The influence of arterial hypoxemia upon labile phosphates and upon extracellular and intracellular lactate and pyruvate concentrations in the rat brain

Perinatal cerebral ischaemia and developmental neurologic disorders

In infants who die during the first months of life necrotic foci are almost invariably found. They are, as a rule, located in border zones between vascular territories, suggesting cerebral ischemia as a pathogenetic mechanism. Normally, the brain is protected against changes in perfusion pressure by autoregulation of cerebral perfusion. In utero, this mechanism is extremely fragile, and normal birth is a sufficient hypoxic insult to abolish autoregulation. Abolished autoregulation has been demonstrated in distressed newborns: Even mild hypotension, which is a common occurrence in these infants during the first few hours of life, is sufficient to induce ischemia. Follow-up studies at the ages of one and four years have shown neonatal ischemia to be the decisive factor in the development of atrophic encephalopathy and motor and cognitive dysfunction. This clarification of the pathogenetic process has important implications for prevention, which is briefly discussed.

The effect of graded hypothermia on hypoxic-ischemic brain damage: a neuropathologic study in the neonatal rat

To investigate the relationship between neuropathologic damage and cerebral metabolic alterations during hypothermia in the neonatal animal, 7 day old Sprague-Dawley rats were subjected to unilateral common carotid artery ligation and hypoxia at 37 degrees C, 29 degrees C, and 21 degrees C. At 37 degrees C, animals had extensive infarction of tectum and ipsilateral cerebral hemisphere, and marked depletion of brain ATP. At 29 degrees C, there was no significant change in brain ATP; neuropathologic damage was limited to a few areas of necrosis in the deeper layers of cerebral cortex. No histologic injury was seen in the 21 degrees C group of rats. Profound hypothermia may prevent cerebral edema and visible neuropathologic damage associated with hypoxic-ischemic injury by decreasing cerebral metabolic demands. Moderate hypothermia confers a partial, but incomplete degree of protection; whereas during normothermia, the full extent of hypoxicischemic injury is manifest.

Reversible uncoupling of oxidative phosphorylation at low oxygen tension

The stoichiometry of oxidative phosphorylation at low oxygen tension (less than 3 torr; O2 less than 5 microM) has been measured in rat liver mitochondria. In a steady-state model in which respiration rate was experimentally controlled by either oxygen or substrate (succinate) limitation, flux-dependent variation in the phosphorylation efficiency (P/O ratio) of stimulated mitochondrial respiration was evaluated. P/O ratio remained constant over a wide range of respiration rates in mitochondria limited only by substrate availability. In contrast, oxygen-limited mitochondria demonstrated a continuous decline in P/O ratio as respiration was increasingly restricted. Significant differences in the two test conditions were demonstrated throughout the range of analysis. The effect of oxygen limitation on phosphorylation efficiency was shown to be completely reversed by restoring zero-order kinetics associated with high oxygen tension. These findings are discussed in regard to a proposed uncoupling of mitochondrial coupling site II at low oxygen tension arising as a consequence of energy-dissipating electron flux through the ubiquinone-cytochrome b-c1 region of the respiratory chain (complex III).

Hypoxic-ischemic cerebral necrosis in midgestational sheep fetuses: physiopathologic correlations

Thirty-eight midgestational sheep fetuses were exposed 120 min to marked hypoxia. The brains in eight that reduced their mean arterial blood pressure to less than 30 mm Hg were markedly damaged. In these same fetuses the serum lactic acid concentrations were elevated during exposure to hypoxia to excessively high values (greater than 15 mM) and remained elevated for a prolonged period during recovery. Twenty-one fetuses exposed to the same magnitude of hypoxia that maintained their blood pressure unchanged showed less marked elevations of serum lactate concentrations and remained brain-intact. Greater quantities of pentobarbital administered to the ewes during hypoxia seemed to protect the brain from hypoxia and this effect was dose-dependent. Exposure of midgestational sheep fetuses to marked hypoxia associated with reductions in cerebral blood flow due to decreased blood pressure and impaired cerebral autoregulation caused major focal cerebral necrosis.

Temporary incomplete ischemia of the legs induced by aortic clamping in man. Metabolic and hemodynamic effects of temporary extracorporeal by-pass

The effects on central hemodynamics and skeletal muscle metabolism during surgery for abdominal aortic aneurysm were compared in 6 patients given a preoperative adrenergic block (group B) and in 6 patients who additionally had a temporary brachio-femoro-femoral by-pass during the aortic clamping (group B + S). The cardiac output, heart rate, arterial and pulmonary artery pressures and the cardiac filling pressure were studied. Biopsy specimens from the lateral vastus muscle and blood samples from the radial artery and the iliac vein were taken before aortic clamping and also before and 30 minutes, 4 and 16 hours after the aortic declamping. Intramuscular temperature and pH were measured. The glycogen, glucose, lactate, pyruvate, ATP, ADP, AMP, phosphocreatine (PCr) and creatine (Cr) contents of the muscle and the lactate and pyruvate concentration in iliac venous and radial arterial blood were determined, using enzymatic fluorometric techniques. In group B, aortic clamping induced severe temporary incomplete ischemia with a 300% increase in lactate/pyruvate (L/P) ratio and a fall in intramuscular pH (pHm). The adenylate energy charge (EC) decreased, but the creatine (PCr + CR) and the adenylate (ATP + ADP + AMP) pool remained unchanged. After aortic declamping, the L/P ratio, EC and pHm regained their preclamping values, but the pools of energy phosphate compounds were reduced, indicating dysfunction or damage of the muscle cells. In group B + S there were no major muscle metabolic changes during clamping or after declamping of the aorta. In group B the systemic vascular resistance (SVR), mean arterial blood pressure (MAP) and left ventricular stroke work (LVSW) increased during the occlusion. On release of the clamp, cardiac output rose, possibly due to the sudden reduction of SVR. A temporary marked fall of MAP occurred. In group B + S, no increase of SVR, MAP or LVSW was observed during aortic clamping. After the declamping, only a minor MAP drop was observed. In both groups, a brief rise in pulmonary vascular resistance after the aortic declamping suggested transient pulmonary microembolism. If a high-risk patient is to undergo reconstructive surgery of the abdominal aorta and/or technical difficulties can be expected to necessitate prolonged cross-clamping during the operation, a temporary extracorporeal by-pass may be a favorable adjuvant, improving cardiac performance and preventing derangement of muscle metabolism.

Dissociation between epileptic seizures induced by convulsant drugs and alteration in the concentrations of pyridoxal phosphate in rat brain regions

Allylglycine increased the concentration of pyridoxal phosphate in cerebral cortex from 1011.4 +/- 25.0 to 1318.0 +/- 66.3 and decreased it in cerebellum from 1289.0 +/- 49 to 1147.7 +/- 119.4 ng/g wet tissue during the preictal period. Mercaptopropionic acid increased the concentration of pyridoxal phosphate in cerebellum from 1525 +/- 91 to 1985.7 +/- 275 ng/g wet tissue. Similar effects were noted in hippocampus and cerebral cortex. Picrotoxin increased the concentration of pyridoxal phosphate in hippocampus from 938.7 +/- 44 to 1043 +/- 118 but decreased it in cerebral cortex from 1124.52 +/- 124 to 979.4 +/- 15 ng/g wet brain. The effects of strychnine were identical to those of allylglycine. Bicuculline reduced the concentration of pyridoxal phosphate in cerebral cortex from 1184 +/- 61 to 1075.14 +/- 78 ng/g wet brain.

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 young-adult and normally aged brain. Its blood flow and oxidative metabolism. A review--part I

Blood flow and oxidative metabolism of the mature and healthy young-adult human brain account for about 20% of the cardiac output and about 20 and 25% of the requirements of oxygen and glucose, respectively, for the whole body. Normal cerebral aging is associated with only smaller reductions in the cerebral metabolic rates of oxygen and glucose while cerebral blood flow would seem to be unchanged. The age-dependent reduction in oxidative brain metabolism may be related to a decline in glycolytic flux due to a diminution of enzyme activities also involving acetylcholine synthesis. This metabolic reduction with age may be tentatively accounted for by a physiologically occurring loss of neurons, dendrites and dendritic spines in distinct brain areas. The mechanisms of autoregulation of cerebral blood flow, of CO2 reactivity of the brain vessels, of arterial hypoxemia on cerebral blood flow and their effects on oxidative and energy metabolism are well documented in young-adult brain. There is, however, no or only minimal information on the responsiveness of the normally aged brain to changes of these important biological parameters controlling and influencing brain blood flow and metabolism.

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.

Reduction of the cerebral protective effect of hypothermia by oligemic hypotension during hypoxia in the rat

The effect of arterial hypotension on cerebral cortical tissue levels of adenosine triphosphate (ATP), phosphocreatine (PGr), lactate, and reduced nicotinamide adenine dinucleotide (NADH) was studied in male Wistar rats with unilateral carotid ligation exposed to arterial by hypoxia (PaO2 25 torr) for 20 min. while the body temperature was maintained at 32 degrees C and 27 degrees C. Brain metabolite levels were normal in normotensive hypothermic animals exposed to hypoxia, but reduction in arterial pressure to 75 torr caused a significant (p less than 0.05) decrease in ATP and PCr values and a significant increase in lactate and NADH levels. These changes were comparable to those of normothermic normotensive, hypoxic animals. Furthermore, there was no significant differences in the brain metabolite levels between the two hypotensive hypoxic groups. These results indicate that arterial hypotension severely alters the cerebral protective effect of hypothermia against injury caused by hypoxia, and that further reduction in body temperature (from 32 degrees C to 27 degrees C) will not prevent the harmful effect of hypoxia upon the brain in hypotensive rats.

Influence of age upon the cerebral metabolic changes induced by acute hypoxia on the synaptosomes from dog brain

The synaptosomal fraction obtained from the motor area of the cerebral cortex of normocapnic, normoxic or hypoxic "young adult," "mature" and "senescent" beagle dogs is incubated and analyzed for : ATP, ADP, AMP, creatine phosphate, pyruvate and lactate. The data are compared with those obtained from the whole controlateral cortical motor area, by the surface freezing technique. After hypoxic hypoxia /15 min; PaO2 = 17-19 mm Hg), the metabolite contents and ratios are differently affected by ageing when the evaluations are performed in the incubated synaptosomal preparation or in the controlateral whole cerebral tissue. In fact, ageing does not affect so much the cerebral changes that occur in the overall energetic state during the hypoxic assault in vivo, but rather those that the synaptosomes remember the tend to reverse during the subsequent incubation in vitro. The protective action of several drugs on the synaptosomal phosphorylation state is tested. Phenobarbital shows a quite broad, age-independent spectrum of action. (-)Eburnamonine and dihydroergocristine exhibits a more limited, age-dependent effectiveness, but are devoid of anesthetic action. Papaverine proves unable to affect the tested biochemical parameters.

Cerebrospinal fluid markers of disturbed brain cell metabolism

Reversibly and irreversibly disturbed brain cell metabolism may be monitored in an indirect way by the analyses of enzymes in the CSF according to the hypothesis of cell swelling induced by energy shortage. Adenylate kinase fulfils the criteria for an ideal CSF marker with the exception that it is not organspecific, which necessitates precautions to avoid influence of AK in erythrocytes and serum. When taking such limitating factors into account, AK determinations may be diagnostically useful in combination with radiological and clinical observations. Besides, it is possible that a combination of AK analyses and clinical signs are useful in the prognostication in individual patients suffering from global cerebral ischemia and cerebral infarction.

Drug action on the metabolic changes induced by acute hypoxia on synaptosomes from the cerebral cortex

The synaptosomal fractions obtained from the motor area of the cerebral cortex of normocapnic, normoxic, or hypoxic, untreated beagle dogs and of pentobarbital (Nembutal)- or cytidine diphosphate (CDP)-choline-treated dogs were incubated and analyzed for ATP, ADP, AMP, creatine phosphate, pyruvate, and lactate. The data were compared with data obtained by the surface freezing technique from the whole contralateral cortical area. The in vivo intracarotid perfusion of the drug differentially affected the content of the metabolites and their ratio. This occurred whether the evaluations were performed in the incubated synaptosomal preparations or in whole cerebral tissue, both during normoxia and after hypoxia (15 min; PaO2 = 17-19 mm Hg). Thus, intracarotid perfusion of nembutal increased the synaptosomal phosphorylation state both in normoxic and in hypoxic animals, whereas the effect on the metabolism of the contralateral cortical motor area as a whole was in all cases less than that observed in the synaptosomal fraction. Perfusion with CDP-choline increased synaptosomal phosphorylation after the hypoxic condition, but had no effect in normoxia or on the whole cortical tissue of the motor area. The possibility of obtaining a cerebral sparing action by utilizing molecules devoid of anesthetic action is suggested.

Oxygen dependence of glucose and acetylcholine metabolism in slices and synaptosomes from rat brain

Previous studies have shown that a reduction in the O2 tension of the blood from 120 torr to 57 torr (hypoxic hypoxia) decreases brain acetylcholine (ACh) synthesis. To determine if this decrease is due to a direct impairment of ACh metabolism or to an indirect effect mediated by other neurotransmitter systems, we studied ACh formation in rat brain slices and synaptosomes. At O2 tensions ranging from 760 to less than 1 torr, 14CO2 production and [14C]ACh synthesis from [U-14C]glucose, the levels of lactate and ATP, and the ATP/ADP ratio were determined. In slices, the first decreases were observed in the rate of 14CO2 production and [14C]ACh synthesis at an O2 tension of 152 torr. The ATP level started to decline at 53-38 torr, and a reduction in the ATP/ADP ratio was first found at and below 19 torr. Lactate formation was maximally stimulated at 38-19 torr. Synaptosomes responded differently than brain slices to reduced O2 tensions. In synaptosomes, 14CO2 production and [14C]ACh synthesis from [U-14C]glucose, the levels of lactate and ATP, and the ATP/ADP ratio were unaltered if a minimum O2 tension of 19 torr was maintained. Despite the difference in sensitivities to decreases in O2 levels, there is a curvilinear relationship between [U-14C]glucose decarboxylation and [14C]ACh synthesis at various O2 tensions for both tissue preparations with a high coefficient of determination (R2 = 0.970). The difference in the metabolic sensitivity of slices and synaptosomes to a reduced O2 level may be explained by the greater distance O2 must diffuse in slices. The results are discussed in comparison with hypoxia in vivo.

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.

The influence of hypoxia on the concentrations of cyclic nucleotides in the rat brain

In order to study the influence of hypoxia on cyclic nucleotides in the brain, we reduced arterial Po(2) for 15-30 min in lightly anaesthetised and artificially ventilated rats to obtain values ranging from about 45 to about 10 mm Hg. In an additional group (arterial Po(2) 18-22 mm Hg), the tissue hypoxia was aggravated by moderate arterial hypotension (mean arterial blood pressure about 80 mm Hg). In all animals, electrocortical activity was recorded. Cyclic GMP concentrations in cerebral cortex were unchanged in all groups but one. In that group, in which tissue hypoxia was severe enough to induce a suppression-burst EEG pattern and a measurable reduction in the adenylate energy charge, cyclic GMP concentrations were slightly increased (p less than 0.05). Cyclic AMP concentrations remained unaltered at all degrees of hypoxia studied. It is concluded that changes in cyclic nucleotides in brain tissue occur first at such severe degrees of hypoxia of the duration studied that function and metabolism are profoundly altered.

Ammonia and disinhibition in cat motor cortex by ammonium acetate, monofluoroacetate and insulin-induced hypoglycemia

Ammonia intoxication abolished the suppression of action potential generation by cortical postsynaptic inhibition due to the inactivation of neuronal Cl- extrusion. The disinhibition by ammonia intoxication occurred when ammonia concentrations in the cerebral cortex were increased to 320% of normal. Fluoroacetate poisoning and insulin-induced hypoglycemia, which are known to increase ammonia concentrations in the CNS and previously have been shown to inactivate Cl- extrusion in spinal motoneurons, abolished the suppression of action potential generation by cortical postsynaptic inhibition like ammonia intoxication. This disinhibition occurred at unchanged cerebral ammonia concentrations. The effect of fluoroacetate and insulin induced hypoglycemia on cortical postsynaptic inhibition is either due to a direct, i.e. not ammonia mediated, inactivation of neuronal Cl- extrusion or due to a disturbance of the synaptic mechanisms mediated by the transmitter of cortical inhibition, GABA. Toxic-metabolic encephalopathies which increase cerebral ammonia concentrations beyond 320% of normal may produce a dysfunction of the CNS due to inactivation of neuronal Cl- extrusion leading to ineffective cortical inhibition. However, in fluoroacetate poisoning and insulin-induced hypoglycemia increased ammonia concentrations in the CNS have only a secondary role in initiating a dysfunction of the CNS since disinhibition occurs before ammonia concentrations increase.

Impaired synthesis of acetylcholine by mild hypoxic hypoxia or nitrous oxide

The effect of mild hypoxic hypoxia on brain metabolism and acetylcholine synthesis was studied in awake, restrained rats. Since many studies of hypoxia are done with animals anesthetized with nitrous oxide (N2O), the effects of N2O were evaluated. N2O (70%) increased the cerebral cortical blood flow by 33% and the cortical metabolic rate of oxygen by 26%. In addition, the synthesis of acetylcholine in N2O-anesthetized animals, measured with [U-14C]glucose and [1-2H2,2-2H2]choline, decreased by 45 and 53%, respectively. Consequently, mild hypoxia was studied in unanesthetized rats. Control rats breathing 30% O2 (partial pressure of oxygen, PaO2 = 120 mm Hg) were compared with rats exposed to 15% O2 (PaO2 = 57 mm Hg) or 10% O2 (PaO2 = 42 mm Hg). The synthesis of acetylcholine, measured with [U-14C]glucose, was decreased by 35 and 54% with 15% O2 and 10% O2, respectively; acetylcholine synthesis, measured with [1-2H2,2-2H2]choline, was decreased by 50 and 68% with 15% O2 and 10% O2, respectively. Animals breathing either 15% or 10% O2 had normal cerebral metabolic rates of oxygen but had increased brain lactates and increased cortical blood flows compared with animals breathing 30% O2. These results show that even mild hypoxic hypoxia impairs acetylcholine synthesis, which in turn may account for the early symptoms of brain dysfunction associated with hypoxia.

Influence of nitrous oxide on local cerebral blood flow in awake, minimally restrained rats

In order to evaluate the effect of 70-80% N2O on local cerebral blood flow (1-CBF) in the rat brain, we developed a procedure for measuring CBF by an autoradiographic [14C]iodoantipyrine technique in awake, minimally restrained animals. Results on 1-CBF, as measured in 22 different structures, showed little variability between animals. In the majority of structures analyzed, 70-80% N2O failed to alter 1-CBF. These included all cerebral cortical and most subcortical structures. However, nitrous oxide reduced CBF in the inferior colliculus and the superior olive, in two of the limbic structures analyzed, and in the hypothalamus. In no structure, except the striatum (p less than 0.05), was a significant increase in 1-CBF obtained in N2O-breathing animals. However, the results suggest that CBF may have been increased in the auditory cortex. Immobilization was found to reduce 1-CBF in the cerebellum, inferior colliculus, superior olive, hippocampus, and septal nuclei. The results also suggest that the procedure somewhat increased CBF in frontal and parietal cortex. When the results obtained in awake, air-breathing animals were compared with those obtained in immobilized animals ventilated on N2O, there was no significant increase in any of the structures analyzed, although there were suggested increases in all cortical areas except the visual cortex. However, the data showed that ventilation with 70-80% N2O significantly decreased CBF in several structures (inferior colliculus, superior olive, hippocampus, amygdala, septal nuclei, and hypothalamus). In some of these, the effects of 70-80% N2O and of immobilization were obviously additive.

Canine cerebral metabolism and blood flow during hypoxemia and normoxic recovery from hypoxemia

There are conflicting reports regarding the effects of hypoxemia on the cerebral metabolic rate for oxygen (CMRO2). Accordingly, we examined the changes in CMRO2 during normoxia, progressive hypoxia (PaO2 of 37, 27, and 23 mm Hg), and normoxic recovery from hypoxia. Measurements were made in dogs anesthetized with nitrous oxide (60-70%) and halothane (less than 0.1%) in oxygen. Arterial-cerebral venous blood oxygen content differences and cerebral blood flow (CBF) were measured simultaneously, the latter by a technique (collection of sagittal sinus outflow) previously validated for conditions of near-maximal CBF. The duration of each of the three hypoxic exposures was approximately 10 min. CMRO2 was significantly decreased (14%) only when the arterial blood oxygen tension was reduced to 23 mm Hg. CBF increased progressively to a maximum of 153% of control. Posthypoxemic brain biopsy values for cerebral metabolites obtained 40 min after normoxemia had been restored were normal. These results, in conjunction with an unchanged CMRO2 at 40 min normoxic recovery, suggest that no gross irreversible brain cell damage occurred. We conclude that with progressive hypoxemia, CMRO2 remains stable until oxygen demand exceeds oxygen delivery, resulting thereafter in a progressive reduction in CMRO2.

Brain metabolism after 30 minutes of hypoxic or anoxic perfusion or ischemia

In 48 separate experiments, isolated canine brain preparations were subjected to 30 min of either hypoxic (PaO2 congruent to 20 mmHg) perfusion, anoxic (PaO2 < 10 mmHg) perfusion, or total ischemia followed by reperfusion for up to 2 h with normal oxygenated blood. Unlike ischemia and anoxia, energy metabolism was sufficient during hypoxia to maintain substantial levels of ATP (48% of normal), sustain normal ion gradients, and prevent edema formation. Posthypoxia metabolism was adequate to clear accumulated lactate, enable recovery of normal tissue glucose levels, and allow return to normal levels of glycolytic intermediates. Although not as complete as that following hypoxia, recovery from cerebral edema and restoration of metabolism were better in ischemic than anoxic cortex. The reduced oxygen uptake in all groups during reoxygenation (55% of normal) indicates that all have a diminished capacity for energy metabolism. The ATP levels recovered more rapidly after 15 min of reoxygenation in the anoxic (57% of normal) than in the ischemic (21% of normal) group. Thus ATP does not appear to be directly related to recovery from edema.

Regional changes in monoamine synthesis in the developing rat brain during hypoxia

4, 14 and 28 days old rats were exposed to hypoxic environment of 6% O2-94% N2 for 30 min. Tyrosine hydroxylase and tryptophan hydroxylase activity was studied in different brain regions (hemispheres, striatum, midbrain and brainstem in vivo by measuring the accumulation of dihydroxyphenylalanine (Dopa) and 5-hydroxytryptophan (5-HTP) respectively, after inhibition of aromatic L-amino acid decarobyxlase with NSD 1015. Tyrosine and tryptophan levels in the different brain regions were measured simultaneously. The tyrosine and tryptophan levels in the various brain parts were generally not influenced during exposure to hypoxia. Tyrosine hydroxylase activity decreased in most areas in the 4 and 14 days old rats, and all brain areas studied in the 28 days old rats. Tryptophan hydroxylase activity decreased markedly in all brain areas at all ages studied. It is concluded that the enzymes tyrosine hydroxylase as well as tryptophan hydroxylase seem to be equally affected during hypoxia in the different brain regions studied.

A new model of bilateral hemispheric ischemia in the unanesthetized rat

A new model of transient, bilateral hemispheric ischemia in the unanesthetized rat is described. During ether anesthesia the rat's vertebral arteries were electrocauterized through the alar foramina of the first cervical vertebra and reversible clasps placed loosely around the common carotid arteries. Twenty-four hr later, the awake rats were restrained and the carotid clasps tightened to produce 4-vessel occlusion. The carotid clasps were removed after 10, 20 or 30 min of 4-vessel occlusion and the animals killed by perfusion fixation 72 hr later. Rats which convulsed during the ischemic or post-ischemic period were excluded from further study. All rats subjected to 20 or 30 min of 4-vessel occlusion demonstrated ischemic neuronal damage. The H1 and paramedian hippocampus, striatum and layers 3, 5 and 6 of the posterior neocortex were the regions most frequently damaged. The advantages of this model are the ease of preparation of large numbers of animals, a high rate of predictable ischemic neuronal damage, a low incidence of seizures and the absence of anesthesia.

Energy utilization and changes in some intermediates of glucose metabolism in normal and hypoxic rat brain after decapitation

Energy metabolism was studied in the cerebral cortex of rats during and after hypoxia induced by breathing a gas mixture of 7% O2 in N2 for 2 h. Cortical energy stores (2ATP + ADP + phosphocreatine) remained unchanged after hypoxic treatment. Lactate rose over four-fold. Pyruvate, glucose and glucose 6-phosphate concentrations also increased significantly. Metabolic activity in the cortex expressed as the utilization of high-energy phosphates 5, 10 and 30 s after decapitation was decreased by 30% after hypoxia and remained lowered for 3 h during recovery. This was accompanied by elevated glucose consumption and lactate production, suggesting that the maintenance of the energy balance after hypoxia was partly due to activation of the glycolytic pathway. During the recovery period, these metabolic abnormalities returned towards control values, but, after 6 h of recovery the high-energy phosphate utilization increased transitorily above the control values.

Impaired autoregulation of cerebral blood flow in the distressed newborn infant

Cerebral blood flow was measured, using the 133Xe clearance technique, a few hours after birth in 19 infants with varying degrees of respiratory distress syndrome. Ten of these infants had had asphyxia at birth. The least affected infants with normotension (systolic blood pressure 60 to 65 mm Hg) had CBF values of about 40 ml/100 gm/minute. Hypotensive infants with asphyxia at birth or RDS or both had values for CBF of about 20 ml/100 gm/minute, or less. CBF was strongly correlated with the arterial blood pressure, showing a linear relationship that was identical in infants with asphyxia at birth and infants with RDS only. CBF varied considerably with spontaneous variations in blood pressure, suggesting that autoregulation was lacking. This finding may explain why distressed premature infants are prone to develop massive capillary bleeding in the germinal layer with penetration to the ventricles.

Brain energy metabolism in global brain oedema

Different degrees of severity in global brain oedema were induced by varying amounts of water intoxication (50, 100, 150, and 200 ml Aqua dest./kg b.wt. intravenously) in groups of six cats, which were functionally nephrectomized. Animals loaded with physiological saline and sham-operated served as controls. Two hours following the water load, the tissue concentrations of CrP, ATP, ADP, AMP, pyruvate, glucose, and lactate were determined by optical enzymatic analysis. The results show disturbances in brain energy metabolism dependent on the severity of the brain oedema. The high energy compounds and in consequence the ATP/ADP-ratio, and respectively the energy charge potential, fall in direct relationship to the severity of the brain oedema. Lactate and lactate-pyruvate ratio increase. The energy source of the cell, glucose as well as pyruvate, significantly falls in the group with severe brain oedema. The results of the brain energy metabolism were compared with our previous study concerning the brain water content, rCBF and CPP in global brain oedema (Meinig et al. 1973). The results show that the disturbances of energy metabolism are directly related to the rCBF and are not dependent on CPP over a wide range.

Cerebral glucose and energy metabolism, cerebral oxygen consumption, and blood flow in arterial hypoxaemia

The influence of moderately reduced arterial oxygen tension (aPO2 of about 45 Torr) on the metabolism and the blood flow of the brain was tested in 20 anaesthetized, artificially ventilated normotensive, normocapnic beagle dogs. It is demonstrated that the decrease in systemic oxygen delivery to the brain is countered by an appropriate increase in flow (CBF being 60.3 ml/100 g min at normoxia and 84.5 mg/100 g min in hypoxaemia) which maintained the cerebral oxygen consumption unchanged (CMRO2 3.80 versus 3.32 ml/100 g min). The cortical tissue content of energy-rich phosphates such as ATP, ADP, AMP, and phosphocreatine was also found to be unaltered. Neuropathological examinations excluded any hypoxic cell damage. This reactive vasodilatory reaction of the cerebral vessels is apparently a sensitive regulatory process which protects the brain against marked oxygen lack. However, a normal carbohydrate metabolism is not restored by this cerebrovascular mechanism. For, significantly increased CMRlactate (0.32 versus 1.46 ml/100 g min) indicated raised cerebral glycolysis, and the tissue metabolites of glucose suggested an increased glycolytic flux in the brain. It is concluded that in moderate arterial hypoxaemia, which is not uncommon in clinical practice, cerebral blood flow plays an effective homeostatic role in preventing a disturbance of the energy metabolism of the brain.

Cerebral energy metabolism during recovery from carbon monoxide hypoxia-oligemia

The effects of 1 h exposure to 1% CO and right common carotid artery clamping upon the cerebral energy metabolism, perfusion and histology were studied in anesthetized (70%N(2)O) and unanesthetized rats following reoxygenation for 0-6 h. At 0 h recovery the clamped hemispheres showed decreases of ATP, PCr and glycogen, and increases of ADP, AMP, inorganic phosphate and lactate which indicated an advanced derangement of the tissues' energy homeostasis. Exposure in unanesthetized animals was associated with lower levels of ATP and glycogen, and with larger increases of lactate in the clamped hemisphere which suggested the presence of a more intense hypoxic-oligemic insult. The metabolic differences in the clamped hemisphere of unanesthetized and anesthetized animals became more marked after 1 h reoxygenation, with anesthetized animals showing large restitutions of ATP, PCr, glycogen and lactate, whereas in unanesthetized animals these metabolites showed severe persistent defects over the 6 h reoxygenation period. Since the arterial oxygen content and tension, and perfusion patterns were equivalent in the 2 series at 0 and 1 h recovery, it is concluded that even minor anesthetic agents such as 70%N(2)O or other non-hypoxic variables during exposure or recovery may cause significant alterations in the metabolic restitution of the hypoxic-oligemic brain.

Dose dependent reduction of glucose utilization by pentobarbital in rat brain

A new method of determining the rate of glucose utilization in brain regions of individual rats has been used to measure the dose dependency of the reduction of the metabolic activity of the cerebral cortex by pentobarbital. Cerebral cortical glucose utilization is depressed to a basal level of 44% of the control rate when cerebral pentobarbital levels exceed 50 microgram per g of tissue. The major portion of this effect occurs between the cerebral pentobarbital range of 10--20 microgram per g, which can be achieved by 1/5 to 1/10 the normal anesthetic intraperitoneal dosage. If a depression of brain metabolism is responsible for the previously reported protection of the brain from ischemic damage, these data suggest a substantial reduction of brain metabolic rate is achieved in the rat at a barbiturate dosage which may be therapeutically relevant in the human after acute brain ischemia.

Low cerebral blood flow in hypotensive perinatal distress

Hypoxic brain injury is the most important neurological problem in the neonatal period and accounts for more neurological deficits in children than any other lesion. The neurological deficits are notably mental retardation, epilepsy and cerebral palsy. The pathogenesis has hitherto been poorly understood. Arterial hypoxia has been taken as the obvious mechanism but this does not fully explain the patho-anatomical findings. In the present investigation we have examined the arterial blood pressure and the cerebral blood flow in eight infants a few hours after birth. The 133Xe clearance technique was used for the cerebral blood flow measurements. The study confirmed that perinatal distress may be associated with low arterial blood pressure, and it was shown that cerebral blood flow is very low, 20 ml/100 g/min or less, in hypotensive perinatal distress. It is concluded that cerebral ischaemia plays a crucial role in the development of perinatal hypoxic brain injury.

Sequential cerebral biochemical and physiological events in controlled hypoxemia

Effects of controlled hypoxemia on cerebral functional activity were studied in rats using cyclic adenosine monophosphate (cAMP) and aminergic neurotransmitters in the brain tissue as special references. Evidence is presented that: (1) mild hypoxemic stress (PaO2 60 to 40 torr) may activate cerebral glycolysis with no evidence of anaerobic metabolism but that further reduction of PaO2 impairs cellular respiration, as evidenced by accumulation of glycolytic products; (2) glycogenolysis in the brain tissue, leakage of potassium ions from the brain cell, increase in brain water, and suppression of neural functional activity occur concomitant with accumulation of cAMP and prior to the fall of adenosine triphosphate; (3) the diminution of cerebral high-energy phosphates during hypoxia is associated with and may be caused by hypoxemia-induced neuroglycopenia and occurs at PaO2 15 torr; (4) induced hypoxemia per se does not affect the level or aminergic neurotransmitter substances in brain tissue.

Effect of hypercapnia and hypocapnia on tryptophan and tyrosine hydroxylation in rat brain

The effects of induced hypo- and hypercapnia upon the rate of hydroxylation of tryptophan and tyrosine in the rat brain were studied by measuring the accumulation of 5-HTP and DOPA following administration of the aromatic L-aminoacid decarboxylase inhibitor 3-hydroxybenzylhydrazine HCl (NSD 1015). The results suggest that the hydroxylation of tryptophan varies directly with the tissue Po2. On the other hand, the hydroxylation of tyrosine did not show a simple relationship to Po2 but appeared to be influenced by pH changes.

Effect of acute ammonia intoxication on cerebral metabolism in rats with portacaval shunts

Rats were made chronically hyperammonemic by portal-systemic shunting and, 8 wk later, were subjected to acute ammonia intoxication by the intraperitoneal injection of 5.2 mmol/kg of ammonium acetate. In free-ranging animals, ammonia treatment induced a brief period of precoma (10-15 min) that progressed into deep, anesthetic coma lasting for several hours and was associated with a high mortality. In paralyzed, artificially ventilated animals that were lightly anesthetized with nitrous oxide, acute ammonia intoxication caused major disturbances of cerebral carbohydrate, amino acid, and energy metabolism that correlated in time with the change in functional state. At 10 min after injection (precoma), the concentrations of most glycolytic intermediates were increased, as was the lactate/pyruvate ratio. Citrate declined, despite a twofold rise in pyruvate, suggesting that the conversion of pyruvate to citrate had been impaired. Concentrations of phosphocreatine, and of the putative neurotransmitters, glutamate and aspartate, declined during precoma, but the concentrations of the adenine nucleotides in the cerebral hemispheres, cerebellum, and brain stem remained within normal limits. At 60 min after injection (coma), ATP declined in all regions of brain; the reduction in total high-energy phosphates was most notable in the brain stem. The findings indicate that cerebral dysfunction in chronic, relapsing ammonia intoxication is not due to primary energy failure. Rather, it is suggested that ammonia-induced depletion of glutamic and aspartic acids, and inhibition of the malate-asparate hydrogen shuttle are the dominant neurochemical lesions.

Cerebral utilization of glucose, ketone bodies and oxygen in starving infant rats and the effect of intrauterine growth retardation

Cerebral arteriovenous differences of acetoacetate, D-beta-hydroxybutyrate, glucose, lactate and oxygen and brain DNA content was measured at 20 days of age in intrauterine growth retarded (IUGR) rats and normal littermates after 48 and 72 h of starvation. Cerebral blood flow (CBF) was measured with labeled microspheres in other comparable groups of IUGR and control rats. CBF was similar in IUGR and normal littermates (0.57+/-0.09 and 0.58+/-0.10 ml/min respectively). After 48 h of starvation, arterial glucose was significantly lower in IUGR than control animals but the arterial concentrations of ketone bodies were similar. After 48 h of starvation, cerebral arteriovenous difference of beta-hydroxybutyrate was significantly higher in control than IUGR rats also when expressed per mg brain DNA as was the fractional uptake of D-beta-hydroxybutyrate. After 72 h of starvation, arterial concentrations of ketone bodies were significantly lower in IUGR rats than controls but the fractional uptake of D-beta-hydroxybutyrate was increased compared to IUGR rats starved for 48 h. The average percentage of calculated total substrate uptake (mumol/min) accounted for by ketone bodies increased in control animals from 31.1% after 48 h of starvation to 41.0% after 72 h of starvation. In IUGR rats these percentage values were 26.5 and 25.7 respectively. After 72 h of starvation the fraction of total cerebral uptake of substrates accounted for by ketone bodies was significantly higher in control that IUGR rats. As total cerebral uptake of substrates was similar between IUGR and control animals it is concluded that IUGR rats are more dependent on glucose as a substrate for the brain during starvation.

Cerebral blood flow and oxidative brain metabolism during and after moderate and profound arterial hypoxaemia

In anaesthetized artificially ventilated dogs, the effect of graded arterial hypoxaemia on cerebral blood flow (CBF) and on the oxidative carbohydrate metabolism of the brain was tested. It is shown that the hypoxic vasodilatory influence on cerebral vessels is present even at moderate systemic hypoxaemia, provide that PaCO2 is kept within normal limits. At PaO2 of about 50 Torr, CBF increased from 56.6 to 89.7 ml/100g/min. With increasing cerebral hyperamia (CBF increased to 110.9 ml/100g/min, at PaO2 of 30 Torr), CMRO2 (4.2 ml/100g/min) was not significantly raised above its normal level (4.7 ml/100g/min) even with profound arterial hypoxaemia. This shows that CMRO2 levels are poor indices of hypoxic hypoxia. A disproportionately high increase in cerebral glucose uptake (CMR glucose levels rose from 4.4 to 10.4 mg/100g/min) and enhanced cerebral glycolysis (CMR lactate changed from 0.2 to 1.6 mg/100g/min) at moderately reduced PaO2 (50 Torr) indicated early metabolic changes which became more marked with further falls in arterial oxygen tension. However, 60 minutes after restoration of a normal PaO2 level, CBF and brain metabolism were found to have completely recovered. It is concluded that a short period of profound systemic hypoxaemia does not produce long lasting metabolic and circulatory disorders of the brain provided the cerebral perfusion pressure does not vary, and is kept at normal levels.

The effect of hepatectomy on the energy state and on acid-base variables of the rat brain

The influence of hepatectomy on cerebral energy metabolism was studied in rats in which an end-to-side portacaval anastomosis had been performed one week earlier. The hepatectomy gave rise to an exaggeration of the increase in tissue ammonia content resulting from the shunt. Three hours after the hepatectomy there was a fall in tissue glutamate content but no change in alpha-ketoglutarate. The energy state of the tissue, as evaluated from the concentrations of ATP, ADP, and AMP, was unchanged. There was a significant increase in cerebrospinal fluid pH but no change in intracellular pH.

XIII. Cerebral circulation and metabolism in stroke. Cerebral circulation and metabolism in stroke study group

An understanding of the cerebral circulation is so fundamental to comprehension of the pathogenesis of stroke that cerebral blood flow and metabolism merit review in this series of reports. The authors recognize that the research described here is very technical in nature and may appear to have little practical application to clinical medicine. Nevertheless, these matters are basic to the development of precise methods for the measurement of regional cerebral blood flow in man which could be used to monitor the therapy of stroke with greater success than is possible at present.

Brain energy metabolism during the process of dying and after cardiopulmonary resuscitation

In order to study the problem of how fast and to what degree severe hypoxic brain tissue changes are reversed after reoxygenation, we challenged the viability of the brain by exposing experimental animals to anoxia of such a duration that cardiopulmonary resuscitation was just possible. The brain tissue concentrations of glucose, lactate, pyruvate and of ATP, ADP, AMP and phosphocreatine were determined. Two series of experiments were carried out. In the first, groups of rats deprived of oxygen for 1, 2, 4 and 6 min were studied in order to show brain tissue changes in the period of impending death as well as the changes coinciding with the onset of clinical death (blood pressure zero). In addition, one group maintained at a rectal temperature of 37 +/- 0.5 degrees C and ventilated for 60 min with an oxygen free gas mixture was included aimed at representing a state of irreversibility. In the second series, restitution after 6 min of no oxygen supply was studied 10 min, 1/2 h, 1 h, and 2 h after cardiopulmonary resuscitation. Attempts were also made to correlate biochemical changes to EEG status and to clinical recovery. The restitution study showed that oxidative phosphorylation of the brain tissue was rapidly resumed with normalization of the adenylate energy charge in all animals in which the pump function of the heart could be restored by our artificial means. However, there was a strikingly poor correlation between recovery of mitochondrial function and restitution of EEG or clinical recovery. Thus, it seems likely that a delayed functional restitution is not due to energy failure but to other biochemical changes or to biophysical alterations not revealed by the present type of study.

Adenyl cyclase system and cerebral energy state

The energy charge potential of the cerebral adenylate system, the cerebral lactate: pyruvate system, and the cerebral glycogen level were used to characterize the cerebral energy state of the brain. Cerebral adenyl cyclase activity and cyclic AMP concentrations were investigated to evaluate their possible influence on the regulatory processes in brain metabolism. These biochemical parameters were evaluated on the cortical motor area of the brain of the beagle dog in hypovolaemic hypotension during acute hypoxia (obtained by altering the composition of the inhalation mixture) and during the post-hypoxic recovery (three minutes after the restoration of normal ventilation). This experimental model of acute hypoxia was also used for investigating the action of some substances acting at beta-adrenergic receptor level or at vascular level. The substances were perfused into the carotid artery at the rate of 0.5 ml/min for three to six minutes. During the first stage of hypoxia, the adenyl cyclase system is probably an important biochemical regulatory factor. This system becomes less important when a critical threshold is crossed (oxygen arterial partial pressure less than 25-20 mmHg) below which other factors become rate-limiting. A beta-receptor stimulating agent enhanced the mechanisms of physiological post-hypoxic recovery; inhibition of this action was obtained with a beta-receptor blocking agent. Under these experimental conditions, no correlation existed between the vascular effects of the agents and brain metabolism.

Lactate and pyruvate concentrations, and acid-base balance of cerebrospinal fluid in experimentally induced intracerebral and subarachnoid hemorrhage in dogs

The effect of blood injected into either subarachnoid space or subcortical brain tissue upon lactate and pyruvate concentrations as well as acid-base balance of cerebrospinal fluid (CSF) was studied in the anesthetized dog. CSF lactate and lactate/pyruvate ratio (L/P ratio) increased progressively following the intracranial injection of blood and reached the maximum level at six hours after injection. These changes were significantly greater in animals with intracerebral hematoma than in those with subarachnoid hemorrhage (SAH). An increase in CSF lactate and L/P ratio in hemorrhagic CSF seems to be caused by two different factors. Shed blood cells per se produce lactate and pyruvate, and blood in the subarachnoid space and intracerebral hematomas cause secondary changes in brain tissue metabolism by a probable reduction of cerebral blood flow. Therefore, an increase in CSF lactate with a concomitant rise in CSF L/P ratio is a useful indicator for brain tissue hypoxia, even when CSF is hemorrhagic. The association of an increase in CSF lactate to a disproportionate decrease in CSF HCO-3 was also observed in these animals.