Dimethylsulfoxide and ethanol, commonly used diluents, prevent dilation of pial arterioles by openers of K(ATP) ion channels.

Ethanol and dimethylsulfoxide are commonly used as diluents for water-insoluble drugs. Both are antioxidants. An earlier study of cats presented pharmacological evidence indicating that oxidants could open the K(ATP) ion channel in cerebral surface arterioles [pial arterioles] and that antioxidants including dimethylsulfoxide and L-cysteine prevented opening of these channels. Ethanol was not tested. The present study extends the older observations to a second species, the rat, and examines ethanol as well as dimethylsulfoxide and L-cysteine. A microscope and image splitter were used to measure arteriolar diameters under a closed cranial window in pentobarbital-anesthetized, paralyzed rats. Drugs were topically applied. Dose-dependent dilations produced by two well-established openers of the K(ATP) ion channel were inhibited in dose-dependent manner by ethanol at doses from 0.01% to 0.075%. Above this dose, the effect disappeared. Dilation by sodium nitroprusside was not affected. Dimethylsulfoxide and L-cysteine inhibited dilation produced by pinacidil. Dimethylsulfoxide inhibited pinacidil in a dose-dependent manner at doses from 0.01% to 0.2%. L-Cysteine inhibited pinacidil. Since all the inhibitory drugs have antioxidant properties, their effect may be a reflection of that property as suggested in an earlier paper. Ethanol and dimethylsulfoxide inhibited in doses frequently present when these agents are used as solvents. When investigators use these solvents to dissolve water-insoluble, topically applied drugs, we suggest that they first test the possibility that their observations are being made under conditions in which opening of the K(ATP) ion channel is inhibited.

Evidence for a K(ATP) ion channel link in the inhibition of hypercapnic dilation of pial arterioles by 7-nitroindazole and tetrodotoxin.

7-Nitroindazole, an inhibitor of neuronal nitric oxide synthase, reportedly inhibits hypercapnic dilation, but tetrodotoxin, an inhibitor of neuronal transmission, reportedly does not. Thus, evidence does not uniformly support the hypothesis of a neurogenic link to the hypercapnic response. Others suggest the hypercapnic response is mediated by a K(ATP) ion channel. In the following studies, we observed that topically administered tetrodotoxin inhibited dilations produced by hypercapnia. In addition, topical tetrodotoxin and either topical or intraperitoneal 7-nitroindazole, inhibited dilations produced by the K(ATP) channel openers, cromakalim and pinacidil. Inhibition of hypercapnic dilation and inhibition of dilation by the openers of the K(ATP) channel was immediately reversed by either L-lysine or L-arginine, amino acids previously shown to facilitate opening of the channel. The data strongly supports the previous conclusion that there is a K(ATP) ion channel link in the response of pial arterioles to hypercapnia. The location of the channel is not established by these data, nor is it known whether the action of tetrodotoxin on the channel was direct or indirect.

Blockade of ATP-sensitive potassium channels in cerebral arterioles inhibits vasoconstriction from hypocapnic alkalosis in cats.

BACKGROUND AND PURPOSE: Recent studies have shown that the cerebral arteriolar dilation from hypercapnic acidosis is blocked by agents which inhibit KATP channels. These findings suggested that this response is due to opening of KATP channels. Because the repose to CO2 is a continuum, with hypercapnic acidosis causing vasodilation and hypocapnic alkalosis causing vasoconstriction, it would be expected that the response to hypocapnic alkalosis would be due to closing of KATP channels. There are no studies of the effect of inhibition of KATP channels on the response to hypocapnic alkalosis. METHODS: We investigated the effect of 3 agents that in earlier studies were found to inhibit KATP channels--NG-nitro-L-arginine, hydroxylysine, and glyburide--on the cerebral arteriolar constriction caused by graded hypocapnia induced by hyperventilation in anesthetized cats equipped with cranial windows. RESULTS: Hypocapnic alkalosis caused dose-dependent vasoconstriction that was inhibited completely by each of the 3 inhibitors of KATP channels. The blockade induced by these agents was eliminated in the presence of topical L-lysine (5 micromol/L). CONCLUSIONS: The findings show that agents which inhibit ATP-sensitive potassium channels in cerebral arterioles inhibit the vasoconstriction from hypocapnic alkalosis. These and earlier results showing that inhibition of KATP channels inhibited dilation from hypercapnic acidosis demonstrate that the response to CO2 in cerebral arterioles is mediated by the opening and closing of KATP channels.

Cerebral arteriolar dilations by KATP channel activators need L-lysine or L-arginine.

We investigated the effects of various amino acids on responses to ATP-sensitive potassium (KATP) channel openers in anesthetized cats equipped with cranial windows. The application of pinacidil by superfusion caused transient vasodilation, whereas there was sustained vasodilation from the application of stationary solution of pinacidil. In the presence of L-arginine or L-lysine, pinacidil by superfusion led to sustained vasodilation, suggesting that the rapid flow of fluid displaced these amino acids from binding on the channel and that such binding was essential for opening the channel. NG-nitro-L-arginine blocked responses to pinacidil, and this blockade was reversed by L-lysine or L-arginine but not by D-arginine, D-lysine, methyl-L-arginine, glycine, L-histidine, dimethylarginine, dimethyl-L-arginine, or hydroxylysine. The blockade of responses to pinacidil induced by glyburide was also reversed completely by L-arginine or L-lysine but not by D-arginine, suggesting that these amino acids act on the sulfonylurea receptor. Hydroxylysine but not methyl-L-lysine, dimethylarginine, or dimethyl-L-arginine blocked responses to pinacidil. The findings show that KATP channels in cerebral arterioles need L-lysine or L-arginine to open in response to agonists.

Antioxidants inhibit ATP-sensitive potassium channels in cerebral arterioles.

BACKGROUND AND PURPOSE: Hydrogen peroxide and peroxynitrite are capable of generating hydroxyl radical and are commonly suspected as sources of this radical in tissues. It would be useful to distinguish the source of hydroxyl radical in pathophysiological conditions and to clarify the mechanisms by which antioxidants modify vascular actions of oxidants. METHODS: We investigated the effect of three antioxidants--dimethylsulfoxide (DMSO), salicylate, and L-cysteine--on the cerebral arteriolar dilation caused by topical application of hydrogen peroxide and peroxynitrite in anesthetized cats equipped with cranial windows. We also tested the effect of these antioxidants on the vasodilation caused by pinacidil and cromakalim, two known openers of ATP-sensitive potassium channels. RESULTS: DMSO was more effective in inhibiting dilation from hydrogen peroxide, whereas salicylate and L-cysteine were more effective in inhibiting dilation from peroxynitrite. All three antioxidants inhibited dilation in concentrations that were remarkably low (< 1 mmol/L). All three antioxidants inhibited vasodilation from two known potassium channel openers, pinacidil and cromakalim. Their effect was specific because they did not affect dilation from adenosine or nitroprusside. CONCLUSIONS: The findings show that antioxidants block ATP-sensitive potassium channels in cerebral arterioles. This appears to be the mechanism by which antioxidants inhibit the dilation from hydrogen peroxide and peroxynitrite and not through scavenging of a common intermediate, ie, hydroxyl radical. The differences between effectiveness in inhibiting dilation from hydrogen peroxide and peroxynitrite by various antioxidants suggest that hydrogen peroxide and peroxynitrite act at two different sites, one in a water-soluble environment and the other in a lipid-soluble environment.

Arginine analogues inhibit responses mediated by ATP-sensitive K+ channels.

Because arginine analogues have been reported to block the vasodilator response to hypercapnia, we investigated the effect of nitro-L-arginine (L-NNA) on the dilation of pial arterioles to arterial hypercapnia induced by inhalation of 3, 5, and 7% CO2 in anesthetized cats equipped with cranial windows. L-NNA at 250 microM, but not at lower concentrations, significantly reduced hypercapnia-induced dilation. This effect could be reversed by L-arginine. However, hypercapnic hyperemia is not the result of increased guanosine 3',5'-cyclic monophosphate via the usual NO-mediated activation of guanylate cyclase, because application of LY-83583, which blocks guanylate cyclase, did not alter the vessel response to CO2. L-NNA at 250 microM also abolished the pial arteriolar dilation in response to cromakalim, minoxidil, and pinacidil, three known openers of ATP-sensitive K+ channels, and this effect could be reversed by L-arginine. Application of glyburide, which blocks ATP-sensitive K+ channels, also reduced the response to CO2. Subsequent application of L-NNA in these experiments had no additional effect. Vasodilation induced by sodium nitroprusside and 3-morpholinosydnonimine, two known NO donors, was unaffected by glyburide. NG-monomethyl-L-arginine had effects similar to those of L-NNA in the cat and rat at concentrations as low as 20 microM. Our findings suggest that arginine analogues inhibit hypercapnic vasodilation by blocking ATP-sensitive K+ channels, independently of activation of guanylate cyclase via increased production of NO. Furthermore, the data suggest that ATP-sensitive K+ channels may have an arginine site that influences their function.

Mechanisms of cerebral vasodilation by superoxide, hydrogen peroxide, and peroxynitrite.

We investigated the role of potassium channels in the vasodilator action of hydrogen peroxide, peroxynitrite, and superoxide on cerebral arterioles. We studied the effect of topical application of these agents in anesthetized cats equipped with cranial windows. Hydrogen peroxide and peroxynitrite induced dose-dependent dilation that was inhibited by glyburide, an inhibitor of ATP-sensitive potassium channels. Superoxide, generated by xanthine oxidase acting on xanthine in the presence of catalase, also induced dose-dependent dilation of cerebral arterioles that was unaffected by glyburide but inhibited completely by tetraethylammonium chloride, an inhibitor of calcium-activated potassium channels. The vasodilations from hydrogen peroxide, peroxynitrite, or superoxide were unaffected by inhibition of soluble guanylate cyclase with LY-83583. The findings provide pharmacological evidence that hydrogen peroxide and peroxynitrite reversibly dilate cerebral arterioles by activating ATP-sensitive potassium channels, probably through an oxidant mechanism, whereas superoxide dilates cerebral arterioles by opening calcium-activated potassium channels. Activation of soluble guanylate cyclase is not a mediator of the vasodilator action of these agents in cerebral arterioles.

Nitric oxide and S-nitroso-L-cysteine as endothelium-derived relaxing factors from acetylcholine in cerebral vessels in cats.

BACKGROUND AND PURPOSE: The predominant view is that the endothelium-derived relaxing factor generated by acetylcholine from blood vessels is nitric oxide. However, there is evidence suggesting that certain nitric oxide-containing compounds such as nitrosothiols resemble the endothelium-derived relaxing factor generated by acetylcholine more closely than does nitric oxide itself. Accordingly, we compared the effects of nitric oxide and S-nitroso-L-cysteine on cerebral arteriolar caliber in relation to the associated increments in nitrite concentration in the effluent. METHODS: Acetylcholine, nitric oxide, and S-nitroso-L-cysteine were administered by continuous superfusion in oxygen-free solution through the space under a cranial window in anesthetized cats. Nitrite concentration was measured in the effluent. The degree of vasodilation induced was evaluated in relation to the increment in nitrite concentration. RESULTS: All agents induced dose-dependent vasodilation and dose-dependent increments in nitrite concentration in the effluent. For any given degree of vasodilation, the increments in nitrite concentration were equivalent during acetylcholine or S-nitroso-L-cysteine infusion, whereas the nitrite concentrations were 10 times higher during nitric oxide infusion. After administration of nitroarginine, a competitive inhibitor of nitric oxide synthesis from arginine, there was depression in the vasodilation as well as the increment in nitrite concentration induced by acetylcholine. CONCLUSIONS: S-Nitroso-L-cysteine resembles endothelium-derived relaxing factor from acetylcholine more closely than does nitric oxide.

Effect of local change in O2 saturation of hemoglobin on cerebral vasodilation from hypoxia and hypotension.

We tested the effect of certain newly synthesized allosteric modifiers of hemoglobin on the dilation induced by arterial hypoxia, arterial hypotension, and arterial hypercapnia in cerebral arterioles of anesthetized cats equipped with cranial windows for the observation of the cerebral microcirculation. The allosteric modifiers of hemoglobin are isomers of 2-(aryloxy)-2-methylpropionic acid. They shift the oxygen dissociation of hemoglobin to the right, thereby facilitating the local release of oxygen. When these compounds were applied topically by superfusion at a rate of 1 ml/min in a concentration of 0.1 mM, they had no significant effect on baseline arteriolar diameter but reduced significantly the vasodilation from arterial hypoxia and arterial hypotension. They did not influence the vasodilation from arterial hypercapnia. Spectrophotometric measurements of optical densities from pial veins 50-80 microns in diameter indicated that the superfusion with the allosteric compounds reduced hemoglobin oxygen saturation both during room air breathing and during hypoxia. We conclude that the vasodilations from arterial hypoxia and arterial hypotension are mediated by local oxygen-dependent mechanisms. The allosteric modifiers of hemoglobin may be useful as tools in investigating oxygen-dependent mechanisms.

Hydroxyl radical-dependent inactivation of guanylate cyclase in cerebral arterioles by methylene blue and by LY83583.

BACKGROUND AND PURPOSE: Methylene blue and 6-anilino,5,8-quinolinedione (LY83583) are used extensively to block activation of guanylate cyclase. Both agents generate oxygen radicals. Therefore, it appeared profitable to investigate whether the generation of oxygen radicals by these agents is responsible for the blockade of responses to nitrodilators that act via activation of guanylate cyclase to relax vascular smooth muscle and cause vasodilation. METHODS: We tested in anesthetized cats equipped with cranial windows responses to topical application of nitroglycerin, nitroprusside, and adenosine before and during topical application of methylene blue (5 microM). Responses to the vasoactive agents were tested during application of methylene blue after permeabilization of the cell membrane with a detergent to allow methylene blue to enter vascular smooth muscle. Responses were also tested in the presence of superoxide dismutase, catalase, deferoxamine, or dimethyl sulfoxide to scavenge reactive products of oxygen metabolism or to eliminate catalytic iron. In additional experiments we tested the effects of topical application of nitroprusside or adenosine before and after application of LY83583. The responses to the vasoactive agents were also tested in the presence of superoxide dismutase, catalase, or dimethyl sulfoxide in addition to LY83583. We also tested responses to calcitonin gene-related peptide before and in the presence of LY83583 with or without superoxide dismutase. RESULTS: Methylene blue eliminated the arteriolar dilation in response to nitroprusside and nitroglycerin after permeabilization of the cell membrane with a detergent but not before. The responses to adenosine were unaffected. The blockade induced by methylene blue was reversed by superoxide dismutase, catalase, or dimethyl sulfoxide but not by deferoxamine. LY83583 blocked responses to nitroprusside but not to adenosine. The blockade was eliminated by superoxide dismutase, catalase, or dimethyl sulfoxide. LY83583 blocked the vasodilation induced by calcitonin gene-related peptide. This blockade was reversed by superoxide dismutase. CONCLUSIONS: Methylene blue and LY83583 prevent the activation of soluble guanylate cyclase by nitrodilators or by calcitonin gene-related peptide by generating oxygen radicals. The mediator of this response is the hydroxyl radical. Methylene blue does not enter the vascular smooth muscle of cerebral arterioles unless the cell membrane is permeabilized.

Oxygen radicals in cerebral ischemia.

Superoxide production was measured as the superoxide dismutase (SOD)-inhibitable portion of nitro blue tetrazolium (NBT) reduction after cerebral ischemia-reperfusion in anesthetized cats equipped with cranial windows. Significant superoxide production was found in the early reperfusion period and continued for more than 1 h after ischemia. Superoxide was not detected in control animals not subjected to ischemia, during ischemia, and at 120 min of reperfusion. After ischemia, the vasoconstrictor response to arterial hypocapnia was reduced. This effect was prevented by pretreatment with SOD plus catalase or by deferoxamine. The response to topical acetylcholine was converted to vasoconstriction after ischemia. The normal vasodilator response reappeared spontaneously at 120 min of reperfusion. The vasodilator response to acetylcholine was preserved in animals pretreated with SOD plus catalase. Blood-brain barrier permeability to labeled albumin and horseradish peroxidase was increased after ischemia. These effects were minimized by pretreatment with SOD and catalase. We conclude that superoxide generation occurs during reperfusion after cerebral ischemia for a fairly long period and that superoxide and its derivatives are responsible at least in part for the vasodilation and the abnormal reactivity as well as for the increase in blood-brain barrier permeability to macromolecules seen after ischemia. Furthermore, the findings suggest that the agent responsible for the vascular abnormalities is hydroxyl radical generated via the iron-catalyzed Haber-Weiss reaction.

Effects in cats of inhibition of nitric oxide synthesis on cerebral vasodilation and endothelium-derived relaxing factor from acetylcholine.

BACKGROUND AND PURPOSE: We investigated the chemical identity of the endothelium-derived relaxing factor generated by acetylcholine in cerebral microvessels by studying the effects and mechanism of action of inhibitors of nitric oxide synthesis from arginine on the vasodilation and endothelium-derived relaxing factor production induced by topical application of acetylcholine in cerebral arterioles. METHODS: We determined cerebral arteriolar dilation and endothelium-derived relaxing factor production by bioassay in anesthetized cats equipped with cranial windows during superfusion of 10(-7) M acetylcholine before and after administration of either NG-monomethyl L-arginine or NG-nitro-L-arginine, two inhibitors of nitric oxide synthesis. RESULTS: NG-Nitro-L-arginine abolished the vasodilation from acetylcholine and eliminated the production of endothelium-derived relaxing factor in the bioassay experiments. NG-Monomethyl L-arginine had no effect on the response to acetylcholine in the absence of pretreatment. However, after pretreatment with the detergent sodium dodecyl sulfate to increase cell membrane permeability, the inhibitor had effects identical to those of NG-nitro-L-arginine. L-Arginine reversed the effects of the inhibitors of nitric oxide synthesis. Neither inhibitor affected baseline vascular caliber, nor did they generate a vasoconstrictor agent in the bioassay experiments. The two inhibitors of nitric oxide synthesis did not affect the response to nitroprusside or adenosine, showing that the effect on responses to acetylcholine was specific. Also, the blockade of the response to acetylcholine induced by the inhibitors of nitric oxide synthesis was unaffected by treatment with superoxide dismutase and catalase, showing that the effect was not mediated by oxygen radicals. CONCLUSION: The endothelium-derived relaxing factor generated by acetylcholine in cerebral arterioles of cats is either nitric oxide or a nitric oxide-containing substance. The effect of these inhibitors on the response to acetylcholine is mediated by inhibition of the synthesis of nitric oxide. There is no involvement of radicals, and no vasoconstrictor agent is generated.

Cytochemical detection of superoxide in cerebral inflammation and ischemia in vivo.

We used a cytochemical technique for the detection of superoxide in cerebral inflammation and ischemia-reperfusion in anesthetized cats. The technique is based on the oxidation of Mn2+ to Mn3+ by superoxide; Mn3+, in turn, oxidizes diaminobenzidine. The oxidized diaminobenzidine forms an osmiophilic electron-dense product that is detected by electron microscopy. The reagents, manganese chloride (2 mM) and diaminobenzidine (2 mg/ml), were placed topically on the brain surface of anesthetized cats equipped with cranial windows. Inflammation was induced by topical carrageenan with or without phorbol 12-myristate 13-acetate to activate leukocytes. In inflammation, superoxide was detected in the plasma membrane and in the phagocytic vacuoles of leukocytes. In ischemia-reperfusion, superoxide was identified in the meninges in association with blood vessels. It was located primarily in the extracellular space and occasionally in endothelial and vascular smooth muscle cells. In both inflammation and ischemia, the reaction product was eliminated by superoxide dismutase or by the omission of either manganese or diaminobenzidine. It was unaffected by sodium azide, which inhibits peroxidases. No superoxide was detected in the brain parenchyma. The findings confirm the generation of superoxide is cerebral ischemia-reperfusion and show that it is produced in cerebral vessels.

Superoxide production in experimental seizures in cats.

BACKGROUND AND PURPOSE: Seizures cause cerebrovascular responses similar to those seen in conditions such as acute hypertension, ischemia/reperfusion, or fluid-percussion brain injury, which are associated with the generation of superoxide. Accordingly, we studied production of superoxide in experimental seizures. METHODS: Superoxide production was measured in anesthetized cats equipped with double cranial windows using the superoxide dismutase-inhibitable reduction of nitro blue tetrazolium as a measure of superoxide production. Seizures were induced by intravenous bicuculline. The contribution of hypertension associated with seizures was studied by maintaining arterial blood pressure constant by bleeding. RESULTS: Significant superoxide dismutase-inhibitable reduction of nitro blue tetrazolium indicative of superoxide production was found during seizures with or without control of arterial blood pressure (1.10 +/- 0.27 and 1.29 +/- 0.16 nmol/l/min, respectively). CONCLUSIONS: The results show that experimental seizures are associated with superoxide generation that is independent of the rise in arterial blood pressure. It is likely that superoxide generation is due to the metabolic changes that occur during seizures.

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.

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.

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.

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.