Analysis of the depressant effect of the endothelium on contractions of rabbit isolated basilar artery to 5-hydroxytryptamine

Response of Rabbit Ear and Femoral Arteries to 5-Hydroxytryptamine During Cooling

The effects of cooling on the response of cutaneous and non-cutaneous arteries to 5-hydroxytryptamine (5-HT) were analysed.

Segments 2-mm long from rabbit central ear (cutaneous) and femoral (non-cutaneous) arteries were prepared for isometric tension recording in an organ bath at 37 and 24°C (cooling). 5-HT (10−9-3 times 10−4 M) induced concentration-dependent contraction of the arteries. The sensitivity and maximal contraction of ear arteries and only the maximal contraction of femoral arteries to this amine were reduced at 24°C.

Endothelium removal or pretreatment with the nitric oxide synthase inhibitor NG-nitro-l-arginine methyl ester (l-NAME, 10−5  m) did not affect the response at 37°C but reversed the decreased sensitivity at 24°C in ear arteries, and neither procedure modified the reactivity at 24 or 37°C in femoral arteries to 5-HT. At both temperatures, the response of ear arteries to 5-HT was shifted to the right by phentolamine (10−6M) more than by the 5-HT antagonist, ketanserin (3 times 10−7M), and that of femoral arteries was shifted to the right by ketanserin or the 5-HT1/5-HT2 antagonist methysergide (3 times 10−7 M) more than by phentolamine, in arteries with and without endothelium.

These data concur with the proposition that the contraction to 5-HT is mediated mainly by α-adrenergic receptors in ear arteries and mainly by 5-HT-ergic receptors in femoral arteries, and suggest that cooling reduces the sensitivity of cutaneous, but not of deep arteries to 5-HT, probably by endothelium-nitric oxide-dependent mechanisms.

Enhancement of mesenteric artery contraction to 5-HT depends on Rho kinase and Src kinase pathways in the ob/ob mouse model of type 2 diabetes

BACKGROUND AND PURPOSE

Arteries from hypertensive subjects are reportedly hyperresponsive to 5-hydroxytryptamine (5-HT), but it remains unclear whether this is true in chronic type 2 diabetes. We have assessed responses to 5-HT shown by mesenteric arteries from type 2 diabetic ob/ob mice (27-32 weeks old) and have identified the molecular mechanisms involved.

EXPERIMENTAL APPROACH

Contractions of mesenteric rings to 5-HT were examined in vitro. Activation of mesenteric RhoA, Rho kinase and Src was measured by Western blotting or by modified enzyme-linked immunosorbent assay.

KEY RESULTS

Concentration-dependent contractions to 5-HT were greater in mesenteric rings from the ob/ob than in those from the age-matched control ('Lean') group. In each group, there was no significant change in the 5-HT-induced contractions after inhibition of nitric oxide synthase (with N(G)-nitro-L-arginine), of cyclooxygenase (with indomethacin) or of protein kinase C (with chelerythrine). However inhibition of the MEK/ERK pathway (with PD98059) decreased the response to 5-HT. Although the diabetes-related enhancement of the 5-HT response was preserved with each of these inhibitors, enhancement was abolished by a Rho kinase inhibitor (Y27632) and by Src kinase inhibitors (PP1 analogue or Src kinase inhibitor I). 5-HT-induced activation of RhoA, Rho kinase and Src kinase in mesenteric arteries was greater in the ob/ob than in the Lean group, but the expression of RhoA, Rho kinase isoforms and Src did not differ between these groups.

CONCLUSIONS AND IMPLICATIONS

These results suggest that the enhancement of 5-HT-induced contraction in mesenteric arteries from ob/ob mice may be attributable to increased activation of RhoA/Rho kinase and Src kinase.

Impaired Nitric Oxide Function in the Basilar Artery of the Obese Zucker Rat

The effect of insulin-resistance syndrome on vascular function has been examined in isolated basilar arteries using the obese Zucker rat (OZR) and age-matched lean littermate controls (lean Zucker rat; LZR) at 36 weeks of age. The OZR showed significantly reduced oral glucose tolerance and increased body weight, blood pressure, proteinuria, plasma levels of triglycerides, cholesterol, and insulin compared with the LZR. The contractile response to serotonin was significantly increased in the OZR. Furthermore, contractions to serotonin in LZR but not OZR were enhanced in the presence of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (NAME). Relaxations to acetylcholine (ACh), histamine, and A23187 were significantly reduced in precontracted arteries from the OZR. In the presence of NAME, histamine responses were significantly reduced whereas ACh and A23187 responses were almost abolished. Relaxations to free-radical nitric oxide (NO) and papaverine were not different in arteries from the OZR, even though responses to sodium nitroprusside were reduced in the OZR. Western blot and immunofluorescent quantitative analyses of eNOS content in cerebral microvessel fractions and basilar artery preparations, respectively, were not significantly different between OZR and LZR. The results suggest impairment in endothelial function resulting in reduced NO function in the basilar artery from the OZR.

Critical role of endothelial nitric oxide synthase and cyclooxygenase in response of rabbit basilar artery to serotonin

The modes of action of serotonin (5-HT) on the tone of the rabbit basilar artery were investigated in vitro with the aim of determining the exact role of the endothelium. After sacrificing the animal under pentobarbital anesthesia, 3-mm segments of the artery were removed and mounted in a 5-ml myograph for isometric tension recording. Vessels precontracted by histamine were relaxed by acetylcholine. Mean maximum relaxation at 10(-4) M was reduced from 79% to 22% (P < 0.001) by 10(-5) M N-nitro-L-arginine (L-NA), and from 73% to 63% (NS) by 3.12(-6) M indomethacin. Intact non-precontracted vessels were contracted by 5-HT (10(-9) M to 10(-5) M): 10(-5) M L-NA significantly increased the contractile force (approximately twofold), whereas 3.10(-6) M indomethacin significantly decreased it (to approximately 35%). In histamine-precontracted vessels, 5-HT induced at low concentrations (3.10(-9) M to 3.10(-8) M) a reduction in tone and induced an increase in tone at higher concentrations. At 10(-5) M, L-NA abolished the relaxant phase of the response, whereas 3.10(-6) M indomethacin potentiated it. In uridine triphosphate-precontracted segments, there was not a net reduction in tone under 5-HT at 3.10(-9) to 3.10(-8) M, but further contraction appeared at higher concentrations. The presence of 10(-5) M L-NA significantly increased the contraction to 5-HT, but 3.10(-6) M indomethacin did not significantly reduce it. Endothelial lesion reduced by about 50% the contractile response of L-NA-treated arteries to 5-HT; and conversely, endothelial lesion increased approximately twofold the contraction of indomethacin-treated arteries to 5-HT. We conclude that 5-HT causes the release from the endothelium of two vasoactive factors, one of which is probably the vasodilator nitric oxide, but the size of the relaxation may depend on the prevailing level of nitric oxide synthase activation. The second factor is a cyclooxygenase-dependent contractile agent. However, the contraction to 5-HT was not modified by the presence of the thromboxane synthase inhibitor CGS 13080 (10(-4) M), suggesting that thromboxane A2 is not the main contractile agent released.

Conditions permitting suppression of stretch-induced and vasoconstrictor tone by basal nitric oxide activity in porcine cerebral artery

This study examined the ability of basal nitric oxide activity to suppress intrinsic and vasoconstrictor tone in isolated rings of porcine cerebral artery. Following stretch of approximately 1 g, N(G)-nitro-L-arginine methyl ester (L-NAME, 100 microM) produced a rise in tone in endothelium-containing but not endothelium-denuded rings. Thus, intrinsic tone was present and was powerfully suppressed by basal nitric oxide activity. Nevertheless, when concentration-response curves were constructed to U46619 and 5-hydroxytryptamine (5-HT), no endothelium-dependent depression of vasoconstriction was observed. It therefore appeared that basal nitric oxide activity was able to suppress intrinsic but not vasoconstrictor tone in these vessels. Stretch-tension curves generated following the application of stretch over the range 0 - 5. 5 g on endothelium-denuded rings showed that tension was stretch-induced. Experiments conducted in the presence of L-NAME (100 microM) revealed that the level of tone present in endothelium-containing rings was substantially higher than in endothelium-denuded rings across the entire range of stretch. When endothelium-containing and endothelium-denuded rings were set at similar levels of stretch-induced tone, rather than similar levels of stretch, the presence of the endothelium now depressed significantly vasoconstrictor responses to U46619 and 5-HT. Thus, when endothelium-containing and endothelium-denuded rings of porcine cerebral artery are set at similar points along their respective stretch-tension curves, rather than at similar levels of stretch, basal nitric oxide activity can be seen to inhibit both stretch-induced and vasoconstrictor tone.

Effect of long-term ouabain treatment on contractile responses of rat aortae

Male Sprague-Dawley rats were infused with 50 microg/kg/day of ouabain for 4 weeks to address the question whether prolonged exposure to the drug affects blood pressure, the in vitro contractile responses to agonists and high K+ of their aortae, and the influence of endothelium on these responses. Systolic blood pressure was not affected by ouabain treatment. The responsiveness of endothelium-intact aortae from ouabain-treated rats to endothelin-1 increased, that to phenylephrine decreased, and that to high K+ was unchanged, as compared with control. The responses of endothelium-free aortae to endothelin-1, phenylephrine, and high K+ were lower in ouabain-treated than in control rats. The removal of endothelium increased the response to phenylephrine and decreased that to high K+ in either control or ouabain-treated rat aortae, whereas it did not affect the response to endothelin-1 in control rat aortae and decreased it in ouabain-treated rat aortae. The response to caffeine was unaffected by either ouabain treatment or endothelium removal. Thus rat ouabain long-term treatment induces opposing effects on the responsiveness of their intact aortae to an alpha-adrenergic agonist and endothelin-1. If these effects observed in the ex vivo experiments occur also in vivo on rat microvasculature, they could balance out and contribute to the lack of effect on systolic blood pressure of prolonged ouabain treatment.

Endothelin and subarachnoid hemorrhage: an overview

INTRODUCTION

Delayed cerebral vasospasm occurring after subarachnoid hemorrhage (SAH) is still responsible for a considerable percentage of the morbidity and mortality in patients with aneurysms. It has been suggested that the pathogenesis of delayed cerebral vasospasm is related to a number of pathological processes, including endothelial damage and smooth muscle cell contraction resulting from spasmogenic substances generated during lysis of subarachnoid blood clots, changes in vascular responsiveness, and inflammatory or immunological reactions of the vascular wall. It has been recognized that the endothelium plays an important role in the regulation of the cerebral vascular tone. In 1988, endothelin (ET)-1, a potent vasoconstrictor, was isolated from cultured porcine aortic endothelial cells.

RESULTS

ET-1, which is one of three distinct isoforms of ETs (ET-1, ET-2, and ET-3), has a more marked effect on cerebral arteries than do the other two isoforms. Elevated levels of ETs have been demonstrated in the cerebrospinal fluid and plasma of patients after SAH and cerebral infarction. ETs act by at least three different receptor subtypes, the ET(A) receptor, which is localized in vascular smooth muscle cells and mediates vasoconstriction, and two different ET(B) receptor subtypes. The ET(B1) receptor subtype is present in vascular endothelial cells and mediates the endothelium-dependent vasodilation. The ET(B2) receptor subtype is present in smooth muscle cells causing vasoconstriction. ET-1 acts from the adventitial but not from the luminal side of cerebral arteries. In vivo and in vitro ET-1 causes a dose-dependent and long-lasting vasoconstriction, similar to cerebral vasospasm after SAH. The vasoconstriction caused by ET-1 can be reversed by selective ET(A) receptor antagonists or combined ET(A) and ET(B) receptor antagonists.

CONCLUSION

The results of current clinical and experimental investigations support the hypothesis that ET-1 is a major cause of cerebral vasospasm after SAH. Other studies indicate that SAH causes complex changes in the ET system and increased ET-1 levels after SAH, which are not solely responsible for the development of vasospasm but may occur after cerebral ischemia. Further investigations are therefore needed to clarify these different hypotheses.

Plasma 5-hydroxytryptamine constricts equine digital blood vessels in vitro: implications for pathogenesis of acute laminitis

Cumulative concentration response curves to 5-hydroxytryptamine (5-HT; 10(-10)-10(-4) mol/l) were constructed using isolated rings of equine digital, facial, tail and coronary arteries (endothelium intact). 5-HT was 17.7 and 41 times more potent as a vasoconstrictor of digital arteries than facial and tail arteries respectively. Removal of the endothelium increased the vasoconstrictor potency of 5-HT in the facial artery by 3.7-fold (P<0.05) but did not alter the sensitivity of digital arteries to 5-HT. Coronary arteries failed to contract to 5-HT. Coronary arteries pre-contracted with U44069 showed concentration dependent relaxation to 5-HT, a response which was partially dependent on the presence of the endothelium. No vasorelaxant effects were found in the digital or facial arteries. The concentration of 5-HT in platelet poor and platelet rich equine plasma was found to be 6.70+/-1.1 x 10(-8) mol/l and 1.77+/-0.36 x 10(-6) mol/l (mean +/-s.e.) respectively by high performance liquid chromatography (HPLC). Plasma which contained no detectable platelets had a 5-HT concentration of 1.12+/-0.48 x 10(-8) mol/l. Isolated digital arteries constricted when exposed to dilutions of platelet poor and platelet depleted equine plasma. These plasma induced contractions were almost completely inhibited by 5-HT receptor antagonists, ketanserin and methiothepin. The change in isometric tension in rings of equine digital artery in vitro was therefore used as a bioassay for plasma 5-HT and the results obtained by this method showed an excellent correlation (r2 = 97.2%, P<0.001) with the concentration estimated by HPLC. Circulating free concentrations of 5-HT in normal horses may be sufficient to constrict digital blood vessels partially in vivo but are well below the threshold for contraction of other peripheral blood vessels examined.

Constrictor responses of the rat basilar artery during diabetes mellitus

Diabetes mellitus produces abnormalities of the endothelium and impairs endothelium-dependent dilatation of large and small cerebral blood vessels. However, the effect of diabetes mellitus on cerebral vasoconstriction and the modulatory influence of nitric oxide on cerebral vasoconstriction is unclear. Thus, the first goal of this study was to examine the effect of diabetes mellitus on constrictor responses of the basilar artery in vivo. Our second goal was to examine a potential role for nitric oxide in modulating constrictor responses of the basilar artery. A craniotomy was performed over the ventral medulla to expose the basilar artery. The diameter of the basilar artery was measured using intravital microscopy in nondiabetic and diabetic (3-4 months after injection of streptozotocin; 50-60 mg/kg i.p.) rats in response to angiotensin II, arginine vasopressin, endothelin-1, and the thromboxane analogue, U-46619. Topical application of angiotensin II (10 and 100 nM) produced only minimal changes in diameter of the basilar artery which were similar in nondiabetic and diabetic rats (p>0.05). Arginine vasopressin (0.1 and 1.0 nM), endothelin-1 (10 and 50 nM), and U-46619 (10 and 100 nM) produced marked dose-related constriction of the basilar artery which also was similar in nondiabetic and diabetic rats (p>0.05). Next, we examine whether the synthesis/release of nitric oxide played a role in constriction of the basilar artery in response to the agonists. We found that L-NMMA (1.0 microM) did not alter constrictor responses of the basilar artery in nondiabetic and diabetic rats. Thus, responses of the basilar artery to important vasoactive agonists are not altered by diabetes mellitus. In addition, it does not appear that the synthesis/release of nitric oxide modulates constrictor responses of the basilar artery to angiotensin II, arginine vasopressin, endothelin-1 and U-46619. We suggest that preservation of vasoconstrictor responses, coupled with impaired vasodilator responses, may contribute to the pathogenesis of cerebrovascular abnormalities associated with diabetes mellitus.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Differential modulation by the endothelium of contractile responses to 5-hydroxytryptamine, noradrenaline, and histamine in the rabbit isolated basilar artery

1. To assess modification by endothelium, we determined the contractile responses of intact rings from rabbit basilar artery to histamine, noradrenaline and 5-hydroxytryptamine (5-HT) and compared them with Triton X-100-treated and NG-nitro-L-arginine (L-NNA)-treated preparations. 2. Among the agonists examined histamine caused the strongest contraction of vascular smooth muscles and noradrenaline the weakest one. The present results are consistent with the basal release of endothelial-derived relaxing factor (EDRF). 3. 5-HT caused a much greater modification than the one expected from basal release of EDRF, suggesting that the release is induced by 5-HT, in contrast to the previous results. Our findings also indicate that 5-HT could induce the release of both contracting factor (EDCF) and EDRF from the endothelium.

Reactive oxygen species-induced impairment of endothelium-dependent relaxation in rat aortic rings: protection by L-arginine

The protective effect of L-arginine against reactive oxygen species-induced impairment of endothelium-dependent vasorelaxation was investigated in isolated ring preparations of rat aorta. The aortic rings were subjected to reactive oxygen species generated by the electrolysis of the bathing solution or incubation with H2O2. Endothelium-dependent relaxation in response to acetylcholine of precontracted aortic rings was attenuated when the rings were exposed to reactive oxygen species or H2O2. Incubation prior to electrolysis with either L-arginine, the endogenous precursor of nitric oxide (NO), or sodium nitroprusside, an exogenous donor of NO, protected the aortic rings against the impairment of endothelium-dependent relaxation. However, D-arginine and glycine, amino acids which do not produce NO, also afforded protection in this model. Therefore, not only the increased synthesis of NO but also the oxidation of L-arginine, with concomitant disproportionation of reactive oxygen species, may be responsible for the protective effect against reactive oxygen species-induced loss of the endothelial response to acetylcholine in isolated rat aorta.

The influence of hyperthyroidism on pharmacologically induced contractions of isolated resistance arteries

We investigated the effect of hyperthyroidism on the responses of small mesenteric resistance arteries to several contractile and dilator agents. Hyperthyroidism was established by feeding rats for 28 days with 5 mg/kg L-thyroxine-containing rat chow. This treatment produced a stable hyperthyroid state, as indicated by the increased serum T4 levels (236 +/- 7 vs. 60 +/- 2; T4-treated vs. control). Preparations of small mesenteric arteries were mounted in an isometric wire myograph. Subsequently, concentration-effect curves were determined for K+, Ca2+, methoxamine, phenylephrine, 5-hydroxytryptamine (5-HT), 9,11-dideoxy-11 alpha, 9 alpha-epoxymethano-prostaglandin F2 alpha (U46619), methacholine and nitroprusside. Our results indicate that hyperthyroidism does not induce major changes in the sensitivity of isolated resistance vessels to K+, Ca+, the alpha-adrenoceptor agonist, methacholine and nitroprusside. Furthermore, neither the affinity of alpha-receptors for methoxamine, nor the alpha-receptor reserve was influenced by the hyperthyroid state of the animal. A clearly sensitizing influence of hyperthyroidism was found for the vasoconstrictor effects of both 5-HT (6.57 +/- 0.04 vs. 6.29 +/- 0.06; hyperthyroid vs. control) and the thromboxane A2 receptor agonist U46619 (6.78 +/- 0.13 vs. 6.30 +/- 0.09; hyperthyroid vs. control). Sensitization to both 5-HT and U46619 was abolished in the presence of N omega-nitro-L-arginine methylester HCl (L-NAME, 0.1 mM). 5-HT-induced contractions in vessels from hyperthyroid rats were diminished by prior incubation with indomethacin (10 microM). The present results indicate that during hyperthyroidism resistance vessels are sensitized to both 5-HT and U46619. This sensitization involves the nitric oxide/L-arginine pathway and probably also certain steps in the cyclooxygenase pathway.

Effect of NG-nitro-L-arginine methylester (L-NAME) on functional and biochemical alpha 1-adrenoceptor-mediated responses in rat blood vessels

1. The modulation by NG-nitro-L-arginine methylester (L-NAME) of alpha 1-adrenoceptor-mediated contraction was investigated on isolated segments of rat tail artery and aorta. The influence of L-NAME on inositol phosphates accumulation by alpha 1-adrenoceptor agonists was also investigated to elucidate the intracellular mechanism responsible for this modulation. 2. In aorta but not in tail artery L-NAME (30 microM) enhanced the sensitivity (3.3 times) and the maximum contraction (Emax) induced by the full agonist, phenylephrine. 3. St-587, a partial alpha 1-adrenoceptor agonist, behaved as a weak agonist in the aorta (22.2% of phenylephrine Emax). However, when the same agonist was studied in tail artery rings a maximum contraction that was 78.4% of the phenylephrine induced Emax was reached. 4. L-NAME increased (3.3 times) the Emax for St-587 contraction in the aorta but not in the tail artery. Sensitivity to St-587 was slightly but significantly (P < 0.001) enhanced (1.9 times) by L-NAME in tail artery segments. 5. Contractile responses to phenylephrine after partial alkylation with phenoxybenzamine were analyzed by the nested hyperbolic null method. To elicit 50% of Emax for contraction only 1.1% of the receptors in the tail artery and 21% of the receptors in the aorta need to be occupied. These results indicate a higher receptor reserve for the tail artery than the aorta. 6. In the tail artery but not in the aorta, St-587 activates phosphoinositide turnover. The presence of L-NAME was without effect on inositol phosphates accumulation induced by this partial alpha 1-adrenoceptor agonist. 7. The maximum contraction induced by phenylephrine, after partial alpha-adrenoceptor alkylation, was enhanced by L-NAME in tail artery rings. However, the NO synthase inhibitor was unable to modify the phenylephrine-induced accumulation of inositol phosphates in the presence of phenoxybenzamine. 8. These results indicate that the differences in St-587-induced contraction and the modulation by L-NAME of alpha 1-adrenoceptor-mediated contraction observed between the tail artery and aorta are associated with differences in receptor reserve. In addition, our biochemical studies indicate that the potentiating effect of L-NAME is independent of intracellular calcium release via phosphatidylinositol turnover.

The vasomotor effects of 5-hydroxytryptamine on equine basilar arteries in vitro

The vasomotor effects of 5-hydroxytryptamine (5-HT) on isolated equine basilar arteries were studied. 5-HT induced contractions of equine basilar arteries in a concentration-dependent manner, with a pEC50 value (with 95% confidence limits) of 7.35 (7.08-7.62). Similar results were obtained with endothelium-denuded basilar arteries. Contractions were not competitively inhibited by the 5-HT2 receptor antagonist ketanserin at low concentrations of 5-HT. Conversely, at high concentrations of 5-HT, contractions were inhibited by ketanserin in a concentration-dependent manner, with a pA2 value of 8.91 (8.62-9.20). The 5-HT1 and 5-HT2 receptor antagonist methiothepin shifted the concentration-response curve of 5-HT downwards and to the right in a concentration-dependent manner. In the presence of 10(-6) mol/L ketanserin, however, methiothepin antagonized 5-HT-induced contractions competitively with a pA2 value of 7.95 (7.59-8.31). The 5-HT3 receptor antagonist MDL 72222 had no effect on 5-HT-induced contractions. The findings of this study indicate that 5-HT1 and 5-HT2 receptors are located in equine basilar arterial smooth muscle cells, and that stimulation of these receptors results in contraction.

The role of endothelin and nitric oxide in modulation of normal and spastic cerebral vascular tone in the dog

To investigate the roles of endothelin and nitric oxide (NO) in the regulation of cerebral vascular tone under basal conditions and in cerebral vasospasm following subarachnoid hemorrhage in dogs, we used BQ-123 (cyclo(-D-Trp-D-Asp-L-Pro-D-Val-L-Leu-) sodium salt), an endothelin ETA receptor antagonist, L-arginine, a substrate for the formation of NO, and NG-nitro-L-arginine methyl ester, an NO synthesis inhibitor, and measured the angiographic diameter of the basilar artery in vivo. In normal dogs, intracisternal (i.c.) injection of BQ-123 (0.6 mg/kg) produced a 29.4 +/- 6.11% (P < 0.01) increase in the basal diameter 24 h after injection. NG-nitro-L-arginine methyl ester (0.6 mg/kg i.c.) produced a 19.3 +/- 2.93% (P < 0.05) decrease in the basal diameter 2 h after injection. This decrease was significantly attenuated by both BQ-123 (0.06-0.6 mg/kg i.c.) and L-arginine (6 mg/kg i.c.), but not by D-arginine. In the two-hemorrhage canine model, BQ-123 significantly inhibited the development of cerebral vasospasm (36.9 +/- 4.11% decrease on day 5 and 42.0 +/- 4.54% decrease on day 6 in controls vs 21.7 +/- 4.75% decrease (P < 0.05) on day 5 and 20.8 +/- 4.14% decrease (P < 0.05) on day 6 for 0.6 mg/kg i.c.) significantly attenuated the cerebral vasospasm on day 4 from a mg/kg i.c.). Furthermore, in this model, L-arginine (6 30.9 +/- 5.78% decrease (before)) to a 12.6 +/- 5.99% decrease (after). The immunoreactive endothelin-1 levels in the endothelial layer and the adventitia of the basilar artery were much higher on days 3 and 7 after the injection of autologous blood than on day 0 before blood injection. These results suggest that endogenous endothelin and NO both participate in regulating the basal tone of cerebral arteries, and, therefore, the development of cerebral vasospasm following subarachnoid hemorrhage may be at least partially attributed to an impairment of the balanced action of endothelin and NO. Furthermore, endothelin ETA antagonists or NO products may be useful in the treatment of cerebral vasospasm following subarachnoid hemorrhage.

Endothelium-derived relaxing factor released by 5-HT: distinct from nitric oxide in basilar arteries of normotensive and hypertensive rats

1. The role of the endothelium in cerebrovascular responses to 5-hydroxytryptamine (5-HT) was investigated in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) in vitro. 2. Cumulative addition of 5-HT caused concentration-dependent contractions in ring preparations of SHR basilar arteries; the contractile response was smaller in WKY basilar arteries. 3. Removal of the endothelium enhanced markedly the contractile responses to 5-HT in WKY arteries but had only a slight effect in SHR arteries. The responsiveness to 5-HT in WKY arteries after removal of endothelium was comparable to that in SHR arteries. 4. The endothelium-dependent relaxation induced by acetylcholine in WKY basilar arteries was almost abolished by treatment with 10 microM methylene blue or 10 microM NG-nitro-L-arginine (L-NOARG). However, the response to 5-HT was not affected by treatment with methylene blue, L-NOARG or indomethacin. 5. Application of 10-20 mM K+ or 3.2 mM tetraethylammonium (TEA) did not change significantly, or only increased slightly, the resting tension, but markedly enhanced the contractile response to 5-HT in WKY arteries with endothelium. In contrast, the submaximal response to 5-HT in SHR arteries with endothelium was significantly enhanced by 0.3 mM TEA. 6. In the presence of 1 mM TEA, the application of 10 microM L-NOARG further enhanced the responses of 5-HT in WKY arteries with endothelium. In SHR arteries with endothelium, 10 microM L-NOARG per se enhanced slightly but significantly the responses to 5-HT. 7. These results indicate that 5-HT-induced contraction in basilar arteries is substantially attenuated by an endothelium-dependent mechanism in WKY, but to a much lesser extent in SHR. The major relaxing factor released by 5-HT from endothelium in WKY is distinct from NO and may exert its effect by activating K+ channels.

Mediation by 5-HT1D receptors of 5-hydroxytryptamine-induced contractions of rabbit middle and posterior cerebral arteries

1. 5-Hydroxytryptamine (5-HT) receptor-mediated contraction of endothelium denuded rabbit middle (MCA) and posterior (PCA) cerebral arteries was characterized by use of selective agonists and antagonists for different 5-HT receptor subtypes. 2. 5-HT and various 5-HT receptor agonists contracted the arteries with the following rank order of potency in MCA: 5-carboxamidotryptamine (5-CT) > 5-HT > 5-methoxytryptamine (5-MeOT) > sumatriptan > alpha-methyl-5-HT (alpha-Me-5-HT) >> 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and in PCA: 5-CT > 5-HT > sumatriptan > 5-MeOT > alpha-Me-5-HT >> 8-OH-DPAT. With few exceptions, the maximal contractile responses of these agonists were similar to that induced by 5-HT. 3. The selective antagonists of 5-HT2A/2C (ketanserin), 5-HT4 (SDZ 205-557) and 5-HT1A/1B (S-(-)-propranolol) sites were devoid of inhibitory effect on 5-HT-mediated contraction in both MCA and PCA, thus excluding activation of the corresponding receptors. 4. In both arteries, the contraction-response curve to 5-HT was unaffected by the 5-HT3 receptor antagonist, ICS 205-930 (0.01 and 0.1 microM) whilst a small (3 and 6 fold displacement) was seen with MDL 72222 (0.1 and 1 microM). 5. The mixed 5-HT1-like/5-HT2A receptor antagonist, methiothepin (0.001-0.1 microM), was a potent antagonist of 5-HT-induced contractions in both arteries, giving pA2 values of 9.4 +/- 0.7 and 9.6 +/- 0.8 in MCA and PCA, respectively. 6. Rauwolscine (O.1-10 MicroM) and yohimbine (0.3, 3 MicroM) inhibited contractions to 5-HT in a competitive manner, pA2 values of 7.1 +/- 0.6 and 6.7 +/-0.6 were determined for rauwolscine in MCA and PCA,respectively. An apparent pA2 value of 6.9 +/-0.2 was calculated for yohimbine (3 MicroM) in both MCA and PCA.7. In conclusion, these results suggest that the contractile response to 5-HT in rabbit isolated MCA and PCA is predominantly mediated by the 5-HTID receptor subtype, although a small contribution by 5-HT3 receptors cannot be excluded.

Enhancement of noradrenergic constriction of large coronary arteries by inhibition of nitric oxide synthesis in anaesthetized dogs

1. Coronary vascular responses to bilateral carotid occlusion (BCO) and the intravenous infusion of tyramine (Tyr, 20 micrograms kg-1 min-1) and noradrenaline (NA, 0.5 microgram kg-1 min-1) were examined after bilateral vagotomy and antagonism of beta-adrenoceptors. BCO, Tyr and NA decreased large coronary artery diameter and increased mean coronary resistance and systemic arterial pressure without affecting heart rate. 2. Inhibition of nitric oxide (NO) synthase with NG-nitro-L-arginine (L-NNA, 5 and 15 mg kg-1) significantly increased mean arterial pressure and decreased heart rate and large coronary artery diameter. Mean coronary resistance was unaffected by either dose of L-NNA. L-NNA significantly reduced depressor and coronary vasodilator responses to the endothelium-dependent vasodilator acetylcholine (ACh, 10 micrograms kg-1, i.v.). Systemic and coronary vasodilator responses to sodium nitroprusside (SNP, 5 micrograms kg-1) were unaffected by L-NNA with the exception that the dilatation of the large coronary artery was significantly enhanced by the higher dose. 3. L-NNA significantly enhanced constriction of the large coronary arteries caused by BCO, Tyr and NA but did not affect the increases in mean coronary resistance or systemic arterial pressure. 4. Inhibition of NO synthesis enhances adrenergic constriction of large coronary arteries caused by both neuronally released and exogenous noradrenaline. In contrast, L-NNA did not affect adrenergic constriction of coronary or systemic resistance vessels. Endothelium-derived NO may play an important role in the modulation of noradrenergic vasoconstriction in coronary conductance arteries.

Nitric oxide and the cerebral circulation

BACKGROUND

Nitric oxide (NO) is a potent vasodilator that was initially described as the mediator of endothelium-dependent relaxation (endothelium-derived relaxing factor, EDRF). It is now known that NO is produced by a variety of other cell types.

SUMMARY OF REVIEW

Endothelium produces NO (EDRF) under basal conditions and in response to a variety of vasoactive stimuli in large cerebral arteries and the cerebral microcirculation. Endothelium-dependent relaxation is impaired in the presence of several pathophysiological conditions. This impairment may contribute to cerebral ischemia or stroke. Activation of glutamate receptors appears to be a major stimulus for production of NO by neurons. Neuronally derived NO may mediate local increases in cerebral blood flow during increases in cerebral metabolism. NO synthase-containing neurons also innervate large cerebral arteries and cerebral arterioles on the brain surface. Activation of parasympathetic fibers that innervate cerebral vessels produces NO-dependent increases in cerebral blood flow. Increases in cerebral blood flow during hypercapnia also appear to be dependent on production of NO. Astrocytes may release some NO constitutively, but astrocytes and microglia can release relatively large quantities of NO after induction of NO synthase in response to endotoxin or some cytokines. Expression of inducible NO synthase, perhaps in response to local production of cytokines, may exert cytotoxic effects in brain during or after ischemia.

CONCLUSIONS

Because endothelium, neurons, and glia can all produce NO in response to some stimuli, the influence of NO on the cerebral circulation appears to be very important. Under normal conditions, constitutively produced NO influences basal cerebral vascular tone and mediates vascular responses to a diverse group of stimuli. The inducible form of NO synthase produces much greater amounts of NO that may be an important mediator of cytotoxicity in brain.

Enhancement of depolarization-induced contractions after endothelium denudation is not related to an impaired production of nitric oxide or prostacyclin in the rabbit basilar artery

Enhancement of the extracellular potassium ion (K+) concentration combined with endothelial injury have been suggested to occur during cerebral ischaemia-reperfusion and vasospasm after subarachnoid hemorrhage. The effect of potassium (K+) depolarization was therefore investigated in isolated segments of the rabbit basilar artery with and without an intact endothelial cell layer. Addition of potassium chloride to the organ bath induced a concentration-dependent contraction. Endothelial denudation of the artery resulted in an unstable baseline tension and a leftward shift of the K+ concentration-response curve. The K+ concentration eliciting half maximum contraction decreased from 26 mmol l-1 in the presence to 12 mmol l-1 in the absence of an intact endothelium. Nimodipine (3 x 10(-7) mol l-1) or exposure to a calcium-free medium abolished the spontaneous as well as K(+)-induced contractions. N omega-nitro-L-arginine (10(-4) mol l-1), indomethacin (3 x 10(-6) mol l-1) and glibenclamide (10(-5) mol l-1) did not affect the contractile response to K+ in intact arteries. However, N omega-nitro-L-arginine increased the baseline tension, and this effect could not be reproduced with N omega-nitro-D-arginine. Pinacidil (10(-6) mol l-1) abolished the spontaneous contractile activity in endothelium-denuded arteries and reduce the K+ sensitivity to the same level as in intact arteries. Tetraethylammonium (3 mmol l-1) and ouabain (10(-5) mol l-1) increased the basal tension and shifted the K+ concentration-response curve to the left.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-derived vasoactive factors and regulation of the cerebral circulation

Vasoactive factors produced and released by endothelium exert a powerful influence on vascular tone in the cerebral circulation. Endothelium-derived relaxing factor (EDRF), which has been identified as nitric oxide (NO) or an NO-containing compound, is produced under basal conditions in cerebral blood vessels. EDRF mediates endothelium-dependent relaxation in response to a number of stimuli in the cerebral circulation. The influence of NO on the cerebral circulation appears to be particularly important and complex because both neurons and glia, in addition to endothelium, produce NO in response to some stimuli. Neuronally derived NO may mediate local vasodilation in response to increased neuronal activity. In addition to EDRF, cerebral endothelium may produce other relaxing factors, including prostacyclin, endothelium-derived hyperpolarizing factor, and oxygen-derived free radicals. Several pathophysiological conditions are associated with impaired endothelium-dependent responses that may involve the decreased production of EDRF and release of endothelium-derived contracting factors, such as the cyclooxygenase products of arachidonic acid and endothelin. The release of endothelin, an extremely potent and long-lasting vasoconstrictor peptide, may contribute to vasospasm after subarachnoid hemorrhage.

Endothelium-dependent contractions to NG-nitro-L-arginine methyl ester in the porcine isolated splenic artery are sensitive to cyclooxygenase and lipoxygenase inhibitors

The nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), produced large endothelium-dependent contractions in isolated segments of the porcine splenic artery, equivalent to approximately 30% of the maximum responses to 5-hydroxytryptamine (5-HT). These responses were inhibited by 1mM L-arginine, but not by either 1mM D-arginine or the superoxide anion scavenger, superoxide dismutase. However, L-NAME-induced contractions were markedly inhibited by the cyclooxygenase inhibitor, flurbiprofen, and the lipoxygenase inhibitor, 2,3,5-tri-methyl-6-(12-hydroxy-5,10-dodecadiynyl)1,4-benzoquinone (AA-861). The combined cyclooxygenase and lipoxygenase inhibitor 3-amino-1-[m-(trifluoromethyl)phenyl]-2-pyrazoline (BW-755C) abolished L-NAME-induced contractions. These findings suggest that suppression of endothelial nitric oxide synthase in the porcine isolated splenic artery results in activation of arachidonic metabolism and production of vasoconstrictor eicosanoids.

Regulation of the cerebral circulation by endothelium

Endothelium exerts an important influence on cerebral vascular tone through the production and release of a diverse group of vasoactive factors. Relaxing factors produced by endothelium include nitric oxide (or a nitric oxide-containing compound), a hyperpolarizing factor, and prostacyclin. Endothelium-derived contracting factors include cyclooxygenase products of arachidonic acid and endothelins. Several pathophysiological conditions are associated with increased formation of endothelium-derived contracting factors. Such endothelial dysfunction in the cerebral circulation may shift the balance of vascular tone toward constriction and may potentially contribute to the onset or maintainance of cerebral ischemia and stroke.