Glucose-associated alterations in ischemic brain metabolism of neonatal piglets

Brain-Oriented Strategies for Neuroprotection of Asphyxiated Newborns in the First Hours of Life

Perinatal asphyxia represents the first cause of severe neurological disabilities and the second cause of neonatal death in term-born babies. Currently, no treatment can prevent immediate cell death from necrosis, but some therapeutic interventions, such as therapeutic hypothermia (TH), can reduce delayed cell death from apoptosis. TH significantly improves the combined outcome of mortality or major neurodevelopmental disability, but the number of patients to be treated is 7 to get 1 child with no adverse neurological outcome. The aim of this educational review is to analyze the other care strategies to be implemented to improve the neurological outcome of children with hypoxic ischemic encephalopathy (HIE). Hypocapnia, hypoglycemia, pain control, and functional brain monitoring are recognized as appropriate approaches to improve outcome in critically ill infants with HIE. Pharmacologic neuroprotective adjuncts are currently under investigation. New drugs such as allopurinol and melatonin seem to provide positive effects although more randomized controlled trials are required to establish the effective therapeutic scheme. In the meantime, sustaining the respiratory, metabolic, and cardiovascular system during TH can be a valuable aid in managing and treating the patient with HIE in an optimal way.

Nutrition and management of glycemia in neonates with neonatal encephalopathy treated with hypothermia

Adequate nutrition and glycemic homeostasis are increasingly recognized as potentially neuroprotective for the developing brain. In the context of hypoxia-ischemia, evidence is scarce regarding optimal nutritional support and administration route, as well as the short- and long-term consequences of such interventions. In this review, we summarize current knowledge on disturbances of brain metabolism of glucose and substrates by hypoxia-ischemia, and compound effects of these mechanisms on brain injury characterized by specific patterns on EEG and MRI. Risks and benefits of nutrition delivery via parenteral or enteral routes are examined. Nutrition could mitigate adverse neurodevelopmental outcomes, and the impact of nutritional strategies and specific nutritional interventions are reviewed. Limited literature highlights the need for further studies to understand the changes in energy metabolism during and after hypoxic-ischemic injury, to optimize nutritional regimens and glucose management, and to inform the neuroprotective role of nutrition.

Antenatal dexamethasone before asphyxia promotes cystic neural injury in preterm fetal sheep by inducing hyperglycemia

Antenatal glucocorticoid therapy significantly improves the short-term systemic outcomes of prematurely born infants, but there is limited information available on their impact on neurodevelopmental outcomes in at-risk preterm babies exposed to perinatal asphyxia. Preterm fetal sheep (0.7 of gestation) were exposed to a maternal injection of 12 mg dexamethasone or saline followed 4 h later by asphyxia induced by 25 min of complete umbilical cord occlusion. In a subsequent study, fetuses received titrated glucose infusions followed 4 h later by asphyxia to examine the hypothesis that hyperglycemia mediated the effects of dexamethasone. Post-mortems were performed 7 days after asphyxia for cerebral histology. Maternal dexamethasone before asphyxia was associated with severe, cystic brain injury compared to diffuse injury after saline injection, with increased numbers of seizures, worse recovery of brain activity, and increased arterial glucose levels before, during, and after asphyxia. Glucose infusions before asphyxia replicated these adverse outcomes, with a strong correlation between greater increases in glucose before asphyxia and greater neural injury. These findings strongly suggest that dexamethasone exposure and hyperglycemia can transform diffuse injury into cystic brain injury after asphyxia in preterm fetal sheep.

Therapeutic Hypothermia for Birth Asphyxia in Neonates

India contributes to the highest neonatal mortality globally. Birth asphyxia is one of the leading causes of neonatal mortality in India. A large number of neonates who suffer from birth asphyxia progress to Hypoxic Ischemic Encephalopathy (HIE). The risk of a neonate progressing to severe form of HIE is many times higher in the low and middle income countries (LMICs) with ill developed health infrastructure. Till date LMICs have had a low institutional delivery rate, poor regionalization of care, lack of adequate transport facilities and ill equipped neonatal intensive care facilities. This has lead to a tremendous burden on the health care systems with a cohort of developmentally challenged neonates surviving into adulthood. Recently, Therapeutic Hypothermia (TH) has emerged as an evidence based intervention to reduce mortality and neurodevelopmental disability associated with asphyxia induced encephalopathy. TH has become the gold standard in the management of such cases in the western world. Extension of this knowledge to the LMICs and countries like India require a better understanding of the unique sociocultural issues associated with asphyxial brain injury in neonates. The high incidence of sepsis and presence of economic constraints make this problem more complex in such countries. The current review has tried to address these issues and looked at the basics of this complex topic from the perspective of a general pediatrician.

Perioperative central nervous system injury in neonates

Anaesthetic-induced developmental neurotoxicity (AIDN) has been clearly established in laboratory animal models. The possibility of neurotoxicity during uneventful anaesthetic procedures in human neonates or infants has led to serious questions about the safety of paediatric anaesthesia. However, the applicability of animal data to clinical anaesthesia practice remains uncertain. The spectre of cerebral injury due to cerebral hypoperfusion, metabolic derangements, coexisting disease, and surgery itself further muddles the picture. Given the potential magnitude of the public health importance of this issue, the clinician should be cognisant of the literature and ongoing investigations on AIDN, and raise awareness of the risks of both surgery and anaesthesia.

Neuroprotective effect of acid-sensing ion channel inhibitor psalmotoxin-1 after hypoxia-ischemia in newborn piglet striatum

Na+,Ca2+-permeable acid-sensing ion channel 1a (ASIC1a) is involved in the pathophysiologic process of adult focal brain ischemia. However, little is known about its role in the pathogenesis of global cerebral ischemia or newborn hypoxia-ischemia (H-I). Here, using a newborn piglet model of asphyxia-induced cardiac arrest, we investigated the effect of ASIC1a-specific blocker psalmotoxin-1 on neuronal injury. During asphyxia and the first 30min of recovery, brain tissue pH fell below 7.0, the approximate activation pH of ASIC1a. Psalmotoxin-1 injection at 20min before hypoxia, but not at 20min of recovery, partially protected the striatonigral and striatopallidal neurons in putamen. Psalmotoxin-1 pretreatment largely attenuated the increased protein kinase A-dependent phosphorylation of DARPP-32 and N-methyl-d-aspartate (NMDA) receptor NR1 subunit and decreased nitrative and oxidative damage to proteins at 3h of recovery. Pretreatment with NMDA receptor antagonist MK-801 also provided partial neuroprotection in putamen, and combined pretreatment with psalmotoxin-1 and MK-801 yielded additive neuroprotection. These results indicate that ASIC1a activation contributes to neuronal death in newborn putamen after H-I through mechanisms that may involve protein kinase A-dependent phosphorylation of NMDA receptor and nitrative and oxidative stress.

Perioperative hyperglycemia: effect on outcome after infant congenital heart surgery

BACKGROUND

Studies demonstrate that cardiopulmonary bypass (CPB) causes intraoperative and postoperative hyperglycemia. Hyperglycemia has been associated with morbidity and mortality after infant cardiac surgery. We studied the effects on early postoperative outcomes of glucose (GLU) changes during and after pediatric cardiac surgery.

METHODS

The records of 144 infants less than 10 kg who underwent CPB for a variety of congenital cardiac procedures were reviewed. The GLU values (at multiple intervals during and after surgery), age, weight, CPB time, ultrafiltration volume, and risk adjustment for congenital heart surgery (RACHS-1) score were recorded. Univariate and multivariate linear and binary logistic regression were used to examine the dependence of the composite outcome mortality or postoperative infection, the mechanical ventilation time (VENT time), and the length of stay (LOS), on these variables.

RESULTS

The RACHS-1 score was the only significant predictor of the composite variable "mortality or infection" (p = 0.008). Glucose at any time was not a significant factor predicting this outcome. Lower pre-CPB GLU, younger age, and higher RACHS-1 score were significant predictors of greater LOS and VENT time.

CONCLUSIONS

In this study, post-CPB and postoperative hyperglycemia were not risk factors for postoperative morbidity and mortality after infant cardiac surgery.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Neonatal encephalopathy: treatment with hypothermia

In this article, the role of hypothermia and neuroprotection for neonatal encephalopathy will be discussed. The incidence of encephalopathy due to hypoxia ischemia as well as the pathophysiology will be presented. The diagnosis of encephalopathy in full-term neonates will be discussed. The current management of brain injury that occurs with hypoxia ischemia and the role of hypothermia in preventing brain injury in fetal and neonatal animal models will be reviewed. The current data from randomized control trials of hypothermia as neuroprotection for full-term infants will be presented along with the results of meta-analyses of these trials. Lastly, the status of ongoing neonatal hypothermia trials will be summarized.

Hypothermia as a treatment for birth asphyxia

This chapter will report to the frequency of neonatal hypoxic-ischemic encephalopathy. The pathophysiology and the childhood outcome of encephalopathy due to hypoxia-ischemia will be examined. The limitations of current therapy for this condition and new therapies will be evaluated. Hypothermia seems to offer the most promise as a therapy for neuroprotection in hypoxic-ischemic encephalopathy. The evidence-based trials of hypothermia will be reviewed along with recommendations regarding clinical applications for this therapy and need for long-term follow-up of children receiving this therapy.

Glucose management during and after intensive delivery room resuscitation

Hypoxic-ischemic encephalopathy remains a major cause of morbidity and mortality in preterm and full-term infants. Experimental data from animal studies suggest that interventions that improve survival of injured neurons and prevent delayed neuronal loss may decrease hypoxic ischemic brain injury. Considerable attention has focused on optimizing management of newborns in the period immediately after resuscitation from perinatal asphyxia to minimize delayed neuronal death. The evidence regarding the role of glucose in modifying post-asphyxia brain injury and resuscitation was reviewed to better define optimal glucose management after perinatal asphyxia and resuscitation.

Glucose transport to the brain: a systems model

Glucose transport to the brain involves sophisticated interactions of solutes, transporters, enzymes, and cell signaling processes, within an intricate spatial architecture. The dynamics of the transport are influenced by the adaptive nature of the blood-brain barrier (BBB), the semi-impermeable membranes of brain capillaries. As both the gate and the gatekeeper between blood-borne nutrients and brain tissue, the BBB helps govern brain homeostasis. Glucose in the blood must cross the BBB's luminal and abluminal membranes to reach neural tissue. A robust representation of the glucose transport mechanism can highlight a target for brain therapeutic intervention, help characterize mechanisms behind several disease phenotypes, or suggest a new delivery route for drugs. The challenge for researchers is understanding the relationships between influential physiological variables in vivo, and using that knowledge to predict how alterations or interventions affect glucose transport. This paper reviews factors influencing glucose transport and approaches to representing blood-to-brain glucose transport including in vitro, in vivo, and kinetic models. Applications for different models are highlighted, while their limitations in answering arising questions about the human in vivo BBB lead to a discussion of an alternate approach. A developing complex systems simulation is introduced, initiating a single platform to represent the dynamics of glucose transport across the adapting human blood-brain barrier.

Effects of exogenous glucose on brain ischemia in ovine fetuses

We examined the effects of prolonged moderate hyperglycemia with and without an additional rapid glucose injection on ischemic brain injury in the fetus. Twenty-five ewes (117-124 d of gestation) were assigned to one of four groups: 1) glucose-infused fetuses exposed to 30 min of carotid artery occlusion followed by 48 h of reperfusion (I/R-Glu, n = 8); 2) glucose-infused plus rapid glucose injection given 100 min before 30 min of occlusion followed by 48 h of reperfusion (I/R-GluR, n = 4); 3) placebo-infused exposed to 30 min of occlusion and 48 h of reperfusion (I/R-PL, n = 8); and 4) glucose-infused sham occlusion and 48 h of sham reperfusion (control, n = 5). After baseline measurements, fetuses were infused with glucose (9-16 mg/kg/min) for 48 h before and after carotid occlusion or sham treatment. The I/R-PL group received 0.9% NaCl. Brain pathologic outcome was determined. Serial sections stained with Luxol fast blue-hematoxylin and eosin were scored for white matter, cerebral cortical, and hippocampal lesions. These areas received graded pathologic scores of 0 to 5, reflecting the amount of injury, where 0 = 0%, 1 = 1-25%, 2 = 26-50%, 3 = 51-75%, 4 = 76-95%, and 5 = 96-100% of the area damaged. Comparisons of the pathologic scores for cerebral cortex (CC), white matter (WM), and hippocampus (H) demonstrated that the I/R-GluR (CC: 4.56 +/- 0.11, WM: 4.50 +/- 0.11, H: 3.44 +/- 0.48, mean +/- SEM) had more (p < 0.05) damage than the I/R-Glu (CC: 2.46 +/- 0.47, WM: 1.97 +/- 0.37, H: 1.81 +/- 0.36) and control (CC: 1.12 +/- 0.13, WM: 0.82 +/- 0.34, H: 0.80 +/- 0.34) groups. The pathologic scores in the I/R-Glu were (p < 0.05) greater than the control, but not the I/R-PL (CC: 2.12 +/- 0.35, WM: 2.20 +/- 0.44, H: 1.59 +/- 0.41) group. We conclude that exposure to prolonged moderate hyperglycemia before ischemia and during reperfusion does not affect the extent of brain injury, but exposure to an additional acute increase in plasma glucose concentration before ischemia is extremely detrimental to the fetal brain.

Initial hypoglycemia and neonatal brain injury in term infants with severe fetal acidemia

OBJECTIVE

To determine the potential contribution of initial hypoglycemia to the development of neonatal brain injury in term infants with severe fetal acidemia.

METHODS

A retrospective chart review was conducted of 185 term infants who were admitted to the neonatal intensive care unit between January 1993 and December 2002 with an umbilical arterial pH <7.00. Short-term neurologic outcome measures include death as a consequence of severe encephalopathy and evidence of moderate to severe encephalopathy with or without seizures. Hypoglycemia was defined as an initial blood glucose < or =40 mg/dL.

RESULTS

Forty-one (22%) infants developed an abnormal neurologic outcome, including 14 (34%) with severe hypoxic ischemic encephalopathy who died, 24 (59%) with moderate to severe hypoxic ischemic encephalopathy, and 3 (7%) with seizures. Twenty-seven (14.5%) of the 185 infants had an initial blood sugar < or =40 mg/dL. Fifteen (56%) of 27 infants with a blood sugar < or =40 mg/dL versus 26 (16%) of 158 infants with a blood sugar >40 mg/dL had an abnormal neurologic outcome (odds ratio [OR]: 6.3; 95% confidence interval [CI]: 2.6-15.3). Infants with abnormal outcomes and a blood sugar < or =40 mg/dL versus >40 mg/dL had a higher pH (6.86 +/- 0.07 vs 6.75 +/- 0.09), a lesser base deficit (-19 +/- 4 vs -23.8 +/- 4 mEq/L), and lower mean arterial blood pressure (34 +/- 10 vs 45 +/- 14 mm Hg), respectively. There was no difference between groups in the proportion of infants who required cardiopulmonary resuscitation (7 [46%] vs 15 [57%]) and those with a 5-minute Apgar score <5 (11 [73%] vs 22 [85%]). By multivariate logistic analysis, 4 variables were significantly associated with abnormal outcome: initial blood glucose < or =40 mg/dL versus >40 mg/dL (OR: 18.5; 95% CI: 3.1-111.9), cord arterial pH < or =6.90 versus >6.90 (OR: 9.8; 95% CI: 2.1-44.7), a 5-minute Apgar score < or =5 versus >5 (OR: 6.4; 95% CI: 1.7-24.5), and the requirement for intubation with or without cardiopulmonary resuscitation versus neither (OR: 4.7; 95% CI: 1.2-17.9).

CONCLUSION

Initial hypoglycemia is an important risk factor for perinatal brain injury, particularly in depressed term infants who require resuscitation and have severe fetal acidemia. It remains unclear, however, whether earlier detection of hypoglycemia, such as in the delivery room, in this population could modify subsequent neurologic outcome.

Effects of fasting and insulin-induced hypoglycemia on brain cell membrane function and energy metabolism during hypoxia-ischemia in newborn piglets

This study was done to determine the effects of 12 h fasting-induced mild hypoglycemia (blood glucose 60 mg/dl) and insulin-induced moderate hypoglycemia (blood glucose 35 mg/dl) on brain cell membrane function and energy metabolism during hypoxia-ischemia in newborn piglets. Sixty-three ventilated piglets were divided into six groups; normoglycemic control (NC, n=8), fasting-induced mildly hypoglycemic control (FC, n=10), insulin-induced moderately hypoglycemic control (IC, n=10), normoglycemic/hypoxic-ischemic (NH, n=11), fasting-induced mildly hypoglycemic/hypoxic-ischemic (FH, n=12) and insulin-induced moderately hypoglycemic/hypoxic-ischemic (IH, n=12) group. Cerebral hypoxia-ischemia was induced by occlusion of bilateral common carotid arteries and simultaneous breathing with 8% oxygen for 30 min. The brain lactate level was elevated in NH group and this change was attenuated in FH and IH groups. The extent of cerebral lactic acidosis during hypoxic-ischemic insult showed significant positive correlation with blood glucose level (r=0.55, p<0.001). Cerebral Na+, K+-ATPase activity and concentrations of high-energy phosphate compounds were reduced in NH group and these changes were not ameliorated in FH or IH group. Cortical levels of conjugated dienes, measured as an index of lipid peroxidation of brain cell membrane, were significantly elevated in NH, FH and IH groups compared with NC, FC and IC groups and these increases were more profound in FH and IH with respect to NH. Blood glucose concentration showed significant inverse correlation with levels of conjugated dienes (r=-0.35, p<0.05). These findings suggest that, unlike in adults, mild or moderate hypoglycemia, regardless of methods of induction such as fasting or insulin-induced, during cerebral hypoxia-ischemia is not beneficial and may even be harmful in neonates.

Maturational changes in cerebral lactate and acid clearance following ischemia measured in vivo using magnetic resonance spectroscopy and microdialysis

Intraischemic hyperglycemia has different effects on neurologic outcome in mature vs. immature brain, and may reflect differences in the extent or duration of cerebral lactic acidosis. We examined the hypotheses that post-ischemic lactate and acid clearance rates depend on the severity of intraischemic cerebral acidosis, and that rates of clearance change as a function of brain maturation. In vivo 31P and 1H magnetic resonance spectroscopy (MRS) was used to compare intracellular acid and lactate clearance rates in newborn and 1-month old swine following a 14-min episode of transient near-complete global ischemia. In the same animals, in vivo microdialysis was used to determine if extracellular lactate clearance changed as a function of cerebral lactic acidosis or differed between age groups following ischemia. Plasma glucose concentration was altered in individual animals to study a range of intraischemic cerebral lactic acidosis. For both age-groups, maximal brain acidosis and lactosis occurred in the post-ischemia interval, indicating a delay in the re-establishment of oxidative metabolism following ischemia. Clearance half-lives of both cerebral acidosis and lactosis increase as a function of increased intraischemic cerebral acidosis. For either age group, the clearance half-life for acidosis was faster than the half-life for lactate. However, the subgroup of 1-month old swine who experienced severe cerebral acidosis (i.e., pH<6.1) had a longer cerebral lactate clearance half-life as compared to the subgroup of newborn animals with a similar severity of acidosis. In both age groups, there were comparable maximal increases in extracellular lactate concentrations in the post-ischemic period and similar rates of decline from the maximum. These results demonstrate that post-ischemic lactate and acid clearance are altered by the extent of intraischemic acidosis, and the extent of post-ischemic uncoupling between brain acid and lactate clearance increases with advancing age. The transmembrane clearance of lactate was not a prominent mechanism that differentiated lactate clearance rates between newborn and 1-month old swine.

Poor outcome after hypoxia-ischemia in newborns is associated with physiological abnormalities during early recovery. Possible relevance to secondary brain injury after head trauma in infants

"Secondary hypoxia/ischemia" (i.e. regional impairment of oxygen and substrate delivery) results in secondary deterioration after traumatic brain injury in adults as well as in children and infants. However, detailed analysis regarding critical physiological abnormalities resulting from hypoxia/ischemia in the immature brain, e.g. acid-base-status, serum glucose levels and brain temperature, and their influence on outcome, are only available from non-traumatic experimental models. In recent studies on hypoxic/asphyxic cardiac arrest in neonatal piglets, we were able to predict short-term outcome using specific physiologic abnormalities immediately after the insult. Severe acidosis, low serum glucose levels and fever after resuscitation were associated with an adverse neurologic recovery one day after the insult. The occurrence of clinically apparent seizure activity during later recovery increased mortality (epileptic state), and survivors had greater neocortical and striatal brain damage. Brain damage after transient hypoxia/ischemia and "secondary brain injury" after head trauma may have some mechanistic overlap, and these findings on physiological predictors of outcome may also apply to pathologic conditions in the post-traumatic immature brain. Evaluation of data from other models of brain injury will be important to develop candidate treatment strategies for head-injured infants and children and may help to initiate specific studies about the possible role of these physiological predictors of brain damage in the traumatically injured immature brain.

Deleterious brain cell membrane effects after NMDA receptor antagonist administration to newborn piglets

Previous studies have shown that administration of the N-methyl-d-aspartate (NMDA) receptor antagonist 3-(2-carboxypiperazin-4-yl)-1-phosphonic acid (CPP) reduces NMDA-mediated neurotoxicity in animal models of hypoxia/ischemia but also may induce brain tissue vacuolization and alter glucose metabolism. The present study tests the hypothesis that CPP administration alters brain cell membrane structure and function in the cerebral cortex of normoxic newborn piglets through the generation of oxygen free radicals and induction of lipid peroxidation. Twenty six anesthetized, ventilated newborn piglets-13 treated with 2 mg/kg i.v. CPP and 13 untreated controls-were studied. ATP and phosphocreatine (PCr) levels were measured as an index of cellular energy metabolism and tissue glucose levels determined. Na+, K+-ATPase activity was measured as an index of brain cell membrane function and the lipid peroxidation products conjugated dienes (CD) and fluorescent compounds (FC) measured. Free radical generation was detected on cortical biopsies homogenized with alpha-phenyl-N-tert-butyl-nitrone (PBN) through electron spin resonance spectroscopy. Signal height of spectrum was divided by dry tissue weight and expressed as mm/g tissue. In the two groups brain tissue ATP and PCr levels were not different. Tissue glucose levels were higher in the CPP group (24+/-5 mg/dl) than in controls (14+/-3 mg/dl), p<0.05, whereas Na+,K+-ATPase activity was lower in the CPP group than in controls (34+/-4 vs. 43+/-6 micromol Pi/mg protein/h), p<0.05. Lipid peroxidation products were higher in the CPP group (CD: 57+/-19 nmol/g brain, FC: 1.5+/-0.3 microg/g brain) than in controls (CD: 0+/-0 nmol/g brain, FC: 0.9+/-0.2 microg/g brain), p<0. 05. Free radical intensity was higher in the CPP group (493+/-397 mm/g tissue) than in controls (51+/-83 mm/g tissue), p<0.05. In vitro administration of CPP to brain cell membranes did not change Na+,K+-ATPase activity or the generation of lipid peroxidation products. The data demonstrate that administration of CPP induces lipid peroxidation, results in free radical generation, decreases brain cell membrane Na+,K+-ATPase activity and alters glucose metabolism in the cerebral cortex of newborn piglets. Since CPP is a potent antagonist of the NMDA receptor, we speculate that CPP generates free radicals through a pathway independent of the NMDA receptor by altering cellular metabolism and possibly glucose utilization during normoxia in newborn piglets.

Effect of hyperglycemia on brain cell membrane function and energy metabolism during hypoxia-ischemia in newborn piglets

The purpose of this study was to test the hypothesis that hyperglycemia ameliorates changes in brain cell membrane function and preserves cerebral high energy phosphates during hypoxia-ischemia in newborn piglets. A total of 42 ventilated piglets were divided into 4 groups, normoglycemic/normoxic(group 1, n=9), hyperglycemic/normoxic(group 2, n=8), normoglycemic/hypoxic-ischemic(group 3, n=13) and hyperglycemic/hypoxic-ischemic(group 4, n=12) group. Cerebral hypoxia-ischemia was induced by occlusion of bilateral common carotid arteries and simultaneous breathing with 8% oxygen for 30 min. Hyperglycemia (blood glucose 350-400 mg/dl) was maintained for 90 min before and throughout hypoxia-ischemia using modified glucose clamp technique. Changes in cytochrome aa3 were continuously monitored using near infrared spectroscopy. Blood and CSF glucose and lactate were monitored. Na+, K+-ATPase activity, lipid peroxidation products (conjugated dienes), tissue high energy phosphates (ATP and phosphocreatine) levels and brain glucose and lactate levels were determined biochemically in the cerebral cortex. During hypoxia-ischemia, glucose levels in blood and CSF were significantly elevated in hyperglycemic/hypoxic-ischemic group compared with normoglycemic/hypoxic-ischemic group, but lactate levels in blood and CSF were not different between two groups. At the end of hypoxia-ischemia of group 3 and 4, triangle up Cyt aa3, Na+, K+-ATPase activity, ATP and phosphocreatine values in brain were significantly decreased compared with normoxic groups 1 and 2, but were not different between groups 3 and 4. Levels of conjugated dienes and brain lactate were significantly increased in groups 3 and 4 compared with groups 1 and 2, and were significantly elevated in group 4 than in group 3 (0.30+/-0.11 vs. 0.09+/-0.02 micromol g-1 protein, 26.4+/-7.6 vs. 13.1+/-2.6 mmol kg-1, p<0.05). These findings suggest that hyperglycemia does not reduce the changes in brain cell membrane function and does not preserve cerebral high energy phosphates during hypoxia-ischemia in newborn piglets. We speculate that hyperglycemia may be harmful during hypoxia-ischemia due to increased levels of lipid peroxidation in newborn piglet.

Calculation of intracellular cerebral [Mg2+] during hypoxic ischemia by in vivo 31P NMR

Several algorithms for the calculation of ionized intracellular magnesium concentration from the chemical shifts of MgATP were compared, using in vivo 31P NMR data obtained from swine brain during and following hypoxic ischemia plus i.v. MgSO4 infusion. This analysis reveals that both the absolute ionized intracellular magnesium and relative changes in magnesium may vary widely between algorithms used. The calculated intracellular pH, used in algorithms to determine ionized magnesium concentration was found to be a critical parameter that governs the extent of these differences.

Effect of hypoglycemia on postischemic cortical blood flow, hypercapnic reactivity, and interstitial adenosine concentration

Hypoglycemia increases the vulnerability of the perinatal brain to asphyxia, but it is not known if hypoglycemia-induced changes in cerebral hemodynamics and vascular reactivity underlie this vulnerability. This study tested the hypothesis that hypoglycemia exacerbates postischemic hypoperfusion, and impairs postischemic CO2 reactivity. The authors also examined the hypothesis that postischemic hypoperfusion is associated with a reduction in the interstitial concentration of the vasodilator metabolite adenosine. Global cerebral ischemia of 10 minutes duration was induced in newborn pigs anesthetized with isoflurane by occlusion of subclavian and brachiocephalic arteries; cortical cerebral blood flow (CBF) and interstitial adenosine concentration were evaluated simultaneously using the combined hydrogen clearance/microdialysis technique. Hypoglycemia (blood glucose < 25 mg/dl) was induced by regular insulin (25 IU/kg) administered intravenously 2 hours prior to induction of ischemia. In the eight normoglycemic animals, baseline CBF was 38 +/- 4 ml/min/100 gm and baseline adenosine concentration was 1.2 +/- 0.1 microM; in the eight hypoglycemic animals, these values were 39% (p < 0.05) and 62% (p < 0.05) greater, respectively, under baseline conditions. At 1 hour of postischemic reperfusion in normoglycemic animals, CBF was reduced 39% relative to the preischemic baseline (p < 0.01), concomitant with a 27% reduction (p < 0.05) in adenosine concentration, suggesting that this lowered concentration may underlie delayed hypoperfusion. These postischemic reductions in CBF and interstitial adenosine concentration were significantly greater in hypoglycemic animals, with CBF and adenosine concentration reduced 70% (p < 0.001) and 71% (p < 0.01), respectively, relative to baseline. In nine animals preischemic reactivity to hypercapnia was unaffected by hypoglycemia. Postischemic hypercapnic reactivity was retained in the eight normoglycemic animals, but was attenuated 73% (p < 0.05) in hypoglycemic animals. Thus, in the newborn pig, hypoglycemia exacerbates postischemic cortical hypoperfusion and impairs postischemic cerebrovascular reactivity to hypercapnia.

The effects of systemic glucose concentration on brain metabolism following repeated brain ischemia

Since systemic glucose concentration is an important determinant of ischemic brain metabolism in neonates, we sought to determine if the systemic glucose concentration influences brain metabolic alterations following repeated partial ischemia. A group of hyperglycemic piglets (n = 12) were compared to a group of modestly hypoglycemic piglets (n = 12) using in vivo 2H and 31P magnetic resonance spectroscopy to simultaneously measure cerebral blood flow and phosphorylated metabolites before, during and 30 min after two 10-min episodes of ischemia (i.e. Recovery 1 and 2). For both groups, beta-ATP levels at Recovery 1 and 2 were lower than Control (91 +/- 11 and 83 +/- 15% of Control, respectively for both groups combined, P = 0.002 vs Control). Inorganic phosphorus was elevated in hyperglycemic piglets at Recovery 1 and 2 (117 +/- 15 and 118 +/- 10% of Control). In contrast, in modestly hypoglycemic piglets inorganic phosphorus progressively rose from Recovery 1 (131 +/- 24% of Control) to Recovery 2 (149 +/- 37% of Control), and differed from the hyperglycemic group (P = 0.02). These changes did not correlate with post-ischemic cerebral blood flow, cerebral O2 delivery or cerebral glucose delivery. In both groups phosphocreatine and intracellular pH returned to Control values during Recovery 1 and 2. The progressive increase in inorganic phosphorus post-ischemia in hypoglycemic piglets suggests that modest hypoglycemia during and following repeated partial ischemia adversely affects immediate brain metabolic recovery.