Comparison of the inhibitory effects of antithrombin III, alpha 2-macroglobulin, and thrombin in human basilar arteries: relevance to cerebral vasospasm

Inflammatory and autoimmune diseases of the nervous system; possibilities of laboratory diagnostic methods in cerebrospinal fluid

Contemporary aspects of cerebrospinal fluid analysis are discussed, including the relationship to neuro-infective, autoimmune and other neurological diseases. The actual state of cerebrospinal fluid microbiological and cytological investigation and analysis of cerebrospinal fluid protein fractions are described in detail.

Role of thrombin in CNS damage associated with intracerebral haemorrhage: opportunity for pharmacological intervention?

Intracerebral haemorrhage (ICH) results in high mortality and morbidity. The most important causes of neurological deterioration after ICH are progression of oedema and injury to nerve cells and axons surrounding the haematoma, as well as haematoma enlargement. Recent studies have indicated that thrombin, formed upon clotting of the haematoma, plays an important role in these processes. As opposed to conventional therapeutic approaches, administration of a thrombin inhibitor could effectively limit oedema formation and neuronal damage, improving survival and functional outcome. A small, preliminary clinical trial has suggested that antithrombin therapy with intravenously administered argatroban may be useful in treatment of ICH. Randomised, controlled studies are needed to confirm these initial findings.

Protease-activated receptor (PAR) 1 but not PAR2 or PAR4 mediates endothelium-dependent relaxation to thrombin and trypsin in human pulmonary arteries

Endothelial protease-activated receptors (PARs) may be important sensors of vascular inflammation and injury. Activation of endothelial PAR1 and PAR2 causes nitric oxide-mediated arterial smooth muscle relaxation in a number of species and PAR4 activation causes similar responses in isolated rat aorta. However, it is unclear whether these receptors mediate such responses in human arteries because the most potent activators of PAR1, PAR2, and PAR4, thrombin and trypsin, cause endothelium-dependent relaxation of human coronary arteries through a common PAR1-like receptor. This study aimed to determine whether this unique pharmacology of PARs in human coronary arteries extends to human pulmonary arteries. PAR1 and PAR2 mRNA and protein were detected in human pulmonary arteries via reverse transcription polymerase chain reaction and immunohistochemistry, respectively. PAR4 mRNA was also detected in human pulmonary arteries. Contracted human pulmonary artery ring segments suspended for isometric tension measurement relaxed in a concentration- and endothelium-dependent manner to thrombin (0.001-0.1 U/ml), trypsin (0.01-1 U/ml), and the PAR1-activating peptide, SFLLRN (0.1-10 microM). By contrast, the PAR2- and PAR4-activating peptides, SLIGKV and GYPGQV, respectively, caused neither contraction nor relaxation of precontracted human pulmonary arteries. Relaxations to thrombin and trypsin cross-desensitized, while tachyphylaxis to SFLLRN abolished subsequent relaxations to both thrombin and trypsin. We conclude that human pulmonary arteries express PAR1, PAR2, and PAR4, but that only PAR1, or a PAR1-like receptor, is coupled to endothelium-dependent relaxation.

Thrombin reduces cerebral arterial contractions caused by cerebrospinal fluid from patients with subarachnoid hemorrhage

BACKGROUND AND PURPOSE

We observed that the second application of cerebrospinal fluid (CSF) from subarachnoid hemorrhage (SAH) patients onto cerebral arterial segments in vitro produces a greater contraction than does the initial application. It was hypothesized that the difference between the first and second applications of SAH CSF was due to the activity of thrombin.

METHODS

Canine vertebrobasilar artery was removed under general anesthesia, cut into rings, and suspended in tissue culture baths so as to measure isometric tension. CSF was taken from patients 1 to 3 days after SAH via ventricular drains. CSF was administered in 10(-5) to 10(-1) dilutions. The thrombin antagonist hirudin (5 U) was administered before CSF in some experiments. The arterial tension response to pure oxyhemoglobin (10(-4) to 3.2 g/dL) and thrombin (10(-4) to 3.2 U/mL), administered alone or in combination, was also examined.

RESULTS

Hirudin increased arterial tension generated on the initial application of SAH CSF but had no effect on the tension generated by the second application of the SAH CSF, suggesting that thrombin limits the tension generated by vasoconstrictive agents in the CSF. Thrombin and pure oxyhemoglobin administered together produced less tension than that generated in response to oxyhemoglobin administered alone; no additive response was observed by coadministering the 2 vasoconstrictive agents.

CONCLUSIONS

In the presence of oxyhemoglobin, thrombin acts to reduce cerebral arterial tension. This interaction between thrombin and hemoglobin may account for the observation that the second application of CSF from SAH patients onto cerebral arterial segments in vitro produces a greater contraction than does the initial application.

Evidence for selective effects of chronic hypertension on cerebral artery vasodilatation to protease-activated receptor-2 activation

BACKGROUND AND PURPOSE

Protease-activated receptor-2 (PAR-2) can be activated after proteolysis of the amino terminal of the receptor by trypsin or by synthetic peptides with a sequence corresponding to the endogenous tethered ligand exposed by trypsin (eg, SLIGRL-NH(2)). PAR-2 mediates nitric oxide (NO)-dependent dilatation in cerebral arteries, but it is unknown whether PAR-2 function is altered in cardiovascular diseases. Since hypertension selectively impairs NO-mediated cerebral vasodilatation in response to acetylcholine and bradykinin, we sought to determine whether PAR-2-mediated vasodilatation is similarly adversely affected by this disease state.

METHODS

We studied basilar artery responses in Wistar-Kyoto rats (WKY) (normotensive) and spontaneously hypertensive rats (SHR) in vivo (cranial window preparation) and in vitro (isolated arterial rings). The vasodilator effects of acetylcholine, sodium nitroprusside, and activators of PAR-2 and protease-activated receptor-1 (PAR-1) were compared in WKY versus SHR. Immunohistochemical localization of PAR-2 was also assessed in the basilar artery.

RESULTS

Increases in basilar artery diameter in response to acetylcholine were 65% to 85% smaller in SHR versus WKY, whereas responses to sodium nitroprusside were not different. In contrast to acetylcholine, vasodilatation in vivo to SLIGRL-NH(2) was largely preserved in SHR, and SLIGRL-NH(2) was approximately 3-fold more potent in causing vasorelaxation in SHR versus WKY in vitro. In both strains, responses to SLIGRL-NH(2) were abolished by N(G)-nitro-L-arginine, an inhibitor of NO synthesis. Activators of PAR-1 had little or no effect on the rat basilar artery. PAR-2-like immunoreactivity was observed in both the endothelial and smooth muscle cells of the basilar artery in both strains of rat.

CONCLUSIONS

These data indicate that NO-mediated vasodilatation to PAR-2 activation is selectively preserved or augmented in SHR and may suggest protective roles for PAR-2 in the cerebral circulation during chronic hypertension.

Diffusion MR imaging during acute subarachnoid hemorrhage in rats

BACKGROUND AND PURPOSE

We analyzed the temporal and spatial pattern of water diffusion changes during acute subarachnoid hemorrhage (SAH) in rat brain to identify factors contributing to the acute pathophysiology of SAH.

METHODS

Subarachnoid hemorrhage was remotely induced via perforation of the circle of Willis with an endovascular suture during MR imaging. A fast echo-planar imaging technique was used to acquire 60 maps of the apparent diffusion coefficient (ADC) beginning 1 min before and continuing for 11 min after induction of SAH. A high-resolution spin-echo diffusion sequence was used to follow diffusion changes over 6 h after SAH. Sham-operated control (n=3), nonheparinized (n=6), and heparinized (n=5) groups were studied.

RESULTS

Sham-operated control animals did not show ADC changes over time. In both SAH groups, however, a sharp decline of ADC within 2 min of SAH was consistently observed in the ipsilateral somatosensory cortex. These decreases in diffusion then spread within minutes over the ipsilateral hemisphere. Similar ADC decreases on the contralateral side started with a further time delay of 1 to 3 min. From 30 min onward, the extent of the diffusion abnormality decreased progressively in the nonheparinized animals. No recovery was observed in heparinized rats.

CONCLUSIONS

MR diffusion imaging allows new insight into the pathophysiology of acute SAH: The spatial and temporal pattern of diffusion changes suggests the initial occurrence of acute vasospasm and subsequently "spreading depolarization" of brain tissue. Persistent hemorrhage in heparinized animals was reflected by early decline of ADC values throughout the entire brain.

Atypical protease-activated receptor mediates endothelium-dependent relaxation of human coronary arteries

Protease-activated receptors (PARs) are a family of G protein-coupled receptors activated by a tethered ligand sequence within the amino terminal that are revealed by site-specific proteolysis. The thrombin-sensitive PAR-1 and trypsin-activated PAR-2 mediate endothelium-dependent vascular relaxation in a number of species. Because both thrombin and trypsin-like enzymes have been implicated in coronary artery disease, the purpose of this study was to investigate whether similar receptors are present in human coronary arteries. Thrombin (0.001 to 0.1 U/mL) and trypsin (0.001 to 1 U/mL) caused concentration- and endothelium-dependent relaxations of human coronary artery ring segments suspended in organ chambers for isometric tension recording and contracted with the thromboxane A2 mimetic U46619. These relaxations were dependent on the catalytic activity of each enzyme and were inhibited by the NO synthase inhibitor NG-nitro-L-arginine (100 micromol/L) and the NO scavenger oxyhemoglobin (20 micromol/L). The synthetic PAR-1 tethered ligand sequence SFLLRN-NH2 (0.01 to 10 micromol/L) also caused endothelium-dependent relaxation of U46619-contracted human coronary arteries; however, the equivalent PAR-2 ligand SLIGKV-NH2 caused almost no relaxation. In addition, desensitization to either thrombin or trypsin resulted in cross-desensitization to the other enzyme but had only a minimal affect on the response to SFLLRN-NH2. Therefore, we conclude that human coronary artery endothelial cells possess a PAR-1-like receptor that is potently activated by thrombin, trypsin, and SFLLRN-NH2 to cause NO-mediated vascular relaxation. Once cleaved, this receptor is recycled in a truncated form, able to respond to exogenous application of only its tethered ligand sequence, suggesting the presence of another endogenous activator possibly acting independently of receptor cleavage.

Mechanisms of edema formation after intracerebral hemorrhage: effects of thrombin on cerebral blood flow, blood-brain barrier permeability, and cell survival in a rat model

Recently, the authors showed that thrombin contributes to the formation of brain edema following intracerebral hemorrhage. The current study examines whether the action of thrombin is due to an effect on cerebral blood flow (CBF), vasoreactivity, blood-brain barrier (BBB) function, or cell viability. In vivo solutions of thrombin were infused stereotactically into the right basal ganglia of rats. The animals were sacrificed 24 hours later; CBF and BBB permeability were measured. The actions of thrombin on vasoreactivity were examined in vitro by superfusing thrombin on cortical brain slices while monitoring microvessel diameter with videomicroscopy. In separate experiments C6 glioma cells were exposed to various concentrations of thrombin, and lactate dehydrogenase release, a marker of cell death, was measured. The results indicate that thrombin induces BBB disruption as well as death of parenchymal cells, whereas CBF and vasoreactivity are not altered. The authors conclude that cell toxicity and BBB disruption by thrombin are triggering mechanisms for the edema formation that follows intracerebral hemorrhage.

Antithrombin III prevents renal dysfunction and hypertension induced by enhanced intravascular coagulation in pregnant rats: pharmacological confirmation of the benefits of treatment with antithrombin III in preeclampsia

We tested the hypothesis that enhanced intravascular coagulation in pregnancy could produce clinical symptoms similar to those of preeclampsia, such as hypertension, proteinuria, and edema. Having confirmed this, we then examined whether the pathological changes caused by intravascular coagulation could be suppressed by administration of antithrombin III (AT III), an endogenous inhibitor active to thrombin and factor X a. Intravascular coagulation was induced in Wistar rats on day 16-20 of pregnancy by 1-h arterial infusion of tissue thromboplastin (TP) through the left ventricle of the heart. One hour after the end of the infusion period, organ blood flows were measured by the radioactive ((57)Co-labeled) microsphere method, and fibrin deposition in organs was measured by radiolabeling with [(125)I] fibrinogen injected before TP infusion. Infusion of TP produced fibrin deposition in the kidney, lung, and liver, but not in the myometrium and placenta, as well as an 80% decrease in renal blood flow (RBF), with oliguria and proteinuria. TP also caused an increase in blood pressure (BP) accompanied by an increase in plasma renin activity (PRA), both of which were suppressed by bilateral nephrectomy before TP infusion. The prophylactic administration of AT III concentrates (60 or 300 U/kg intravenously (i.v.), followed by infusion of 30 or 150 U/kg/2 h, respectively) prevented all pathological changes in a dose-dependent manner. AT III increased placental blood flow regardless of the state of coagulation. These findings suggest that intravascular coagulation plays a significant part in the pathophysiology of preeclampsia and that AT III concentrates may have therapeutic potential in the treatment of this condition.

Edema from intracerebral hemorrhage: the role of thrombin

The mechanism by which intracerebral hemorrhage leads to the formation of brain edema is unknown. This study assesses the components of blood to determine if any are toxic to surrounding brain. Various solutions were infused stereotactically into the right basal ganglia of rats. The animals were sacrificed 24 hours later; brain edema and ion contents were measured. Whole blood caused an increase in brain water content and ion changes consistent with brain edema. Concentrated blood cells, serum from clotted blood, and plasma from unclotted blood all failed to provoke edema formation when infused directly into the brain. On the other hand, activation of the coagulation cascade by adding prothrombinase to plasma did produce brain edema. The edema response to whole blood could be prevented by adding a specific thrombin inhibitor, hirudin, to the injected blood. This study indicates that thrombin plays an important role in edema formation from an intracerebral blood clot.

Cerebral vasoconstrictor mediators

Endogenous cerebral vasoconstrictor mediators regulate vascular resistance and blood flow in the brain as a whole and in various regions and participate in the pathogenesis of cerebral circulatory disturbances. Vasoconstrictors are effective in the treatment of diseases associated with cerebral vasodilatation. There are variations in the response of cerebral arteries from primate and subprimate mammals; therefore, information as to similarities and differences in their response is quite important in evaluating the physiological role, involvement in pathogenesis and therapeutic usefulness of the mediators in healthy men and patients. In this review we described characteristics of the action of vasoconstrictors (amines, peptides, prostanoids, and others) on isolated cerebral arteries from mammals, including humans and monkeys.

Does thrombin prevent cerebral vasospasm following aneurysmal subarachnoid haemorrhage?

Fibrinopeptide A (FPA) levels which have been shown to be a quantitative index of thrombin generation, were measured in blood and cerebrospinal fluid (CSF) samples from patients following subarachnoid haemorrhage (SAH) and from a control population. The levels found in samples obtained in patients following SAH are compared with those found in controls and also correlated with clinical grade on admission as assessed by the Glasgow Coma Score and the World Federation of Neurological Surgeons' grading system, and with the amount of subarachnoid blood seen on CT, the occurrence of ischaemic deterioration, the occurrence of low-density change on CT, the presence of vasospasm on angiography, clinical outcome as assessed by the Glasgow Outcome Score 3 months following the ictus, and the incidence of ischaemia as a cause of death or disability as assessed 3 months following the ictus. The levels of FPA found in blood and CSF from patients following SAH were significantly raised when compared with those found in controls. There was significant correlation between blood FPA levels and the amount of subarachnoid blood seen on initial CT. CSF FPA levels had a statistically significant correlation with outcome as assessed at 3 months post-ictus. No statistically significant correlation was found between blood or CSF FPA levels and any of the other variables studied.

Identification of capric acid as a potent vasorelaxant of human basilar arteries

To determine whether naturally occurring fatty acids, especially saturated ones, might act directly as vasodilators, segments of human basilar arteries and umbilical arteries were precontracted submaximally with prostaglandin F2 alpha and then exposed to different saturated fatty acids (C4 through C16) or unsaturated fatty acids (C14:1, C18:1, C18:2, and C18:3) at concentrations from 4 microM to 4 mM. The results showed caprate (C10) to be the most potent vasorelaxant and basilar arteries to be more responsive (EC50 = 63 microM) than umbilical arteries (EC50 = 780 microM). Caprate also inhibited contractions elicited by KCl, serotonin, and the thromboxane analogue U46619. The relaxation was independent of the endothelium, and potency was not related to the weak capacity of caprate to inhibit Ca(2+)-induced contractions of K(+)-depolarized basilar arteries. The pattern of potencies for the arteries differed, but among unsaturated fatty acids the monounsaturated (C14:1, C18:1) were more potent than the polyunsaturated (C18:2, C18:3). Comparing the potencies obtained with the concentrations reported for the free fatty acid content of arteries, brain, and plasma indicates that these lipids could influence vasomotion in health and disease.

Capric acid as a potent dilator of canine vessels in vitro and in vivo

1. Pharmacodynamic effects of even numbered saturated fatty acids, C4-C16, were determined on isolated canine basilar and femoral arteries precontracted with PGF2 alpha. 2. The fatty acids relaxed the precontracted vessels. 3. The basilar artery was the most sensitive vessel and caprate (C10) was the most potent acid with an EC50 of 49 microM. 4. The relaxant effect was endothelium-independent. 5. Contractions elicited by norepinephrine, serotonin, and U46619 were also inhibited. 6. Caprate (C10) given intra-arterially increased femoral blood flow in a dose-dependent manner and the dose computed to increase blood flow 50% was 1.27 microM/kg.

Effect of aging on endothelium-dependent vascular relaxation of isolated human basilar artery to thrombin and bradykinin

Using strips of human basilar arteries mounted in organ chambers to record isometric tension, we investigated vascular reactivity to thrombin and bradykinin. Both agents produced endothelium-dependent relaxation of basilar artery strips precontracted with phenylephrine but had no effect on resting tension in strips with or without endothelium. The relaxations caused by thrombin were abolished by antithrombin III/heparin, hirudin, and MD805. Thrombin but not bradykinin caused complete tachyphylaxis toward a second exposure. Indomethacin did not inhibit the relaxations induced by thrombin or bradykinin, whereas bromophenacyl bromide and methylene blue did. Aging decreased the relaxation induced by thrombin but did not affect the concentration needed to reach 50% maximal relaxation, nor did it affect the maximal relaxation in response to bradykinin, calcium ionophore A23187, and sodium nitroprusside. Our results suggest that thrombin and bradykinin produce endothelium-dependent relaxations mediated by an endothelium-derived relaxing substance and that the relaxation caused by thrombin is mediated by a proteolytic action on endothelial cells. The decrease in relaxations in response to thrombin with increasing age might be due to a decrease in the number or sensitivity of thrombin receptors on endothelial cells.

Effect of intracisternal antithrombin III on subarachnoid hemorrhage-induced arterial narrowing

The ability of antithrombin III, an endogenous plasma glycoprotein, to reverse the arterial narrowing in a rabbit model of cerebral vasospasm was evaluated. The vasodilator activity of antithrombin III on rabbit arteries was first assessed in vitro using a myograph-arterial ring preparation. Antithrombin III (10 IU/ml) induced a 55.4% +/- 2.66% (mean +/- standard error of the mean) relaxation in basilar artery precontracted with serotonin (5-HT) in five specimens as compared with a 9.8% +/- 1.6% relaxation of common carotid artery in six specimens. For in vivo analysis, 21 New Zealand White male rabbits were separated into three groups: Group 1 served as normal controls; Group 2 received a subarachnoid blood injection (SAH) and were sacrificed on Day 3 thereafter; and Group 3 animals were subjected to SAH, then received a 2-hour intracisternal infusion of antithrombin III (100 IU) in saline prior to sacrifice on Day 3. Basilar artery caliber was determined using a morphometric method to analyze perfusion-fixed arterial segments. Control basilar artery diameter in Group 1 was 0.64 +/- 0.02 mm. In Group 2 a 27% reduction in arterial caliber to 0.47 +/- 0.03 mm was observed by Day 3 post SAH (p less than 0.0001). Group 3 animals had a mean basilar artery diameter of 0.68 +/- 0.02 mm. This was significantly larger than the untreated SAH rabbits in Group 2 (p less than 0.0001), but not different from control artery diameters in Group 1. The findings demonstrate that antithrombin III in saline has a significant ability to reverse delayed narrowing of the rabbit basilar artery after SAH.

Pharmacodynamic effects of tosyl-arginine methyl ester (TAME) on isolated human arteries

1. The effects of the competitive proteinase inhibitor TAME on isolated human umbilical and basilar arteries were studied. 2. Most experiments were performed on umbilical arteries and showed that 5 x 10(-4) M or 5 x 10(-3) M TAME significantly inhibited contractions elicited by prostaglandin F2 alpha, bradykinin, serotonin, and histamine. The inhibition was not endothelium-dependent. 3. The contraction elicited by prostaglandin E2 in basilar arteries was inhibited by TAME in a dose-dependent manner indicating that inhibition did not depend on the origin of the vessel. 4. Inhibition by 5 x 10(-7) M TAME of the contraction produced by KCl in the basilar artery was limited to low concentrations of KCl (10 and 30 mM). TAME did not significantly inhibit contractions of umbilical arteries to KCl. 5. At 5 U/ml (approximately 8.6 x 10(-8) M) antithrombin III markedly inhibited contractions of basilar arteries to prostaglandin E2 but had no effect of contractions elicited by 10(-6) M serotonin in the umbilical artery, nor did aprotinin (5 x 10(-4) M). 6. Although vessels of different origin may differ in sensitivity, the manifest effect of antiproteinases on arteries is inhibition.

Responses of human basilar arteries to vasoactive intestinal polypeptide

The responses to 9 X 10(-7) M vasoactive intestinal polypeptide (VIP) by isolated human basilar arteries of 30 individuals were studied to further elucidate the role the peptide might play in modifying cerebrovascular tone normally and in disease. In most experiments the artery was precontracted with prostaglandin F2 alpha (PGF2 alpha), either with 1 or 2 X 10(-6) M or with 10(-5) M PGF2 alpha. The course of action to VIP was observed for 15 min following its application to the contracted vessel. Some arteries failed to respond to VIP (13%), otherwise the arteries relaxed 44% when the contraction was induced by 10(-5) M PGF2 alpha and 67.6% after the lower concentrations of PGF2 alpha. There was no significant decrement in the vasorelaxant effect of VIP throughout the period of observation. A second and third application of VIP to the precontracted artery produced significantly less of an effect than the first, but no consistent progressive pattern of tachyphylaxis was evident. In additional experiments, indomethacin (10(-5) M) did not prevent the vasorelaxant effect of VIP, suggesting that prostanoid synthesis was not involved. Pretreatment of the artery with VIP did not prevent the contractions generated by 10, 30, 50 and 90 mM KCl while antithrombin III (1.2 X 10(-7) M) did, indicating fundamental differences between these two vasorelaxants. In conclusion, VIP will inhibit contraction of isolated human cerebral arteries for prolong periods and could be a significant factor regulating cerebral blood flow in humans.