Effects of surgery and asphyxia on levels of nucleosides, purine bases, and lactate in cerebrospinal fluid of fetal lambs

Purine and pyrimidine metabolism and electrocortical brain activity during hypotension in near-term lambs

BACKGROUND

Insufficient cerebral O2 supply leads to cellular energy failure and loss of brain cell function. The relationship between the severity of cellular energy failure due to hemorrhagic hypotension and the loss of electrocortical brain activity (ECBA), as a measure of brain cell function, is not yet fully elucidated in near-term born lambs.

OBJECTIVES

To study the relationship between cerebral purine and pyrimidine metabolism, as a measure of brain cell energy failure, and brain cell function after hemorrhagic hypotension in near-term born lambs.

METHODS

Eight near-term lambs (term 147 days) were delivered at 131 days of gestation. After a stabilization period, mean arterial blood pressure was reduced till 30% of baseline by withdrawal of blood. Cerebrospinal fluid (CSF) was obtained at the end of the hypotensive period (2.5 h). CSF from 8 siblings was used for comparison. HPLC was used to determine purine and pyrimidine metabolites in CSF, as a measure of cellular energy failure. ECBA was calculated as the root mean square value of a band-filtered (2-16 Hz) one-channel EEG.

RESULTS

Values of guanosine, inosine, hypoxanthine, xanthine and uridine were significantly higher, while ECBA was significantly lower after hemorrhagic hypotension than control values. The concentrations of inosine, hypoxanthine, xanthine and uridine were significantly negatively linearly related to ECBA.

CONCLUSIONS

Brain cell function is negatively related to concentrations of inosine, hypoxanthine, xanthine and uridine in the CSF after hemorrhagic hypotension in near-term born lambs.

Purine and pyrimidine metabolism and electrocortical brain activity during hypoxemia in near-term lambs

Insufficient cerebral O(2) supply leads to brain cell damage and loss of brain cell function. The relationship between the severity of hypoxemic brain cell damage and the loss of electrocortical brain activity (ECBA), as measure of brain cell function, is not yet fully elucidated in near-term newborns. We hypothesized that there is a strong relationship between cerebral purine and pyrimidine metabolism, as measures of brain cell damage, and brain cell function during hypoxemia. Nine near-term lambs (term, 147 d) were delivered at 131 (range, 120-141) d of gestation. After a stabilization period, prolonged hypoxemia (fraction of inspired oxygen, 0.10; duration, 2.5 h) was induced. Mean values of carotid artery blood flow, as a measure of cerebral blood flow, and ECBA were calculated over the last 3 min of hypoxemia. At the end of the hypoxemic period, cerebral arterial and venous blood gases were determined and CSF was obtained. CSF from 11 normoxemic siblings was used for baseline values. HPLC was used to determine purine and pyrimidine metabolites in CSF, as measures of brain cell damage. Concentrations of purine and pyrimidine metabolites were significantly higher in hypoxemic lambs than in their siblings, whereas ECBA was lower in hypoxemic lambs. Significant negative linear relationships were found between purine and pyrimidine metabolite concentrations and, respectively, cerebral O(2) supply, cerebral O(2) consumption, and ECBA. We conclude that brain cell function is related to concentrations of purine and pyrimidine metabolites in the CSF. Reduction of ECBA indeed reflects the measure of brain damage due to hypoxemia in near-term newborn lambs.

1H-NMR spectroscopy of cerebrospinal fluid of fetal sheep during hypoxia-induced acidemia and recovery

The purpose of the study was to investigate the sequence of processes occurring during and after hypoxia-induced acidemia. We used proton nuclear magnetic resonance spectroscopy, which provides an overview of metabolites in cerebrospinal fluid (CSF), reflecting neuronal metabolism and damage. The pathophysiological condition of acute fetal asphyxia was mimicked by reducing maternal uterine blood flow in 14 unanesthetized pregnant ewes. CSF metabolites were measured during hypoxia-induced acidemia, and during the following recovery period, including the periods at 24 and 48 h after the hypoxic insult. Maximum values of the following CSF metabolites were reached during severe hypoxia (pH <or= 7.00): glucose, lactate, pyruvate, hypoxanthine, alanine, beta-hydroxybutyrate, choline, creatine, myo-inositol, citrate, succinate, valine, and an unknown metabolite characterized by a resonance at 1.56 ppm in the proton nuclear magnetic resonance spectrum. Twenty-four hours after the hypoxic insult, myo-inositol was increased, and alanine was decreased 48 h after the hypoxic insult, both compared with control values. Choline levels in CSF had a linear relationship with arterial pH (r = 0.26, p < 0.005). During severe hypoxia, CSF levels of succinate and choline are increased. Increased CSF levels of succinate may indicate dysfunction of the mitochondrial respiratory chain, whereas elevated CSF choline levels may indicate disrupted cell membranes. The increase of the CSF myo-inositol level after 24 and 48 h may indicate osmolytic cell changes causing cell edema. Decreased alanine level may represent changes in the source of excitatory amino acid synthesis.

Concentrations of nucleotides, nucleosides, purine bases, oxypurines, uric acid, and neuron-specific enolase in the cerebrospinal fluid of children with sepsis

To determine the effects of sepsis on cerebral energy metabolism, the cerebrospinal fluid adenosine monophosphate, inosine monophosphate, inosine, adenosine, guanosine, hypoxanthine, xanthine, and urate concentrations were determined by high-performance liquid chromatography and the neuron-specific enolase levels by means of an enzyme immunoassay method in 32 children with sepsis, without meningitis, aged between 2 months and 13 years, and in 160 age-matched controls. The septic group had significantly higher cerebrospinal fluid concentrations of inosine, adenosine, xanthine, and urate than controls. These results suggest that sepsis could provoke some degree of neuronal hypoxia and significant alterations of cerebral energy metabolism homeostasis.

Hypoxia in fetal lambs: a study with (1)H-MNR spectroscopy of cerebrospinal fluid

In fetal lambs, severe hypoxia (SH) will lead to brain damage. Mild hypoxia (MH) is thought to be relatively safe for the fetal brain because compensating mechanisms are activated. We questioned whether MH, leading to mild acidosis, induces changes in cerebral metabolism. Metabolites in cerebrospinal fluid (CSF) samples, as analyzed by proton magnetic resonance spectroscopy, were studied in two groups of seven anesthetized near-term fetal lambs. In group I, SH leading to acidosis with an arterial pH <7.1 was achieved. In group II, MH with an intended pH of 7.23--7.27 was reached [start of MH (SMH)], and maintained during 2 h [end of MH (EMH)]. During SH, choline levels in CSF, a possible indicator of cell membrane damage, were increased. Both during SH and at EMH, CSF levels of lactic acid, alanine, phenylalanine, tyrosine, lysine, branched chain amino acids, and hypoxanthine were increased compared with control values and with SMH, respectively. At EMH, the hypoxanthine CSF-to-blood ratio was increased as compared with SMH. These results indicate that prolonged MH leads to energy degradation in the fetal lamb brain and may not be as safe as assumed.

Intrapartum fetal hypoxia and biochemical markers: a review

Intrapartum fetal hypoxia is a rare event, although fetal intrapartum surveillance is discussed as a subject of major importance. This is mainly because of consequences of fetal hypoxia that may lead to cerebral palsy. A fetus suffering from hypoxia initially compensates by producing energy through anaerobic metabolism. At some stage, the fetus becomes decompensated and basic cellular functions fail, with risks of permanent morbidity or mortality. How long a fetus can survive on anaerobic metabolism differs because metabolic reserves differ, i.e., growth-restricted fetuses might deteriorate at an earlier stage. An increasing body of evidence has clarified brain-damaging mechanisms. Neuronal loss occurs in two phases: during the primary hypoxic event and later during the reperfusion/reoxygenation phase. Animal studies have suggested the possibility of prophylactic treatment to prevent neuronal loss after the hypoxic event. Intrapartum diagnostic tools should aim for detecting fetal hypoxemia/hypoxia when the fetus is still compensated. This may be achieved by assessment of biochemical data such as pH, lactate, and oxygen saturation, with the aim of prophylactic intervention before the fetus becomes decompensated. The measurement of cord blood levels of oxygen free radicals and excitatory amino acids at the time of birth may prove to be helpful in determining the risk of brain damage and evaluating the effect of prophylactic treatments to prevent or ameliorate brain injury from hypoxia.

Plasma hypoxanthine reacts more abruptly to changes in oxygenation than base deficit and uric acid in newborn piglets

Previously, high postmortem concentrations of hypoxanthine have been found in vitreous humor of children dying from sudden infant death syndrome (SIDS). We wanted to investigate further the accumulation of hypoxanthine in vitreous humor during hypoxia. Twenty-four piglets aged 9-15 days were exposed to continuous hypoxemia (180 min 11% O2, n = 6), long interval intermittent hypoxemia (60 min 11% O2, 20 min room air, n = 7) or short interval intermittent hypoxemia (10 min 9% O2, 10 min room air with (n = 6) or without (n = 5) superimposed ligation of both carotid arteries). The increase in vitreous humor Hyp was four-fold higher (p < 0.01) with ligation of the carotid arteries (14 +/- 2.4 to 38 +/- 8.9 mumol/l) than without ligation (15 +/- 2.8 to 21 +/- 5.9 mumol/l). During continuous hypoxemia, plasma Hyp (r = 0.85), Xa (r = 0.89) uric acid (UA) (r = 0.85), and base deficit (BD) (r = 0.78) increased almost linearly (p < 0.001). Plasma Hyp responded more abruptly to changes in oxygenation than base deficit (BD) and UA. Ligation of the carotid arteries had a strong impact on Hyp accumulation in vitreous humor, suggesting that vitreous humor Hyp is not merely a filtration product of plasma Hyp, but reflects local hypoxia/ischemia in the eye.

Magnesium sulfate therapy during asphyxia in near-term fetal lambs does not compromise the fetus but does not reduce cerebral injury

OBJECTIVE

Our purpose was to investigate (1) the safety of fetal magnesium sulfate treatment and (2) possible beneficial effects on the brain during perinatal asphyxia.

STUDY DESIGN

In 20 chronically instrumented fetal lambs (gestational age 125.8 +/- 3.5 days) four total umbilical cord occlusions for 5 minutes were repeated at 30-minute intervals. Fetuses received either saline solution (n = 11) or magnesium sulfate (n = 9) as a bolus of 300 mg intravenously 2 hours before occlusions, followed by an infusion of 100 mg/hr until 1 hour after occlusions.

RESULTS

In the treated fetuses plasma magnesium levels rose from 0.85 +/- 0.20 to 2.23 +/- 0.40 mmol/ L. Occlusions induced asphyxia, associated with mortality; 4 of 11 fetuses in the control group versus 1 of 9 in the magnesium-treated group died (not significant). Fetal electroencephalographic activity decreased and cerebral impedance increased during occlusions. Maximum spike and seizure activity occurred 5 to 10 hours after asphyxia. Neuronal loss was primarily localized in the corpus striatum. Magnesium caused no alterations in blood pressure, heart rate, or cerebral and peripheral blood flow, nor did it influence electrophysiologic responses or neuronal loss.

CONCLUSIONS

Administration of magnesium sulfate was safe but did not offer significant cerebral protection from asphyxia in the near-term fetal lamb.

Cerebrospinal fluid lactate and glutamine are reduced in multiple sclerosis

OBJECTIVES

To analyse various metabolites in human cerebrospinal fluid from healthy controls and patients with multiple sclerosis.

PATIENTS AND METHODS

Cerebrospinal fluid was obtained from patients by lumbar puncture, frozen, redissolved, and analysed for metabolites by proton nuclear magnetic resonance spectroscopy.

RESULTS

Significantly lower values for lactate and glutamine were found in patients with multiple sclerosis in comparison with controls. No significant differences were found between patients with the relapsing-remitting and chronic progressive forms of the disease for any of the metabolites measured.

CONCLUSION

There is a concomitant reduction in both lactate and glutamine in the cerebrospinal fluid of patients with multiple sclerosis compared to controls. This may be related to altered astrocytic metabolism during the disease. The results clearly show the diagnostic potential of magnetic resonance spectroscopy in diseases such as multiple sclerosis.

Cerebral metabolism within 18 hours of birth asphyxia: a proton magnetic resonance spectroscopy study

Proton magnetic resonance spectroscopy (1H MRS) was performed within 18 h of birth (median 13, range 4-18 h) on 16 term infants with clinical features of birth asphyxia. Ten infants with no evidence of birth asphyxia were studied as controls at 5-18 (median 8) h after birth. To detect delayed impairments in cerebral energy metabolism, 15 infants suspected of asphyxia underwent 31P MRS at 33-106 (median 62) h of age. Choline, creatine, and N-acetylaspartate (NAA) were detected in spectra located to the basal ganglia in all infants. Lactate was detected in 15 of the 16 infants suspected of asphyxia, but in only 4 of the 10 controls (p < 0.05, chi 2). Glutamine and glutamate (Glx) was detected in 11 infants suspected of asphyxia and in three controls, but this difference was not significant at the 5% level. The spectra revealed no other significant differences between asphyxiated infants and controls. In the asphyxiated infants, there was a negative correlation between the ratio of lactate to creatine in the first 18 h of life and phosphocreatine/inorganic phosphate (PCr/ P(i)) at 33-106 h (p < 0.001). Five severely asphyxiated infants had PCr/P(i) < 0.75 (median 0.53, range 0.14-0.65), indicating a poor neurodevelopmental prognosis, and a further infant died before PCr/Pi could be measured. Ten infants had PCr/P(i) > 0.75 (1.03, 0.76-1.49). Median lactate/creatine was 1.47 (range 0.67-3.81) in the six severely affected subjects, 0.38 (0-1.51) in the latter group, and 0 (0-0.6) in controls (p < 0.0005, Kruskall-Wallis). These results suggest that, after birth asphyxia, cerebral energy metabolism is abnormal during the period when 31P MRS characteristically gives normal results. 1H MRS might be of value in predicting which infants are likely to suffer a decline in cerebral high energy phosphate concentrations and subsequent neurodevelopmental impairment.

Neuronal death after perinatal asphyxia

During perinatal asphyxia several mechanisms aim to limit cerebral damage. However, when the degree of asphyxia passes beyond a certain threshold, brain damage is inevitable. This review focuses on the various factors determining the final cerebral outcome. Metabolic and biochemical events, such as the intracellular level of calcium, the formation of oxygen derived free radicals, the release of excitotoxic neurotransmitters and the interrelationship of these parameters are discussed. Furthermore, steps possibly useful to pharmacologic intervention aiming to reduce cerebral damage are presented.