Constriction Of Pial Arterioles Research Articles

Optical coherence tomography of arteriolar diameter and capillary perfusion during spreading depolarizations

Spreading depolarization (SD) is associated with profound oligemia and reduced oxygen availability in the mouse cortex during the depolarization phase. Coincident pial arteriolar constriction has been implicated as the primary mechanism for the oligemia. However, where in the vascular bed the hemodynamic response starts has been unclear. To resolve the origin of the hemodynamic response, we used optical coherence tomography (OCT) to simultaneously monitor changes in the vascular tree from capillary bed to pial arteries in mice during two consecutive SDs 15 minutes apart. We found that capillary flow dropped several seconds before pial arteriolar constriction. Moreover, penetrating arterioles constricted before pial arteries suggesting upstream propagation of constriction. Smaller caliber distal pial arteries constricted stronger than larger caliber proximal arterioles, suggesting that the farther the constriction propagates, the weaker it gets. Altogether, our data indicate that the hemodynamic response to cortical SD originates in the capillary bed.

H2S mediates the vasodilator effect of endothelin-1 in the cerebral circulation.

H2S is an endogenous gasotransmitter that increases cerebral blood flow. In the cerebral vascular endothelium, H2S is produced by cystathionine δ-lyase (CSE). Endothelin-1 (ET-1) has constrictor and dilator influences on the cerebral circulation. The mechanism of the vasodilation caused by ET-1 may involve endothelium-derived factors. We hypothesize that ET-1-elicited dilation of pial arterioles requires an elevation of H2S production in the cerebral vascular endothelium. We investigated the effects of ET-1 on CSE-catalyzed brain H2S production and pial arteriolar diameter using cranial windows in newborn pigs in vivo. H2S was measured in periarachnoid cerebrospinal fluid. ET-1 (10-12-10-8 M) caused an elevation of H2S that was reduced by the CSE inhibitors propargylglycine (PPG) and β-cyano-l-alanine (BCA). Low doses of ET-1 (10-12-10-11 M) produced vasodilation of pial arterioles that was blocked PPG and BCA, suggesting the importance of H2S influences. The vasodilator effects of H2S may require activation of smooth muscle cell membrane ATP-sensitive K+ (KATP) channels and large-conductance Ca2+-activated K+ (BK) channels. The KATP inhibitor glibenclamide and the BK inhibitor paxilline blocked CSE/H2S-dependent dilation of pial arterioles to ET-1. In contrast, the vasoconstrictor response of pial arterioles to 10-8 M ET-1 was not modulated by PPG, BCA, glibenclamide, or paxilline and, therefore, was independent of CSE/H2S influences. Pial arteriolar constriction response to higher levels of ET-1 was independent of CSE/H2S and KATP/BKCa channel activation. These data suggest that H2S is an endothelium-derived factor that mediates the vasodilator effects of ET-1 in the cerebral circulation via a mechanism that involves activation of KATP and BK channels in vascular smooth muscle. NEW & NOTEWORTHY Disorders of the cerebral circulation in newborn infants may lead to lifelong neurological disabilities. We report that vasoactive peptide endothelin-1 exhibits vasodilator properties in the neonatal cerebral circulation by stimulating production of H2S, an endothelium-derived messenger with vasodilator properties. The ability of endothelin-1 to stimulate brain production of H2S may counteract the reduction in cerebral blood flow and prevent the cerebral vascular dysfunction caused by stroke, asphyxia, cerebral hypoxia, ischemia, and vasospasm.

Cerebral microcirculatory alterations and the no-reflow phenomenon in vivo after experimental pediatric cardiac arrest

Decreased cerebral blood flow (CBF) after cardiac arrest (CA) contributes to secondary ischemic injury in infants and children. We previously reported cortical hypoperfusion with tissue hypoxia early in a pediatric rat model of asphyxial CA. In order to identify specific alterations as potential therapeutic targets to improve cortical hypoperfusion post-CA, we characterize the CBF alterations at the cortical microvascular level in vivo using multiphoton microscopy. We hypothesize that microvascular constriction and disturbances of capillary red blood cell (RBC) flow contribute to cortical hypoperfusion post-CA. After resuscitation from 9 min asphyxial CA, transient dilation of capillaries and venules at 5 min was followed by pial arteriolar constriction at 30 and 60 min (19.6 ± 1.3, 19.3 ± 1.2 µm at 30, 60 min vs. 22.0 ± 1.2 µm at baseline, p < 0.05). At the capillary level, microcirculatory disturbances were highly heterogeneous, with RBC stasis observed in 25.4% of capillaries at 30 min post-CA. Overall, the capillary plasma mean transit time was increased post-CA by 139.7 ± 51.5%, p < 0.05. In conclusion, pial arteriolar constriction, the no-reflow phenomenon and increased plasma transit time were observed post-CA. Our results detail the microvascular disturbances in a pediatric asphyxial CA model and provide a powerful platform for assessing specific vascular-targeted therapies.

Constriction and dysfunction of pial arterioles after regional hemorrhage in the subarachnoid space

Increasing evidence indicates that poor outcomes after brain hemorrhage, especially after subarachnoid hemorrhage (SAH), can be attributed largely to dysfunction of the cerebral microcirculation. However, the cause of this dysfunction remains unclear. Here, we investigated changes in the cerebral microcirculation after regional hemorrhage in the subarachnoid space using the closed cranial window technique in mice. A single pial arteriole on the surface of the brain was punctured to induce a regional hemorrhage in the subarachnoid space. Physiological parameters were monitored during the procedure, and microvessel diameter was measured after hemorrhage. The vasoreactivity of the arterioles in response to hypercapnia as well as to topical application of the vasodilator acetylcholine (ACh) and S-nitroso-N-acetyl-penicillamine (SNAP) were assessed. The constriction of pial arterioles was detected without changes in other physiological parameters. Decreased reactivity of pial arterioles to all of the applied vasodilatory stimuli was observed after hemorrhage. Our results indicate that regional hemorrhage in the subarachnoid space can induce the vasospasm of microvessels and also reduce the vasoreactivity of pial arterioles.

Onset of pulmonary ventilation in fetal sheep produces pial arteriolar constriction dependent on cytochrome p450 ω-hydroxylase activity

With the onset of ventilation at birth, cerebral blood flow decreases as oxygenation increases, but the mechanism of cerebral vasoconstriction is unknown. Cytochrome P-450 omega-hydroxylase activity metabolizes arachidonic acid to 20-HETE, a potent vasoconstrictor, in a physiologically relevant O(2)-dependent manner. We tested the hypothesis that the omega-hydroxylase inhibitor, 17-octadecynoic acid (17-ODYA), reduces cerebral vasoconstriction during in utero ventilation with O(2) in fetal sheep. In anesthetized pregnant sheep near term, the fetal head was exposed with the rest of the body remaining in utero. Pial arteriolar diameter was measured by intravital microscopy through a closed cranial window superfused with vehicle or 17-ODYA. Mechanical ventilation of the fetal lungs with a high O(2) mixture to increase arterial Po(2) from approximately 20 to approximately 90 Torr markedly decreased pial arteriolar diameter by 24 + or - 3% (+ or - SE) without a change in arterial pressure. In contrast, superfusion of 17-ODYA completely blocked the decrease in diameter (2 + or - 3%) with increased oxygenation. Vasoconstriction to hypocapnia was intact after returning to the baseline intrauterine oxygenation state, thereby indicating that the effect of 17-ODYA was selective for increased oxygenation. In cerebral arteries isolated from fetal sheep, increasing oxygenation increased 20-HETE production. We conclude that cytochrome P-450 omega-hydroxylase activity makes an important contribution to cerebral vasoconstriction associated with the onset of ventilation at birth.

The Direct Effects of Propofol on Pial Microvessels in Rabbits

Propofol is widely used for neurosurgical anesthesia; however, its effects on the pial microvasculature are unknown. We therefore evaluated the direct effects of propofol on pial microvessels in rabbits. Pial microcirculation was visualized using a closed cranial window technique in 20 Japanese white rabbits. In the first experiment (n=14), after baseline hemodynamic measurements, the cranial window was superfused with 5 increasing concentrations of propofol (10, 10, 10, 10, 10 mol/L; n=8) or intralipid (at comparable concentrations; n=6) dissolved in artificial cerebrospinal fluid for 7 minutes each. A typical anesthetic concentration of 5 microg/mL corresponds to 10 mol/L. In the second experiment (n=6), phenylephrine 10 mol/L and nitroglycerin 10 mol/L were applied topically for 7 minutes under pentobarbital anesthesia. In the third experiment (n=3), electroencephalogram and bispectral index were measured under pentobarbital anesthesia. Diameters of selected pial arterioles and venules were visualized with a microscope-video capture unit combination and subsequently measured with a digital video analyzer. Topical application of propofol at 10, 10, 10, or 10 mol/L did not alter the diameters of the pial microvessels; however, at 10 mol/L propofol induced dilation in large and small arterioles, along with venular dilation. Intralipid alone did not have any significant effect on vessel diameters. Phenylephrine and nitroglycerin produced pial arteriolar and venular constriction and dilation, respectively. Phenylephrine constricted and nitroglycerin dilated pial arterioles and venules. Pentobarbital did not produce either burst suppression or an isoelectric electroencephalogram. The results confirm our hypothesis: clinically relevant concentrations of propofol, that is, approximately 10 mol/L, do not dilate pial arterioles or venules.

In Vivo Cerebrovascular Effects of Cocaine- and Amphetamine-Regulated Transcript (CART) Peptide

Cocaine- and amphetamine-regulated transcript (CART) and its associated peptides have been implicated in a number of physiologic processes including modulation of the hypothalamo-pituitary-adrenal (HPA) axis and cardiovascular regulation. Recently, we reported that in isolated cerebral arterioles, CART peptide (CARTp) acts directly to produce endothelium-dependent constriction via the endothelin signaling pathway. We used the rat closed cranial window model to determine the in vivo effects of CARTp on pial arteriolar diameter. Intravenous administration of 30 microg/kg CARTp produced a significant pressor effect and constriction of pial arterioles. The pressor response to systemic CARTp was blocked by the beta-adrenergic receptor antagonist propranolol (2 mg/kg IV). Direct application of 0.1 nM-1 microM CARTp to pial arterioles produced a dose-dependent and long-lasting constriction to approximately 88% of baseline diameter. The constriction response to topically applied 100 nM CARTp was blocked by both the endothelin A (ETA) receptor antagonist BQ-123 (10 microM) and the inhibitor of endothelin-converting enzyme, phosphoramidon (100 nM). These results demonstrate for the first time that CARTp constricts cerebral vessels in vivo, an action mediated by its effects on the endothelin system, specifically via activation of ETA receptors. This supports the notion that CARTp plays a physiologic role in cerebrovascular regulation, particularly during times of HPA axis activation.

Insensitivity of cerebral oxygen transport to oxygen affinity of hemoglobin-based oxygen carriers

The cerebrovascular effects of exchange transfusion of various cell-free hemoglobins that possess different oxygen affinities are reviewed. Reducing hematocrit by transfusion of a non-oxygen-carrying solution dilates pial arterioles on the brain surface and increases cerebral blood flow to maintain a constant bulk oxygen transport to the brain. In contrast, transfusion of hemoglobins with P 50 of 4–34 Torr causes constriction of pial arterioles that offsets the decrease in blood viscosity to maintain cerebral blood flow and oxygen transport. The autoregulatory constriction is dependent on synthesis of 20-HETE from arachidonic acid. This oxygen-dependent reaction is apparently enhanced by facilitated oxygen diffusion from the red cell to the endothelium arising from increased plasma oxygen solubility in the presence of low or high-affinity hemoglobin. Exchange transfusion of recombinant hemoglobin polymers with P 50 of 3 and 18 Torr reduces infarct volume from experimental stroke. Cell-free hemoglobins do not require a P 50 as high as red blood cell hemoglobin to facilitate oxygen delivery.

The Comparative Effects of Intravenous Nicardipine and Prostaglandin E1 on the Cerebral Pial Arteriolar Constriction Seen After Unclamping of an Aortic Cross-Clamp in Rabbits

The potent vasodilators nicardipine and prostaglandin E1 (PGE1) are useful for the treatment of systemic hypertension or pulmonary hypertension during aortic surgery. We measured cerebral pial arteriolar diameters, using a rabbit closed cranial window preparation: before (baseline) and 15 min after the start of an IV infusion (preclamp) (0.9% saline [control group], nicardipine [at 0.1, 1.0, or 10 microg x kg(-1) x min(-1)], or PGE1 [at 0.1 or 1.0 microg x kg(-1) x min(-1)]), just after aortic clamping, 20 min after clamping, and at 0-60 min after unclamping. In the control group, a significant decrease in diameter persisted for at least 60 min after unclamping (maximum [at 60 min], -16% for large [> or =75 microm], and -27% for small [<75 microm] arterioles versus baseline). Although the aortic unclamping-induced vasoconstriction was unaffected under the smallest dose of nicardipine, it was significantly attenuated under larger doses in both large and small arterioles (residual vasoconstriction, -10% and -6% for large and -18% and -10% for small arterioles; at 60 min). The pial arteriolar constriction observed at 5 min or more after unclamping in the control group was not altered by PGE1 in either large or small arterioles. The larger doses of nicardipine, but neither dose of PGE1, attenuated aortic unclamping-induced sustained cerebral pial arteriolar constriction.

Both Milrinone and Colforsin Daropate Attenuate the Sustained Pial Arteriolar Constriction Seen After Unclamping of an Abdominal Aortic Cross-Clamp in Rabbits

We previously reported that unclamping of an abdominal aortic cross-clamp causes initial dilation of pial arteries followed by sustained constriction. Both milrinone and colforsin daropate have a vasodilator action, and both have been used in such critical conditions as abdominal aortic aneurysmectomy. We measured cerebral pial arteriolar diameters using a rabbit closed cranial window preparation before (baseline) and 15 min after the start of an IV infusion of 0.9% saline (control group), milrinone, or colforsin daropate (0.05 and 0.5 microg . /kg(-1) . min(-1)) (pre-clamp), just after aortic clamping, 20 min after clamping, and at 0 to 60 min after unclamping. In the control group, a significant decrease in diameter persisted for at least 60 min after unclamping (maximum, -15% for large and -26% for small arterioles versus baseline). These values were significantly smaller after both doses of milrinone and the larger dose of colforsin daropate (-5% and -8%, 10% and 12%, and -2% and -5%, respectively vs baseline, at 60 min). In a second experiment, changes in regional cerebral blood flow and tissue oxygen tension reflected changes in vascular variables. Thus, sustained cerebral pial arteriolar constriction induced by aortic unclamping can be attenuated by IV milrinone or colforsin daropate.

Cerebrovascular response to decreased hematocrit: effect of cell-free hemoglobin, plasma viscosity, and CO2.

The effect of transfusing a nonextravasating, zero-link polymer of cell-free hemoglobin on pial arteriolar diameter, cerebral blood flow (CBF), and O2 transport (CBF x arterial O2 content) was compared with that of transfusing an albumin solution at equivalent reductions in hematocrit (approximately 19%) in anesthetized cats. The influence of viscosity was assessed by coinfusion of a high-viscosity solution of polyvinylpyrrolidone (PVP), which increased plasma viscosity two- to threefold. Exchange transfusion of a 5% albumin solution resulted in pial arteriolar dilation, increased CBF, and unchanged O2 transport, whereas there were no significant changes over time in a control group. Exchange transfusion of a 12% polymeric hemoglobin solution resulted in pial arteriolar constriction and unchanged CBF and O2 transport. Coinfusion of PVP with albumin produced pial arteriolar dilation that was similar to that obtained with transfusion of albumin alone. In contrast, coinfusion of PVP with hemoglobin converted the constrictor response to a dilator response that prevented a decrease in CBF. Pial arteriolar dilation to hypercapnia was unimpaired in groups transfused with albumin or hemoglobin alone but was attenuated in the largest vessels in albumin and hemoglobin groups coinfused with PVP. Unexpectedly, hypocapnic vasoconstriction was blunted in all groups after transfusion of albumin or hemoglobin alone or with PVP. We conclude that 1) the increase in arteriolar diameter after albumin transfusion represents a compensatory response that prevents decreased O2 transport at reduced O2-carrying capacity, 2) the decrease in diameter associated with near-normal O2-carrying capacity after cell-free polymeric hemoglobin transfusion represents a compensatory mechanism that prevents increased O2 transport at reduced blood viscosity, 3) pial arterioles are capable of dilating to an increase in plasma viscosity when hemoglobin is present in the plasma, 4) decreasing hematocrit does not impair pial arteriolar dilation to hypercapnia unless plasma viscosity is increased, and 5) pial arteriolar constriction to hypocapnia is impaired at reduced hematocrit independently of O2-carrying capacity.

Changes in cerebral microcirculation during and after abdominal aortic cross-clamping in rabbits: the role of thromboxane A2 receptor.

Little is known about any changes in cerebral hemodynamics, during and after abdominal aortic cross-clamping and unclamping, especially in the cerebral microcirculation. We studied the effects of abdominal aortic cross-clamping and unclamping on cerebral pial vessel diameter in the presence or absence of the thromboxane (Tx)A(2) receptor antagonist using a closed cranial window in 27 rabbits. Although infrarenal aortic cross-clamping did not affect pial vessel diameter, release of a 20-min aortic cross-clamp caused pial arterioles to dilate and then constrict. A significant constriction persisted for at least 60 min (maximum, -17% for large [> or =75 micro m] and -28% for small arterioles [<75 micro m] compared with baseline). Topical administration of a TxA(2) receptor antagonist, seratrodast, at 10(-7) M and 10(-6) M, significantly attenuated the constriction of large and small arterioles (at 60 min, -9% and -13% constriction for 10(-7) M, and -6% and -7% for 10(-6) M). Release of a 20-min aortic cross-clamp induced a sustained pial arteriolar constriction. Because this unclamping-induced vasoconstriction was attenuated by topical administration of seratrodast, it was likely partially mediated via the washout of TxA(2) produced in the ischemic region during the clamp and after cross-clamp release. Abdominal aortic unclamping after a 20-min clamp caused an initial dilation followed by a sustained constriction of pial arterioles. Seratrodast, a thromboxane A(2) receptor antagonist, attenuated the vasoconstriction suggesting that it is at least partly mediated by thromboxane A(2) washed out from the region rendered ischemic by clamping.

Melatonin increases pial artery tone and decreases the lower limit of cerebral blood flow autoregulation.

We studied a possible link between the melatonin-induced increase in cerebral arteriolar tone and the melatonin-induced shift in cerebral blood flow (CBF) autoregulation to a lower pressure level. Using the cranial window technique, we showed that intravenous infusion of melatonin constricted pial arterioles (-5.1 +/- 1.3 and -5.4 +/- 0.7 microm at 60 and 600 ng/kg/h, respectively). Perivascular application of luzindole alone had no significant effect but abolished vasoconstriction induced by melatonin (-0.5 +/- 0.7 and + 3.0 +/- 1.2 microm at 60 and 600 ng/kg/h respectively). Using a combination of the hydrogen clearance and cranial window techniques, we showed that intravenous infusion of melatonin had no effect on baseline CBF but shifted the lower limit (LL) of CBF autoregulation (stepwise hypotension) to a lower pressure (90 +/- 2 mmHg in vehicle vs. 71 +/- 3 and 51 +/- 5 mmHg, both P < 0.05, after melatonin at 60 and 600 ng/kg/h, respectively). As melatonin had no effect on systemic blood pressure yet shifted the LL of CBF autoregulation, the security margin increased (28 +/- 5 in controls vs. 38 +/- 3 and 55 +/- 5% after melatonin at 60 and 600 ng/kg/h, respectively, both P < 0.05). The higher i.v. infusion rate of melatonin increased the relative arteriolar dilatory response to hypotension but did not increase absolute diameter at any given pressure level. Our results show that melatonin shifts the LL of CBF autoregulation to a lower systemic pressure level. This effect does not appear to be explained by the effect of melatonin on cerebral arteriolar diameter.

Leukocyte adherence contributes to disruption of the blood–brain barrier during activation of mast cells

The goal of this study was to examine the role of leukocytes in disruption of the blood–brain barrier during activation of mast cells using compound 48/80. We examined the pial microcirculation in rats using intravital fluorescence microscopy. Permeability of the blood–brain barrier (clearance of fluorescent-labeled dextran; molecular weight 10 000 daltons; FITC-dextran-10 K) was determined while suffusing with vehicle or compound 48/80 (10 or 25 μg/ml). During superfusion with vehicle (saline), clearance of FITC-dextran-10 K from pial vessels was modest and remained relatively constant during the experimental period (0.52±0.05 ml/s×10 −6 at 80 min). In addition, diameter of pial arterioles remained constant (32±5 μm) while suffusing with vehicle. In contrast, topical application of compound 48/80 produced marked disruption of the blood–brain barrier to FITC-dextran-10 K. For example, suffusion with compound 48/80 (25 μg/ml) increased clearance of FITC-dextran-10 K about 4-fold to 2.26±0.25 ml/s×10 −6 at 80 min. In addition, superfusion with compound 48/80 (25 μg/ml) constricted pial arterioles by 26±9% at 80 min. To determine a potential role for leukocyte adhesion to endothelium in disruption of the blood–brain barrier during suffusion with compound 48/80, we examined permeability during suffusion with compound 48/80 (25 μg/ml) in the presence of WT.3 (2 mg/kg i.v.), a monoclonal antibody directed against the functional epitope of the leukocyte adhesive glycoprotein (CD18; LFA-1β). We found that infusion of WT.3 markedly attenuated disruption of the blood–brain barrier to FITC-dextran-10 K in response to compound 48/80. The clearance of FITC-dextran-10 K during superfusion with compound 48/80 in the presence of WT.3 was 1.29±0.14 ml/s×10 −6 at 80 min ( P<0.05). Thus, the findings of the present study suggest that application of compound 48/80, to degranulate mast cells, activates the adhesion of leukocytes to cerebral venular endothelium which contributes to disruption of the blood–brain barrier.

Administration of Selective Endothelin Receptor Type a Antagonist Ro 61-1790 Does Not Improve Outcome in Focal Cerebral Ischemia in Cat

The authors examined the effect of selective endothelin (ET) receptor type A (ET(A)) antagonism on histological and functional recovery in cat at 24 hours after reversible middle cerebral artery occlusion (MCAO). A novel and specific ET(A) antagonist, Ro 61-1790 [5-methylpyridine-2-sulfonic acid-6-(2-hydroxyethoxy)-5-(2-methoxyphenoxy)-2-(2-1H-tetrazol-5-y l-pyridin-4-yl)-pyrimidin-4-ylamide sodium salt (1:2)] (Roche, Basel, Switzerland), was used at doses that produced steady-state plasma concentrations and abolished ET-induced pial arteriolar vasoconstriction. In a cranial window preparation, 8 nmol/L ET constricted pial arterioles by 33 +/- 18% (mean +/- SD), but this response was ablated by intravenous Ro 61-1790 treatment (10-mg/kg bolus, 4-mg/kg/h infusion). In additional animal cohorts, halothane-anesthetized cats were treated with 90 minutes of MCAO and 24 hours of reperfusion. Animals received Ro 61-1790 infusion beginning at the onset of reperfusion and continuing for 6 or 24 hours (n = 41). Control cats were treated with 0.9% saline by intravenous infusion throughout reperfusion. There was no difference in injury volume or neurologic evaluation score in saline-treated cats (n = 11; caudate 24 +/- 28%, cortical injury 7.5 +/- 5% of ipsilateral structure; score 52 +/- 8) versus the results in cats treated with Ro 61-1790 for either 24 hours (n = 6; caudate 22 +/- 23%, cortex 6 +/- 5%, injury volume of ipsilateral structure; score 55 +/- 3) or 6 hours (n = 11; caudate 33 +/- 30%, cortex 12 +/- 14%, injury volume of ipsilateral structure; score 50 +/- 10). Mortality was greatest in the 24-hour drug treatment group. These data suggest that blockade of ET(A) receptor activity is not beneficial to tissue or functional outcomes from experimental stroke in cat.

Enhanced pial arteriolar sensitivity to bioactive agents following exposure to endothelin-1

Effects of prior exposure of pial arterioles to endothelin-1 (ET-1) (10 −9) M) on the constriction induced by the by-products of hemolyzed blood (5-HT, LTC 4, LPA, and thromboxane analog U-46619) were examined. Piglets (age: 1–3 d) anesthetized with a mixture of ketamine hydrochloride and acepromazine were implanted with cranial windows, and anesthesia was maintained with α-chloralose. Topical applications of the by-products of hemolyzed blood mildly constricted pial arterioles. Following prior exposure of the microvessels to ET-1, application of the by-products of hemolyzed blood produced significantly potentiated and long-lasting constrictions compared to the controls. In another experiment, pretreatment of pial arterioles with U-46619 (10 −8 M) also potentiated the constriction induced by ET-1. The constriction produced was fast and longer-lasting. Thus, these data show that by-products of hemolyzed blood, though not potent vasoconstrictors per se, potently constricted pial arterioles in the presence of ET-1. The same agents in the CSF can also potentiate constriction induced by ET-1. Hence, by-products of hemolyzed blood may play a significant role in the initiation and maintenance of cerebral arterial narrowing observed following intracranial bleeding.

Preserved hypocapnic pial arteriolar constriction during hyperammonemia by glutamine synthetase inhibition.

Ammonia intoxication, which results in astrocytic edema and glutamine accumulation, blocks cerebral vasodilation during hypercapnia but not during hypoxia. Ammonia's effect on blood flow during hypocapnia is unclear, with some brain regions showing a paradoxical increase in flow. Here, we studied the responses to hypocapnia of pial arterioles not surrounded by astrocytic end feet to avoid mechanical compression by local edema. Blood flow was measured by microspheres in pentobarbital sodium-anesthetized rats equipped with closed cranial windows that permitted intravital microscopy. The normal pial arterial constriction in hypocapnia (12 +/- 1%; mean +/- SE) was blocked (2 +/- 1%) during a 6-h intravenous infusion of ammonium acetate, with some regions (cerebrum, midbrain) showing increased flow during hypocapnia. After pretreatment with methionine sulfoximine (MSO), which inhibits glutamine synthesis, the normal hypocapnic constrictor response was retained in pial arterioles (11 +/- 2%) during hyperammonemia. The increase in the calculated cerebrovascular resistance also was retained. An analog of MSO that does not block glutamine synthesis (buthionine sulfoximine) was ineffective in maintaining hypocapnic reactivity. In a sodium acetate-treated control group, MSO did not alter the pial arteriolar response. Normal vasoconstrictive ability was shown during ammonium infusion in response to U-46619, a thromboxane analog. We conclude that the inhibition of hypocapnic responsivity induced by ammonium is not due to paralysis of the pial arteriolar smooth muscle or to vascular compression by swollen astrocytes but is in some way due to glutamine metabolically produced from the ammonium.

Endothelin-mediated vascular tone following focal cerebral ischaemia in the cat.

The actions of Bosentan and PD155080, nonpeptide endothelin receptor antagonists, were examined in feline pial arterioles in situ following middle cerebral artery (MCA) occlusion to gain insight into the cerebrovascular influence of endogenous endothelins in focal cerebral ischaemia. Immediately following permanent MCA occlusion, all pial arterioles overlying the suprsylvian and ectosylvian gyri displayed marked dilatations, which were maintained in a population of vessel but differentiated into sustained constrictions in others. Perivascular subarachnoid microinjections of Bosentan (30 microM), PD155080 (30 microM), and artificial CSF (pH 7.2) were performed between 30 and 210 min following MCA occlusion. The perivascular microapplication of Bosentan (30 microM) and PD155080 (30 microM) around pial vessels overlying the suprasylvian and ectosylvian gyri, which are within the territory of the occluded MCA, elicited in increase in the calibre of postocclusion dilated and constricted pial arterioles. The perivascular microapplication of PD155080 (30 microM) around postocclusion constricted arterioles overlying the ectosylvian and suprasylvian gyri elicited an increase in the calibre of arterioles (69 +/- 49% from preinjection baseline; n = 8). The perivascular microapplication of Bosentan (30 microM) around postocclusion constricted arterioles overlying the ectosylvian and suprasylvian gyri also elicited an increase in the calibre of arterioles (68 +/- 60% from preinjection baseline; n = 13). In contrast, the microapplication of CSF (pH 7.2) elicited small reductions in pial arteriolar calibre of postocclusion constricted arterioles (-8 +/- 13% from preinjection baseline; n = 8). The perivascular microapplication of PD155080 (30 microM) around postocclusion dilated pial arterioles overlying the ectosylvian and suprasylvian gyri elicited an increase in the calibre of arterioles (11 +/- 10% from preinjection baseline; n = 38). The perivascular microapplication of Bosentan (30 microM) around postocclusion dilated arterioles elicited an increase in the calibre of arterioles (16 +/- 15% from preinjection baseline; n = 36). In contrast, the microapplication of CSF (pH 7.2) elicited small reductions in pial arteriolar calibre of postocclusion dilated arterioles (-9 +/- 6% from preinjection baseline; n = 44). Perivascular microapplication of Bosentan or PD155080 had minimal effect on the calibre of pial arterioles on the parasagittal gyrus (anterior cerebral artery territory), although these arterioles had also displayed sustained dilatation following MCA occlusion. These results indicate that contractile factors (whose effects can be reversed with endothelin receptor antagonists) constrict or impair dilatation of cortical resistance arterioles in an acute cerebral ischaemic episode.

Different pial arteriolar responses to acetylcholine in the newborn and juvenile pig.

Using the closed cranial window technique, the present study was designed to test the hypothesis that the pial arteriolar response to acetylcholine is age dependent. In newborn pigs (1-5 days old) pretreated with the phosphodiesterase inhibitor isobutyl methyl xanthine (IBMX), acetylcholine (10(-5) M) produced pial arteriolar constriction with no change in CSF cyclic GMP (cGMP) that was blocked by indomethacin (5 mg/kg i.v.). In contrast, in indomethacin- and IBMX-treated juvenile pigs (3-4 weeks old), acetylcholine (10(-) M) increased the pial arteriolar diameter by 17 +/- 1% and increased CSF cGMP by 2.1 +/- 0.3-fold. Similar vascular and biochemical changes for acetylcholine were observed in juvenile pigs pretreated with only IBMX. In the absence of IBMX, acetylcholine produced modest pial constriction in juvenile pigs. In the IBMX-pretreated juvenile pigs, L-nitroarginine (LNA; 10(-6) M) decreased pial arteriolar diameter by 15 +/- 2% and blocked acetylcholine-induced dilation and associated changes in CSF cGMP. A23187, a calcium ionophore, and sodium nitroprusside (SNP) elicited similar dilation and changes in CSF cGMP in both age groups. LNA blocked A23187 dilation, but SNP dilation was unchanged. L-Arginine (10(-3) M) partially restored acetylcholine- and A23187-induced dilation to indomethacin- and LNA-pretreated juvenile pigs. These data show that acetylcholine produces dilation in the juvenile pig through the production of the putative endothelium-derived relaxing factor (EDRF) nitric oxide but does not do so in the new born period. We speculate that contributions of EDRF to the acetylcholine-induced changes in pial arteriolar diameter develop with age.

Subarachnoid blood causes pial arteriolar constriction in newborn pigs.

The present study was designed to determine in newborn animals the delayed effect of subarachnoid blood on pial arteriolar diameter and eicosanoid concentrations in cortical periarachnoid fluid. Forty-eight to 96 hours after subarachnoid blood installation, closed cranial windows were implanted over the cerebral area exposed to blood in anesthetized, artificially ventilated newborn piglets. All pial arterioles greater than 60 microns in diameter were measured, and cortical periarachnoid fluid was collected for the determination of eicosanoids. Subarachnoid blood resulted in a 20% to 30% decrease in the average diameter of pial arterioles exposed to blood for 48 to 96 hours, a decreased number of large pial arterioles (greater than 200 microns), and an increased number of small arterioles (60 to 100 microns). No changes in dilator prostanoids (prostacyclin [as 6-keto-prostaglandin F1 alpha] and prostaglandin E2) were detected. Concentrations of vasoconstrictor prostanoids in cortical cerebrospinal fluid increased. Thromboxane B2 increased to 430 +/- 70 pg/mL, and prostaglandin F2 alpha increased to 1370 +/- 180 pg/mL compared with 250 +/- 20 and 860 +/- 70 pg/mL, respectively, in the control group. The concentration of peptidoleukotrienes increased to 400 to 600 pg/mL 72 to 96 hours after blood installation, while the level in the control group was less than 80 pg/mL. The altered balance between vasodilator and vasoconstrictor eicosanoids could contribute to cerebral vasoconstriction after subarachnoid blood installation in newborn pigs.