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(Stroke. 1996;27:114-121.)
© 1996 American Heart Association, Inc.
Articles |
Amelioration of Impaired Cerebral Metabolism After Severe Acidotic Ischemia by Tirilazad Posttreatment in Dogs
From the Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins Medical Institutions, Baltimore, Md (H.K., R.C.K., P.D.H., R.J.T.), and CNS Diseases Research Unit, The Upjohn Company, Kalamazoo, Mich (E.D.H.).
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Abstract |
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Background and Purpose Acidosis may contribute to ischemic injury by mobilizing iron because the iron chelator deferoxamine improves early metabolic recovery from hyperglycemic ischemia. Mobilized iron may then promote oxygen radical-induced lipid peroxidative injury during reperfusion. We tested the hypothesis that administration of the antioxidant tirilazad at the start of reperfusion improves early metabolic recovery after severe acidotic ischemia and ameliorates depletion of the endogenous antioxidant glutathione.
Methods In anesthetized dogs, arterial glucose concentration was increased to 500 to 600 mg/dL and global incomplete cerebral ischemia was produced for 30 minutes by ventricular fluid infusion to reduce perfusion pressure to 10 to 12 mm Hg. Metabolic recovery and intracellular pH were measured by phosphorus MR spectroscopy. In the first experiment, four groups of eight dogs each received either vehicle or 0.25, 1, or 2.5 mg/kg of tirilazad mesylate at reperfusion. Cerebral blood flow was measured with microspheres. In the second experiment, two groups of eight dogs each received either vehicle or 2.5 mg/kg of tirilazad at reperfusion, and cortical glutathione was measured at 3 hours of reperfusion.
Results Cerebral blood flow decreased to approximately 6 mL/min per 100 g and intracellular pH decreased to approximately 5.6 during ischemia in all groups. In the vehicle group, ATP recovery was transient and pH remained less than 6.0. Cerebral blood flow, O2 consumption, and ATP eventually declined to near-zero levels by 3 hours. Recovery was improved by tirilazad posttreatment in a dose-dependent fashion. At the highest dose, cerebral blood flow and O2 consumption were sustained near preischemic levels, and five of eight dogs had recovery of ATP greater than 50% and of pH greater than 6.7. Recovery of ATP and phosphocreatine became significantly greater than that in the vehicle group by 17 minutes of reperfusion despite similar levels of early hyperemia, indicating that the drug was acting before the onset of hypoperfusion. Cortical glutathione concentration in the vehicle group was 27% less than that in the tirilazad group and 34% less than that in nonischemic controls.
Conclusions Decreased depletion of the endogenous antioxidant glutathione is consistent with tirilazad acting as an antioxidant in vivo. Improvement in high-energy phosphate recovery 17 minutes after starting tirilazad infusion during reperfusion is consistent with an early onset of a functionally significant oxygen radical injury. Thus, severe acidosis appears to contribute to early ischemic injury through an oxygen radical mechanism sufficient to impede metabolic recovery.
Key Words: acidosis • free radicals • lipid peroxidation • metabolism • dogs
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Introduction |
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TopAbstractIntroductionMaterials and Methods Results Discussion References |
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Increasing the severity of lactic acidosis during global ischemia by hyperglycemia exacerbates neuronal injury.1 2 3 4 However, the mechanism whereby acidosis augments ischemic injury is uncertain. Rehncrona et al5 suggested that severe lactic acidosis destabilizes iron bound to protein via carbonate bridges and that mobilized iron enhances oxygen radical injury. This hypothesis is supported by our recent observations that treatment with the iron chelator deferoxamine improved early recovery of high-energy phosphates and intracellular pH measured by phosphorus MR spectroscopy (MRS) after 30 minutes of hyperglycemic, incomplete cerebral ischemia.6 Ordinarily, this severity and duration of hyperglycemic ischemia results in only partial and transient recovery of ATP, persistent reductions in intracellular pH below 6.0, and delayed intracranial hypertension and hypoperfusion.7 Moreover, pretreatment with the antioxidant tirilazad mesylate (U74006F) also ameliorated secondary metabolic decay during reperfusion in this model of hyperglycemic ischemia.8 This drug is an inhibitor of lipid peroxidation in vitro that works by a combination of chemical radical scavenging and membrane stabilizing mechanisms.9 10 11 Because lipid peroxidation is presumed to occur most intensely during the reoxygenation period after ischemia, tirilazad should be effective if it is administered at the time of reperfusion.
The purpose of the present study was threefold. First, we determined whether treatment with tirilazad at the start of reperfusion after 30 minutes of severe incomplete cerebral ischemia accompanied by hyperglycemia improves metabolic and intracellular pH recovery during early reperfusion and prevents delayed hypoperfusion. Second, we determined whether there is an optimal dose of tirilazad for this effect. Third, we determined whether the endogenous cytosolic antioxidant glutathione is depleted after hyperglycemic ischemia and whether tirilazad treatment attenuates this depletion, providing evidence of an in vivo antioxidant mechanism of action.
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Materials and Methods |
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TopAbstractIntroductionMaterials and Methods Results Discussion References |
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Experimental procedures were approved by the institutional animal care and use committee. Male dogs weighing approximately 10 kg were anesthetized with a high dose of fentanyl (50 µg/kg IV) plus a low dose of pentobarbital (10 mg/kg IV). Supplemental pentobarbital (3 mg/kg per hour) was infused continuously throughout the experiment. End-tidal CO2 was controlled with mechanical ventilation, and oxygenation was maintained by increasing inspired O2 to 25% to 30%. Catheterization included femoral veins for drug administration and a femoral artery for blood pressure monitoring. To measure cerebral blood flow (CBF), the left ventricle was catheterized via a femoral artery for injecting radiolabeled microspheres, and an axillary artery was catheterized for withdrawing the reference sample at a rate of 2.5 mL/min during and after the injection, as previously described.7 12 The microspheres were 15±0.5 µm in diameter and were labeled with either 153Gd, 114mIn, 113Sn, 103Ru, 95Nb, or 46Sc (Dupont-NEN Products). Pancuronium bromide (0.3 mg/kg IV) was administered for muscle paralysis, and the temporalis muscle was fully retracted. Burr holes were made in the skull for insertion of catheters into the sagittal sinus and lateral ventricle, insertion of a thermistor into the epidural space, and placement of an electrode near the right somatosensory cortex for recording evoked potentials during left foreleg stimulation.7 12 Blood was analyzed for pH, partial pressure of CO2 and O2 (ABL3, Radiometer), O2 content (CO-oximeter 182, Instrumentation Laboratories), and glucose (model 23A, Yellow Springs). Cerebral O2 consumption was calculated as the product of blood flow to the cerebrum and the arterial-sagittal sinus O2 content difference.
The crown of bone along the sagittal suture was thinned with rongeurs
and a drill, and a two-turn surface coil 5 cm in diameter was
secured over the midline. The dogs were placed on a warm-water
circulating blanket and wrapped in a plastic bag to maintain
normothermia. Fiberglass insulation was placed above the head and coil
to reduce radiant heat loss. Phosphorus MRS was obtained at 32.5 MHz.
Experiments were conducted in a 25-cm-bore, 1.89-T magnet (Oxford
Instruments). Phosphorus spectra were obtained at 32.5 MHz with a
Vivospec spectrometer (Otsuka Electronics), and areas under each peak
were integrated as previously described.7 12
Intracellular
pH was calculated from the chemical shift (
) of Pi:
pH=6.73+log [(-3.07)/(5.68-)].
Intracellular
bicarbonate ion concentration was estimated from the
Henderson-Hasselbalch equation with the use of the MRS-derived pH and
sagittal sinus blood PCO2 as an approximation
of tissue
PCO2.7 12 13
A solution of 10% dextrose was infused intravenously to increase arterial glucose concentration to approximately 500 to 600 mg/dL before ischemia. The infusion was stopped at reperfusion. Global incomplete cerebral ischemia was produced for 30 minutes by infusion of 38°C artificial cerebrospinal fluid into the lateral ventricle. The infusion was adjusted to keep intracranial pressure 10 mm Hg less than mean arterial blood pressure while the latter spontaneously changed. This procedure maintains CBF relatively constant during ischemia.12 13 To start reperfusion, ventricular fluid infusion was stopped and intracranial pressure rapidly decreased. After 3 hours of reperfusion, the heart was arrested by either intravenous potassium chloride injection or by perfusion with fixatives.
In the first experiment, the dose response to tirilazad posttreatment was determined in four groups of eight dogs each. Dogs were randomly assigned to receive either 0, 0.25, 1.0, or 2.5 mg/kg IV of tirilazad mesylate over 3 minutes starting at reperfusion. The concentration of tirilazad was 1.5 mg/mL. The vehicle was a citrate buffer (20 mmol/L citric acid monohydrate, 32 mmol/L sodium citrate dihydrate, 77 mmol/L sodium chloride; pH=3). The 0-mg/kg group received 0.67 mL/kg of vehicle. In addition, the three groups receiving tirilazad also received a continuous infusion of 0.2 mg/kg per hour of tirilazad mesylate throughout reperfusion, whereas the control group received a continuous infusion of vehicle. Dogs with CBF less than 1 or more than 12 mL/min per 100 g during ischemia were excluded. Investigators were blinded to treatment and remained blinded until data from all dogs were analyzed. Data among the four groups were compared by ANOVA. At individual time points, mean values in the three tirilazad dose groups were compared with the vehicle group by Dunnett's test. Within each group, comparisons were made to preischemic baseline values by repeated-measures ANOVA and Dunnett's test. Values are presented as mean±SEM, and P<.05 was considered significant.
In the second experiment, cortical glutathione concentration was measured at 180 minutes of reperfusion. Hyperglycemic ischemia was produced, and MRS measurements were obtained as in the first experiment. However, radioactive microspheres were not used to avoid radioactivity in the tissue. One group of 8 dogs received vehicle, and one group of 8 dogs received tirilazad at a dose of 2.5 mg/kg at reperfusion plus 0.2 mg/kg per hour during 3 hours of reperfusion. Only the highest dose of tirilazad was tested in this experiment because this dose was found to be the most effective in restoring energy metabolism. In addition, 8 dogs subjected to the same surgical procedures and duration of anesthesia served as nonischemic time controls (MRS was not performed on nonischemic time controls). At the end of the experiment, the skull was rapidly removed, and the brain was cut into large blocks, quickly frozen in liquid nitrogen, and stored at -70°C.
Measurements of total glutathione and oxidized glutathione were made by high-performance liquid chromatography with fluorescent detection. The assay for oxidized and total glutathione has been described in detail elsewhere.14 15 Briefly, 20 to 40 mg of cortical tissue was weighed and homogenized in 300 µL of 25 mmol/L phosphate buffer (pH 6.0) by sonication at 4°C. Samples were centrifuged at 15 000 rpm at 4°C for 12 minutes. For total glutathione measurement, 100 µL of supernatant was mixed with dithiothreitol. After 30 minutes, the samples were precipitated by 250 µL of 2.5% 5-sulfosalicylic acid. A 200-µL aliquot of the supernatant was derivatized with 200 µL of o-phthalaldehyde and diluted to 800 µL with phosphate buffer (pH 7.0). A 100-µL sample was injected onto a high-performance liquid chromatograph with a C18 reverse-phase column and monitored by a Waters 470 fluorescence detector with excitation at 340 nm and emission at 420 nmol/L. The mobile phase was methanol/0.15 mol/L sodium acetate (20:80, pH 7.0) at a flow rate of 1.0 mL/min. Oxidized and total glutathione values among the nonischemic controls and postischemic vehicle and tirilazad groups were compared by ANOVA and Newman-Keuls multiple range test.
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Results |
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TopAbstractIntroductionMaterials and Methods Results Discussion References |
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Arterial glucose concentration was increased to 500 to 600 mg/dL before ischemia, maintained during 30 minutes of ischemia, and allowed to spontaneously decrease during reperfusion in the vehicle group and in the three groups receiving different doses of tirilazad at reperfusion (Table 1
).
Moderate arterial acidemia occurred during and after
ischemia in all groups. Normocapnia was maintained and
arterial PO2 was kept at a level
sufficient to maintain oxyhemoglobin saturation. Arterial
hemoglobin concentration increased in all groups, resulting in a
proportional increase in O2 content.
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Epidural temperature was maintained near normal levels throughout the
experiment (Table 2). Mean arterial pressure
was maintained through early reperfusion but declined significantly by
60 minutes in the vehicle group, by 120 minutes in the low-dose
group, and by 180 minutes in the two higher-dose groups.
Intracranial pressure, which was increased to similar levels among
groups to produce ischemia, rapidly recovered to near baseline
levels during early reperfusion (Fig 1). During later
reperfusion, however, significant increases in intracranial pressure
occurred in the vehicle group and in the two lower-dose groups.
Consequently, cerebral perfusion pressure, which was decreased to 10 to
12 mm Hg during ischemia, recovered to similar values in all
groups during early reperfusion but declined during prolonged
reperfusion. Perfusion pressure in the highest-dose group became
significantly greater than that in the vehicle group beyond 120 minutes
(Fig 1).
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During ischemia CBF was reduced to approximately 6 mL/min per
100 g in all groups (Fig 2). Postischemic
hyperemia was present in all groups. However, by 180
minutes hypoperfusion was significant in the vehicle and
lowest-dose groups in parallel with the low perfusion pressure. In
the highest-dose group, CBF was maintained at a higher level than
CBF in the vehicle group. Cerebral O2 consumption was
reduced by approximately 80% by mid-ischemia and largely
recovered during early reperfusion. By 180 minutes, O2
consumption was depressed below baseline values in the vehicle and two
lower-dose groups, whereas O2 consumption in the
highest-dose group was sustained greater than that in the vehicle
group (Fig 2).
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Cerebral ATP and phosphocreatine were depleted in all groups by
equivalent amounts during ischemia (Fig 3). In
the vehicle and lowest-dose groups, modest recovery of
high-energy phosphates occurred during early reperfusion followed
by a decline to nearly undetectable levels in most dogs by 180 minutes.
In the 1-mg/kg dose group, recovery was significantly improved as early
as 45 minutes for ATP and 120 minutes for phosphocreatine. In the
highest-dose group, both ATP recovery and phosphocreatine recovery
were improved as early as 17.5 minutes and remained significantly
greater than values in the vehicle group throughout most of reperfusion
(Fig 3). However, recovery was incomplete at 180 minutes, with
only
five of eight dogs having greater than 50% recovery of ATP and two
dogs having undetectable ATP.
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Intracellular pH decreased to approximately 5.6, and estimated
intracellular bicarbonate concentration decreased to less than 1 mmol/L
by the end of ischemia in all groups (Fig 4).
These values are similar to those previously shown to be associated
with poor recovery when acidosis is augmented by
hyperglycemia.7 8 In the vehicle group, pH remained
less
than 6.0 and bicarbonate remained less than 1.5 mmol/L throughout
reperfusion. Recovery of pH and bicarbonate were significantly improved
in the two highest-dose groups. With the highest dose, values at
180 minutes were not significantly different from baseline.
Intracellular pH was greater than 6.7 in five of eight dogs but less
than 6.0 in two dogs at 180 minutes.
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Persistent acidosis during early reperfusion may contribute to the
inability to restore energy metabolism. In Fig 5, ATP recovery
at 180 minutes of reperfusion is plotted
against intracellular pH at 30 minutes of reperfusion for the
individual dogs of the four groups. Dogs in which intracellular pH
remained less than approximately 6.0 by 30 minutes of reperfusion had
poor ATP recovery, whereas the majority of the dogs that had pH greater
than 6.4 by 30 minutes had greater than 50% ATP recovery at 180
minutes. Of those with pH greater than 6.4 at 30 minutes but poor ATP
recovery, most were in the intermediate dose groups, where pH recovery
could not be sustained.
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Somatosensory evoked potentials remained isoelectric in the vehicle
group. In the high-dose tirilazad group, 6 of 8 dogs had detectable
evoked potentials (
10% of baseline) by 45 minutes of reperfusion.
However, evoked potential amplitude remained less than baseline values
(Table 2
).
In another set of vehicle-treated dogs, total glutathione
concentration measured in the cortex at 3 hours of reperfusion was 34%
less than that in nonischemic time controls (Table 3). In dogs
treated with 2.5 mg/kg of tirilazad at
reperfusion, glutathione was greater than that in the
vehicle-treated group and not different from
nonischemic control values. The amount of glutathione in
the oxidized form was not significantly different among groups whether
expressed in absolute concentration or as a percentage of total
glutathione. Physiological parameters
during and after hyperglycemic ischemia in this experiment were
similar to those obtained in the first experiment. As in the first
experiment, recovery of ATP, phosphocreatine, and intracellular pH were
significantly greater in the high-dose tirilazad group than in the
vehicle group (Table 3).
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Discussion |
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TopAbstractIntroductionMaterials and Methods Results Discussion References |
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The major findings of this study are (1) that administration of the antioxidant tirilazad at the onset of reperfusion after severe hyperglycemic cerebral ischemia improves early metabolic recovery in a dose-dependent fashion and (2) that the highest dose tested (2.5 mg/kg) preserves cortical glutathione concentration.
We purposely chose a model of severe global cerebral ischemia
(
80% reduction of CBF) of prolonged duration (30 minutes) with
marked hyperglycemia (500 to 600 mg/dL) to achieve severe tissue
acidosis. In this model, acidosis becomes the primary factor limiting
metabolic recovery. With 30 minutes of normoglycemic
ischemia in this model, cerebral pH falls to approximately 6.1
to 6.2, with intracellular bicarbonate estimated to be approximately
1.7 to 2.0 mmol/L during ischemia; ATP recovers approximately
85% and pH fully recovers during reperfusion.7 16
However, augmenting the decrease in pH to approximately 5.6 and in
bicarbonate to approximately 0.8 mmol/L with hyperglycemia results in
persistent acidosis during reperfusion and only transient ATP
recovery.7 8 Shortening ischemic duration to 20
minutes and using more moderate hyperglycemia (300 mg/dL) results in
intermediate decreases in cerebral pH to 5.9 and in bicarbonate to 1.3
mmol/L; recovery of ATP was intermediate (58%).17 Thus,
titrating the severity of acidosis below a pH of 6.2 and bicarbonate of
2 mmol/L results in a graded reduction in early metabolic
recovery.
This deficit in early metabolic recovery depended not only
on intraischemic pH but also on intraischemic
bicarbonate concentration. Superimposing severe hypercapnia during
normoglycemic ischemia augmented the decrease in pH to 5.7, but
bicarbonate was not severely depleted (6 mmol/L).13 In
this case, ATP and pH fully recovered during reperfusion. Rehncrona et
al5 observed that lactic acidosis but not carbonic
acidosis of brain homogenates augmented lipid peroxidation,
and this augmentation was inhibited by the iron chelator
deferoxamine. Based on their findings, one interpretation
of our previous results was that bicarbonate depletion associated with
lactic acidosis but not carbonic acidosis permitted proton attack of
carbonate bridges of protein-bound iron. This interpretation is
supported by the findings that pretreatment with
tirilazad8 or free deferoxamine but not
iron-loaded deferoxamine6 markedly
improved early metabolic recovery from hyperglycemic
ischemia. Accordingly, we hypothesized that severe acidosis
during ischemia mobilizes iron, which then promotes oxygen
radical injury upon reperfusion.
If this hypothesis is true, then administration of the antioxidant tirilazad at the start of reperfusion should be as effective as pretreatment. We found that infusion of tirilazad at the start of reperfusion partially restored cerebral metabolism in the majority of dogs. However, the loading dose required to restore metabolism with posttreatment (180-minute recovery of ATP=49±13% at 2.5 mg/kg; ATP=31±10% at 1 mg/kg) was greater than that previously observed with pretreatment (ATP recovery=70±16% at 1 mg/kg). Oxygen radical damage may begin within minutes of reperfusion. In the present study, the loading dose was delivered over the first 3 minutes of reperfusion, and tissue levels may have been inadequate at early critical times. Early reperfusion injury is supported by the significant improvements in high-energy phosphate recovery as early as 17 minutes of reperfusion in the present study with tirilazad posttreatment and 30 minutes with pretreatment.8 The ability to generate somatosensory evoked potentials, albeit at reduced amplitude, within 45 minutes of reperfusion in the majority of dogs treated with the highest dose of tirilazad indicates that the early restoration of energy metabolism was sufficient to restore ionic gradients necessary for action potential conduction and synaptic transmission, at least at subcortical synapses. More rapid metabolic and electrophysiological recovery with tirilazad has also been noted during the first 12 minutes of reperfusion after normoglycemic ischemia.16 18 Thus, the drug appears to be acting during the early minutes of reperfusion and may require early delivery to the tissue.
Assuming that tirilazad acts by inhibiting lipid peroxidation in vivo as it does in vitro,9 10 11 then these results are consistent with the hypothesis that acidosis promotes peroxidation at an early stage of reperfusion. Other evidence that tirilazad acts as an antioxidant in vivo includes its ability to reduce hydroxyl radical concentrations detected by the salicylate trap technique after head injury19 and to preserve the endogenous antioxidants vitamins C and E after reperfusion following prolonged hemispheric cerebral ischemia in the gerbil.20 21 The present results showing that the depletion of the endogenous antioxidant glutathione is prevented by tirilazad is consistent with the hypothesis that tirilazad breaks the chain of free radical propagation in membranes and thereby preserves endogenous antioxidants.
In contrast, Lundgren et al22 could not detect evidence of early free radical damage after 15 minutes of hyperglycemic ischemia. Glutathione and vitamin E depletion was not greater than that with normoglycemic ischemia. However, these results are not strictly comparable to our study because we used a longer ischemic duration of 30 minutes in which tissue acidosis is more severe and persists throughout reperfusion, leading to rapid metabolic deterioration. In addition, Lundgren et al22 could not exclude free radical damage in a small compartment such as the endothelium or perivascular astrocytes. The vasculature may be an important site of superoxide production after normoglycemic ischemia,23 and acidotic mobilization of iron could augment oxygen radical propagation at this site.
If the vasculature was a major site of oxygen radical damage, one might
anticipate impaired reflow. However, during the first 30 minutes of
reperfusion, we found no differences in CBF between the vehicle and
high-dose tirilazad groups that would account for early differences
in ATP and phosphocreatine. Only at later time points, when
intracranial pressure increased and arterial pressure
decreased, was CBF better maintained with tirilazad. However, these
observations do not exclude a major site of action in the vasculature
or in perivascular astrocytes because tirilazad is
hydrophobic11 24 and may remain in relatively high
concentrations in endothelial and astrocyte membranes.
Ultrastructural changes in endothelium25
and astrocytes3 are evident during early reperfusion after
hyperglycemic ischemia, and blood-brain barrier damage can
become widespread.26 Thus, it is possible that tirilazad
could act on endothelium and astrocytes to improve the
homogeneity of capillary reflow and to improve blood-brain barrier
transport of acid equivalents. The association of improved ATP recovery
with early improvement of tissue pH (Fig 5) supports an action
of the
drug on improved transport or metabolic consumption of acid
equivalents. In addition, early pH recovery may prevent further
mobilization of iron and reduce further oxygen radical injury.
Although several studies have found neuroprotection and improved outcome from normoglycemic ischemia with tirilazad in doses of 1.5 to 3.0 mg/kg,27 28 29 30 few have examined a range of doses. At a dose of 3 mg/kg after 10 minutes of carotid occlusion plus hypotension in rats, Lesiuk et al28 reported less neuronal injury in the cortex but no improvement in the hippocampal CA1 region. In contrast, after 5 or 15 minutes of four-vessel occlusion in the normoglycemic rat, Buchan et al31 failed to find neuroprotection in the cortex or hippocampal CA1 region after single doses of 3 or 10 mg/kg or multiple doses of 10 mg/kg. Oxygen radical injury to the vasculature and astrocytes is not considered to be a major cause of neuronal injury in these experimental models where selective neuronal vulnerability is prominent. With 24 hours of permanent focal ischemia, Park and Hall32 observed significant reductions in infarct volume with multiple dosing of 1 or 3 mg/kg. The greatest effect was at 3 mg/kg. In our study of global, hyperglycemic ischemia, the most consistent improvement in metabolism was observed with 2.5 mg/kg at reperfusion plus 0.2 mg/kg per hour. Perhaps higher loading and maintenance dosing would achieve more complete metabolic recovery.
Because of the severity of the ischemic insult, we did not attempt long-term recovery for histological analysis. Although tirilazad improved early metabolic recovery, ATP and phosphocreatine were below baseline levels. Thus, a significant portion of the cells presumably were nonviable. It is also possible that the remaining viable cells with ATP would eventually die if tirilazad is acting to merely delay lethal levels of brain swelling. Nevertheless, our results suggest that tirilazad might extend the therapeutic window after severe acidotic ischemia for other classes of drugs.
In summary, the present results with tirilazad posttreatment taken together with previous results of deferoxamine pretreatment are consistent with the following sequence: (1) the additional acidosis that occurs when hyperglycemia accompanies incomplete cerebral ischemia promotes iron mobilization during ischemia and reperfusion; (2) critical levels of mobilized iron in one or more compartments promote oxygen radical injury during reoxygenation sufficient to impair energy metabolism and ion and proton transport; and (3) impaired energy metabolism and ion transport lead to a vicious cycle of cell swelling, increased intracranial pressure, depressed arterial pressure regulation, and impeded microcirculatory perfusion. Early administration of tirilazad at reperfusion appears to block this chain of events in the majority of dogs and may extend the therapeutic window for other neuroprotective drugs.
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Acknowledgments |
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This study was supported in part by grants from the National Institutes of Health (NS20020 and NR03521). The authors are grateful to Judy Klaus and Kathleen Blizzard for their fine technical assistance and to Lisa DeLoriers for preparing the manuscript.
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Footnotes |
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Reprint requests to Richard J. Traystman, PhD, Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins Medical Institutions, 600 N Wolfe St, Blalock 1408, Baltimore, MD 21287-4961.
The Upjohn Company, which manufactures tirilazad, supported the purchase of supplies and nuclear MR time in this study. The investigators were blinded to drug treatment until nuclear MR and other data were analyzed and all experiments were completed. E.D.H., coauthor, is an employee of Upjohn. He analyzed the tissue samples for glutathione and was blinded to drug treatment until all tissue was analyzed.
Received August 21, 1995; revision received October 13, 1995; accepted October 16, 1995.
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