Cerebral metabolic and circulatory changes induced by hypoxia in starved rats

The combined detection of umbilical cord nucleated red blood cells and lactate: early prediction of neonatal hypoxic ischemic encephalopathy

OBJECTIVE

To establish a simple and quick method that could be used to predict the occurrence of hypoxic ischemic encephalopathy (HIE) as early as possible by investigating the variations of nucleated red blood cells per 100 white blood cell (NRBC/100 WBC) counts and lactate levels in cord blood.

METHODS

In 46 cases of acute fetal distress (AFD) and 54 cases of chronic fetal distress (CFD) neonates we measured the percentage (NRBCs/100 WBC) and lactate in the umbilical blood.

RESULTS

Both lactate levels and NRBC/100WBC counts were higher in CFD and AFD groups than in controls (both P<0.01). The numbers of NRBC/100 WBC and the values of lactate in moderate-severe HIE group were higher than in mild-HIE group (P=0.002 and P=0.042, respectively). The combined sensitivity and specificity was 94% and 96% at 15NRBC/100WBC and 4.25 mmol/L level by combined detecting NRBC and lactate to predict HIE. Three infants (including 1 death and 2 survivors) had the highest levels of NRBC/100WBC and lactate in cord blood, and the 2 survivors had the lowest mental development index (MDI) and psychomotor development index (PDI).

CONCLUSIONS

Combined detection with NRBC/100WBC and lactate allows early prediction of development and severity of HIE. The levels of these parameters are related to the neurodevelopment outcome of HIE infants.

Glycolysis and perinatal hypoxic-ischemic brain damage

To ascertain the regulation of glycolysis during perinatal hypoxia-ischemia, 7-day postnatal rats were subjected to unilateral common carotid artery ligation followed by hypoxia with 8% oxygen for up to 90 min. Brain concentrations of glucose, lactate, and key glycolytic intermediates were determined at specific intervals of hypoxia. During hypoxia-ischemia, anaerobic glycolysis increased to approximately 62% of its maximal capacity, which equates to a 135% stimulation of the glycolytic flux. The key regulatory enzymes, hexokinase, phosphofructokinase and pyruvate kinase, were all stimulated during hypoxia-ischemia, and there were no enzymatic rate limitations. The major rate-limiting step for glycolysis was the transport of glucose across the blood-brain barrier into the brain.

Decreased energy metabolism in brain stem during central respiratory depression in response to hypoxia

Metabolic changes in the brain stem were measured at the time when oxygen deprivation-induced respiratory depression occurred. Eucapnic ventilation with 8% oxygen in vagotomized urethan-anesthetized rats resulted in cessation of respiratory drive, monitored by recording diaphragm electromyographic activity, on average within 11 min (range 5-27 min), presumably via central depressant mechanisms. At that time, the brain stems were frozen in situ for metabolic analyses. By using 20-microns lyophilized sections from frozen-fixed brain stem, microregional analyses of ATP, phosphocreatine, lactate, and intracellular pH were made from 1) the ventral portion of the nucleus gigantocellularis and the parapyramidal nucleus; 2) the compact and ventral portions of the nucleus ambiguus; 3) midline neurons; 4) nucleus tractus solitarii; and 5) the spinal trigeminal nucleus. At the time of respiratory depression, lactate was elevated threefold in all regions. Both ATP and phosphocreatine were decreased to 50 and 25% of control, respectively. Intracellular pH was more acidic by 0.2-0.4 unit in these regions but was relatively preserved in the chemosensitive regions near the ventral and dorsal medullary surfaces. These results show that hypoxia-induced respiratory depression was accompanied by metabolic changes within brain stem regions involved in respiratory and cardiovascular control. Thus it appears that there was significant energy deficiency in the brain stem after hypoxia-induce respiratory depression had occurred.

Paradoxical mitochondrial oxidation in perinatal hypoxic-ischemic brain damage

Measurements of cytoplasmic and mitochondrial markers of the oxidation-reduction (redox) state of brain tissue were conducted in a perinatal animal model of cerebral hypoxia-ischemia to ascertain underlying biochemical mechanisms whereby ischemia (reduced oxygen and substrate supply) causes brain damage. Seven-day postnatal rats underwent unilateral common carotid artery ligation followed by exposure to 8% oxygen at 37 degrees C for 3 h. During the course of hypoxia-ischemia, the rat pups were quick frozen in liquid nitrogen and their brains processed for the enzymatic, fluorometric measurement of cerebral metabolites necessary for the calculation of intracellular pH and cytoplasmic and mitochondrial redox states. The results showed an early mitochondrial reduction followed by re-oxidation during the course of hypoxia-ischemia. The oxidation reflected a partial depletion in accumulated reducing equivalents and coincides temporally with the duration of hypoxia-ischemia required to convert selective neuronal necrosis into cerebral infarction. The findings suggest that perinatal cerebral hypoxia-ischemia is characterized more by a limitation of substrate than of oxygen supply to the brain, which may explain why glucose supplementation of the immature animal improves neuropathologic outcome, in contrast to adults.

Cerebral mitochondrial redox states during metabolic stress in the immature rat

The brain mitochondrial NAD+/NADH ratio, as a reflection of the oxidation-reduction (redox) state of cellular compartment, was determined under conditions of hypoxia, anoxia, hypoxia-ischemia, complete ischemia and hypoglycemia in immature rats. NAD+/NADH ratios were calculated from changes in the concentrations of specific oxidative substrates and calculated intracellular pH during cerebral metabolic stress. The results suggest that the use of the acetoacetate/beta-hydroxybutyrate substrate couple provides a more accurate prediction of the mitochondrial redox state under adverse conditions than use of the alpha-ketoglutarate/glutamate couple. It is possible that the mitochondrial oxidation seen with the latter substrate couple during cerebral metabolic stress might reflect a population of cells (neurons or glia) which are substrate-deprived relative to the rest of the brain in the setting of metabolic stress produced by oxygen deficiency.

Effects of hypobaric hypoxia on the human auditory brainstem responses

Auditory brainstem responses (ABRs) were recorded in six volunteers before, during and after 90-min exposure to hypobaric hypoxia (5,184 m; barometric pressure = 405 mmHg) in an altitude chamber. Waves I, III and V absolute and interpeak latencies were analysed. The main result of the experiment was a significant shortening of the brainstem transmission time (I-V interval) in the recovery from hypoxia compared with the basal condition. This finding could be explained with a slow decay of the compensatory mechanisms acting during hypoxia and/or a transient neuronal hyperexcitability at the end of the hypoxic stress.

Lesions of the rostral ventrolateral medulla reduce the cerebrovascular response to hypoxia

Sympathoexcitatory neurons of the rostral ventrolateral medulla are tonically active and required for maintenance of resting levels of arterial pressure. They are also selectively excited by hypoxia and responsible for the associated sympathoexcitation. Since electrical or chemical stimulation of RVL will increase regional cerebral blood flow (rCBF) independently of changes in regional cerebral glucose utilization (rCGU) we investigated whether the RVL was also required to maintain resting levels of rCBF and also participated in the cerebrovascular vasodilation elicited by hypoxia. Rats were anesthetized (chloralose; 40 mg/kg, s.c.), paralyzed (tubocurarine) and ventilated (100% O2). rCBF was measured in 10 dissected brain regions using [14C]iodoantipyrine; rCGU was measured by 2-deoxy-D-[14C]glucose. In controls (n = 6) rCBF ranged from 56 +/- 5 in corpus callosum to 101 +/- 6 ml/min x 100 g in inferior colliculus. Hypoxic-hypoxia (PaO2 = 36 +/- 1 mmHg, n = 6) increased rCBF in all structures maximally, at 204% of control, in occipital cortex. Hypercapnia (PaCO2 = 63.5 +/- 0.9, n = 5) also increased rCBF (P < 0.01) maximally to 299% of control in superior colliculus. Spinal cord transection with maintenance of arterial pressure did not affect resting rCBF and increased the vasodilation to hypoxia (PaO2 = 39 +/- 1 mmHg, n = 5) from 2- to 3-fold in all structures (P < 0.01). Bilateral lesions within the RVL had no effect on resting rCBF or rCGU. However, they significantly reduced, in all areas by 50-69% (P < 0.01, n = 5), the cerebrovascular dilation elicited by hypoxia but not hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischemic brain damage is not ameliorated by 1,3-butanediol in hyperglycemic rats

BACKGROUND AND PURPOSE

Treatment with the ketone body precursor 1,3-butanediol has been suggested to ameliorate hypoxic/ischemic brain damage. Butanediol could provide an alternative energy substrate for the brain, thereby decreasing the amount of glycolytically produced lactate. Hyperglycemia aggravates brain damage after brain ischemia and causes fatal postischemic seizures, probably by increasing the production of lactate and decreasing the pH. We studied whether butanediol treatment altered the adverse consequences following ischemia complicated by hyperglycemia.

METHODS

Hyperglycemic adult male rats were given 25 or 50 mmol.kg-1 body wt butanediol intravenously 30 minutes before 10 minutes of transient forebrain ischemia. Morphological evaluation was performed following perfusion-fixation after 15 hours of recovery. Blood concentrations of beta-hydroxybutyrate, acetoacetate, glucose, and lactate and brain tissue concentrations of energy metabolites were measured before and after ischemia.

RESULTS

Blood levels of ketone bodies increased in the butanediol-treated rats. Ischemia decreased the blood levels of acetoacetate but increased the levels of beta-hydroxybutyrate by a similar amount, resulting in unchanged high levels of total ketone bodies in the animals that received butanediol. Brain tissue levels of glucose, energy metabolites, and lactate showed no difference between butanediol- and saline-treated rats. Furthermore, compared with saline-treated animals butanediol-treated rats showed no decrease in brain damage and no attenuation in the development of postischemic seizures.

CONCLUSIONS

The ketone body precursor 1,3-butanediol offers no protective effect in transient forebrain ischemia complicated by hyperglycemia.

Acidosis and ischemic brain damage

It is now widely accepted that acidosis is an important component of the pathogenetic events that lead to ischemic brain damage. The objective with this article is to recall the evolution of the concept, to describe the conditions under which tissue acidosis arises and causes enhanced brain damage, and to probe into the cellular and molecular mechanisms involved.

Lactate and pH in the brain: association and dissociation in different pathophysiological states

Brain tissue pH and lactate content were measured in rats under three different experimental conditions, namely: during complete global cerebral ischemia; after reversible near-complete cerebral ischemia; and in experimental brain tumors. At the end of the experiments brains were frozen with liquid nitrogen. A series of 20-microns thick coronal sections was prepared in a cryostat and then used for the regional determination of tissue pH (umbelliferone technique) and tissue lactate (bioluminescent technique). In addition, tissue samples were taken for the quantitative measurement of brain lactate (enzymatic fluorometric technique). The relationship between lactate content and tissue pH was different for each of the three experimental models studied: only after short-term global cerebral ischemia did an increase in the lactate content correlate with a decrease in tissue pH (r = 0.94; p less than 0.001). A highly significant increase in the lactate content (p less than 0.001) was accompanied by physiological pH values (6.96 +/- 0.08 in comparison to 6.97 +/- 0.04 in controls) during recirculation after transient cerebral ischemia and in brain tumors even by an alkaline pH shift. In view of these observations the term "lactacidosis" should not be used without measuring both the lactate content and the pH. The observed dissociation between pH and lactate is due to the fact that both parameters are regulated independently. During anaerobiosis the main source of proton production is ATP hydrolysis rather than glycolysis. It is, therefore, suggested that the terms "acidosis" and "lactosis" should be used instead of "lactacidosis."

Influence of blood glucose concentration on brain lactate accumulation during severe hypoxia and subsequent recovery of brain energy metabolism

The effects of hypoxaemia on regional cerebral blood flow (CBF) and brain cortical metabolite concentrations were investigated at different blood glucose concentrations in rats under nitrous oxide anaesthesia. Tissue hypoxia of 15-min duration was induced by a combination of arterial hypoxaemia, hypotension, and clamping of the right carotid artery. Blood glucose concentrations were manipulated by varying the food intake in the 24 h before the experiment, and by glucose administration. Cortical CBF doubled during hypoxia on the intact side, but did not differ significantly from control values on the clamped side. In the clamped hemisphere there was a substantial decrease in adenylate energy charge. At brain tissue glucose concentration of 1 mumol g-1 and above, there was an inverse correlation between adenylate energy charge and brain lactate concentration. In starved animals with mean brain glucose of 0.32 +/- 0.00 mumol g-1, lactate concentration was significantly lower, in spite of equally severe disruption of energy state. Recovery of brain adenylate energy charge was worse in fed and glucose-infused groups than in the fasted group. These results demonstrate that limitation of substrate supply during severe hypoxia in the rat allows enhanced recovery of brain energy metabolism following the hypoxic episode.

The calculation of the mitochondrial free [NAD+]/[NADH][H+] ratio in brain: effect of electroconvulsive seizure

This study is an investigation into the validity of calculating the mitochondrial redox state in brain in vivo using models of seizure and anoxia in rats. At six intervals following electroconvulsive seizure (0.5-10 min) and after 5 min of complete anoxia, multiple metabolites were measured in freeze-blown or freeze-clamped brain. From substrate ratios, the apparent changes in the mitochondrial free [NAD+]/[NADH] [H+] ratio were calculated from the L-glutamate dehydrogenase reaction [EC 1.4.1.3] and compared with shifts in the oxidized to reduced ratio of total ubiquinone (a component of the mitochondrial phosphorylation chain). During complete anoxia the calculated mitochondrial free [NAD+]/[NADH] [H+] ratio and the ubiquinone redox ratio both became more reduced by a factor of approximately 7. In contrast, following seizure the two indicators of the mitochondrial redox state moved in opposite directions. Mainly because of a large increase in tissue NH4+, the calculated mitochondrial free [NAD+]/[NADH] [H+] ratio paradoxically became more oxidized, plateauing between 2 and 10 min post seizure at a value approximately double that of the control. At the same time, however, the ubiquinone redox state fell to one-half the control value at two min and moved back towards normal between 5 and 10 min after the onset of the seizure. The results have been taken to be evidence against the applicability of the calculation of the mitochondrial free [NAD+]/[NADH] [H+] ratio from the L-glutamate dehydrogenase reaction in brain at least under conditions of rapid change. The results also suggest the possibility that the NH4+ produced during seizure is extra-mitochondrial and has relatively little tendency to diffuse into the matrix.

Decreases in amino acids and acetylcholine metabolism during hypoxia

Hypoxia impairs brain function by incompletely defined mechanisms. Mild hypoxia, which impairs memory and judgment, decreases acetylcholine (ACh) synthesis, but not the levels of ATP or the adenylate energy charge. However, the effects of mild hypoxia on the synthesis of the glucose-derived amino acids [alanine, aspartate, gamma-amino butyric acid (GABA), glutamate, glutamine, and serine] have not been characterized. Thus, we examined the incorporation of [U-14C]glucose into these amino acids and ACh during anemic hypoxia (injection of NaNO2), hypoxic hypoxia (15 or 10% O2), and hypoxic hypoxia plus hypercarbia (15 or 10% O2 with 5% CO2). In general, the synthesis of the amino acids and of ACh declined in parallel with each type of hypoxia we studied. For example, anemic hypoxia (75 mg/kg of NaNO2) decreased the incorporation of [U-14C]glucose into the amino acids and into ACh similarly. [Percent inhibition: ACh (57.4), alanine (34.4), aspartate (49.2), GABA (61.9), glutamine (59.2), glutamate (51.0), and serine (36.7)]. A comparison of several levels (37.5, 75, 150, 225 mg/kg of NaNO2) of anemic hypoxia showed a parallel decreased in the flux of glucose into ACh and into the amino acids whose synthesis depends on mitochondrial oxidation: GABA (r = 0.98), glutamate (r = 0.99), aspartate (r = 0.96), and glutamine (r = 0.97). The synthesis of the amino acids not dependent on mitochondrial oxidation did not correlate as well with changes in ACh metabolism: serine (r = 0.68) and alanine (r = 0.76). The decreases in glucose incorporation into ACh and into the amino acids with hypoxic hypoxia (15% or 10% O2) or hypoxic hypoxia with 5% CO2 were very similar to those with the two lowest levels of anemic hypoxic. Thus, and explanation of the brain's sensitivity to a decrease in oxygen availability must include the alterations in the metabolism of the amino acid neurotransmitters as well as ACh.

High affinity choline transport and acetylCoA production in brain and their roles in the regulation of acetylcholine synthesis

This review describes recent advances made in the understanding of the regulation of acetylcholine synthesis in brain with regard to the availability of its two precursors, choline and acetylCoA. Choline availability appears to be regulated by the high affinity choline transport system. Investigations of the localization and inhibition of this system are reviewed. Procedures for measuring high affinity choline transport and their shortcomings are described. The kinetics and effects of previous in vivo and in vitro treatments on high affinity choline transport are reviewed. Kinetic and direct coupling of the transport and acetylation of choline are discussed. Recent investigations of the source of acetylCoA used for the synthesis of acetylcholine are reviewed. Three sources of acetylCoA have recently received support: citrate conversion catalyzed by citrate lyase, direct release of acetylCoA from mitochondria following its synthesis from pyruvate catalyzed by pyruvate dehydrogenase, and production of acetylCoA by cytoplasmic pyruvate dehydrogenase. Investigations indicating that acetylCoA availability may limit acetylcholine synthesis are reviewed. A model for the regulation of acetylcholine synthesis which incorporates most of the reviewed material is presented.