Pial vessel caliber and cerebral blood flow become dissociated during ischemia-reperfusion in cats

The relationship between pial arteriolar caliber and cerebral blood flow (CBF) was examined in 11 cats subjected to reperfusion for up to 120 min after 10 min of global cerebral ischemia induced by four-vessel occlusion and systemic hypotension. Thirty minutes after reperfusion CBF, as assessed by radiolabeled microsphere injection, had increased to 588% of control in middle cerebral artery (MSEC) cortical gray matter territory. The caliber of MSEC pial arterioles measured using the closed cranial window technique (greater than 33 to less than 213 microns) increased to 172% of baseline. By 60 min of reperfusion, CBF was 76% of basal levels, but pial arterioles remained 133% of baseline. After 120 min, CBF approximated baseline values, but pial dilatation persisted (115% of control). Intracranial pressure measurements did not differ significantly from resting values. At 45 min and beyond, total cerebrovascular resistance did not differ from resting values. The coexistence of vasodilatation within pial arterioles and normal blood flow in cortical gray matter indicates that pial vessels (greater than 33 microns) cannot be responsible for normal blood flow restoration following postocclusive hyperemia. Resistance during the posthyperemic phase must be increased selectively within parenchymal vessels to account for normal total cerebrovascular resistance, pial vessel dilatation, and normal-low parenchymal blood flow. Whether obstruction rather than vasoconstriction explains the resistance changes within intraparenchymal vessels remains for further study.

Chronic parasympathetic sectioning decreases regional cerebral blood flow during hemorrhagic hypotension and increases infarct size after middle cerebral artery occlusion in spontaneously hypertensive rats

Regional cerebral blood flow (rCBF) during controlled hemorrhagic hypotension (140-20 mm Hg) was assessed 10-14 days after chronic unilateral sectioning of parasympathetic and/or sensory fibers innervating pial vessels in spontaneously hypertensive rats (SHR). rCBF was measured in the cortical barrel fields bilaterally by laser Doppler blood flowmetry. Immunohistochemistry of middle cerebral artery (MCA) whole mount preparations was used to verify the surgical lesion. During hemorrhagic hypotension, rCBF was equivalent on the two sides in shams, after selective sensory denervation, or in parasympathetically sectioned animals exhibiting small decreases (less than or equal to 30%) in immunoreactive vasoactive intestinal peptide (VIP)-containing fibers. After chronic parasympathetic denervation, decreases in perfusion pressure were accompanied by greater reductions in rCBF on the lesioned side; changes in vascular resistance were also attenuated on that side. The rCBF response to hypercapnia (PaCO2 50 mm Hg), however, was symmetrical and robust. To examine the effects of impaired neurogenic vasodilation on the pathophysiology of cerebral ischemia, infarct size was measured 24 h following tandem MCA occlusion in denervated animals. Infarction volume was larger after selective parasympathetic sectioning (sham, 156 +/- 27 vs. 196 +/- 32 mm3, respectively) but only in those denervated animals demonstrating greater than or equal to 40% decrease in immunoreactive VIP-containing fibers within the ipsilateral MCA. Lower than expected blood flow/perfusion pressure in the cortex distal to an occluded blood vessel may relate the observed blood flow responses to the occurrence of larger cortical infarcts in parasympathetically denervated animals. If true, the findings suggest a novel role for neurogenic vasodilation in the pathophysiology of cerebral ischemia and in rCBF regulation within the periinfarction zone.

Calcitonin gene-related peptide mediates nitroglycerin and sodium nitroprusside-induced vasodilation in feline cerebral arterioles

The cerebral vasodilator response induced by topical nitroglycerin and nitroprusside was examined in cats equipped with cranial windows for the observation of the cerebral microcirculation. In cats subjected to chronic unilateral trigeminal ganglionectomy, the vasodilator responses to nitroprusside and nitroglycerin were markedly depressed on the denervated side. Application of a selective calcitonin gene-related peptide (CGRP) antagonist [CGRP(8-37)] on the innervated side reduced the response to nitrodilators to the same extent as seen on the denervated side. The vasodilator response to acetylcholine was unaffected by trigeminal ganglionectomy. CGRP(8-37) almost abolished the vasodilator response to nitroglycerin and sodium nitroprusside and to CGRP, but did not affect the response to adenosine or to adenosine diphosphate. Pretreatment with LY83583, a drug that lowers cyclic GMP levels, diminished the vasodilation to CGRP and to nitroprusside but not to adenosine. We conclude that the nitrovasodilators activate sensory fibers to release CGRP, which in turn relaxes cerebral vascular smooth muscle by activating guanylate cyclase. Hence, nitrovasodilators possess a novel mechanism of action within the cephalic circulation which may explain both the occurrence of vasodilation and headache.

Effect of hematocrit variations on cerebral blood flow and basilar artery diameter in vivo

Despite observations that pial arterioles constrict with decreased blood viscosity or hemodilution, several investigators have found an inverse relationship between cerebral blood flow (CBF) and hematocrit (Hct) under physiological conditions. To investigate whether this is due to a dilation of the more proximal large cerebral arteries, in vivo responses of CBF and basilar artery to hemodilution and hemoconcentration were studied in 21 anesthetized normal cats, using a closed clival window model. An inverse correlation between Hct and CBF was found, but CBF responses were smaller than previously reported data suggest. Varying Hct between 60 and 120% of baseline caused CBF to vary between 140 and 90%, approximately. Moderate hemodilution was associated with a significant decrease (-4.4%) in basilar artery diameter (P less than 0.05), but other Hct manipulations had no consistent effect on basilar artery diameter. It is concluded that dilation of large cerebral arteries cannot account for the decreased cerebrovascular resistance following hemodilution but that a disproportionate reduction of in vivo viscosity must be responsible. Pial arteriolar constriction after hemodilution therefore probably reflects a normal autoregulatory adjustment of vasomotor tone to altered blood rheology, whereas changes in large artery caliber may serve to modulate microvascular pressure.

Endothelium-dependent responses after experimental brain injury

We examined the effect of fluid percussion brain injury on the responses to topical application of acetylcholine and serotonin, two vasoactive agents that have endothelium-dependent effects, in anesthetized cats equipped with cranial windows. Before brain injury, topical acetylcholine dilated both small and large arterioles. Thirty minutes after brain injury, acetylcholine constricted small arterioles, and the vasodilator response of large vessels was abolished. Subsequent application either of superoxide dismutase plus catalase to eliminate superoxide and hydrogen peroxide or of deferoxamine, an agent that scavenges iron and inhibits the production of hydroxyl radical via the Haber-Weiss reaction, restored the normal vasodilator responses to acetylcholine. Serotonin constricted both large and small arterioles before brain injury. After brain injury, small arterioles responded with a small vasodilation, and the response of large arterioles was abolished. After application of superoxide dismutase and catalase, the normal vasoconstrictor response to serotonin was restored. The results show that endothelium-dependent vasodilation from acetylcholine is eliminated by brain injury by a mechanism that involves the generation of oxygen radicals, and, more specifically, the production of hydroxyl radical. The results with serotonin are explained by the elimination by oxygen radicals of a vasoconstrictor agent generated by this agent, perhaps an endothelium-derived contracting factor.

The role of neuroeffector mechanisms in cerebral hyperperfusion syndromes

Cerebral hyperperfusion, a state in which blood flow exceeds the metabolic needs of brain, may complicate a number of neurological and neurosurgical conditions. It may account for the propensity with which hemorrhage, cerebral edema, or seizures follow embolic stroke, carotid endarterectomy, or the excision of large arteriovenous malformations, and for some of the morbidity that accompanies acute severe head injury, prolonged seizures, and acute severe hypertension. Hyperperfusion syndromes have in common acute increases in blood pressure, vasodilatation, breakdown of the blood-brain barrier, and the development of cerebral edema. These common features suggest the possibility that they share the same pathogenic mechanisms. It was believed until recently that reactive hyperemia was caused primarily by the generation of vasoactive metabolites, which induced vasodilatation through relaxation of vascular smooth muscle. However, the authors have recently established that the release of vasoactive neuropeptides from perivascular sensory nerves via axon reflex-like mechanisms has a significant bearing upon a number of hyperperfusion syndromes. In this article, the authors summarize their data and discuss possible therapeutic implications for blockade of these nerves or their constituent neuropeptides.

Adverse effects of prolonged hyperventilation in patients with severe head injury: a randomized clinical trial

There is still controversy over whether or not patients should be hyperventilated after traumatic brain injury, and a randomized trial has never been conducted. The theoretical advantages of hyperventilation are cerebral vasoconstriction for intracranial pressure (ICP) control and reversal of brain and cerebrospinal fluid (CSF) acidosis. Possible disadvantages include cerebral vasoconstriction to such an extent that cerebral ischemia ensues, and only a short-lived effect on CSF pH with a loss of HCO3-buffer from CSF. The latter disadvantage might be overcome by the addition of the buffer tromethamine (THAM), which has shown some promise in experimental and clinical use. Accordingly, a trial was performed with patients randomly assigned to receive normal ventilation (PaCO2 35 +/- 2 mm Hg (mean +/- standard deviation): control group), hyperventilation (PaCO2 25 +/- 2 mm Hg: HV group), or hyperventilation plus THAM (PaCO2 25 +/- 2 mm Hg: HV + THAM group). Stratification into subgroups of patients with motor scores of 1-3 and 4-5 took place. Outcome was assessed according to the Glasgow Outcome Scale at 3, 6, and 12 months. There were 41 patients in the control group, 36 in the HV group, and 36 in the HV + THAM group. The mean Glasgow Coma Scale score for each group was 5.7 +/- 1.7, 5.6 +/- 1.7, and 5.9 +/- 1.7, respectively; this score and other indicators of severity of injury were not significantly different. A 100% follow-up review was obtained. At 3 and 6 months after injury the number of patients with a favorable outcome (good or moderately disabled) was significantly (p less than 0.05) lower in the hyperventilated patients than in the control and HV + THAM groups. This occurred only in patients with a motor score of 4-5. At 12 months posttrauma this difference was not significant (p = 0.13). Biochemical data indicated that hyperventilation could not sustain alkalinization in the CSF, although THAM could. Accordingly, cerebral blood flow (CBF) was lower in the HV + THAM group than in the control and HV groups, but neither CBF nor arteriovenous difference of oxygen data indicated the occurrence of cerebral ischemia in any of the three groups. Although mean ICP could be kept well below 25 mm Hg in all three groups, the course of ICP was most stable in the HV + THAM group. It is concluded that prophylactic hyperventilation is deleterious in head-injured patients with motor scores of 4-5.(ABSTRACT TRUNCATED AT 400 WORDS)

Description of a closed window technique for in vivo study of the feline basilar artery

Recent interest in the regulatory functions of large cerebral arteries has led to many studies addressing the specific reactivity of these vessels. Current data originate mainly from in vitro experiments, as in vivo studies of larger intracranial cerebral arteries have been cumbersome so far due to the lack of a suitable animal model. We provide a detailed technical description of a closed transclival window method for in vivo study of the basilar artery in cats. We present our experience with this preparation in 29 animals, which shows that the technique is feasible and allows repeated, accurate, and reproducible measurements of the basilar artery, although possible depressive effects of the anesthesia on vascular reactivity have to be taken into account. With hyperventilation, the basilar artery constricted by 12.2 +/- 7.6% of the baseline diameter. The cerebral blood flow response to hypocapnia with this preparation was 2.0 +/- 0.4%/torr PaCO2. An exudative clouding of the window occurred in some cats but had no apparent effect on vascular reactivity. We also discuss possible pitfalls in the surgical preparation.

Neuroeffector functions of sensory nerve fibers in the cerebral circulation after global cerebral ischemia

The importance of trigeminal neuroeffector mechanisms in the regulation of postischemic cerebral blood flow (CBF) was evaluated in cats subjected to chronic unilateral denervation of cortical sensory nerve fibers by trigeminal ganglionectomy or by the topical application of capsaicin to a cortical branch of the middle cerebral artery. CBF was determined using isotopically labeled microspheres before and at intervals after reperfusion following 10 min of global cerebral ischemia induced by four vessel occlusion combined with systemic hypotension. Postocclusive hyperemia 30 min after reperfusion in cortical gray matter ipsilateral to the side of denervation was attenuated by up to 58% (176 vs. 91 ml/100 g per min; p less than 0.05), but resting CBF, the duration of hyperemia, and the cerebrovascular response to hypercapnia were unaffected. These data underline the influence of neurogenic mechanisms in the regulation of postischemic CBF. Blockade of this axon reflex-like mechanism may reduce the morbidity associated with several hyperperfusion syndromes.

Chronic trigeminal ganglionectomy or topical capsaicin application to pial vessels attenuates postocclusive cortical hyperemia but does not influence postischemic hypoperfusion

Marked hyperemia accompanies reperfusion after ischemia in the brain, and may account for the propensity of cerebral hemorrhage to follow embolic stroke or carotid endarterectomy, and for the morbidity that follows head injury or the ligation of large arteriovenous malformations. To evaluate the contribution of trigeminal sensory fibers to the hyperemic response, CBF was determined in 12 symmetrical brain regions, using microspheres with up to five different isotopic labels, in four groups of cats. Measurements were made at 15-min intervals for up to 2 h of reperfusion after global cerebral ischemia induced by four-vessel occlusion combined with systemic hypotension of either 10- or 20-min duration. In normal animals, hyperemia in cortical gray matter 30 min after reperfusion was significantly greater after 20 min (n = 10) than after 10 min (n = 7) of ischemia (312 ml/100 g/min versus 245 ml/100 g/min; p less than 0.01). CBF returned to preischemic levels approximately 45 min after reperfusion and was reduced to approximately 65% of basal CBF for the remaining 75 min. In cats subjected to chronic trigeminal ganglionectomy (n = 15), postocclusive hyperemia in cortical gray matter was attenuated by up to 48% on the denervated side (249 versus 150 ml/100 g/min; p less than 0.01) after 10 min of ischemia. This effect was maximal in the middle cerebral artery (MCA) territory, and was confined to regions known to receive a trigeminal innervation. In these animals, substance P (SP) levels in the MCA were reduced by 64% (p less than 0.01), and the density of nerve fibers containing calcitonin gene-related peptide (but not vasoactive intestinal polypeptide or neuropeptide Y) was decreased markedly on the lesioned side. Topical application of capsaicin (100 nM; 50 microliters) to the middle or posterior temporal branch of the MCA 10-14 days before ischemia decreased SP levels by 36%. Postocclusive hyperemia in cortical gray matter was attenuated throughout the ipsilateral hemisphere by up to 58%, but the cerebral vascular response to hypercapnia (PaCO2 = 60 mm Hg) was unimpaired. The duration of hyperemia and the severity of the delayed hypoperfusion were not influenced by trigeminalectomy, capsaicin application, or the intravenous administration of ATP. These data demonstrate the importance of neurogenic mechanisms in the development of postischemic hyperperfusion, and suggest the potential utility of strategies aimed at blocking axon reflex-like mechanisms to reduce severe cortical hyperemia.

Postischemic cerebral blood flow and neuroeffector mechanisms

The influence of neuroeffector mechanisms in the regulation of postischemic cerebral blood flow was investigated by microsphere determination in 8 cats after chronic unilateral vascular deafferentation by trigeminal ganglionectomy. The animals were subjected to 90 min of reperfusion following 10 min of global ischemia induced by 4-vessel occlusion and systemic hypotension. Cortical hyperemia 30 min after reperfusion was attenuated by up to 48% in cortical gray matter ipsilateral to the side of trigeminal ganglionectomy (p less than 0.01). Axon reflex mechanisms involving the release of neuropeptides from peripheral sensory nerve fibers, such as substance P (SP), calcitonin gene-related peptide (CGRP) and neurokinin A (NKA), mediate this response. SP and NKA cause vasodilation by endothelium-dependent mechanisms (endothelium-dependent relaxing factor), whereas CGRP relaxes vascular smooth muscle by direct receptor interactions. Studies were therefore undertaken to determine the extent to which endothelium-dependent mechanisms mediate the hyperemia following global cerebral ischemia. In 7 intact cats, the postischemic response of pial arterioles to the topical application of acetylcholine (ACh; 10(-7) M), an endothelial-dependent vasodilator, was measured using a closed cranial window technique. Although ACh increased pial arteriolar caliber by 17% under resting conditions, the same dose elicited a vasoconstrictor response (87% of pre-ACh diameter 30 min after reperfusion) for the first 60 min of reperfusion after 10 min of ischemia. ACh-induced vasodilation was restored by 75 min (105%), but was less than control even at 120 min (109 vs. 117%; p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Neurogenic control of the cerebral circulation during global ischemia

The influence of the trigeminal nerve on the cerebral circulation was investigated in chronically denervated cats during and after reversible four-vessel occlusion for 10 minutes combined with controlled hypotension (50 mm Hg). Postocclusive hyperemia 30 minutes after reperfusion was attenuated by up to 48% in cortical gray matter of the anterior, middle, and posterior cerebral artery territories on the side of trigeminal ganglionectomy. Similar results were observed for denervation accomplished by direct surgical ablation and by the topical application of capsaicin to a cortical branch of the middle cerebral artery. Denervation did not alter basal cerebral blood flow or the duration of hyperemia, nor did it impair the cerebrovascular response to hypercapnia. These data demonstrate the importance of neurogenic mechanisms in the development of postischemic hyperperfusion and suggest that strategies directed at blocking axon reflex-like mechanisms may be beneficial in reducing the morbidity that follows severe cortical hyperemia.

Endothelium-derived relaxing factors. A perspective from in vivo data

We review below published studies of endothelium-dependent vasodilation in vivo. Endothelium-dependent vasodilation has been demonstrated in conduit arteries in vivo and in the cerebral, coronary, mesenteric, and femoral vascular beds as well as in the microcirculation of the brain and the microcirculation of cremaster muscle. The available evidence, although not complete, strongly suggests that the endothelium-derived relaxing factor generated by acetylcholine in the cerebral microcirculation is a nitrosothiol. The endothelium-derived relaxing factor generated by bradykinin in this vascular bed is an oxygen radical generated in association with enhanced arachidonate metabolism via cyclooxygenase. In the microcirculation of skeletal muscle, on the other hand, the vasodilation from bradykinin is mediated partly by prostacyclin and partly by an endothelium-derived relaxing factor similar to that generated by acetylcholine. Basal secretion of endothelium-derived relaxing factor is controversial in vivo but is usually present in vitro. On the other hand, it appears that endothelium-derived relaxing factor mediates flow-dependent vasodilation in both large vessels and in the microcirculation in vivo. The generation and release of endothelium-derived relaxing factor from endothelium may be abnormal in a variety of conditions including acute and chronic hypertension, atherosclerosis, and ischemia followed by reperfusion. Several mechanisms for these abnormalities have been identified. These include inability to generate endothelium-derived relaxing factor or destruction of endothelium-derived relaxing factor by oxidants after its release in the extracellular space. These abnormalities in endothelium-dependent relaxation may contribute to the vascular abnormalities in these conditions.

H2O2 and endothelium-dependent cerebral arteriolar dilation. Implications for the identity of endothelium-derived relaxing factor generated by acetylcholine

We studied the mechanism of the vasodilator effect of H2O2 on cerebral arterioles and its effect on endothelium-dependent responses to acetylcholine. Topical application of H2O2 (0.1-1 microM) on the brain surface of anesthetized cats equipped with cranial windows induced dose-dependent arteriolar dilation, which was markedly inhibited by topical deferoxamine, showing that it was probably mediated by generation of hydroxyl radical. Higher concentrations of H2O2 (3 microM) also induced dilation, which was unaffected by deferoxamine, indicating the participation of other mechanisms. After topical application of H2O2, endothelium-dependent responses to acetylcholine were eliminated or converted to vasoconstriction, and in bioassay experiments, acetylcholine-mediated endothelium-derived relaxing factor (EDRF) was absent. Superoxide dismutase plus catalase restored the appearance of transferable EDRF after 1 microM H2O2 but not after 3 microM H2O2. Application of H2O2 in the assay window eliminated the responses to nitroprusside and nitric oxide but did not affect responses to adenosine, to EDRF from the donor window, or responses to S-nitroso-L-cysteine. The inhibiting effect of H2O2 on the response to nitroprusside was partially eliminated after topical application of N-acetyl-L-cysteine. The results show that H2O2 inhibits the vasodilator action of nitroprusside and nitric oxide probably because it oxidizes thiols in vascular smooth muscle and prevents the formation of a nitrosothiol. EDRF from acetylcholine and S-nitroso-L-cysteine still produce dilation in the presence of the blockade induced by H2O2. The findings suggest strongly that the EDRF from acetylcholine in cerebral vessels is a nitrosothiol like S-nitroso-L-cysteine.

Captopril and enalaprilat do not scavenge the superoxide anion

The ability of captopril and enalaprilat, 2 angiotensin-converting enzyme (ACE) inhibitors, to scavenge superoxide anion radical was examined. With use of a number of superoxide-generating systems, such as xanthine-xanthine oxidase, phorbol myristate acetate-activated neutrophils, auto-oxidizing dihydroxyfumarate, and auto-oxidation of epinephrine to adrenochrome, captopril was seen not to scavenge superoxide directly, because it did not inhibit superoxide-dependent cytochrome c or nitro-blue tetrazolium reduction. Superoxide-dependent cytochrome c reduction was inhibited only when captopril was preincubated with a lower concentration of cytochrome c (22 microM). This effect was due to a decrease in the concentration of cytochrome c, because captopril reduced cytochrome c directly. When this effect was compensated for, no cytochrome c reduction induced by superoxide was observed. Captopril inhibited the auto-oxidation of epinephrine to adrenochrome at pH 10.2 where this auto-oxidation is superoxide-dependent, and at pH 7.8 where it is superoxide-independent and superoxide dismutase insensitive. It appears that captopril, in this respect, acted as a nonspecific antioxidant, probably by reducing an intermediate in the complex oxidation of epinephrine to adrenochrome. Therefore, caution may be used in interpreting the role of captopril in the attenuation of reperfusion-induced myocardial dysfunction and in attributing this effect to the inhibition of free radical mechanism.

Differences in endothelium-dependent cerebral dilation by bradykinin and acetylcholine

We compared the mechanism of action of acetylcholine and bradykinin, two agents that cause endothelium-dependent relaxation, on cerebral arterioles of cats equipped with cranial windows for the observation of the cerebral microcirculation. The vasodilation caused by bradykinin was eliminated by cyclooxygenase inhibition with topical indomethacin, it was reduced by topical deferoxamine, an agent that scavenges iron and thereby inhibits the production of hydroxyl radical via the Haber-Weiss reaction, and it was eliminated by 3-amino-1,2,4-triazole, an agent that inhibited superoxide production by cyclooxygenase. The vasodilation from acetylcholine was not affected by these agents. Acetylcholine induced a transferable, short-lived vasodilator material in bioassay experiments, whereas bradykinin did not. Bradykinin or acetylcholine, when applied topically by themselves, induced arteriolar dilation; when applied together, they did not. The findings are consistent with the view that the cerebral arteriolar dilation from bradykinin is caused by oxygen radicals generated in association with accelerated arachidonate metabolism via cyclooxygenase, whereas the dilation from acetylcholine is caused by an endothelium-derived relaxing factor (EDRF) similar to that generated by this agent in large vessels in vitro. The EDRF from acetylcholine and the radicals from bradykinin interact and inactivate each other.

Stable xenon versus radiolabeled microsphere cerebral blood flow measurements in baboons

Regional cerebral blood flow was simultaneously determined using the stable xenon computed tomographic and the radioactive microsphere techniques over a wide range of blood flow rates (less than 10-greater than 300 ml/100 g/min) in 12 baboons under conditions of normocapnia, hypocapnia, and hypercapnia. A total of 31 pairs of determinations were made. After anesthetic and surgical preparation of the baboons, cerebral blood flow was repeatedly determined using the stable xenon technique during saturation with 50% xenon in oxygen. Concurrently, cerebral blood flow was determined before and during xenon administration using 15-microns microspheres. In Group 1 (n = 7), xenon and microsphere determinations were made repeatedly during normocapnia. In Group 2 (n = 5), cerebral blood flow was determined using both techniques in each baboon during hypocapnia (PaCO2 = 20 mm Hg), normocapnia (PaCO2 = 40 mm Hg), and hypercapnia (PaCO2 = 60 mm Hg). Xenon and microsphere values in Group 1 were significantly correlated (r = 0.69, p less than 0.01). In Group 2, values from both techniques also correlated closely across all levels of PaCO2 (r = 0.92, p less than 0.001). No significant differences existed between the slopes or y intercepts of the regression lines for either group and the line of identity. Our data indicate that the stable xenon technique yields cerebral blood flow values that correlate well with values determined using radioactive microspheres across a wide range of cerebral blood flow rates.

Postocclusive cerebral hyperemia is markedly attenuated by chronic trigeminal ganglionectomy

Marked hyperemia may develop in brain following temporary cessation of blood flow and is associated with the morbidity following cardiac arrest, stroke, and head injury. Regional cerebral blood flow was measured using radiolabeled microspheres and compared in 10 symmetrical regions after chronic unilateral trigeminal ganglionectomy (n = 8), trigeminal rhizotomy (n = 4), or sham operation (n = 4) following 10 min of combined brachiocephalic-left subclavian occlusion and hypotension (mean arterial blood pressure less than 50 mmHg) in cats. Blood flow was symmetrical at rest in the three groups and was undetectable during the ischemic period. Within 30 min after re-establishing flow, values in cortical gray matter increased symmetrically to approximately 250 ml.100 g-1.min-1 in the rhizotomy and the sham groups. Increases of similar magnitude were measured on the intact side following trigeminal ganglionectomy but flow was attenuated by greater than 50% ipsilateral to the ganglionectomy. Marked hyperemia developed during reperfusion in thalamus, caudate, deep cortical white matter, midbrain, and pons, but no asymmetries were present in the three groups. These data suggest that cortical hyperemia is mediated by trigeminal neurogenic mechanisms via axonal reflexlike mechanisms and suggest the importance of therapeutic strategies based on blockade of this nerve or its constituent neuropeptides.

Similar responsiveness of smooth muscle of the canine basilar artery to EDRF and nitric oxide

Experiments were designed to compare the reactivity of canine basilar arteries to endothelium-derived relaxing factor (EDRF) and nitric oxide. Preparations with endothelium activated by bradykinin were used to study relaxations induced with EDRF, whereas the inhibitory effect of nitric oxide was studied in preparations without endothelium. All experiments were performed in the presence of indomethacin. EDRF- and nitric oxide-induced relaxations were significantly augmented in the presence of superoxide dismutase plus catalase but were reduced in the presence of methylene blue, LY 83583, and hemoglobin. M & B 22984 did not affect relaxations to either EDRF or nitric oxide. These results indicate that in the canine basilar artery EDRF is not an oxygen-derived free radical. The similar responsiveness of the basilar artery to EDRF and nitric oxide is consistent with the proposal that in the canine basilar artery nitric oxide is the factor.

Combined effect of respirator-induced ventilation and superoxide dismutase in experimental brain injury

The function-specific enzyme superoxide dismutase (SOD) was tested for its protective effect in severe experimental fluid-percussion brain injury (4.45 +/- 0.10 atm) in 30 of 60 randomly selected male Sprague-Dawley rats. A respirator was used only in the event of need. The number of animals with permanent resumption of spontaneous breathing (Type I respiratory response) remained essentially the same in each group. However, when Type II apnea (cannot maintain recovery) and Type III apnea (never recovers from the initial apnea) were terminated with a respirator, all rats with Type II responses from each group were successfully converted to a state of sustained spontaneous breathing. In contrast, only five (41.7%) of the 12 rats with Type III response were salvaged in the control group while five (83.3%) of six Type III rats in the SOD-treated group were saved. The results reveal the nature of the therapeutic effectiveness of superoxide radical scavengers in the overall outcome of head injury in this animal model. While SOD alone did not increase the number of spontaneous survivors, the drug shifted a number of animals from the critically injured rats with Type III respiratory response to the less critical Type II condition. Whereas induced respiration as the sole therapy in the control group lowered the mortality rate to 23.3%, respiratory assistance together with SOD treatment reduced the "mortality" to a single animal with Type III apnea (3.3%) which was alive but still required the respirator after 2 hours (p less than 0.001). The results show that respiratory assistance alone accounted for a 33% decrease in mortality rate and that SOD, given in addition to induced ventilation, further decreased mortality by 20%. Since SOD enzymes are reactively specific for superoxide, the increased survival rate of the brain-injured rat must have been due either to preventing or to minimizing pathophysiological changes, probably in the brain stem, caused by oxygen free radicals.

Effect of indomethacin pretreatment on acute mortality in experimental brain injury

The effect of indomethacin administration on the mortality rate of brain-injured rats was studied in four groups of animals subjected to a level of injury with a fluid-percussion apparatus predetermined to cause 50% mortality (50% lethal dose, or LD50). There were 24 animals in each of the following groups: 1) a control group, on which the LD50 was evaluated; 2) an ethanol-treated group with a mean blood serum level of 0.32 +/- 0.03 gm% (+/- standard error of the mean); 3) an indomethacin-treated group at a dose level of 3 mg/kg body weight administered intraperitoneally 10 to 15 minutes before injury; and 4) an indomethacin/ethanol-treated group. Significant differences in mortality rates were found in these experimental groups; namely, 50%, 58%, 8.3% (p less than 0.005), and 25% (p less than 0.05), respectively. The predetermined LD50 level of a 2.5- to 2.6-atm peak pressure pulse produced immediate apnea in all animals, which was either sustained (Type III), followed by temporary respiratory recovery (Type II), or followed by permanent resumption of breathing (Type I). The most important effect of indomethacin on respiratory function was manifested by a much higher percentage of Type I respiratory responses and a much lower percentage of Type II and III responses (hence a lower mortality rate). There was also a more rapid return to normal breathing in the postapneic period of recovery. Suppression of prostaglandin synthesis and of superoxide anion production at the of trauma may explain, at least in part, these favorable effects of indomethacin.

Effects of anesthesia on cerebral arteriolar responses to hypercapnia

We evaluated the effect of general anesthesia induced by 45 mg/kg iv pentobarbital sodium or by 75 mg/kg iv alpha-chloralose plus 500 mg/kg iv urethan on the response of cerebral arterioles to hypercapnia in rabbits equipped with chronically implanted cranial windows for the observation of the cerebral microcirculation. Both types of anesthetic induced approximately comparable anesthesia and depressed the responsiveness to CO2 to an equal extent. There were no changes in resting vessel diameter or in mean arterial blood pressure induced by either anesthetic, but both anesthetics increased end-expiratory PCO2 during room air breathing. The findings show that anesthetics depress the responsiveness of cerebral arterioles to hypercapnia. A decrease in cerebral metabolism and/or direct effects of the anesthetics on cerebral vessels may be involved.

Recovery of impaired endothelium-dependent relaxation after fluid-percussion brain injury in cats

The effect of a moderate level of fluid-percussion brain injury on acetylcholine-induced cerebral arteriolar vasodilation was examined for 12 hours after trauma in anesthetized cats equipped with cranial windows. The cats were then perfused with aldehydes, and the pial arteries were prepared for electron microscopy. Immediately after brain injury, the normal vasodilator response to topical application of acetylcholine was converted to vasoconstriction. By 4 hours after trauma, the ability of small pial arterioles (diameters less than 100 microns) to dilate after acetylcholine application had returned to the pretrauma level and was observed to be normal at both 8 and 12 hours after trauma (p less than 0.05). The vasodilator response of large caliber arterioles (diameters greater than or equal to 100 microns) at 4, 8, and 12 hours after injury was reduced relative to the pretrauma response but was significantly improved relative to their response at 30 minutes after trauma (p less than 0.05). Moreover, the response of large vessels at 4, 8, and 12 hours in injured animals was equal to that observed in noninjured control animals assessed at 4, 8, and 12 hours after window implantation. At 12 hours after injury, the ultrastuctural characteristics of both large and small vessels resembled their preinjury state. These data suggest that the impairment of acetylcholine-induced endothelium-dependent relaxation observed in cats after fluid-percussion brain injury is not irreversible but returns to normal (small arterioles) or exhibits significant recovery (large arterioles) within 4 hours after injury.

Inhibition by arachidonate of cerebral arteriolar dilation from acetylcholine

After topical application of arachidonate (200 micrograms/ml) on the brain surface of anesthetized cats equipped with cranial windows, the vasodilator response to topical acetylcholine (10(-7) M) was either abolished or converted to vasoconstriction. This effect of arachidonate was prevented by pretreatment with topical deferoxamine (1 mM) to chelate free iron and inhibit the generation of hydroxyl radical from the Haber-Weiss reaction. Posttreatment with deferoxamine or with the combination of superoxide dismutase and catalase did not reestablish the vasodilator response to acetylcholine. Using a bioassay preparation in which one cranial window served as the donor for endothelium-derived relaxing factor (EDRF) while the other window was used to assay EDRF, we found that arachidonate applied to the donor window inhibited the generation and/or release of EDRF. Arachidonate applied to the assay window did not influence the response to EDRF. The results show that arachidonate interferes with the vasodilator response to acetylcholine, primarily because it inhibits the generation and release of EDRF. This effect is caused by injury to the endothelium induced by hydroxyl radical.

Trigeminovascular fibers increase blood flow in cortical gray matter by axon reflex-like mechanisms during acute severe hypertension or seizures

Cerebral blood flow was measured and compared in 10 symmetrical brain regions following unilateral trigeminal ganglionectomy (n = 13), sham operation (n = 6), or trigeminal root section (rhizotomy) (n = 8) in cats. Multiple determinations were obtained in anesthetized and paralyzed animals using radiolabeled microspheres during (i) normocapnia-normotension, (ii) hypercapnia (5% CO2/95% room air), (iii) angiotensin-induced acute severe hypertension (190 greater than mean arterial blood pressure less than 210 mmHg), or (iv) bicuculline-induced seizures. Flow was symmetrical in all brain regions at rest and during increases induced by hypercapnia in the three groups. During severe hypertension or seizures, marked elevations developed bilaterally (approximately 93% and approximately 130%, respectively). In ganglionectomized animals, increases due to hypertension or seizures were attenuated by 28-32% on the denervated side within cortical gray matter regions corresponding to the anterior, middle, and posterior cerebral arteries. Flow was symmetrical within all brain regions in sham-operated animals and in the rhizotomy group, despite comparable increases in regional cerebral blood flow induced by angiotensin. Hence, the trigeminal nerve mediates blood flow adaptations during severe hypertension and seizures. Furthermore, since trigeminal cell bodies and peripheral axons are destroyed or degenerate following ganglionectomy but not following rhizotomy, local "axon reflex-like" mechanisms mediate these increases in cerebral blood flow.