Milde, James H.

Experiments were designed to determine the effect of subarachnoid hemorrhage on endothelium-dependent relaxations in small arteries of the brain stem. A "double-hemorrhage" canine model of the disease was used, and the presence of vasospasm in the basilar artery was confirmed by angiography. Secondary branches of both untreated basilar arteries (inner diameter, 324 +/- 11 microns; n = 12) and arteries exposed to subarachnoid hemorrhage for 7 days (inner diameter, 328 +/- 12 microns; n = 12) were dissected and mounted on glass microcannulas in organ chambers. Changes in the intraluminal diameter of pressurized arteries were measured using a video dimension analyzer. In untreated arteries, 10(-11) to 10(-7) M vasopressin, 10(-10) to 10(-6) M bradykinin, and 10(-9) to 10(-6) M calcium ionophore A23187 caused endothelium-dependent relaxations. At 10(-6) and 3 x 10(-4) M the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) abolished relaxations to vasopressin and produced small but significant rightward shifts of the concentration-response curves to bradykinin and A23187. At 10(-3) M L-arginine prevented the inhibitory effect of L-NAME. Subarachnoid hemorrhage abolished relaxations to vasopressin but did not affect relaxations to bradykinin or A23187. These studies suggest that in small arteries of the brain stem vasopressin causes relaxations by activation of the endothelial L-arginine pathway. This mechanism of relaxation is selectively inhibited by subarachnoid hemorrhage. Preservation of endothelium-dependent relaxations to bradykinin and A23187 is consistent with the concept that small arteries are resistant to vasospasm after subarachnoid hemorrhage.

Regional cerebral ischemia was produced by common carotid artery occlusion in gerbils and by middle cerebral artery occlusion in dogs, cats, and squirrel monkeys. Anesthesia was induced with either pentobarbital or halothane and maintained for two to there hours after vessel occlusion. In acute studies, the effect of regional cerebral ischemia on cerebral concentrations of ATP, phosphocreatine, lactate, and pyruvate was determined at the end of this period in gerbils, cats, and squirrel monkeys. In chronic studies, the degree of neurological deficit and size of cerebral infarction were determined 48 hours after a two-hour to three-hour period of vessel occlusion in cats and squirrel monkeys and permanent occlusion in dogs. In gerbils, dogs, and cats, there were no differences in the metabolic, functional, or pathological effects of anesthesia with pentobarbital or halothane. However, in the squirrel monkey, in acute studies the metabolic alterations were significantly less with pentobarbital, and in chronic studies the frequency and magnitude of functional deficits and cerebral infarction were significantly less. We conclude that pentobarbital does provide a degree of protection during regional cerebral ischemia but that such effects are only consistently demonstrable in primates. In nonprimates, we assume that variability in the collateral circulation renders demonstration of significant differences difficult or impossible.

We examined the 21-aminosteroid U74006F, a potent inhibitor of lipid peroxidation, for potential neuroprotective effects in a canine model of complete cerebral ischemia. Two 1.5-mg/kg boluses were administered to six dogs, the first bolus 15 minutes prior to a 12-minute episode of complete cerebral ischemia and the second bolus after 11 minutes of ischemia, 1 minute prior to reperfusion. Using this dosage regimen, plasma U74006F levels of greater than 0.3 microgram/ml were maintained for up to an hour postischemia. An additional six animals received equal volumes of the citrate vehicle solution. At 24 and 48 hours postischemia, the dogs were neurologically evaluated by an observer blinded as to treatment selection. All six U74006F-treated animals had a normal neurologic outcome at 48 hours postischemia, while the citrate vehicle-treated animals all suffered moderate to severe neurologic deficits. The difference in outcome was significant at both 24 and 48 hours (p less than 0.005). Although U74006F is a 21-aminosteroid, it is not reported to possess glucocorticoid activity. This is supported by the present finding that no changes in plasma glucose concentration were observed following administration of the drug. The systemic vitamin E levels of citrate vehicle-treated animals decreased significantly (from 4.10 +/- 0.46 micrograms/ml to 2.95 +/- 0.38 micrograms/ml, p less than 0.05), whereas the vitamin E levels in U74006F-treated animals did not decrease significantly. These results suggest that U74006F may be of benefit in improving neurologic outcome when administered prior to an episode of complete cerebral ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

The goal of this study was to determine the temperature coefficient (Q 10 ) for canine CMRO 2 at temperatures below 14°C. Eight dogs were anesthetized with halothane for surgical preparation. The animals were placed on total cardiopulmonary bypass and CBF was measured by direct sagittal sinus outflow. Duplicate measurements were taken at 37, 13, and 7°C. The EEG became isoelectric at a temperature of 12.0 ± 0.8°C. The Q 10 between 13 and 7°C was 2.19 ± 0.59. With rewarming to 37°C, cerebral metabolic variables returned to control levels. Brain biopsies taken at the end of the study yielded normal values for brain energy stores. We conclude that the Q 10 for CMRO 2 at temperatures between 7 and 37°C can be profoundly affected by the state of cerebral function as reflected by the EEG. In the absence of EEG activity, an expected Q 10 value of 2.2 reflects only the direct effect of temperature on the rates of biologic reactions.

Nimodipine, a calcium entry blocker, was administered in increasing doses of 0.1–3.0 μg kg −1 min −1 to six dogs after they had recovered consciousness from a surgical preparation that was conducted under general anesthesia and while they were under the influence of total spinal anesthesia. CBF was measured with a sagittal sinus outflow technique and CMRO 2 was calculated as the product of CBF and the arteriovenous O 2 difference. Nimodipine did not influence either CBF or CMRO 2 . There was a decrease in the cortical pyruvate level at the end of the study, but no significant change in phosphocreatine, ATP, lactate, or energy charge when compared with six control dogs. It has previously been reported that nimodipine increases the CBF in global ischemia with a potentially beneficial effect on the neurological outcome. With no effect on normal CBF or metabolism, this suggests that nimodipine may be useful in a variety of ischemic situations without fear of either a steal phenomenon or untoward effects on intracranial pressure.

Ten minutes of complete ischemia was produced in 11 dogs by temporary ligation of the aorta. Immediately before the ischemic episode, the dogs received nimodipine, a new calcium entry blocker, 10 μg kg −1 , i.v., followed by an infusion of 1 μg kg −1 min −1 for 2 h. Post-ischemic cerebral blood flow and metabolism were measured for 120 min in six dogs. Neurologic recovery was evaluated 48 h post-ischemia in five dogs. The results were compared to previously determined controls. Nimodipine nearly doubled cerebral blood flow in the delayed post-ischemic hypoperfusion period, compared to untreated dogs (approximately 45% versus 25% of pre-ischemic control values), but had no significant effect on metabolism. Nimodipine also improved neurologic recovery. Four of five treated dogs were normal and one was moderately damaged, whereas six of seven controls were either severely damaged or dead. This suggests that the delayed hypoperfusion state occurring after complete cerebral ischemia probably does contribute to the ultimate neurologic damage, and that nimodipine offers a potential protective effect.

A small number of animal studies have suggested that during the delayed postischemic hypoperfusion state, CO 2 reactivity of the cerebral vasculature is lost whereas autoregulation is retained. These findings, however, are inconsistent with the bulk of experimental evidence which demonstrates that CO 2 reactivity is more robust and may be retained in pathologic circumstances which abolish autoregulation. These opposing viewpoints were therefore further evaluated in 18 dogs in which complete global ischemia was induced by cerebrospinal fluid (CSF) compression for periods of 12 ( n = 12) and 18 ( n = 6) min. Following 45 min of reperfusion and with onset of the delayed postischemic hypoperfusion state, autoregulation and CO 2 reactivity were evaluated using a continuous measurement of CBF (by sagittal sinus outflow). CO 2 reactivity was tested over a P a co 2 range of 20 to 60 mm Hg; autoregulation was tested over a blood pressure range of 60 to 140 mm Hg. Results demonstrated that after both 12 and 18 min of complete global ischemia, autoregulation and CO 2 reactivity of the cerebral vasculature were both present, but attenuated. In the case of CO 2 reactivity, the slope of the CBF response was decreased approximately 75%. In the case of autoregulation, the response in some dogs was incomplete as compared with their preischemic response.

Following complete global cerebral ischemia and reperfusion, a brief period of reactive hyperemia is followed by a prolonged period of low flow commonly referred to as the delayed postischemic hypoperfusion state. It is generally assumed that this low-flow state may be injurious because of inadequate substrate delivery, thus implying that flow is no longer coupled to metabolic needs. This relationship of CBF to CMRO 2 was examined in six anesthetized dogs that were subjected to 12 min of complete ischemia induced either by CSF compression or aortic occlusion. Following reperfusion and onset of the low-flow state, which stabilized at 45 min postischemia, control normothermic (37°C) measurements of CBF and CMRO 2 were determined. Thereafter, femoral arterial blood was circulated through a heat exchanger (42.5°C), and brain temperature was increased to 40°C and measurements were repeated. The brain was then cooled back to 37°C for a final set of normothermic measurements. Thereafter, brain biopsies were taken to determine the energy state of the brain. CMRO 2 changed ∼6%/°C. CBF paralleled the change in CMRO 2 . Accordingly, the ratio of CBF to CMRO 2 remained constant throughout at a value of 8 to 9, demonstrating maintained coupling. The brain energy state was normal at the end of the study. The authors conclude that postischemic CBF is modulated by the brain's metabolic needs.

The effect of ischemia-reperfusion on endothelium-dependent relaxations and reactivity of vascular smooth-muscle cells was studied in rings of basilar arteries obtained from six dogs exposed to 12 min of complete global cerebral ischemia followed by 100 min of reperfusion. Three sham-operated control dogs served as controls. Ischemia was induced either by an increase in intracranial pressure or by aortic occlusion. The rings were suspended for isometric tension recording in physiological salt solution. Ischemia-reperfusion did not affect endothelium-dependent relaxations to vasopressin and bradykinin. In rings without endothelium relaxations to sodium nitroprusside, molsidomine (SIN-1), and papaverine as well as contractions to 5-hydroxytryptamine and KCl were preserved. These results demonstrate that in large canine cerebral arteries, ischemia-reperfusion of these durations does not affect relaxations mediated by activation of endothelium or direct relaxations and contractions of vascular smooth-muscle cells.