Tirilazad pretreatment improves early cerebral metabolic and blood flow recovery from hyperglycemic ischemia.

Acidosis may augment cerebral ischemic injury by promoting lipid peroxidation. We tested the hypothesis that when acidosis is augmented by hyperglycemia, pretreatment with the 21-aminosteroid tirilazad mesylate (U74006F), a potent inhibitor of lipid peroxidation in vitro, improves early cerebral metabolic recovery. In a randomized, blinded study, anesthetized dogs received either tirilazad mesylate (1 mg/kg plus 0.2 mg/kg/h; n = 8) or vehicle (n = 8). Hyperglycemia (400-500 mg/dl) was produced prior to 30 min of global incomplete cerebral ischemia. Intracellular pH and high energy phosphates were measured by phosphorus magnetic resonance spectroscopy. During ischemia, microsphere-determined CBF decreased to 8 +/- 4 ml min-1 100 g-1 and intracellular pH decreased to 5.6 +/- 0.2 in both groups. During the first 20 min of reperfusion, ATP partially recovered in the vehicle group to 57 +/- 21% of baseline, but then declined progressively in association with elevated intracranial pressure. By 30 min, ATP recovery was greater in the tirilazad group (77 +/- 35 vs. 36 +/- 19%), although postischemic hyperemia was similar. By 45 min, the tirilazad group had a higher intracellular pH (6.5 +/- 0.5 vs. 5.9 +/- 0.6) and a lower intracranial pressure (18 +/- 6 vs. 52 +/- 24 mm Hg). By 180 min, blood flow and ATP were undetectable in seven of eight vehicle-treated dogs, whereas ATP was > 67% and pH was > 6.7 in six of eight tirilazad-treated dogs. Thus, tirilazad acts during early reperfusion to prevent secondary metabolic decay associated with severe acidotic ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related cerebrovascular response to global ischemia in pigs.

We tested the hypothesis that 1- to 2-wk-old pigs (piglet) have improved recovery of cerebral blood flow (CBF), cerebral oxygen consumption (CMRO2), and somatosensory-evoked potentials (SEP) compared with 6- to 8-mo-old pigs (pig) after transient global cerebral ischemia. All animals were anesthetized with pentobarbital sodium. After tracheostomy ventilation was adjusted to maintain normoxia (arterial oxygen pressure, 100-150 mmHg) and normocarbia (arterial carbon dioxide pressure, 35-40 mmHg). Arterial blood gases, blood pressure, and hemoglobin concentration remained within physiological limits throughout the experiment. Cerebral ischemia was produced by sequentially tightening ligatures around the inferior vena cava and ascending aorta. During ischemia the electroencephalogram and SEP became isoelectric within 40 and 120 s, respectively. At 10 min of reperfusion hyperemia occurred in most brain regions (e.g., whole brain: piglet, 270 +/- 45%; pig, 316 +/- 48%). In pigs delayed hypoperfusion occurred in all regions except white matter. In contrast, piglets only had delayed hyperperfusion to the brain stem and caudate nucleus. Throughout reperfusion CMRO2 was decreased in pigs (3.3 +/- 0.4 to 1.9 +/- 0.2 ml.min-1.100 g-1) but was not different from control (2.7 +/- 0.3 ml.min-1.100 g-1) in piglets. By the end of reperfusion SEP amplitude was closer to control in piglets than pigs (55 +/- 9 vs. 32 +/- 4% of control). We conclude that 1- to 2-wk-old piglets have quicker return of CBF, CMRO2, and SEP to control values after global ischemia, which mechanistically may explain previous reports of improved neurological recovery in young animals after transient ischemia.

Deferoxamine reduces early metabolic failure associated with severe cerebral ischemic acidosis in dogs.

BACKGROUND AND PURPOSE: Postischemic metabolic injury may be mediated by acidosis and tissue bicarbonate depletion, with consequent-iron mobilization and oxygen radical formation during reperfusion. We have previously shown that reducing intracellular pH to below 5.7 and bicarbonate ion to below 1 to 2 mmol/L during hyperglycemic ischemia produces a profound secondary deterioration of brain ATP and cerebral blood flow during reperfusion. This study tested the hypothesis that pretreatment with free deferoxamine ameliorates metabolic decay and delayed hypoperfusion after global hyperglycemic ischemia. In addition, deferoxamine conjugated to a high-molecular-weight starch was administered to determine the importance of an intravascular site of action. Iron-loaded deferoxamine was used to determine whether the iron chelation properties of deferoxamine are important to postischemic viability as distinguished from the agent's significant radical scavenging potential. METHODS: Cerebral ATP, phosphocreatine, and pH were measured by 31P magnetic resonance spectroscopy in anesthetized dogs. Tissue bicarbonate concentration was calculated from the Henderson-Hasselbalch equation. Incomplete cerebral ischemia was produced by intracranial pressure elevation for 30 minutes with plasma glucose at 540 +/- 15 mg/dL. Free deferoxamine, saline vehicle, hydroxyethyl starch-conjugated deferoxamine, hydroxyethyl starch vehicle, and deferoxamine loaded with equimolar ferric chloride were administered intravenously in five groups of dogs. The dose of deferoxamine was 50 mg/kg before ischemia, 50 mg/kg at the onset of reperfusion, and 50 mg/kg over the 180-minute reperfusion period. RESULTS: Ischemic hemispheric blood flow (mean, 6 to 8 mL/min per 100 g), intracellular pH (5.7 to 6.0), and bicarbonate levels (1 to 2 mmol/L) were similar in all groups. During reperfusion, cerebral pH and bicarbonate recovered only in the free-deferoxamine group. Both ATP and phosphocreatine initially increased in all groups, but recovery was sustained only in the free-deferoxamine group. Secondary losses of energy phosphates and cerebral oxygen consumption were observed in all other groups, accompanied by progressive reduction of perfusion. CONCLUSIONS: These data support the hypothesis that iron catalyzed oxygen radical production plays an important role in acidosis-mediated mechanisms of ischemic brain injury. The results with free and iron-loaded deferoxamine suggest that iron scavenging is an important, but not necessarily the principal, component of this mechanism. The poor recovery seen with conjugated deferoxamine indicates that the beneficial action of deferoxamine is not localized within the intravascular compartment.

Effect of the 21-aminosteroid tirilazad on cerebral pH and somatosensory evoked potentials after incomplete ischemia.

BACKGROUND AND PURPOSE: Postischemic evoked potential recovery correlates with acidosis during ischemia and early reperfusion. Acidosis promotes lipid peroxidation in vitro. We tested the hypothesis that the 21-aminosteroid tirilazad mesylate (U74006F), an inhibitor of lipid peroxidation in vitro, ameliorates somatosensory evoked potential recovery and acidosis during reperfusion after severe incomplete cerebral ischemia. METHODS: Cerebral perfusion pressure was reduced to 11 +/- 1 mm Hg (+/- SEM) for 30 minutes by cerebral ventricular fluid infusion in anesthetized dogs. Cerebral intracellular pH and high-energy phosphates were measured by magnetic resonance spectroscopy. Dogs were randomized to receive vehicle (citrate buffer; n = 8) or tirilazad (1 mg/kg; n = 8) before ischemia in a blinded study. RESULTS: Cerebral blood flow was reduced to 6 +/- 1 mL/min per 100 g during ischemia, resulting in nearly complete loss of high-energy phosphates and an intracellular pH of 6.0-6.1 in both groups. Initial postischemic hyperemia was similar between groups but lasted longer in the vehicle group. Tirilazad accelerated mean recovery time of intracellular pH from 31 +/- 5 to 15 +/- 3 minutes and of inorganic phosphate from 13 +/- 2 to 6 +/- 1 minutes. Recovery of somatosensory evoked potential amplitude was greater with tirilazad (49 +/- 3%) than vehicle (33 +/- 6%). Fractional cortical water content was less with tirilazad (0.819 +/- 0.003) than vehicle (0.831 +/- 0.002). CONCLUSIONS: Tirilazad attenuates cerebral edema and improves somatosensory evoked potential recovery after incomplete ischemia associated with severe acidosis. Accelerated pH and inorganic phosphate recovery indicates that this antioxidant acts during the early minutes of reperfusion.

Tirilazad mesylate does not improve early cerebral metabolic recovery following compression ischemia in dogs.

BACKGROUND AND PURPOSE: Tirilazad mesylate (U74006F) has been reported to improve recovery following cerebral ischemia. We conducted a randomized blinded study to determine if the drug would improve immediate metabolic recovery after complete cerebral compression ischemia. METHODS: Mongrel dogs were anesthetized with pentobarbital and fentanyl and treated with either vehicle (citrate buffer, n = 8) or tirilazad (1.5 mg/kg i.v. plus 0.18 mg/kg/hr, n = 8). Normothermic complete cerebral compression ischemia was produced for 12 minutes by lateral ventricular fluid infusion to raise intracranial pressure above systolic arterial pressure. Cerebral high-energy phosphate concentrations and intracellular pH were measured by phosphorus magnetic resonance spectroscopy. Cerebral blood flow was measured with radiolabeled microspheres, and oxygen consumption was calculated from sagittal sinus blood samples. Somatosensory evoked potentials were measured throughout the experiment. RESULTS: During ischemia, both groups demonstrated complete loss of high-energy phosphates and a fall in intracellular pH (vehicle, 5.76 +/- 0.23; tirilazad, 5.79 +/- 0.26; mean +/- SEM). At 180 minutes of reperfusion, there were no differences between groups in recovery of intracellular pH (vehicle, 6.89 +/- 0.07; tirilazad, 6.88 +/- 0.18), phosphocreatine concentration (vehicle, 89 +/- 16%; tirilazad, 94 +/- 24% of baseline value), oxygen consumption (vehicle, 2.6 +/- 0.2 ml/min/100 g; tirilazad, 1.8 +/- 0.5 ml/min/100 g), or somatosensory evoked potential amplitude (vehicle, 11 +/- 6%; tirilazad, 7 +/- 4% of baseline value). Forebrain blood flow fell below baseline levels at 180 minutes of reperfusion in the tirilazad-treated animals but not in the vehicle-treated dogs (vehicle, 28 +/- 4 ml/min/100 g; tirilazad, 18 +/- 5 ml/min/100 g). CONCLUSIONS: We conclude that tirilazad pretreatment does not improve immediate metabolic recovery 3 hours following 12 minutes of normothermic complete ischemia produced by cerebral compression.

Dependence of cerebral energy phosphate and evoked potential recovery on end-ischemic pH.

We determined whether the rate of metabolic recovery and electrophysiological deficit after incomplete cerebral ischemia is related to intracellular pH (pHi) achieved at the end of ischemia in a dose-dependent manner. End-ischemic pHi was varied by employing two ischemic durations, 12 and 30 min, and by setting preischemic plasma glucose to approximately 80 or 400 mg/dl. Incomplete global ischemia was produced in anesthetized dogs by transient intracranial hypertension followed by 4 h of reperfusion, and pHi, ATP, and phosphocreatine (PCr) were measured with 31P magnetic resonance spectroscopy. Cerebral blood flow was reduced to approximately 6 ml.min-1.100 g-1 during ischemia. End-ischemic pHi was greater than 5.7 in all animals from various treatment groups except for four of seven dogs treated with 30-min hyperglycemic ischemia. When end-ischemic pHi remained greater than 5.7, there was nearly complete recovery of ATP, PCr, pHi, intracellular bicarbonate concentration [( HCO3-]i), and O2 consumption. Partial recovery of somatosensory-evoked potentials (SEP) occurred in most of these animals. In the 30-min hyperglycemic animals in which pHi fell below 5.5, ATP, PCr, and O2 consumption recovered by only one-half over 60 min of reperfusion and then declined to near-zero levels without SEP recovery. In addition, pHi remained less than 6.0, and [HCO3-]i remained less than 2 mM throughout reperfusion. We conclude that there is an apparent in vivo pHi threshold of approximately 5.5-5.7 during incomplete cerebral ischemia that is associated with an inability to significantly restore pHi and [HCO3-]i and with secondary deterioration of high-energy phosphate levels.