamate binding assays. Brain tissue ATP and PCr levels confirmed tissue hypoxia, and were not preserved by CPP administration. [3H]MK-801 binding assays revealed that CPP treatment attenuated the hypoxia-induced decrease in the number of receptors (Bmax) and receptor binding affinity (Kd) during hypoxia. CPP treatment also decreased receptor affinity (increased Kd) for [3H]MK-801 binding during normoxia and hypoxia. Assays of [3H]glutamate binding revealed that hypoxia decreased both the Bmax and the Kd of the NMDA receptor for [3H]glutamate and both were preserved by CPP treatment prior to hypoxia. CPP had no effect on [3H]glutamate Bmax or Kd during normoxia. We conclude that hypoxia decreases the Bmax and Kd of the NMDA receptor glutamate recognition site for [3H]glutamate and the ion channel site for [3H]MK-801 in newborn piglets. These changes are prevented by CPP administration prior to hypoxia. The different effects of CPP binding during normoxia and hypoxia suggest a use-dependent mechanism for CPP binding during hypoxia, possibly through an hypoxia-induced alteration of the high-affinity binding site for CPP. During both normoxia and hypoxia CPP binding appeared to induce a conformational change in the receptor causing a decrease in binding affinity for [3H]MK-801. CPP administration did not preserve brain tissue ATP or PCr levels during hypoxia and may alter cellular metabolism in addition to its action at the NMDA receptor. However, even with depletion of the energy precursors ATP and PCr, and with higher lactate levels in the CPP-Hx group, CPP was able to maintain NMDA receptor binding characteristics during hypoxia and may decrease excitotoxic cellular damage from hypoxia.

Cytosolic and membrane-bound cerebral nitric oxide synthase activity during hypoxia in cortical tissue of newborn piglets

To determine the role of nitric oxide production during hypoxia, the presence of two forms of neuronal nitric oxide synthase, cytosolic (cNOS) and membrane-bound (memNOS), in cortical tissue of newborn piglets and the effects of hypoxia on the activity of these enzymes were studied. Experiments were performed in 12 anesthetized and ventilated Yorkshire piglets, 2-4 days of age. Hypoxia was induced by decreasing the FiO2 to 0.07. The control group was ventilated maintaining normoxia. After 1 h of normoxic or hypoxic ventilation brain tissue was removed and frozen immediately in liquid nitrogen. Tissue hypoxia was confirmed by analysis of adenosine triphosphate (ATP) and phosphocreatine (PCr): ATP was reduced to 52% and PCr to 28% of control values. cNOS activity was 35.3 +/- 13.7 pmol/mg protein per min in the control group and 28.3 +/- 7.0 in the hypoxia group; memNOS activity was 10.5 +/- 4.5 and 12.0 +/- 3.9 pmol/mg protein per min in the control and hypoxia groups, respectively. Differences in cNOS and memNOS activity between control and hypoxic animals were not significant. The results indicate that both cNOS and memNOS are present in cortical tissue of newborn piglets and that the activity is unaffected by 1 h of tissue hypoxia. We suggest that production of nitric oxide and its derivative peroxynitrite during hypoxia may therefore be a potential mechanism for hypoxia-induced brain cell membrane lipid peroxidation.

The effect of post-asphyxial reoxygenation with 21% vs. 100% oxygen on Na+,K(+)-ATPase activity in striatum of newborn piglets

To compare the effect of 21% vs. 100% oxygen during post-asphyxial reoxygenation on brain cell membrane function in the striatum, 20 anesthetized, ventilated newborn piglets were studied: group 1 (normoxia, n = 5), group 2 (asphyxia, no reoxygenation, n = 5), group 3 (asphyxia followed by reoxygenation with 21% O2, n = 5), and group 4 (asphyxia followed by reoxygenation with 100% O2, n = 5). Asphyxia was induced by a stepwise reduction in FiO2 at 20 min intervals from 21% to 14%, 11%, and 8%. Following a total 60 min of asphyxia, piglets in groups 3 and 4 were recovered for 2 h with either 21% or 100% O2. Na+,K(+)-ATPase activity (mumol Pi/mg protein/h) in striatal cell membranes was 31 +/- 1, 22 +/- 2, 32 +/- 2 and 26 +/- 1 in groups 1, 2, 3 and 4, respectively. Na+,K(+)-ATPase activities in groups 2 and 4 were significantly lower than in groups 1 and 3 (p < 0.01). Piglets recovered post-asphyxia for 2 h with 21% O2 had restoration of Na+,K(+)-ATPase activity to baseline levels, while those treated with 100% O2 during recovery had persistent Na+,K(+)-ATPase inhibition of 16%. This could result from increased free radical production during reoxygenation with 100% O2 which could contribute to post-asphyxial cellular injury in the striatum.

Kainate receptor modification in the fetal guinea pig brain during hypoxia

The present study tests the hypothesis that hypoxia alters the high-affinity kainate receptors in fetal guinea pig brain. Experiments were conducted in normoxic and hypoxic guinea pig fetus at preterm (45 days of gestation) and term (60 days of gestation). Hypoxia in the guinea pig fetus was induced by exposure to maternal hypoxia (FiO2 = 7%) for 60 min. Brain tissue hypoxia in the fetus was documented biochemically by decreased levels of ATP and phosphocreatine. [3H]-Kainate binding characteristics (Bmax = number of receptors, Kd = dissociation constant) were used as indices of kainate receptor modification. P2 membrane fractions were prepared from the cortex of normoxic and hypoxic fetuses and were washed six times prior to performing the binding assays. [3H]kainate binding was performed at 0 degrees C for 30 min in a 500 microliters medium containing 50 mM Tris-HCl buffer, 0.1 mM EDTA (pH 7.4), 300 micrograms protein and varying concentrations of radiolabelled kainate ranging from 1 to 200 nM. Non-specific binding was determined in the presence of 1.0 mM glutamate. During brain development from 45 to 60 days gestation, Bmax value increased from 330 +/- 16 to 417 +/- 10 fmoles/mg protein; however, the Kd was unchanged (8.2 +/- 0.4 vs 8.8 +/- 0.5 nM, respectively). During hypoxia at 60 days, the Kd value significantly increased as compared to normoxic control (15.5 +/- 0.7 vs 8.8 +/- 0.5 nM, respectively), whereas the Bmax was not affected (435 +/- 12 vs 417 +/- 10 fmol/mg protein, respectively). At 45 days, hypoxia also increased the Kd (11.9 +/- 0.6 vs 8.2 +/- 0.4 nM) without affecting the Bmax (290 +/- 15 vs 330 +/- 16 fmol/mg protein, respectively). The results show that the number of kainate receptors increase during gestation without change in affinity and demonstrate that hypoxia modifies the high-affinity kainate receptor sites at both ages; however the effect is much stronger at 60 days (term). The decreased affinity of the site could decrease the kainate receptor-mediated fast kinetics of desensitization and provide a longer period for increased Na(+)-influx, leading to increased accumulation of intracellular Ca2+ by reversal of the Na(+)-Ca2+ exchange mechanism. In addition, Kd values for kainate-type glutamate receptor sites are 30-40 fold lower (i.e. higher affinity) than those for NMDA-displaceable glutamate sites. The higher affinity suggests that the activation of the kainate-type glutamate receptor during hypoxia could precede initiation of NMDA receptor-mediated excitotoxic mechanisms. We propose that hypoxia-induced modification of the high affinity kainate receptor in the fetus is a potential mechanism of neuroexcitotoxicity.

Transcranial optical path length in infants by near-infrared phase-shift spectroscopy

BACKGROUND

Near-infrared spectroscopy (NIRS) is an emerging technique for noninvasive, bedside monitoring of cerebral oxygenation and blood flow. Traditionally, it has relied on the Beer's Law relationship in which the concentration of light-absorbing oxygen-carrying pigments is proportional to their light absorbance, and inversely proportional to an optical path length (a measure of the distance traveled by photons passing through the tissue). In practice, NIRS has been based upon assumptions that mean transcranial optical path length, the average optical path length for a given patient, is constant among patients and independent of the wavelength of light used.

OBJECTIVE

The objective of our study was to measure mean optical transcranial path length in infants as a step in allowing quantitation of cerebral oxygenation.

METHODS

We measured mean transcranial optical path length in 34 infants, aged 1 day to 3 years, using amplitude-modulated phase-shift spectroscopy at 754 nm and 816 nm. Optical transcranial path lengths (mean +/- SEM) were 8.6 +/- 0.9 cm, 11.1 +/- 0.9 cm, and 11.3 +/- 0.9 cm at 754 nm, and 8.8 +/- 0.9 cm, 11.2 +/- 0.8 cm, and 11.1 +/- 0.9 cm at 816 nm, using emitter-detector separations of 1.8, 2.5, and 3.0 cm, respectively. Optical path length increased as emitter-detector separation, head circumference, or age increased. Variance in the ratio of mean optical path lengths at the two different wavelengths exceeded that accounted for by variation in repeated measures alone (p < 0.001), suggesting that optical path length is also not independent of wavelength.

CONCLUSIONS

NIRS instrument emitter-detector geometry, subject age, head size, and wavelength used each influence optical path length. Quantitative NIRS measurements in clinical use may require concurrent measurement of both absorbance and optical path length at each wavelength, or use of newer measures that are not based upon Beer's Law assumptions.

Neuroprotective effect of phenytoin against in utero hypoxic brain injury in fetal guinea pigs

The present study tests the hypothesis that phenytoin, an antiepileptic agent known to block Na+ and Ca2+ channels, will prevent hypoxic brain injury in the fetus by preventing lipid peroxidation and preserving Na+,K(+)-ATPase activity. Studies were performed in 37 fetuses obtained from pregnant guinea pigs at 58-60 days gestation (term). The pregnant guinea pigs were divided into four groups: a normoxic group, a hypoxic group, a normoxic group treated with phenytoin, and a phenytoin treated hypoxic group. There were eight to ten fetal guinea pigs in each group. The treatment groups were given phenytoin 30 mg/kg (50 mg phenytoin/ml solvent) intraperitoneally. Hypoxia was induced by exposing the guinea pigs to 7% oxygen for 60 min. This level of hypoxia has been shown to decrease ATP and phosphocreatine levels by 90%. The fetal brains were harvested and the brain cell membranes were prepared from each group of fetuses. Na+,K(+)-ATPase activity and lipid peroxidation products, measured as relative fluorescent intensity, were determined. The mean Na+,K(+)-ATPase activity in the control, hypoxic, phenytoin-normoxic and phenytoin-hypoxic groups was 56.4 +/- 9.7, 37.9 +/- 10.6, 47.0 +/- 8.4 and 52.0 +/- 9.7 mumol inorganic phosphate/mg protein per h, respectively. The hypoxic group had significantly less Na+,K(+)-ATPase activity than both the normoxic group (P < 0.01), and the phenytoin treated hypoxic group (P < 0.05). There was no significant difference between the normoxic group and either of the phenytoin-treated groups (P = ns).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of in utero hypoxia on the ouabain/strophanthidin binding site of the fetal guinea pig brain cell membrane Na+,K(+)-ATPase

This study investigates the effect of hypoxia on the high affinity strophanthidin/ouabain binding site of brain cell membrane Na+,K(+)-ATPase in 30, 45 and 60 day (term) fetal guinea pigs. Studies were performed on 30 fetuses randomized to either normoxic or hypoxic conditions. The hypoxic fetuses were exposed to maternal hypoxia (FiO2 = 7%) for 60 min. Brain cell membrane fractions were prepared, and the rate of ATP hydrolysis was determined at varying concentrations of strophanthidin. In every experiment with 45 and 60 day brain preparations hypoxia caused a leftward shift in the IC50, but this did not reach the level of statistical significance (4.0 x 10(-5) normoxic, 9.0 x 10(-6) hypoxic, P = 0.069, at 45 days; 9.5 x 10(-6) M normoxic, 8.5 x 10(-6) M hypoxic, P = 0.23, at 60 days). If hypoxia does cause a true left shift this would indicate greater sensitivity of the hypoxic brain to inhibitor. In addition, [3H]ouabain binding studies were performed. In the 30 day normoxic brain preparations, the Kd was 24.7 +/- 5.6 nM, and the Bmax was 0.26 +/- 0.08 pmol/mg protein. At 45 days the ouabain binding sites showed no change in affinity following hypoxia (Kd = 14.6 +/- 1.7 nM normoxic, 13.0 +/- 0.8 nM hypoxic, P = NS); however, there was a significant decrease in receptor number following hypoxia (Bmax = 22.1 +/- 2.2 pmol/mg protein normoxic, 16.9 +/- 0.3 pmol/mg protein hypoxic, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Brain cell membrane function during hypoxia in hyperglycemic newborn piglets

To test the hypothesis that acute hyperglycemia reduces changes in cell membrane structure and function during cerebral hypoxia in the newborn, brain cell membrane Na+,K(+)-ATPase activity and levels of membrane lipid peroxidation products were measured in four groups of anesthetized, ventilated newborn piglets: normoglycemia/normoxia (control, group 1, n = 12), hyperglycemia/normoxia (group 2, n = 6), untreated hypoxia (group 3, n = 10), and hyperglycemia/hypoxia (group 4, n = 7). Hyperglycemia (blood glucose concentration 20 mmol/L) was induced using the glucose clamp technique. The hyperglycemic glucose clamp was maintained for 90 min before onset of hypoxia and throughout the period of hypoxia. Cerebral tissue hypoxia was induced in groups 3 and 4 by reducing fraction of inspired oxygen for 60 min and was documented by a decrease in the ratio of phosphocreatine to inorganic phosphate as measured using 31P-nuclear magnetic resonance spectroscopy. Blood glucose concentration during hypoxia in hyperglycemic hypoxic animals was 20.7 +/- 1.2 mmol/L, compared with 10.3 +/- 1.7 mmol/L in untreated hypoxic piglets (p < 0.05). Peak blood lactate concentrations were not significantly different between the two hypoxic groups (8.4 +/- 2.8 mmol/L versus 7.8 +/- 1.6 mmol/L). In cerebral cortical membranes prepared from the untreated animals, cerebral tissue hypoxia caused a 25% reduction in Na+,K(+)-ATPase activity compared with normoxic controls and an increase in conjugated dienes and fluorescent compounds, markers of lipid peroxidation. In contrast, Na+,K(+)-ATPase activity and levels of lipid peroxidation products in hyperglycemic hypoxic animals were not significantly different from the values in control normoxic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of acute hypoglycemia on the cerebral NMDA receptor in newborn piglets

The effects of acute insulin-induced hypoglycemia on the cerebral NMDA receptor in the newborn were examined by determining [3H]MK-801 binding as an index of NMDA receptor function in 6 control and 7 hypoglycemic piglets. In hypoglycemic animals, the glucose clamp technique with constant insulin infusion was used to maintain a blood glucose concentration of 1.2 mmol/l for 120 min before obtaining cerebral cortex for further analysis; controls received a saline infusion. Concentrations of glucose, lactate, ATP, and PCr were measured in cortex, and Na+,K(+)-ATPase activity was determined in a brain cell membrane preparation. [3H]MK-801 binding was evaluated by: (1) saturation binding assays over the range of 0.5-50 nM [3H]MK-801 in the presence of 100 microM glutamate and glycine; and (2) binding assays at 10 nM [3H]MK-801 in the presence of glutamate and/or glycine at 0, 10, or 100 microM. Blood and brain glucose concentrations were significantly lower in hypoglycemic animals than controls. There was no change in brain ATP with hypoglycemia, but PCr was decreased 80% compared to control (P < 0.05). Na+,K(+)-ATPase activity was 13% lower in hypoglycemic animals (P < 0.05). Based on saturation binding data, hypoglycemia had no effect on the number of functional receptors (Bmax), but the apparent affinity was significantly increased, as indicated by a decrease in the Kd (dissociation constant) from the control value of 8.1 +/- 1.6 nM to 5.5 +/- 2.1 nM (P < 0.05). Augmentation of [3H]MK-801 binding by glutamate and glycine alone or in combination was also significantly greater in the hypoglycemic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of postasphyxial resuscitation with 100% and 21% oxygen on cortical oxygen pressure and striatal dopamine metabolism in newborn piglets

The present study tests the hypothesis that ventilation with 100% O2 during recovery from asphyxia leads to greater disturbance in brain function, as measured by dopamine metabolism, than does ventilation with 21% oxygen. This hypothesis was tested using mechanically ventilated, anesthetized newborn piglets as an animal model. Cortical oxygen pressure was measured by the oxygen-dependent quenching of phosphorescence, striatal blood flow by laser Doppler, and the extra-cellular levels of dopamine and its metabolites by in vivo microdialysis. After establishment of a baseline, both the fraction of inspired oxygen (FiO2) and the ventilator rate were reduced in a stepwise fashion every 20 min over a 1-h period. For the subsequent 2-h recovery, the animals were randomized to breathing 21 or 100% oxygen. It was observed that during asphyxia cortical oxygen pressure decreased from 36 to 7 torr, extracellular dopamine increased 8,300%, and dihydroxyphenylacetic acid and homovanillic acid decreased by 65 and 60%, respectively, compared with controls. During reoxygenation after asphyxia, cortical oxygen pressure was significantly higher in the piglets ventilated with 100% oxygen than in those ventilated with 21% oxygen (19 vs. 11 torr). During the first hour of reoxygenation, extracellular dopamine levels decreased to approximately 200% of control in the 21% oxygen group, whereas these levels were still much higher in the 100% oxygen group (approximately 500% of control). After approximately 2 h of reoxygenation, there was a secondary increase in extracellular dopamine to approximately 750 and approximately 3,000% of baseline for the animals ventilated with 21 and 100%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain cell membrane Na+,K(+)-ATPase activity after inhibition of cerebral nitric oxide synthase by intravenous NG-nitro-L-arginine in newborn piglets

This study investigated the effects of in vivo inhibition of cerebral nitric oxide synthase by intravenous administration of NG-nitro-L-arginine (NNLA) on the cell membrane Na+,K(+)-ATPase activity in the cerebral cortex of newborn piglets. NNLA was administered intravenously to 22 piglets at doses of 5 mg/kg (n = 3), 25 (n = 3), 50 (n = 4), 75 (n = 4), and 100 mg/kg (n = 2). Control animals (n = 6) received normal saline only. 90 min after infusion the cerebrum was obtained. The cerebral nitric oxide synthase activity, determined by measuring the conversion of [3H]-L-arginine into [3H]-L-citrulline in the brain homogenate, decreased from 9.1 +/- 2.0 pmol/mg protein/min in controls to 1.7 +/- 0.6 pmol/mg protein/min after the administration of 75 and 100 mg/kg NNLA. The Na+,K(+)-ATPase activity was measured in the P2 fraction of cortical tissue homogenate. The Na+,K(+)-ATPase activity was within the normal range (48.3 +/- 4.9 mumol/mg protein/h) up to 75 mg/kg of NNLA. At a dose of NNLA of 100 mg/kg, the Na+,K(+)-ATPase activity decreased to 31.5 +/- 0.7 mumol/mg protein/h (p < 0.05). Four animals developed hypoxemia and lactic acidosis. The results demonstrate that inhibition of the cerebral nitric oxide synthase activity in vivo in newborn piglets by intravenous administration of NNLA did not affect the cortical cell membrane Na+,K(+)-ATPase activity up to a dose of 75 mg/kg. Doses of 100 mg/kg decreased the Na+,K(+)-ATPase activity, probably by inducing cerebral hypoxia-ischemia.

Effect of nordihydroguaiaretic acid on cerebral blood flow and metabolism during hypoxia in newborn piglets

Nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, was investigated for its effect on cerebral blood flow (CBF) and cortical oxygen consumption during hypoxia in 9 anesthetized, ventilated newborn piglets. CBF was measured by radioactive microspheres while brain cortical metabolism was evaluated by continuous 31P-NMR spectroscopy. Five piglets were treated with NDGA (3 mg/kg i.v. in 50% ethanol as vehicle) prior to hypoxia and had CBF measured before NDGA (control), 15 min after NDGA (baseline) and then after 15 and 45 min of hypoxia following NDGA. Another 4 piglets were treated with vehicle (2 ml/kg 50% ethanol) under the same protocol. In the NDGA-treated piglets, cerebral cortical O2 consumption for a given PCr/Pi was significantly increased (p < 0.05) compared to non-NDGA. Since NDGA inhibits production of vasoconstricting leukotrienes during hypoxia, cortical capillary beds otherwise constricted may be perfused following NDGA, thus increasing the O2-consuming tissue area.

Proton MR spectroscopy of brain abnormalities in neonates born to HIV-positive mothers

PURPOSE

To examine the sensitivity of proton MR spectroscopy for detecting early central nervous system abnormalities in neonates born to human immunodeficiency virus (HIV)-positive mothers.

METHODS

Asleep, unsedated, and continuously monitored by electrocardiography, 10 newborns, 5 with HIV-positive and 5 with HIV-negative mothers, were studied within the first 10 days of life in a 1.5-T scanner. After T1- and T2-weighted images were obtained, proton spectra were performed using voxels of interest (3.4 cm3) in the deep parietooccipital white matter. Peaks were identified as N-acetyl-aspartate (2.0 ppm), creatine and phosphocreatine (3.0 ppm), choline (3.2 ppm), and inositol (3.5 ppm). Peak areas were used to calculate metabolic ratios: N-acetyl-aspartate to creatine, inositol to creatine, and creatine to choline.

RESULTS

All newborns of HIV-positive mothers had abnormal proton spectra compared with control infants; a nonspecific amino acid peak in the 2.1- to 2.6-ppm area was elevated, broad, and overlapping the N-acetyl-aspartate peak in all the HIV-exposed newborns and in only 1 of the 5 control newborns. The choline-to-creatine ratio was higher in HIV-exposed newborns at 2.3 +/- 0.4 (normal term, 0.9 +/- 0.3), as was the N-acetyl-aspartate-to-creatine ratio at 2.6 +/- 0.9 (for control subjects, 1.2 +/- 0.4). MR images from these brain regions were all considered normal. Because acquired immunodeficiency syndrome develops in only a small fraction of neonates born to HIV-seropositive mothers, the above spectral abnormalities found in all our subjects may result from indirect effects of HIV, such as intrauterine growth retardation.

CONCLUSIONS

These findings indicate that proton MR spectroscopy might play an important role in detecting early central nervous system complications in newborns of HIV-seropositive mothers.

Effect of hemorrhagic hypotension on extracellular level of dopamine, cortical oxygen pressure and blood flow in brain of newborn piglets

The present study describes the relationships between extracellular striatal dopamine, cortical oxygen pressure and blood flow in brain of newborn piglets during hemorrhagic hypotension. Cerebral oxygen pressure was measured optically by the oxygen dependent quenching of phosphorescence; extracellular dopamine by in vivo microdialysis; striatal blood flow was monitored by a laser Doppler. Following a 2 h stabilization period after implanting the microdialysis and laser Doppler probes in the striatum, the mean arterial blood pressure (MABP) was decreased in stepwise manner from 87 +/- 4 Torr (control) to 35 +/- 5 Torr, during 63 min. The whole blood was then reinfused and measurements were continued for 45 min. Statistically significant decrease in blood flow, 10%, was observed when arterial blood pressure decreased to about 53 Torr. With further decrease blood pressure to 35 Torr, blood flow decreased to about 35% of control (P < 0.01). Cortical oxygen pressure decreased almost proportional to decrease in blood pressure. The progressive decrease in MABP from 87 +/- 4 Torr to 65 +/- 6, 52 +/- 7, and 35 +/- 5 Torr resulted in cortical oxygen pressure decreasing from 45 +/- 4 Torr to 33 +/- 3 Torr (P < 0.05), 24 +/- 4 Torr (P < 0.01) and 13 +/- 3 Torr (P < 0.01). The levels of extracellular dopamine in the striatum increased with decreasing cortical oxygen pressure. As cortical oxygen decreased, the extracellular dopamine increased to 230%, 420% and 3200% of control, respectively. Our results show that in mild hypotension total blood flow is well maintained but oxygen pressure in the microvasculature decreases, possibly due to heterogeneity in the regulatory mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effect of alpha-tocopherol on brain cell membrane function during cerebral cortical hypoxia in newborn piglets

Protective effect of alpha-tocopherol on the structure and function of brain cell membranes was investigated by measuring Na+,K(+)-ATPase activity and products of lipid peroxidation (fluorescent compounds) in brain cell membranes obtained from newborn piglets. Four groups of anesthetized, ventilated piglets were studied: five hypoxic piglets and five normoxic piglets were pretreated with free alpha-tocopherol (20 mg/kg/dose i.m.), five additional hypoxic piglets received i.m. placebo and five normoxic piglets served as control. Placebo and alpha-tocopherol were given 48 and 3 h prior to onset of hypoxia. Hypoxic hypoxia was induced and cerebral hypoxia was documented as a decrease in the ratio of phosphocreatine to inorganic phosphate (PCr/P(i)) using 31P NMR spectroscopy. PCr/P(i) decreased from baseline of 2.62 +/- 0.54 to 1.05 +/- 0.27 in alpha-tocopherol-pretreated and from 2.44 +/- 0.48 to 1.14 +/- 0.30 in the placebo-pretreated group during hypoxia. Na+,K(+)-ATPase activity was unchanged in both normoxic and hypoxic alpha-tocopherol-pretreated groups. However, in placebo-pretreated hypoxic group, Na+,K(+)-ATPase activity decreased as compared with control (44.9 +/- 9.7 vs. 61.8 +/- 5.7 mumol P(i)/mg protein/h, P < 0.005). The level of fluorescent compounds increased in placebo-pretreated but not in alpha-tocopherol-pretreated group as compared with control. During hypoxia, serum alpha-tocopherol levels were higher in alpha-tocopherol-pretreated groups as compared with placebo-pretreated hypoxic group. The present data indicates that alpha-tocopherol protects brain cell membranes in newborn piglets from lipid peroxidative damage during tissue hypoxia probably by being incorporated in cell membrane and also as circulating antioxidant.

Brain cell membrane modification following hypercapnia and recovery in newborn piglets

The effect of hypercapnia on brain cell membrane structure and function was studied in anesthetized newborn piglets. Lipid peroxidation products (conjugated dienes and fluorescent compounds), Na+,K(+)-ATPase activity and enzyme affinity to ATP (substrate), K+ and Na+ ions (activators), and strophanthidin (inhibitor) were measured in three groups of animals: controls, those exposed to 90 minutes of PaCO2 > 80 mmHg (hypercapnia) and those exposed the same way, following restoration of normal PaCO2 (recovery). Enzyme activity was unchanged by hypercapnia, but enzyme affinity was altered as indicated by an increase in ATP affinity. Affinities to Na+, K+, and strophanthidin were unchanged. Restoration of normal PaCO2 resulted in an increase in conjugated dienes. The data demonstrate that hypercapnia followed by restoration of normal PaCO2 in healthy term newborn piglets is associated with mild modification of brain cell membrane Na+,K(+)-ATPase, possibly due to lipid peroxidation.

Effect of allopurinol on uric acid levels and brain cell membrane Na+,K(+)-ATPase activity during hypoxia in newborn piglets

Oxygen-free radicals generated by xanthine oxidase during hypoxia-ischemia may result in cellular injury through harmful effects on membrane phospholipids. The present study investigated the effect of administration of allopurinol, an inhibitor of xanthine oxidase, on free-radical generation and brain cell membrane injury during hypoxia by inhibiting the breakdown of hypoxanthine to uric acid. Brain cell membrane Na+,K(+)-ATPase activity and lipid peroxidation products (conjugated dienes and fluorescent compounds) were determined as indices of brain membrane function and structure. Cerebral oxygenation was continuously monitored during hypoxia by 31P-NMR spectroscopy. Plasma and brain tissue levels of uric acid were measured to evaluate xanthine oxidase activity and purine degradation. Na+,K(+)-ATPase activity decreased significantly in both hypoxic groups; however, the allopurinol-treated hypoxic group showed a smaller decrease than the untreated hypoxic group (47.3 +/- 4.9 vs. 42.0 +/- 2.7 mumol Pi/mg protein/h, P < 0.05), respectively. Conjugated dienes increased significantly in the untreated hypoxic compared to control animals (0.070 +/- 0.045 vs. 0.004 +/- 0.006 mumol/g brain, P < 0.05), with the allopurinol-treated animals having intermediate values (0.053 +/- 0.039 mumol/g brain). Fluorescent compounds were lower in the allopurinol-treated hypoxic group compared to the untreated hypoxic group (0.79 +/- 0.19 vs. 1.06 +/- 0.60 micrograms/quinine sulfate/g brain, P < 0.05). Measurements of serum and brain tissue uric acid were significantly lower during hypoxia in the allopurinol-treated compared to the untreated group (30.3 +/- 15.6 vs. 45.7 +/- 10.6 microM (P < 0.05) and 1.69 +/- 0.97 vs. 4.27 +/- 2.37 nmol/g (P < 0.05), respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Functional analysis of spontaneous movements in preterm infants

Spontaneous movements of premature infants between 25 and 34 weeks conceptional age were observed for 1 hr on two or three occasions. Subjects had low-risk prognoses and were clinically stable at the time of testing. Behavioral acts were scored using a 0/1 time sampling technique in 60 continuous, 1-min time blocks. Temporal associations between individual movements were found using chi-square analyses. Some associated behaviors contained combinations consistent with neonatal action patterns, for example, single and bilateral leg kicking, head turning, and mouthing. Features of state organization were also evident in that general motor activity (GM), which has been used as a marker of active sleep (AS) in neonates, was found to cluster temporally with startle, facial, and head movements but not eye movements. Behavioral quiescence (> or = 5 s) was dissociated from AS-related behaviors (GM, facial, head, and eye movements). Combinations of state-segregated behaviors were more likely to exhibit co-occurrence within 1-min intervals in infants 30 weeks conceptional age and older.

Relationship of extracellular dopamine in striatum of newborn piglets to cortical oxygen pressure

The present studies describes the relationship between extracellular dopamine in striatum of newborn piglets and cortical oxygen pressure. The extracellular level of dopamine was measured by in vivo microdialysis and the oxygen pressure in the cortex was measured by phosphorescence lifetime of oxygen probe in the blood. Controlled, graded levels of hypoxic insult to the brain of animals were generated by decreasing of the oxygen fraction in the inspired gas (FiO2) from 21% to 14%, 11%, and 9%. This resulted in decrease in the cortical oxygen pressure from 31-35 Torr to about 24 Torr, 15 Torr and 4 Torr, respectively. The changes in extracellular level of dopamine, DOPAC and HVA were dependent on changes in cortical oxygen pressure. Stepwise decrease in the cortical oxygen pressure (see above) caused increases in extracellular dopamine of about 80%, 200% and 550%, respectively. The levels of DOPAC and HVA progressively decreased and when cortical oxygen decreased to 4-6 Torr were about 50% and 70% of control, respectively. After return of FiO2 to control (21%), the cortical oxygen pressure rapidly increased to above normal, then returned to control values. The extracellular levels of dopamine, DOPAC, and HVA recovered more slowly, attaining control values in about 30 minutes. The data show that extracellular levels of dopamine increase with even very small decreases in oxygen pressure. Thus, there is no "oxygen reserve" which protects dopamine release and metabolism from decrease in oxygen pressure.

Protective effect of MgSO4 infusion on nmda receptor binding characteristics during cerebral cortical hypoxia in the newborn piglet

This study tests the hypothesis that magnesium, a selective non-competitive antagonist of the NMDA receptor, will attenuate hypoxia-induced alteration in NMDA receptors and preserve MK-801 binding characteristics during cerebral hypoxia in vivo. Anesthetized, ventilated and instrumented newborn piglets were divided into three groups: normoxic controls were compared to untreated hypoxic and Mg(2+)-treated hypoxic piglets. Cerebral hypoxia was induced by lowering the FiO2 to 5-7% and confirmed biochemically by a decrease in the levels of phosphocreatine (82% lower than control). The Mg(2+)-treated group received MgSO4 600 mg/kg over 30 min followed by 300 mg/kg administered during 60 min of hypoxia. Plasma Mg2+ concentrations increased from 1.6 +/- 0.1 mg/dl to 17.7 +/- 3.3 mg/dl. 3H-MK-801 binding was used as an index of NMDA receptor modification. The Bmax in control, hypoxic and Mg(2+)-treated hypoxic piglets was 1.09 +/- 0.17, 0.70 +/- 0.25 and 0.96 +/- 0.14 pmoles/mg protein, respectively. The Kd for the same groups were 10.02 +/- 2.04, 4.88 +/- 1.43 and 8.71 +/- 2.23 nM, respectively. The Bmax and Kd in the hypoxic group were significantly lower compared to the control and Mg(2+)-treated hypoxic groups, indicating a preservation of NMDA receptor number and affinity for MK-801 during hypoxia with Mg2+. The activity of Na+, K+ ATPase, a marker of neuronal membrane function, was lower in the hypoxic group compared to the control and Mg(2+)-treated hypoxic groups. These findings show that MgSO4 prevents the hypoxia-induced modification of the NMDA receptor and attenuates neuronal membrane dysfunction.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of brain cell membrane function following cocaine exposure in the fetal guinea pig

The effect of cocaine on brain cell membrane structure and function was studied in the fetal guinea pig. We tested the hypothesis that cocaine, a potent vasoconstrictor, would result in brain cell membrane dysfunction as determined by altered activity of Na+,K(+)-ATPase and the appearance of products of membrane lipid peroxidation (conjugated dienes (CD) and fluorescent compounds (FC)). A total of 14 pregnant guinea pigs were studied at term (60 days). One hour prior to delivery, the pregnant guinea pigs were divided into 3 groups as follows: cocaine, 30 mg/kg i.p., saline placebo i.p., or 7% FiO2 for 1 h. Following cocaine, brain Na+,K(+)-ATPase activity decreased (mean +/- S.D., 25.6 +/- 9.2 vs. 54.6 +/- 3.4 mumol Pi/mg protein/h, cocaine vs. control, respectively, P < 0.01) and was similar to the hypoxia group (21.9 +/- 2.8 mumol Pi/mg protein/h). The products of lipid peroxidation did not change significantly following cocaine whereas hypoxia resulted in a rise in CD from 0 to 0.175 +/- 0.015 mumol/g brain, control vs. hypoxia, (P < 0.01), and FC from 1.13 +/- 0.15 to 1.88 +/- 0.13 micrograms quinine sulfate/g brain, control vs. hypoxia, (P < 0.01). These data show that acute fetal cocaine exposure, unlike hypoxia alone, results in a significant decrease in Na+,(K+)-ATPase activity without a significant increase in the products of lipid peroxidation, suggesting the mechanism by which cocaine affects brain cell membrane integrity is distinct from hypoxia. Inhibition of the enzyme activity may be due to a direct action of cocaine on the enzyme or due to enzyme regulation by cocaine-induced alterations in neurotransmitters.

Hypoxia-induced modification of the N-methyl-D-aspartate receptor in the brain of the newborn piglet

The effect of hypoxia on the N-methyl-D-aspartate (NMDA) receptor/ion channel complex in the brain cell membrane of the newborn piglet was studied. Experiments were conducted on newborn piglets, 2-4 days of age, that were anesthetized and mechanically ventilated. Hypoxic hypoxia was induced in the experimental group by lowering the FiO2 to 5-7%. The control group was ventilated under normoxic conditions. Tissue hypoxia was documented biochemically by decreased levels of ATP and phosphocreatine (PCr) in the hypoxic group (52% and 81% lower than the normoxic group, respectively). [3H]MK-801 binding characteristics (Bmax = number of receptors, Kd = dissociation constant) were used as an index of NMDA receptor modification. In hypoxic brains, Bmax decreased from the control level of 1.13 +/- 0.15 pmol/mg protein to 0.68 +/- 0.23 pmol/mg protein (P < 0.01) and the Kd value decreased (reflecting increased affinity) from 9.46 +/- 1.68 nM in the control brains to 4.87 +/- 1.42 nM (P < 0.01) in the hypoxic brains. The Na+,K(+)-ATPase activity, an index of brain cell membrane function, decreased from a control value of 46.5 +/- 0.4 to 40.5 +/- 2.3 mumol inorganic phosphate (Pi) mg protein/h (P < 0.005) during hypoxia. The results of this study indicate that hypoxia in newborn piglets modifies the NMDA receptor in the cerebral cortex, resulting in an increased affinity of the receptor channel. Hypoxia-induced modification of the NMDA ion/receptor complex may be a potential mechanism of cerebral excitotoxicity.

Cerebral blood flow and metabolism during repeated asphyxias in newborn piglets: influence of theophylline

The effect of theophylline on cerebral blood flow (CBF), oxygen transport, and energy metabolism was investigated during and following brief episodes of asphyxia. CBF was determined by microspheres during control, asphyxia, and recovery with reventilation after a single asphyxia (recovery I) and after 7 repeated asphyxias (recovery II). In addition, cerebral energy metabolism by 31P NMR spectroscopy and cerebral oxygen consumption (CMRO2) in newborn piglets treated with 30 mg/kg theophylline (serum levels 22-25 micrograms/ml) were compared with nontreated piglets. Theophylline increased CMRO2 during recovery I (348 mumol O2/min/100 g vs. 144 for non-theophylline) but not during control, asphyxia, or recovery II. There was no significant difference between the theophylline and non-theophylline groups in depletion of phosphoenergetics as measured by 31P NMR.

Effect of cyclooxygenase inhibition on brain cell membrane lipid peroxidation during hypoxia in newborn piglets

To test the hypothesis that indomethacin, an inhibitor of cyclooxygenase, reduces free radical-induced brain cell membrane changes during cerebral hypoxia, we determined levels of brain cell membrane lipid peroxidation products and Na+,K(+)-ATPase activity as indicators of free radical production and membrane function, respectively, in 29 newborn piglets divided into 4 groups. Eight saline- and 4 indomethacin-treated normoxic animals served as controls; 8 saline-pretreated piglets and 9 piglets pretreated with indomethacin were exposed to hypoxic hypoxia for 60 min. Cerebral hypoxia was documented using 31P-NMR spectroscopy. In saline-pretreated hypoxic animals Na+,K(+)-ATPase activity decreased significantly and levels of membrane lipid peroxidation products increased significantly compared to normoxic controls. Indomethacin pretreatment prevented the hypoxia-induced increase in membrane lipid peroxidation products but had no effect on the decrease in Na+,K(+)-ATPase activity. Thus the apparent reduction in free radical production by indomethacin pretreatment did not prevent the hypoxia-induced change in Na+,K(+)-ATPase activity.

Spontaneous motility in premature infants: features of behavioral activity and rhythmic organization

The spontaneous motor activity of clinically stable premature infants, 26-36 weeks gestational age, was investigated. Movements were recorded using a pressure-sensitive transducer positioned below the infant's head and torso. Behavior samples were digitized every 0.5 s during 2 and 3-hr continuous recording sessions. Time-series analyses revealed prominent motility cycles of circa 80 min and circa 30 min. These results are consistent with periodicities in motility and REM activation observed in full-term neonates. The longer rhythms of 70-100 min of motility found in this study establish that these periods are present at this stage of development independent of maternal zeitgebers. Developmental changes in motility rhythms and movement burst durations were also observed. Bout durations became somewhat longer in older (> 30 weeks) infants, but the relative time devoted to movement per session was comparable in older and younger (< or = 30 weeks) infants.

Nuclear magnetic resonance imaging and spectroscopy following asphyxia

Nuclear magnetic resonance imaging and spectroscopy have added significant new information about the newborn brain during and following asphyxia. NMR imaging has permitted sequential in vivo analysis of CNS maturation in the perinatal period that is superior in anatomic resolution, and especially in the characterization of myelination, to either cranial ultrasound or radiographic computed tomography. As a result, the accurate detection and recognition of the brain lesions associated with hypoxic-ischemic encephalopathy is now possible, including PVL, cerebral infarction, intraparenchymal and intraventricular hemorrhage, and delayed myelination. This has improved our understanding of the associated potential risk for abnormal neuro-developmental outcome with specific lesions. NMR spectroscopy has provided a metabolic window into the biochemical events during and following asphyxia. 31P MRS captures the phosphorous metabolites as levels rise and fall and shift in relation to each other to maintain cellular energy homeostasis in the face of oxygen depletion. Meanwhile, proton NMR spectroscopy promises to sustain the metabolic purview beyond the immediate cellular response to asphyxia to the chronic adaptation phase. Appropriately applied, this noninvasive technology may yet enable us to identify brain injury that is reversible in sufficient time to intervene and to diagnose accurately what is irreversible for timely prognostication. Furthermore, the integration of clinical imaging and spectroscopy capabilities is both feasible and desirable; information provided by each being mutually complementary. Imaging could improve spectroscopy interpretation by identifying the observed tissue, whereas MRS should clarify diagnosis of anatomic lesions detected by MRI. Advances in spatial resolution and speed of data acquisition may soon make integrated MRI/MRS a clinical reality.

Brain cell membrane Na+,K(+)-ATPase modification following hypoxia in the guinea pig fetus

The effect of hypoxia in utero on the affinity of the active sites of Na+,K(+)-ATPase for Na+, K+ and ATP of the fetal guinea pig brain was investigated. Brain cell membranes were prepared from normoxic and hypoxic guinea pig fetuses, and a detailed enzyme kinetics analysis was carried out. In the hypoxic fetal brain membranes the Ka 0.5 for Na+ and K+ increased 104% and 20+, respectively, indicating a decrease in the affinity of the active sites of the enzyme for Na+ and K+. The affinity of the ATP site increased, as indicated by a decrease in the Km of 37% in hypoxic brain. The results indicate that changes in the affinity of active sites would affect the phosphorylation and dephosphorylation mechanisms of the Na+,K(+)-ATPase reaction. The increased affinity for ATP will favor the phosphorylation step, but will be opposed by the decrease in the Na+ affinity. The decreased affinity of the active site for K+ would oppose the dephosphorylation of the enzyme-P complex causing the enzyme to be trapped in an inactive phosphorylated state. The results demonstrated the sensitivity of the Na+,K(+)-ATPase active sites to hypoxia, and illustrated a selective modification of the enzyme active sites under hypoxic conditions, a key mechanism altering the cell membrane function leading to hypoxic brain damage.

Activity of tyrosine hydroxylase in the striatum of newborn piglets in response to hypocapnic hypoxia

The effect of graded hypoxia induced by hyperventilation on the activity of tyrosine hydroxylase was measured in vivo by microdialysis. Microdialysis probes were inserted into the striatum of newborn piglets and perfused with medium containing 3-hydroxybenzylhydrazine, an inhibitor of L-aromatic amino acid decarboxylase. The level of 3,4-dihydroxyphenylalanine (DOPA) measured in the effluent dialysate was then an index of tyrosine hydroxylase activity. The oxygen pressure in the veins and capillaries of the cortex was measured, through a cranial window placed over the parietal cortex, by the phosphorescence lifetime of palladium-meso-tetra(4-carboxyphenyl)porphine added to the blood. After baseline measurements, PaCO2 was decreased from 38 torr (control value) to 19, 13, and 11 torr resulting in decreases in the cortical oxygen pressure from 40 +/- 6 torr to 26 +/- 3, 23 +/- 4, and 20 +/- 4 torr, respectively. Decrease in the oxygen pressure to 26 +/- 3 torr caused a statistically significant increase of 25-30% in the level of DOPA in the effluent perfusate. During the next step of increase in ventilator rate, when oxygen decreased only slightly, the level of DOPA remained at the higher level. Ventilation rates that lowered the oxygen pressure to below 20 torr, however, caused a progressive decrease in the level of DOPA. During recovery, the level of DOPA steadily increased, attaining 160% of control value after 1.5 h. When the oxygen pressure was decreased to 16 +/- 2 torr by a single increase in ventilator rate, the DOPA level decreased in the effluent to 15-20% below control.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of NMDA receptor by in vitro lipid peroxidation in fetal guinea pig brain

The effect of lipid peroxidation on the NMDA receptor and its modulatory sites in fetal guinea pig brain cell membranes was examined. P2 membrane fractions were prepared from the fetal brain tissue and peroxidized in the presence of ferric chloride and ascorbate. [3H]-MK-801-binding studies were performed and Bmax (number of binding sites) and Kd (affinity) values were used as indices of NMDA receptor modification. In lipid-peroxidized membranes the Kd value increased from 6.76 +/- 2.69 in control to 15.12 +/- 7.38 nM (P < 0.01), indicating a decreased affinity of NMDA receptors following lipid peroxidation. However, there was no significant change in Bmax. The glutamate- and glycine-dependent increase in activation was 40% lower in lipid-peroxidized membranes as compared to control. The spermine-dependent activation was also significantly reduced following lipid peroxidation as compared to control suggesting decreased affinity of spermine site. The results of this study indicate that lipid peroxidation modifies recognition, coactivator and spermine sites of NMDA receptor by decreasing its affinity without affecting the number of binding sites. Normal activation of NMDA receptor is important for neuritic growth, synaptogenesis, long-term potentiation and synaptic plasticity. Therefore, we speculate that any clinical condition causing lipid peroxidation of brain cell membranes could jeopardize these maturational processes in the developing brain causing neurological impairment.

Decreased erythrocyte Na+,K(+)-ATPase activity associated with cellular potassium loss in extremely low birth weight infants with nonoliguric hyperkalemia

To determine whether a shift of potassium ions from the intracellular space to the extracellular space accounts, in part, for the hyperkalemia seen in extremely low birth weight infants, we examined potassium concentration in serum and erythrocytes from extremely low birth weight infants with hyperkalemia (n = 12) or with normokalemia (n = 27). In addition, to determine whether the shift of potassium was associated with low sodium-potassium-adenosinetriphosphatase (Na+,K(+)-ATPase) activity, we studied the activity of ATPase in the last 16 infants enrolled in the study. Fluid intake and output were measured during the first 3 days of life. Infants were considered to have hyperkalemia if the serum potassium concentration was 6.8 mmol/L or greater. Blood was obtained daily for intracellular sodium and potassium levels by means of lysis of erythrocytes. The remaining erythrocyte membranes were frozen and analyzed for Na+,K(+)-ATPase activity. There were significantly lower intracellular potassium/serum potassium ratios in the infants with hyperkalemia for each day of the 3-day study (p < 0.001). In the hyperkalemic group, there was lower Na+,K(+)-ATPase activity than in the infants with normokalemia (p = 0.006). Low Na+,K(+)-ATPase activity was associated with lower intracellular potassium/serum potassium ratios (p = 0.006), higher serum potassium values (p = 0.02), and lower intracellular potassium concentration (p = 0.009). The urinary data demonstrated that there was no difference in glomerulotubular balance between the two groups. We conclude that nonoliguric hyperkalemia in extremely low birth weight infants may be due, in part, to a shift of potassium from the intracellular space to the extracellular space associated with a decrease in Na+,K(+)-ATPase activity.

Selective sensitivity of synaptosomal membrane function to cerebral cortical hypoxia in newborn piglets

The effect of hypoxia on the structure and function of the synaptosomal membranes and myelin fraction (glial cells, neuronal cells bodies and axonal membranes) was investigated by measuring Na+,K(+)-ATPase activity and levels of lipid peroxidation products in cerebral cortical synaptosomal membranes and myelin fractions obtained from newborn piglets. Hypoxic hypoxia was induced and cerebral hypoxia was documented as a decrease in the ratio of phosphocreatine to inorganic phosphate (PCr/Pi) using 31P-NMR spectroscopy. PCr/Pi decreased from baseline of 2.93 +/- 0.76 to 0.61 +/- 0.36 during hypoxia. The synaptosomal membrane Na+,K(+)-ATPase activity decreased from a control value of 56.6 +/- 3.7 to 40.4 +/- 6.0 mumol Pi/mg protein/h during hypoxia. The level of conjugated dienes increased from zero (reference value) to 4.5 +/- 2.7 nmol/mg lipid and the level of fluorescent compounds increased from 23.5 +/- 2.2 to 92.6 +/- 46.4 ng quinine sulfate/mg lipid in the synaptosomal membranes during hypoxia. No change in myelin fraction Na+,K(+)-ATPase activity or levels of lipid peroxidation products were noted. These data indicate that synaptosomal membranes, rich in polyunsaturated fatty acids, are more susceptible to oxygen free radical mediated lipid peroxidative damage during hypoxia.

Modification of modulatory sites of NMDA receptor in the fetal guinea pig brain during development

Ontogeny of the NMDA receptor and modification of its modulatory sites in the developing fetus brain was determined. MK-801 binding characteristics in the presence of glycine, glutamate, Mg2+ and spermine were determined and used as an index of NMDA receptor modification. Experiments were performed on guinea pig fetuses at 30, 45, 50, 55, and 60 days (term = 63 days) of gestation. The Bmax value increased approximately three-fold from 30 days to 60 days of gestation. The Kd value decreased during the 45-50 day period and then increased toward the end of gestation. The Bmax value reached its maximum level by 55 days of gestation, indicating the presence of a maximum number of NMDA receptors by this age, while the apparent affinity of the receptor showed its peak at 45-50 days of gestation, indicating a potential role for NMDA receptor during the proliferation period of brain development in the guinea pig fetus. The activation of NMDA receptor in the presence of glutamate (10 microM) and glycine (10 microM), as measured by MK-801 binding, was absent at 30 days gestation, with the earliest observation occurring at 35 days gestation. The spermine dependent activation decreased with gestational age. Mg2+ ions increased MK-801 binding in the range of 1-20 microM concentration. Sensitivity to Mg2+ dependent activation increased with the gestational age (from 10 microM Mg2+ at 45 days to 2.5 microM at 55 and 60 days).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of graded levels of tissue oxygen pressure on dopamine metabolism in the striatum of newborn piglets

The effect of graded levels of tissue hypoxia on the extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid has been monitored in vivo by microdialysis. Reproducible levels of decreased oxygen in the brain were obtained by increasing the rate of ventilation from the control value of 25/min to as high as 95/min. With increasing ventilatory rate, the oxygen pressure in the cortex decreased from approximately 40 torr to 16 torr. As the oxygen pressure decreased stepwise from 40 to 27, 22, and 16 torr, the dopamine levels in the extracellular medium rose by 70, 90, and 150%, respectively, returning to baseline within a few minutes of return to control ventilation rates. Levels of the catabolic products 3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid decreased with decreasing tissue oxygen. Unlike the dopamine levels, these catabolite levels continued to decrease through 30 min of recovery (to 50% of control), returning to baseline only after recovery periods of 1-2 h. These data suggest that hypoxia induces long-term alterations in the neurotransmitter turnover. The marked effects of mild tissue hypoxia (decrease of oxygen from 40 torr to 26 torr) on both the extracellular dopamine concentration and dopamine metabolism indicate that the metabolic consequences of decreased tissue oxygen pressure extend to higher values than generally appreciated.

NMDA receptor modification in the fetal guinea pig brain during hypoxia

The effect of maternal hypoxia on the modification of the fetal brain cell membrane N-methyl-D-aspartate (NMDA) receptor and its modulatory sites was investigated. Experiments were conducted in pregnant guinea pigs of 60 days of gestation. Guinea pig fetuses were exposed to maternal hypoxia (FiO2 = 7%) for 60 minutes. Tissue hypoxia in the fetal brain was documented biochemically by decreased levels of ATP and phosphocreatine (91.3% and 88.6% lower than normoxia, respectively). MK-801 binding characteristics (Bmax = number of receptors, Kd = affinity of receptor) were used as an index of NMDA receptor modification. P2 membrane fraction was prepared from the cortex of normoxic and hypoxic fetal brain and washed thoroughly before carrying out the binding assay. In hypoxic brains, Bmax decreased from the normoxic control level 0.79 +/- 0.03 pmol/mg protein to 0.58 +/- 0.03 pmol/mg protein (P < 0.005) and Kd value decreased (increased affinity) from 8.54 +/- 0.27 nM to 4.01 +/- 0.23 nM (P < 0.005) respectively. The MK-801 binding in the absence of added glutamate and glycine in hypoxic brain was 100% higher as compared to controls, indicating an increased sensitivity of the NMDA receptor to activation. The spermine dependent maximum activation of the NMDA receptor increased to 44% in the hypoxic animals as compared to 25% in controls. The Mg2+ response of the NMDA receptor was not affected by hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of cerebral deoxygenation during deep hypothermic circulatory arrest in neonates

Brain injury associated with neonatal congenital heart operations performed during deep hypothermia and/or total circulatory arrest is often attributed to cerebral hypoxia. We studied the kinetic changes in cerebrovascular hemoglobin O2 saturation (HbO2%) and total hemoglobin concentration (Hbtotal) in 17 neonates undergoing cardiac surgery as they were cooled to 15 degrees C, underwent total circulatory arrest, and were rewarmed. HbO2% and Hbtotal in brain vasculature were monitored noninvasively by near-infrared spectroscopy. Neonates were cooled over 12 min and rewarmed over 15 min while being perfused using cardiopulmonary bypass (CPB). Total circulatory arrest lasted from 20 to 70 min. We found that HbO2% in brain vasculature increased during the initial 8 min of CPB as nasopharyngeal temperature decreased, and then remained constant until circulatory arrest. After the onset of circulatory arrest, cerebrovascular HbO2% decreased curvilinearly for 40 min; no further hemoglobin desaturation was observed from 40 to 70 min of arrest. The changes in cerebrovascular Hbtotal were quite different from those in HbO2%, as Hbtotal decreased during the initial minute of CPB and circulatory arrest and then remained constant until recirculation. Brain intravascular HbO2% and Hbtotal increased within 3 min after the onset of recirculation to prearrest levels, and during rewarming, HbO2% decreased to normothermic baseline values. The results demonstrate that cerebral oxygenation increased during CPB cooling; O2 was consumed by the neonatal brain during the initial 40 min of deep hypothermic circulatory arrest; and cerebral oxygenation was restored on recirculation. These observations may be important in identifying the etiologies of brain injury during neonatal congenital heart surgery.

Adaptive control of inspired oxygen delivery to the neonate

Adaptive adjustment of inspired oxygen (FIO2), based on a desired percent arterial hemoglobin saturation (SO2) was achieved by on-line bedside control of the oxygen concentration delivered to the neonate. Fourteen infants with bronchopulmonary dysplasia (BW, 860 +/- 80 g; GA, 26 +/- 1 weeks; study age, 41 +/- 8 days) receiving oxygen-air mixtures by hood were studied. The desired range of SO2 from 92 to 96% with a target value of 95% was determined by pulse oximetry and maintained with adjustment of FIO2 using three modes: 1) standard neonatal intensive care protocol with oxygen delivery evaluated at 20 minutes intervals; 2) bedside manual control with FIO2 manipulation every 2 to 5 minutes; and 3) adaptive control with on-line adjustment of FIO2 according to a specifically designed adaptive program. Each study period was of 40 minute duration. SO2 values within a steady 94 to 96% range was achieved for 54% of the time with standard protocol, compared to 69% (P less than 0.01) with bedside manual control and 81% (P less than 0.01) with adaptive control. In addition, fluctuations in SO2 values and overshoots were less apparent with adaptive control of oxygen delivery. These data describe adaptive FIO2 control as an efficient alternative technique for achieving a stable desired range of oxygenation in neonates.

Fluctuations in cerebral oxygenation and blood volume during endotracheal suctioning in premature infants

To investigate the effect that suctioning of the endotracheal tube has on the cerebral circulation, we monitored brain intravascular hemoglobin saturation (tHbo2%), cerebral blood volume (CBV), and arterial hemoglobin saturation (Spo2) during suctioning in 12 infants (24 to 33 weeks of gestational age) with respiratory distress syndrome treated with mechanical ventilation. The tHbo2% and CBV values were monitored over the forebrain by dual-wavelength near-infrared spectroscopy, and Spo2 was monitored by pulse oximetry of a finger. The monitored variables were stable during the baseline period. With suctioning, Spo2 decreased from 94% +/- 1% to 84% +/- 1%, tHbo2% decreased, and CBV increased (p less than 0.05). Desaturation in the arterial and cerebral circulations began within 5 seconds of the onset of suctioning. Arterial reoxygenation began with the onset of reventilation, whereas reoxygenation in the brain was delayed by 15 seconds. The Spo2, tHbo2%, and CBV values returned to baseline within 1 minute of reventilation. Studies were repeated in six of the infants after the fraction of inspired oxygen was increased to attain a baseline Spo2 of 100%. In the preoxygenated infants, tHbo2%, CBV, and Spo2 remained constant during suctioning. These studies confirm that endotracheal suctioning results in transient hypoxemia, and demonstrate that this is reflected in the brain by vasodilation and deoxygenation. These effects are preventable by preoxygenation before suctioning.

Effect of hyperventilation on oxygenation of the brain cortex of neonates

A new phosphorescence imaging method (Rumsey et al, Science (1988) 1649) has been used to continuously monitor the oxygen pressure in the blood of the cerebral cortex of newborn pigs. The animals' blood pressure was continuously measured and PaCO2, PaO2 and arterial blood pH were measured periodically. The oxygen pressure in the blood was quantitatively determined for regions of about 100 um square within the image (from a total field of about 3 mm diameter). It was observed that during hyperventilation, which lowered PaCO2 and increased pH of the blood, oxygen pressure decreased in proportion to the decrease in PaCO2. For example, hyperventilation which decreased PaCO2 from its normal value of 40 Torr to 10 Torr caused a rapid (within 5 minutes) decrease in oxygen pressure in the blood of capillaries and veins to approximately 1/4 of normal.

Brain cell membrane dysfunction following acute asphyxia in newborn piglets

Brain cell membrane function during and following single and repeated episodes of asphyxia was investigated. Asphyxia in 24 anesthetized, paralyzed, mechanically-ventilated newborn piglets was produced by stopping ventilation for 2-3 min followed by recovery with reventilation. Measurements of cerebral Na+,K(+)-ATPase activity and of lipid peroxidation products, conjugated dienes and fluorescent compounds, were made during control (n = 12), asphyxia (n = 5), recovery after a single asphyxia event (n = 4), and recovery following 7 repeated asphyxia episodes (n = 3). Cerebral Na+,K(+)-ATPase activity remained unchanged from control during asphyxia (14.57 +/- 2.43 compared to 15.33 +/- 4.27 mumol Pi/mg protein/h, mean +/- SD), but was significantly reduced both during recovery after single (3.87 +/- 1.66) and after repeated (2.59 +/- 1.58) asphyxias, representing a 73 and 82% reduction in enzyme activity, respectively. Conjugated dienes and fluorescent compounds were similarly unchanged during asphyxia compared to control, but increased during recovery from single and from repeated episodes. Decreased cerebral Na+,K(+)-ATPase activity, simultaneous with an increase in lipid peroxidation products, reflects significant cellular membrane damage consistent with oxygen free radical formation during the recovery from acute asphyxia in the newborn piglet.

Effect of graded hypoxia on brain cell membrane injury in newborn piglets

Alterations in brain cell membrane structure and function during cerebral hypoxia were investigated by measuring Na+,K(+)-ATPase activity and levels of lipid peroxidation products in brain cell membranes obtained from newborn piglets following exposure to 60 min of hypoxic hypoxia in vivo. Cerebral hypoxia was documented as a decrease in the ratio of phosphocreatine to inorganic phosphate (PCr/Pi) using 31P-NMR spectroscopy. During hypoxia (FiO2 0.07-0.11), PCr/Pi decreased 28-47% compared to the corresponding baseline value without a decrease in cerebral ATP levels. No change in brain cell membrane Na+,K(+)-ATPase activity was observed for changes in PCr/Pi of less than 30%. When PCr/Pi was at least 30% lower than baseline, Na+,K(+)-ATPase activity decreased linearly as a function of the decrease in PCr/Pi (r = 0.95). Levels of lipid peroxidation products (conjugated dienes and fluorescent compounds) increased significantly as PCr/Pi decreased. These data suggest that below a critical threshold value of oxidative metabolism there are progressive changes in brain cell membrane structure and function during cerebral hypoxia, and demonstrate that membrane alterations occur prior to changes in cellular ATP levels.

Effect of hyperventilation on oxygenation of the brain cortex of newborn piglets

A new phosphorescence imaging method (Rumsey et al. Science Wash. DC 241: 1649-1651, 1988) has been used to continuously monitor the PO2 in the blood of the cerebral cortex of newborn pigs. A window was prepared in the skull and the brain superfused with artificial cerebrospinal fluid. The phosphorescent probe for PO2, Pd-meso-tetra(4-carboxyphenyl)porphine, was injected directly into the systemic blood. The phosphorescence of the probe was imaged, and the lifetimes were measured using flash illumination and a gated video camera. The PO2 in the blood of the veins and capillary beds of the cortex was calculated from the lifetimes. Systemic blood pressure was continuously monitored while the systemic arterial PCO2, PO2, and blood pH were measured periodically. The PO2 in the blood was quantitated for 60- to 200 microns2 regions within the image (from a total field of approximately 3 mm diam). The PO2 in the microvasculature was not uniform across the viewing field but increased or decreased in each region independently of the other regions. Thus at any point in time the PO2 in a region could be substantially above or below the average value. During hyperventilation, which lowered arterial PCO2 and increased pH of the blood, the average PO2 decreased in proportion to the decrease in arterial PCO2. For example, hyperventilation, which decreased arterial PCO2 from its normal value of 40 Torr to 10 Torr, caused a rapid (within 5 min) decrease in PO2 in the blood of capillaries and veins to approximately one-third of normal.

Sympathetic nerve modulation of regional cerebral blood flow during asphyxia in newborn piglets

Regional cerebral blood flow (rCBF) during asphyxia suggests a reflex vasoconstrictor mechanism active principally in brain cortex. Present studies in newborn piglets investigate sympathetic modulation of the cerebrovasculature both during and after acute asphyxia. Unilateral superior cervical sympathetic ganglionectomy (SCSG) was performed in 13 newborn piglets, after which asphyxia was produced by discontinuing ventilation. In 8 animals, blood flow was measured during control and sequentially 1, 2, and 3 min after ventilation was stopped. In 5 piglets with unilateral SCSG, cortical flow decreased in the innervated hemisphere, -34 +/- 14% after 2 min, and -25 +/- 9% at 3 min of asphyxia compared with control (104 +/- 22 ml.min-1.100 g-1; mean +/- SE). In contrast, the sympathetically denervated hemisphere showed -13 +/- 17% at 2 min and +7 +/- 23% at 3 min, representing 45 +/- 6% and 30 +/- 9% left-right (L-R) flow differences, respectively. Bilateral SCSG (3 piglets) similarly attenuated the cortical CBF vasoconstrictor response to asphyxia, +6 +/- 21% at 2 min and -8 +/- 5% at 3 min. Significant innervated-denervated rCBF differences were present during asphyxia in cerebral gray (55% +/- 24), cerebral white (41% +/- 16), caudate (25% +/- 7), hippocampus (36% +/- 12), and choroid plexus (145% +/- 42), indicating sympathetic nerve modulation. Brain stem structures showed increasing rCBF throughout asphyxia and no L-R differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dexamethasone on cerebral prostanoid formation and pial arteriolar reactivity to CO2 in newborn pigs

This study investigates the effect of glucocorticoid treatment on the relationship between arteriolar PCO2 and cortical prostanoid production and on cerebrovascular responsiveness to elevated CO2 in newborn piglets. The response of pial arteries to hypercapnia (fractional inspired CO2 = 0.035 and 0.07) was studied in 18 anesthetized newborn piglets, 9 of which were pretreated with dexamethasone (2 mg.kg-1.day-1 for 36-48 h). Pial arterioles (77-122 microns diam) were monitored using a closed cranial window and intravital microscopy. Perivascular cerebrospinal fluid (CSF) was sampled from the cortical surface and analyzed for 6-keto-prostaglandin F1 alpha and thromboxane B2 (TxB2) using radioimmunoassay. In the dexamethasone-treated animals the increase in arteriolar diameter to CO2 was diminished by approximately 50% for each respective CO2 concentration vs. the control group. Acute sympathetic denervation did not restore the CO2 dilator response. Dexamethasone did not alter baseline cortical CSF prostanoid concentrations but abolished the CO2-induced increase in CSF prostanoids. The dilator response to exogenously applied prostaglandin E2 was inhibited in dexamethasone-treated animals. However, the dilator response to exogenous adenosine and the contractile response to prostaglandin F2 alpha were not altered in the dexamethasone-treated piglets. The data support the concept that metabolites of arachidonic acid participate in the cerebrovascular response to CO2 and suggest that glucocorticoid treatment may influence cerebrovascular tone via this mechanism.

Serial neurobehavioral assessments in preterm infants

The purpose of this study was to compare serial neurobehavioral assessments in a sample of preterm infants with intraventricular hemorrhage (IVH) to those of a group of unaffected preterm infants. Of the 30 infants included in the study, 5 infants had a grade III or IV/VI intraventricular hemorrhage. Infants with IVH demonstrated more abnormalities in mental status and a cluster of abnormal neurologic findings (persistent ankle clonus, tremulousness, and brisk deep tendon reflexes). Of the 30 infants, 20 infants were included in a brain metabolism study. Infants who had experienced an IVH had significantly different brain metabolism findings, as measured by nuclear magnetic resonance spectroscopy (NMR). There was a significant correlation between one measure of brain metabolism (PCr/PI) and the neurobehavioral assessment rating for infants with IVH.

Lipid peroxidation as the mechanism of modification of brain 5'-nucleotidase activity in vitro

The effect of lipid peroxidation on the Mg2(+)-independent and Mg2(+)-dependent activity of brain cell membrane 5'-nucleotidase was determined and the affinity of the active sites of Mg2(+)-dependent enzyme for 5'-AMP (substrate) and Mg2+ (activator) was examined. Brain cell membranes were peroxidized at 37 degrees C in the presence of 100 microM ascorbate and 25 microM FeCl2 (resultant) for 10 min. The activity of 5'-nucleotidase and lipid peroxidation products (thiobarbituric acid reactive substances) were determined. At 10 min, the level of lipid peroxidation products increased from 0.20 +/- 0.10 to 17.5 +/- 1.5 nmoles malonaldehyde/mg membrane protein. The activity of Mg2(+)-independent 5'-nucleotidase increased from 0.201 +/- 0.020 in controls to 0.305 +/- 0.028 mumol Pi/mg protein/hr in peroxidized membranes. In the presence of 10 mM Mg2+, the activity increased by 5.8-fold in the peroxidized membrane preparation in comparison to 14-fold in control. In peroxidized preparation, the affinity of active site of Mg2(+)-dependent 5'-nucleotidase for 5'-AMP tripled, as indicated by a significant decrease in Km (Km = 95 +/- 2 microM AMP for control; Km = 32 +/- 2 microM AMP for peroxidized). Vmax was significantly reduced from 3.35 +/- 0.16 in control to 1.70 +/- 0.9 mumoles Pi/mg protein in peroxidized membranes. The affinity of the active site for Mg2+ significantly increased (Km = 6.17 +/- 0.37 mM Mg2+ for control; Km = 4.0 +/- 0.31 peroxidized).(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-oxidant enzymes in the brain of newborn piglets during ischemia followed by reperfusion

The activity of anti-oxidant enzymes in the brains of newborn piglets were studied under the condition of ischemic hypoxia followed by reperfusion. The activity of superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glucose-6-phosphate dehydrogenase, was determined in the brain tissue of control animals and animals exposed to 60 min of hypoxia followed by 30 min of normoxia. The results showed that the activities of these enzymes were not significantly affected by hypoxia and subsequent reperfusion, suggesting that under these conditions the anti-oxidant system is not a target for, nor is its inhibition a cause of, cellular damage. It is proposed that the anti-oxidant enzyme system in the brain is non-responsive to and may not play a role during hypoxia/ischemia and subsequent reperfusion.