Endothelin receptors mediating contraction of rat and human pulmonary resistance arteries: effect of chronic hypoxia in the rat

1. We examined the endothelin (ET) receptors mediating contractions to ET-1, ET-3 and sarafotoxin S6c (SX6c) in rat pulmonary resistance arteries by use of peptide and non-peptide ET receptor antagonists. Changes induced by pulmonary hypertension were examined in the chronically hypoxic rat. The effect of the mixed ET_A/ET_B receptor antagonist SB 209670 on endothelin-mediated contraction was also examined in human pulmonary resistance arteries.
2. In rat vessels, the order of potency for the endothelin agonists was SX6c=ET-3>ET-1 (pEC₅₀ values in control rats: 9.12±0.10, 8.76±0.14 and 8.12±0.04, respectively). Maximum contractions induced by ET-3 and ET-1 were increased in vessels from chronically hypoxic rats.
3. The ET_A receptor antagonist FR 139317 (1 μM) had no effect on the potency of ET-1 in any vessel studied but abolished the increased response to ET-1 in the chronically hypoxic vessels. The ET_A receptor antagonist BMS 182874 (1 μM) increased the potency of ET-1 in control rat vessels without effecting potency in the pulmonary hypertensive rat vessels.
4. Bosentan (non-peptide mixed ET_A/ET_B receptor antagonist) increased the potency of ET-1 in control rat vessels but was without effect in the pulmonary hypertensive rat vessels. Bosentan (1 μM) inhibited responses to SX6c in control and chronically hypoxic rat vessels with pK_b values of 5.84 and 6.11, respectively. The ET_B receptor antagonist BQ-788 (1 μM) did not inhibit responses to ET-1 in any vessel tested but did inhibit responses to both SX6c and ET-3 (pK_b values in control and chronically hypoxic rat vessels respectively: SX6c 7.15 and 7.22; ET-3: 6.68 and 6.89). BQ-788 (1 μM) added with BMS 182874 (10 μM) did not inhibit responses to ET-1 in control vessels but caused a significant inhibition of responses to ET-1 in chronically hypoxic preparations.
5. SB 209670 inhibited responses to ET-1 in both control and chronically hypoxic vessels with pK_b values of 7.36 and 7.39, respectively. SB 209670 (0.1 and 1 μM) virtually abolished responses to ET-1 in the human pulmonary resistance artery.
6. In conclusion, in rat pulmonary resistance arteries, vasoconstrictions induced by ET-1, SX6c and ET-3 are mediated predominantly by activation of an ET_B–like receptor. However, lack of effect of some antagonists on ET-1 induced vasoconstriction suggests that ET-1 stimulates an atypical ET_B receptor. The increase in potency of ET-1 in the presence of some antagonists suggests the presence of an inhibitory ET_A-like receptor. The influence of this is reduced, or absent, in the chronically hypoxic rats. Increased responses to ET-1 are observed in the chronically hypoxic rat and may be mediated by increased activation of ET_A receptors. SB 209670 is unique in its potency against responses to ET-1 in both control and chronically hypoxic rats, as well as human, isolated pulmonary resistance arteries.

     

Contractile effects of tacrolimus in human and rat isolated renal arteries

1. We tested the vasoactive properties of the immunosuppressive drug FK 506 (tacrolimus) in preconstricted rat and human isolated renal arteries in vitro. 2. In rat renal arteries, tacrolimus (3, 10 microM) showed a direct and dose-dependent contractile effect by maximally 23 microm (10% of the noradrenaline effect), which was only observed in the presence of intact endothelium. Moreover, a lower concentration of tacrolimus (1 microM) potentiated pressor responses to the sympathetic neurotransmitter noradrenaline but not to ATP in this species. ATP- (0.01-10 microM) induced vasodilation was not affected by tacrolimus (1 microM). 3. In contrast, in human interlobar arteries, tacrolimus failed to induce direct vasoconstriction and did not significantly potentiate constrictor responses to noradrenaline. Acetylcholine-(1 microM) induced vasodilation was much smaller in human than in rat renal arteries suggesting the lack of functional endothelium in the human preparation. 4. The findings suggest that tacrolimus releases an endothelium-derived constricting factor in rat renal arteries to increase vascular tone and to potentiate pressor responses to noradrenaline. In human interlobar arteries, this effect of tacrolimus is not observed probably because of the absence of functional endothelium or the necessary mediator mechanism.     

Role of reactive oxygen species and gp91phox in endothelial dysfunction of pulmonary arteries induced by chronic hypoxia

1. This study investigates the role of nitric oxide (NO) and reactive oxygen species (ROS) on endothelial function of pulmonary arteries in a mice model of hypoxia-induced pulmonary hypertension. 2. In pulmonary arteries from control mice, the NO-synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) potentiated contraction to prostaglandin F2alpha (PGF2alpha) and completely abolished relaxation to acetylcholine. In extrapulmonary but not intrapulmonary arteries, acetylcholine-induced relaxation was slightly inhibited by polyethyleneglycol-superoxide dismutase (PEG-SOD) or catalase. 3. In pulmonary arteries from hypoxic mice, ROS levels (evaluated using dihydroethidium staining) were higher than in controls. In these arteries, relaxation to acetylcholine (but not to sodium nitroprusside) was markedly diminished. L-NAME abolished relaxation to acetylcholine, but failed to potentiate PGF2-induced contraction. PEG-SOD or catalase blunted residual relaxation to acetylcholine in extrapulmonary arteries, but did not modify it in intrapulmonary arteries. Hydrogen peroxide elicited comparable (L-NAME-insensitive) relaxations in extra- and intrapulmonary arteries from hypoxic mice. 4. Exposure of gp91phox(-/-) mice to chronic hypoxia also decreased the relaxant effect of acetylcholine in extrapulmonary arteries. However, in intrapulmonary arteries from hypoxic gp91phox(-/-) mice, the effect of acetylcholine was similar to that obtained in mice not exposed to hypoxia. 5. Chronic hypoxia increases ROS levels and impairs endothelial NO-dependent relaxation in mice pulmonary arteries. Mechanisms underlying hypoxia-induced endothelial dysfunction differ along pulmonary arterial bed. In extrapulmonary arteries from hypoxic mice, endothelium-dependent relaxation appears to be mediated by ROS, in a gp91phox-independent manner. In intrapulmonary arteries, endothelial dysfunction depends on gp91phox, the latter being rather the trigger than the mediator of impaired endothelial NO-dependent relaxation     

Contractile response of human omental arteries to endothelin.

The effects of endothelin have been studied in isolated arterial segments (0.8-1 mm in external diam.) of human omental arteries obtained during the course of abdominal operations (15 patients, 7 men and 8 women). Paired segments, one normal and the other de-endothelized, were mounted for isometric recording of tension in organ baths. Endothelin produced concentration-dependent contractions with an EC50 value of 5.4 x 10(-9) M. Removal of endothelium did not affect significantly endothelin-induced contractions (EC50, 6.7 x 10(-9) M). Removal of extracellular calcium or addition of the calcium channel blocker nicardipine (10(-6) M) diminished but did not abolish responses to endothelin. These results indicate that endothelin exerts powerful contractile effects on human isolated omental arteries which are independent of the presence of an intact endothelial cell layer; this contraction cannot be explained solely by voltage-dependent calcium channels.     

Changes in functional and histological distributions of nitric oxide synthase caused by chronic hypoxia in rat small pulmonary arteries

1. Chronic hypoxia (CH) increases lung tissue expression of all types of nitric oxide synthase (NOS) in the rat. However, it remains unknown whether CH-induced changes in functional and histological NOS distributions are correlated in rat small pulmonary arteries. 2. We measured the effects of NOS inhibitors on the internal diameters (ID) of muscular (MPA) and elastic (EPA) pulmonary arteries (100-700 micro m ID) using an X-ray television system on anaesthetized rats. We also conducted NOS immunohistochemical localization on the same vessels. 3. Nonselective NOS inhibitors induced ID reductions in almost all MPA of CH rats (mean reduction, 36+/-3%), as compared to approximately 60% of control rat MPA (mean, 10+/-2%). The inhibitors reduced the ID of almost all EPA with similar mean values (approximately 26%) in both CH and control rats. On the other hand, inducible NOS (iNOS)-selective inhibitors caused ID reductions in approximately 60% of CH rat MPA (mean, 15+/-3%), but did so in only approximately 20% of control rat MPA (mean, 2+/-2%). This inhibition caused only a small reduction (mean, approximately 4%) in both CH and control rat EPA. A neuronal NOS-selective inhibitor had no effect. 4. The percentage of endothelial NOS (eNOS)-positive vessels was approximately 96% in both MPA and EPA from CH rats, whereas it was 51 and 91% in control MPA and EPA, respectively. The percentage for iNOS was approximately 60% in both MPA and EPA from CH rats, but was only approximately 8% in both arteries from control rats. 5. The data indicate that in CH rats, both functional and histological upregulation of eNOS extensively occurs within MPA. iNOS protein increases sporadically among parallel-arranged branches in both MPA and EPA, but its vasodilatory effect is predominantly observed in MPA. Such NOS upregulation may serve to attenuate hypoxic vasoconstriction, which occurs primarily in MPA and inhibit the progress of pulmonary hypertension.     

Ouabain-induced hypertension alters the participation of endothelial factors in alpha-adrenergic responses differently in rat resistance and conductance mesenteric arteries

1. This study compares the role of endothelial factors in alpha-adrenoceptor contractile responses in mesenteric resistance (MRA) and superior (SMA) mesenteric arteries from ouabain-treated (8.0 microg day(-1), 5 weeks) and untreated rats. The role of the renin-angiotensin system was also evaluated. 2. Ouabain treatment increased systolic blood pressure. In addition, ouabain reduced the phenylephrine response in SMA but did not alter noradrenaline responses in MRA. 3. Endothelium removal or the nitric oxide synthase (NOS) inhibitor (l-NAME, 100 microm) increased the responses to alpha-adrenergic agonists in both vessels. After ouabain treatment, both endothelial modulation and the l-NAME effect were increased in SMA, while only the l-NAME effect was increased in MRA. Endothelial NOS expression remained unaltered after ouabain treatment. 4. Indomethacin (10 microm) similarly reduced the noradrenaline contraction in MRA from both groups; in contrast, in SMA, indomethacin only reduced phenylephrine-induced contractions in segments from untreated rats. Co-incubation of l-NAME and indomethacin leftward shifted the concentration-response curves for noradrenaline more in MRA from ouabain-treated rats; tetraethylammonium (2 mm) shifted the noradrenaline curves further leftward only in MRA from untreated rats. 5.Losartan treatment prevents the development of hypertension but not all vascular changes observed after ouabain treatment. 6. In conclusion, a rise in endothelial NO and impaired prostanoid participation might explain the reduction in phenylephrine-induced contraction in SMA after ouabain treatment. An increase in the modulatory effect of endothelial NO and impairment of endothelium-dependent hyperpolarizing factor effect might explain why the ouabain treatment had no effect on noradrenaline responses in MRA.     

Chronic hypoxia differentially alters the responses of pulmonary arteries and veins to endothelin-1 and other agents.

The effects of chronic hypoxia on the responses of rat large pulmonary arteries and veins to vasoactive agents have been examined. Endothelin-1-induced contractions of pulmonary arteries and pulmonary veins were reduced by chronic hypoxia. In contrast, chronic hypoxia augmented sarafotoxin 6c-induced contractile responses in pulmonary veins but not in pulmonary arteries. Chronic hypoxia augmented the constrictor effect of phenylephrine in pulmonary arteries, but not in pulmonary veins. The thromboxane receptor agonist, U46619 (9,11-dideoxy-9alpha,11alpha-epoxy-methanoprostaglandin++ + f2alpha) contracted pulmonary arteries and pulmonary veins, and although maximal responses were not altered in chronically hypoxic preparations, the EC50 value in pulmonary arteries was increased following chronic hypoxia. The relaxant effects of acetylcholine and isoprenaline on pulmonary arteries were potentiated by chronic hypoxia. In contrast, ionomycin-mediated relaxations of pulmonary arteries and pulmonary veins were reduced, while sodium nitroprusside-induced relaxation of pulmonary arteries and veins were not altered by chronic hypoxia. Previous studied have looked primarily at the effects of chronic hypoxia on pulmonary arteries. This data provides evidence that chronic hypoxia also causes selective changes in the reactivity of large pulmonary veins.     

Low-level lead exposure changes endothelial modulation in rat resistance pulmonary arteries

Lead exposure induces hypertension and endothelial dysfunction. However, the effects on the pulmonary vasculature have not been explored. In this study, rats exposed to lead acetate for seven days (4 μg/100 g on the 1st day and 0.05 μg/100 g/day i.m. subsequently) had lead blood level of 3.9 ± 0.7 μg/dL and increased right ventricular pressures. There was an increased Pb deposition and superoxide anions production in the pulmonary arteries, associated with reduced vasoconstriction but unchanged endothelium-dependent vasodilatation to acetylcholine (ACh). In both groups, inhibition of the nitric oxide (NO) synthase with L-NAME blocked the response to ACh, while indomethacin (cycloxygenase inhibitor) had no effect. Incubation with nonspecific potassium channel blocker (tetraethylammonium) reduced the ACh-induced vasodilatation only in the Pb group. Apamin (SK_Ca channel blocker) and 4-aminopyridine (K_v channel blocker), but not iberiotoxin (BK_Ca channel blocker), also inhibited this response in the Pb group. The vasodilatation to exogenous NO was reduced by Pb, while relaxation to the cGMP analogue was similar between groups. Concordantly, the protein level of soluble guanylate cyclase (sGC) was reduced. In conclusion, short-term and low-level exposure to Pb changes pulmonary haemodynamic and increases oxidative stress. The pulmonary vasculature exhibited increased hyperpolarization by the K_v and SK_Ca channels, probably as a compensatory mechanism to the decreased responsiveness to NO.     

Dexfenfluramine-induced contraction of human and rat isolated pulmonary arteries

Mechanisms of dexfenfluramine-induced vasoconstriction were studied in isolated pulmonary arteries suspended in organ baths for isometric tension recording. Dexfenfluramine (10(-7)-10(-4) M) caused concentration-dependent contractions in rat and human pulmonary arteries with and without endothelium. In pulmonary arteries of the rat, the response to dexfenfluramine was nearly abolished by treatment with the alpha-adrenoceptor antagonists, phentolamine (10(-6) M) or prazosin (10(-7) M). In human pulmonary arteries, the concentration-response curve to dexfenfluramine was unaltered by the presence of phentolamine (10(-6) M), prazosin (10(-7) M), ketanserin (10(-6) M), or indomethacin (3x10(-6) M). The results suggest that dexfenfluramine causes contraction of pulmonary vascular smooth muscle by multiple mechanisms, one of which involves activation of alpha-adrenoceptors within the blood vessel wall. The mechanisms by which dexfenfluramine causes pulmonary vasoconstriction may differ between rat and human pulmonary arteries.     

Contractile responses and calcium mobilization in renal arteries of diabetic rats.

The causes of diabetes-associated change of renal artery vasomotion have not been established. We investigated both contractile responses to KCl and norepinephrine (NE) in renal arteries of rats with streptozotocin-induced diabetes and age-matched controls and the effects on Ca2+ mobilization. Renal arteries from diabetics had greater maximum contractile responses to KCl and NE, but the threshold concentrations and EC50 values of KCl and NE were similar in controls and diabetics. The concentration-response for Bay K 8644, a dihydropyridine Ca2+ channel agonist in the presence of 20 mM KCl was significantly greater in diabetics than in controls. The maximum contractile responses to Ca2+ in the presence of 10(-6) M NE were significantly (P less than 0.05) greater in diabetics than in controls. The increased contractile response at low concentrations of Ca2+ (0.01-0.05 mM) was inhibited in both preparations by 10(-6) M nifedipine, but at high concentrations of Ca2+ (0.1-2.5 mM) the inhibition by nifedipine was significantly less in diabetics than in the controls. 45Ca2+ uptake had significantly greater resting levels in diabetics than in controls. The uptake of 45Ca2+ induced by 10(-5) M NE was significantly greater in diabetics than in controls, and 10(-7) M prazosin diminished both responses. The results suggest hyperreactivity of contractile responses to KCl or NE, and hyperpermeability of renal artery smooth muscle membrane to Ca2+ in streptozotocin-induced diabetics.     

Contractile effects of thrombin in porcine pulmonary arteries and the influence of thrombin inhibitors

Thrombin catalyzes not only the conversion of fibrinogen to fibrin but also activates several receptor-mediated cell responses. In ring segments of porcine pulmonary arteries the contractile effect of thrombin was studied in the presence and absence of endothelium. The integrity of endothelium was assessed by the bradykinin-induced relaxation of PGF₂ (3 mol/l)-precontracted vessels which was absent after mechanical removal of endothelium. Thrombin at 0.1 to 10 U/ml (i. e. about 1–100 nmol/1) caused a sustained contraction in endothelium-denuded arteries with a maximum at 20–30 min. In vessels with intact endothelium a significant increase in tension up to 1 U/ml was observed preceded by a transient relaxant response. The contractile effect in vessels with intact endothelium was comparatively weaker. This is probably due to the release of EDRF from endothelial cells since blockade of EDRF synthesis by N^G-nitro-L- arginine augmented the thrombin-induced contractions in arteries with intact endothelium. Indomethacin did not alter the contractile effect. However, in vessels with endothelium and in endothelium-denuded vessels the contractions were reduced when extracellular calcium was omitted. Verapamil (10 gmol/l) significantly diminished the contractile effect only in endothelium-denuded vessels.On preincubation of endothelium-denuded arterial ring segments with myo-[2-³H]inositol the addition of thrombin (10 U/ml) caused an accumulation of [³H]in-ositol-1,4,5-triphosphate (IP3). A maximum was observed after 2 min preceding the maximum increase in contraction. Measurement of thrombin-induced endogenous IP₃ generation by radioreceptor assay yielded the same results.The thrombin-induced contractile effect requires the proteolytic activity of the enzyme. Catalytically inactivated thrombin by d-phenylalanyl-prolyl-arginine chloro-methylketone (PPACK) was ineffective. The potent thrombin inhibitors hirudin (recombinant desulfatohirudin) and Na-(2-naphthylsulfonyl-glycyl)-4-amidino-phenylalanine piperidide (-NAPAP) inhibited the contractile thrombin effect in a concentration-dependent manner.The present studies suggest that proteolytically active thrombin at physiologically relevant concentrations caused a contractile response of vascular smooth muscle cells independent of the endothelium. The thrombin receptor-mediated signal transduction to intracellular sites involves the formation of IP3. The thrombin-induced contractile effect may be of pathophysiologic relevance particularly in vascular regions with endothelial injury. Correspondence to: E. Glusa at the above address     

Contractile responses of rat aorta to phenylephrine and serotonin, and aging

1. The potency (pD2 value) of phenylephrine increased with age from 3 to 10 weeks, but decreased thereafter from 10 to 80 weeks, while the affinity (pKA value) of phenylephrine to alpha 1-adrenoceptors did not alter with aging. 2. The potency (pD2 value) of serotonin did not alter with aging. 3. There is no significant difference between slopes of regression lines between a cytosolic free Ca2+ level ([Ca2+]i) and tension in the presence of phenylephrine in aorta strips from 10- and 60-week-old rats, suggesting that the sensitivity of contractile system to Ca2+ did not alter with aging. 4. Effect of ryanodine on the transient increase of [Ca2+]i and the followed sustained contraction induced by phenylephrine or serotonin in Ca2+ free solution did not alter with aging.     

Contractile responses of human deferential artery and vas deferens to vasopressin

We studied the effects of vasopressin on isolated rings of human deferential artery and vas deferens (prostatic portion) obtained from patients undergoing radical cystectomy (n = 11) or prostatectomy (n = 10). Ring segments of artery or vas deferens were studied in organ bath experiments at optimal resting tension. In artery rings, vasopressin produced concentration-dependent, endothelium-independent contractions with an EC₅₀ of 4.5 × 10⁻¹⁰ M. The presence of N^G-nitro- -arginine methyl ester hydrochloride (10⁻⁴ M), an inhibitor of nitric oxide synthase, did not change significantly (P > 0.05) the vasopressin-induced contraction. In ring preparations of the prostatic part of the vas deferens, vasopressin induced phasic contractions with an EC₅₀ of 7.0 × 10⁻⁹ M. The vasopressin V₁ receptor antagonist, d(CH₂)₅Tyr(Me)AVP (10⁻⁸ and 10⁻⁶), displaced to the right in parallel the control curve to vasopressin in artery and vas deferens rings. These results indicate that vasopressin exerts a powerful constrictor action on human deferential artery and vas deferens by direct stimulation of V₁ receptors. It is concluded that the deferential artery may dampen the passage of blood to the vas deferens in circumstances characterized by increased plasma vasopressin levels.     

钾通道开放剂尼可地尔和钾通道阻断剂氯化铯对缺氧引起离体大鼠肺动脉收缩反应的影响

目的　观察钾通道开放剂尼可地尔和钾通道阻断剂氯化铯对缺氧引起离体大鼠肺动脉环收缩反应的影响。方法　以０．９５Ｎ２＋０．０５ＣＯ２造成肺动脉环的缺氧性收缩反应;机械法去除肺动脉血管内皮细胞。结果　急性缺氧１ｈ后,含内皮与去内皮肺动脉环的ＫＣｌ诱导的最大收缩百分率（ＴＫｍａｘ％）分别为１６％±４％、１８％±５％（Ｐ＞０．０５）。有氧时,钾通道阻滞剂氯化铯（１０、２０ｍｍｏｌ·Ｌ－１）可略微提高肺动脉环的收缩张力,氯化铯４０ｍｍｏｌ·Ｌ－１引起的明显收缩,依赖细胞外Ｃａ２＋内流。缺氧可增强氯化铯的上述作用,氯化铯（１０、２０、４０ｍｍｏｌ·Ｌ－１）诱导肺动脉环产生的ＴＫｍａｘ％分别增加７５０％、３５４％、１７％。钾通道开放剂尼可地尔（３、１０、３０μｍｏｌ·Ｌ－１）可浓度依赖性地抑制缺氧及氯化铯诱导的肺动脉环收缩反应。含内皮与去内皮肺动脉环在上述反应中有相似性的表现。结论　急性缺氧造成非内皮依赖性肺动脉收缩反应,与平滑肌钾通道阻滞有关。氯化铯可增强此反应。尼可地尔对缺氧性肺动脉收缩反应的抑制作用,提示此类药物在临床治疗中的前景。     

慢性低氧大鼠TRPC1表达与肺动脉收缩变化时间曲线关系

目的探讨慢性低氧(chronic hypoxia,CH)上调TRPC1表达和SOCE介导肺动脉(pulmonary arteris,PAs)收缩的时间曲线。方法清洁级♂SD大鼠,常压低氧(氧分压为9.5%¹0.5%)饲养,观察在不同时间点CH对TRPC1表达量和SOCE介导的PAs收缩的影响,描记其时间依赖性曲线。结果 1 CH上调平均右心室收缩压(mRVSP),低氧1 d压力增高明显,7 d达到最大值,21 d后达到稳定;右心室质量指数(RVMI)低氧3 d后开始增高,此后一直呈持续增高状态,直到21 d达到稳定;2半定量RT-PCR检测显示,CH上调TRPC1 mRNA的表达量,低氧1 d上调明显,3 d达到最大值一直维持到7 d,21 d后达到稳定;3 CH上调SOCE介导的PAs收缩,低氧3 d开始上调,7 d达到最大值,21 d后达到稳定。结论 CH早期TRPC1/SOCE明显上调,两者的时间曲线具有相关性,表明TRPC1和SOCE的上调在慢性低氧致肺高压的机制中发挥着重要作用。     

缺氧对15-脂氧酶同工酶在肺动脉中表达的影响

目的比较15-脂氧酶的两种同工酶:15-lipoxygenase-1(15-LO-1),15-lipoxygenase-2(15-LO-2)在正常对照组和慢性缺氧组中肺动脉表达的差异,从而探讨两种同工酶在缺氧性肺血管收缩中的作用。方法18只SD大鼠随机分为两组(n=9),正常对照组和缺氧组。通过动物乏氧箱建立大鼠慢性缺氧模型。分别用免疫组化法、原位杂交法、Western blot法比较两种同工酶在两组中表达的差异。结果①15-LO-1在正常对照组肺动脉有蛋白质的表达,而15-LO-2在正常对照组蛋白质水平不表达。②缺氧时15-LO-1蛋白质表达明显强于在正常对照中的表达(P<0.01)。③缺氧时15-LO-2在蛋白质水平表达,但15-LO-1蛋白质水平表达强于15-LO-2。④15-LO-1,15-LO-2在正常对照组和慢性缺氧组均有mRNA水平的表达,而且在缺氧组中的表达高于对照组。结论缺氧同时诱导15-LO-1,15-LO-2在肺动脉中表达增加,从而使二者在缺氧性肺血管收缩中发挥一定作用。     

Contractile effect of big endothelin-1 and its conversion to endothelin-1 in rabbit cerebral arteries

The effect of big endothelin-1 (big ET-1) and its conversion to endothelin-1 (ET-1) in rabbit cerebral arteries were examined. Big ET-1 and ET-1 induced concentration-dependent contractions in the basilar artery; ET-1 was approximately 8 times more potent than big ET-1. The metalloprotease inhibitor phosphoramidon (30 ol/1) almost abolished the contractile response to big ET-1, whereas the ET-1-induced contraction was unaffected. Removal of the endothelium did not attenuate the big ET-1-induced contraction. ET-1 was approximately 14 times more potent than endothelin-3 (ET-3) to elicit contraction. The contractions induced by big ET-1, ET-1 and ET-3 were all inhibited by the ET_A receptor antagonist BQ 123 (3 mol/l). The ET_B receptor antagonist IRL 1038 (3 mol/l) had no effect on the contractile responses to big ET-1 and ET 1, but produced a small inhibition of the ET-3-induced contraction. Formation of ET-1 was demonstrated in membrane fractions of cerebral arteries incubated with big ET-1 as measured by high pressure liquid chromatography followed by radioimmunoassay. These results suggest that externally applied big ET-1 is converted to ET-1 by a phosphoramidon-sensitive endothelin converting enzyme present in the vascular smooth muscle cells. The ET-1 formed subsequently mediates the big ET-1-induced contraction by activation of mainly ETA receptors, although a small contribution of ETB receptors cannot be excluded.     

Inhibitory role of endothelium-derived relaxing factor in rat and human pulmonary arteries.

1. The inhibitory role of endothelium-derived relaxing factor was studied in both rat and human pulmonary arteries in vitro by inhibiting its synthesis with the L-arginine analogue NG-monomethyl-L-arginine (L-NMMA). 2. In rat pulmonary arteries, L-NMMA pretreatment (10-300 microM) dose-dependently inhibited acetylcholine-induced relaxation (which is endothelium-dependent). NG-monomethyl-D-arginine (D-NMMA, 100 microM) was without effect. L-Arginine, but not D-arginine, dose-dependently reversed this inhibition. L-NMMA had no effect on relaxation induced by sodium nitroprusside. 3. In human small pulmonary arteries L-NMMA (100 microM) pretreatment similarly inhibited the acetylcholine-induced relaxation but had no effect on the sodium nitroprusside-induced relaxation. 4. In both rat and human pulmonary arteries, L-NMMA, but not D-NMMA, always caused contraction of preconstricted tissues whereas it had no effect on baseline tone. In the rat this contraction was completely prevented by prior treatment with L-arginine. 5. L-NMMA (100 microM) pretreatment mimicked the effect of endothelium removal on phenylephrine-induced vasoconstriction, both resulting in an increase in tension development at each concentration of phenylephrine. This enhancement was greatest at low concentrations of phenylephrine but was still present even at the highest concentrations. Pretreatment with L-NMMA (100 microM) also significantly increased the responses to single doses of phenylephrine. 6. These results suggest that endothelium-derived relaxing factor from endothelial cells both mediates the relaxation response to acetylcholine and also acts as a physiological brake against vasoconstriction in pulmonary vessels.