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(Hypertension. 1997;29:763-769.)
© 1997 American Heart Association, Inc.
Articles |
Blood Pressure and Vascular Effects of Endothelin Blockade in Chronic Nitric Oxide–Deficient Hypertension
the Cardiology, Cardiovascular Research, and Hypertension Division (S.S.), University Hospital, Bern, Switzerland.
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Abstract |
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Because nitric oxide inhibits the synthesis and vasoconstrictor effect of endothelin-1, the effect of endothelin-1 may be enhanced under conditions of chronic inhibition of nitric oxide synthesis. We studied the effect of chronic therapy with bosentan, a combined endothelin-A/endothelin-B receptor antagonist, on blood pressure and vascular function and structure of small arteries as well as on the reactivity of the aorta in N
-nitro-L-arginine methyl ester (L-NAME)–induced hypertension. Six-week-old Wistar-Kyoto rats were randomly treated for 6 weeks with placebo (control), L-NAME (70 mg/kg per day), or L-NAME plus bosentan (100 mg/kg per day). The treatments were stopped 2 to 3 days before the in vitro experiments so that only the long-term effects of the drugs could be observed. L-NAME increased systolic blood pressure; bosentan did not prevent this effect although the initial blood pressure rise was delayed (P=NS versus L-NAME group). Bosentan administration did not modify the structural alteration of the resistance vessels induced by L-NAME, nor did it improve endothelium-dependent relaxation of resistance vessels or the aorta. However, bosentan therapy markedly increased endothelium-dependent contraction to acetylcholine, which was slightly enhanced by L-NAME. In contrast, bosentan inhibited aortic endothelium-dependent contractions when applied acutely in vitro. This observation, together with the increased maximal vasoconstriction to the thromboxane A2 receptor agonist U46619 after 2 weeks of bosentan administration, suggests that bosentan also interacts with the receptors mediating endothelium-dependent contractions. In conclusion, our experiments suggest a minor role of endothelin in chronic L-NAME–induced hypertension as well as in the concomitant alterations of vascular structure.
Key Words: endothelins • hypertension, experimental • blood vessels
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Introduction |
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The advent of selective and nonselective specific ET receptor antagonists makes it possible to determine more conclusively the role ET plays in several disease states. Because of the potent and long-lasting vasoconstriction it produces, ET could be involved in the development of hypertension.1 Accordingly, a twofold to threefold increase of ET plasma levels for 8 days in dogs led to a 20–mm Hg increase in mean arterial pressure.2 However, plasma levels of the peptide are generally not increased, and its vascular actions are often depressed in experimental and essential hypertension.1 3 Furthermore, ET receptor antagonists have shown modest or conflicting results in experimental models of hypertension. Bosentan, a combined ETA/ETB receptor antagonist, produced a modest reduction of the elevation of BP and vascular hypertrophy in deoxycorticosterone acetate–salt hypertensive rats treated for 3 weeks.4 In spontaneously hypertensive rats, however, chronic administration of bosentan was not antihypertensive.5 Similarly, 3-day administration of BMS-182874, an orally active selective ETA receptor antagonist, showed some antihypertensive properties in deoxycorticosterone acetate–salt hypertensive rats but not in spontaneously hypertensive rats.6 Three studies using acute administration of BQ-123, a peptidic selective ETA receptor antagonist, showed a slight or no reduction in BP in different models of hypertension.7 8 9
The efficacy of ET receptor antagonists in hypertension may depend on the alteration of the endothelial function and more specifically on the integrity of the L-arginine–NO pathway. Indeed, it has been shown that NO inhibits the synthesis and vasoconstrictor effect of ET in acute in vitro conditions.10 11 This finding has been confirmed in acute in vivo conditions by the demonstration that ET receptor antagonists (nonselective or selective for ETA receptor) administered intravenously were able to prevent part of the BP rise induced by an acute administration of L-arginine analogues.12 13 14 It is therefore possible that these antagonists may prove to be more effective as antihypertensive agents in hypertensive conditions associated with an important endothelial vasodilator dysfunction, in which the vascular effects of ET could be enhanced. To address this hypothesis, we administered bosentan for 6 weeks to rats simultaneously treated with the NO synthase inhibitor L-NAME. We studied the effect of these treatments on the structure and function of basilar and mesenteric resistance arteries. We also examined the vascular reactivity of the aorta to provide a more detailed assessment of the endothelial function during these chronic treatments.
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Animal Preparation
Male Wistar-Kyoto rats 6 weeks of age were obtained from Iffa Credo (L'Arbresle, France) and randomly divided in a control group, a group receiving L-NAME in their drinking water (50 mg/100 mL), and a group receiving L-NAME plus bosentan mixed with the chow (120 mg/100 g chow). The actual dose given for each drug was calculated according to the water or food intake measured three times weekly. The average intakes of L-NAME and bosentan were 70±2 and 100±5 mg/kg per day, respectively. The treatments were carried out for 6 weeks and stopped 2 or 3 days before the in vitro experiment for study of the long-term effects of bosentan without the involvement of the effects of acute receptor occupation. Before the treatments were started and each week thereafter, the rats were weighed and their systolic BP and heart rate were measured by a tail-cuff method with the use of a pulse transducer (model LE 5000, Letica). The average of at least three measurements was taken at each occasion. The final BP determination was made before therapy was discontinued. Seven rats per group were studied. Seven additional rats were treated only with bosentan (120 mg/100 g chow) for 2 to 3 weeks and compared with seven age-matched controls. Bosentan was removed 2 days before the in vitro experiments. All procedures were approved by the Commission for Animal Research of the canton of Bern.
Vessel Preparation
Rats were anesthetized with thiopental (50 mg/kg IP) and killed by decapitation. The brain, aorta, and mesentery were gently removed and immersed in a cold Krebs' solution (control solution) of the following composition (mmol/L): NaCl 118.6, KCl 4.7, CaCl2 2.5, KH2PO4 1.2, MgSO4 1.2, NaHCO3 25.1, edetate calcium disodium 0.026, and glucose 10.1.
The proximal part of the basilar artery and a 2-mm-long segment of the fourth branch of the mesenteric artery, which was closest to the ileum and 2 mm from the intestine, were isolated under a dissection microscope. The arteries were then transferred to a small vessel chamber (Living Systems Instrumentation) filled with warmed (37±0.5°C) and oxygenated (95% O2/5% CO2) control solution circulating from a 250-mL oxygenated reservoir at a flow rate of 50 mL/min. The proximal end of the vessels was inserted and sutured on a small glass cannula (afferent cannula) positioned in the vessel chamber. The other end of the vessel was inserted in an efferent cannula as previously described,15 and the vessel was perfused intraluminally with the control solution to which 1% bovine serum albumin was added. The perfusion pressure was set and maintained at the optimal contractions to either serotonin (basilar artery) or norepinephrine (mesenteric small arteries), which were 35 and 30 mm Hg, respectively.15 16 The arteriograph chamber was placed on the stage of a microscope equipped with a video camera attached to the viewing tube. The signal derived from the video image of the vessel was processed by a video dimension analyzer (Living Systems Instrumentation) for continuous measurement and recording of the lumen diameter.
The thoracic aorta was dissected free and cut into 3.5-mm-long rings that were mounted horizontally between two stirrups in organ chambers filled with 25 mL of control solution (37°C; 95% O2/5% CO2). One stirrup was anchored and the other was connected to a force transducer (UTC2, Gould Statham) for recording of isometric tension. After a 30-minute equilibration period, rings were progressively stretched until the contractile response was maximal to 100 mmol/L KCl Krebs-Ringer solution (mmol/L: NaCl 23.0, KCl 100.0, CaCl2 2.5, MgSO4 1.2, KH2PO4 1.2, NaHCO3 25.1, edetate calcium disodium 0.026, and glucose 10.1). Optimal tension for the vessels averaged 2.5±0.2 g. The aortic rings were allowed to equilibrate for 30 minutes before the experiments.
Protocols
At the end of the 45-minute equilibration period, the intraluminal diameter and media thickness of the basilar artery were measured in calcium-free Krebs' solution to prevent any myogenic tone and later superfused with control solution (normal Krebs') for reactivity experiments. The morphometric measurements of the mesenteric artery were made only in control solution because no myogenic tone is present in these arteries. The following protocols were used in basilar and mesenteric arteries. As in previous studies,15 16 all drugs were administered extraluminally, and each protocol was separated by a 45-minute washout period. First, the vessels were contracted with a single concentration of Ang II (10-7 mol/L). Then, concentration-response curves to serotonin (10-10 to 3x10-6 mol/L) for the basilar artery or norepinephrine (10-9 to 3x10-5 mol/L) for the mesenteric artery were constructed. In the mesenteric arteries, concentration-responses curves to acetylcholine (10-10 to 3x10-5 mol/L) and sodium nitroprusside (10-10 to 3x10-5 mol/L) were constructed in preparations half-maximally precontracted with norepinephrine. Finally, a concentration-response curve to ET-1 (10-11 to 3x10-8 mol/L) was obtained in both arteries.
Endothelium-dependent relaxation to acetylcholine (10-9 to 10-4 mol/L) was studied in precontracted aortic rings (norepinephrine, 2x10-7 mol/L) harvested from the three chronic treatment groups in the presence or absence of SQ30741 (a TxA2-endoperoxide receptor antagonist, at 10-7 mol/L for 30 minutes17 ). To study direct relaxation of vascular smooth muscle, we precontracted vessels with norepinephrine (2x10-7 mol/L) and then relaxed them with sodium nitroprusside (10-10 to 10-5 mol/L). Endothelium-dependent contractions were also tested in quiescent aortic rings. For this, the segments were incubated for 30 minutes with L-NAME (10-4 mol/L) alone or in combination with SQ30741 (10-7 mol/L), and then acetylcholine was added in a cumulative fashion (10-9 to 10-4 mol/L). Control vessels receiving only acetylcholine without preincubation were tested in parallel. Finally, aortic contractions to norepinephrine, ET-1, and Ang I and II were analyzed. Norepinephrine (10-10 to 10-5 mol/L) was added to vessels with or without preincubation with L-NAME (10-4 mol/L), and the same protocol was used with ET-1 (10-11 to 10-7 mol/L). Because of tachyphylaxis, a single dose (10-7 mol/L) of Ang I or Ang II was tested.
Contractions to the TxA2 mimetic U46619 were compared in the aorta of rats treated for 2 weeks with bosentan only and in their respective controls. In these control rats, endothelium-dependent contractions to acetylcholine (L-NAME preincubation) were performed with or without a 30-minute preincubation with bosentan (10-5 mol/L).
Plasma ET-1 Levels
A blood sample was obtained from the rats through a heart puncture before death. The blood was immediately transferred to a tube containing EDTA and centrifuged at 4°C for 5 minutes. The plasma was separated from the blood cells at 4°C and kept at -80°C until the assay.
Extraction was performed by absorption on 500-mg SepPak Vac C18 cartridges (Millipore) as described by Soerensen.18 Columns were preactivated by successive washes with 5 mL of 86% ethanol in 4% acetic acid, 5 mL of methanol, 5 mL of sterile distilled water, and 5 mL of 4% acetic acid. A 2-mL plasma sample acidified with 6 mL of 4% acetic acid was then applied to the column (flow rate, approximately 3 mL/min). The columns were then washed with 18 mL of sterile distilled water and 18 mL of 24% ethanol in 4% acetic acid before ET was eluted with 86% ethanol in 4% acetic acid. The eluate was dried under nitrogen at 37°C and redissolved in 230 µL of assay buffer composed of 0.1% phosphate buffer (pH 7.4), 0.05 mol/L NaCl, 0.1% Triton X-100, 0.02% sodium azide, and 0.1% bovine serum albumin.
The radioimmunoassay of plasma ET was performed with synthetic human/porcine ET-1 (Sigma Chemical Co), a rabbit antibody against synthetic ET (Peninsula Laboratories), and 125I–ET-1 (Amersham). The antibody has 100% cross-reactivity with ET-1, 7% with ET-2 and ET-3, 17% with big ET, and no cross-reactivity with other peptides. The antibody to ET was reconstituted according to the manufacturer's instructions and then further diluted 1:3.5 with the assay buffer before addition of 100 µL to the standards or the reconstituted plasma samples (100 µL) analyzed in duplicate. After 24 hours of incubation, 100 µL of 125I–ET-1 (10 to 12x103 cpm per tube) was added and the incubation allowed to continue for an additional 24 hours. Bound and free antigen were separated by a second antibody method, and the pellets were counted by a gamma counter (Canberra Packard). With this combination of techniques and reagents, the assay sensitivity was increased compared with previously reported protocols, and recovery averaged 78±4% (n=8). The effective range of the standard curve was between 0.16 and 40 pg of ET-1 per tube, with a lower limit of detection of 0.16 pg per tube and an IC50 value of 1.5 pg per tube. The intra-assay and interassay coefficients of variation averaged 8.6% and 13.6%, respectively (n=10).
Drugs
Bosentan (free sulfonamide) was kindly provided by Dr M. Clozel (F Hoffmann–La Roche Ltd). The following drugs were obtained from Sigma Chemical Co, dissolved in distilled water, and diluted with Krebs' solution: acetylcholine chloride, Ang I, Ang II, (-)norepinephrine bitartrate, serotonin, sodium nitroprusside, U46619 (9,11-dideoxy-11
,9-epoxymethanoprostaglandin F2), and L-NAME. ET-1 (Calbiochem-Novabiochem AG) was dissolved in distilled water containing 0.1% bovine serum albumin and then diluted with Krebs' solution containing 0.05% bovine serum albumin. Bosentan and SQ30741 (Squibb Institute for Medical Research) were dissolved in 100% ethanol. To eliminate any possible effect of the vehicle, we performed all experiments with 0.1% ethanol in the organ chamber.
Calculations and Statistical Analysis
Contractions of the small arteries were expressed as a percentage of the decrease in basal intraluminal diameter. Relaxations were expressed as a percentage of the increase in intraluminal diameter from the diameter obtained after precontraction. For the aorta, contractions were expressed as a percentage of the response to KCl (100 mmol/L), and dilatations were expressed as the percentage of recovery from precontraction. Data are shown as mean±SE. The concentration of the agonists causing a half-maximal response (EC50 value) and the maximal response were calculated for both contractions and relaxations by a nonlinear regression analysis. EC50 values were expressed as pD2 (negative logarithm). The pD2 values, maximal responses, and vascular structure variables were compared by ANOVA with Bonferroni's correction for multiple comparisons.19 Paired Student's t test was used when appropriate. A value of P<.05 was considered significant.
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Results |
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BP, Body Weight, and Plasma ET
After 6 weeks of treatment, rats receiving L-NAME alone had a marked increase in systolic BP (final BP, 217±8 mm Hg versus 150±3 in control rats), whereas heart rate was significantly lower than in the control group (Fig 1
). The concomitant administration of L-NAME and bosentan did not modify the final BP of the rats (207±8 mm Hg, P=NS versus L-NAME), although bosentan delayed the initial pressure increase to the second week of treatment compared with the already significant hypertension after the first week of L-NAME treatment (P<.05, paired t test versus baseline). Body weight was similar in all groups (control, 310±4 g; L-NAME, 294±5; L-NAME plus bosentan, 299±8). Plasma ET-1 concentrations were not altered significantly in the chronic L-NAME treatment group (2.45±0.39 versus 1.80±0.30 pg/mL in the control group, P=NS) but were significantly enhanced when bosentan was added to the L-NAME treatment (3.15±0.34 pg/mL, P<.05).
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), L-NAME–treated (
), and L-NAME plus bosentan–treated (
) rats. n=7 per group. *P<.05 vs control for week 6. Note that the y axis does not start at zero.
Vascular Structure of Small Arteries
The lumen diameter of small arteries was reduced significantly in basilar but not mesenteric arteries (P=.055) of rats chronically treated with L-NAME (Table 1). This effect, associated with an increased media thickness, resulted in a significantly increased media-lumen ratio (Table 1), which was mainly due to remodeling (remodeling indexes20 of 76% and 89% for basilar and mesenteric arteries, respectively). Indeed, cross-sectional area did not increase significantly, suggesting that growth was not a major contributor to the increased media-lumen ratio. In addition to its lack of effect on BP, bosentan had no significant effect on the increased media-lumen ratio induced by L-NAME. A strong positive correlation was observed between BP (at 6 weeks) and the media-lumen ratio in both basilar and mesenteric arteries (r=.74 and r=.70, respectively, P<.005; data not shown).
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Reactivity of Small Arteries
In mesenteric small arteries half-maximally contracted with norepinephrine, acetylcholine elicited concentration-dependent relaxations that were blunted by chronic L-NAME treatment (Fig 2A). Bosentan did not prevent this effect of L-NAME. Chronic L-NAME treatment tended to increase the sensitivity of mesenteric resistance arteries to sodium nitroprusside (pD2, 7.94±0.18; P=.06) compared with the control group (pD2, 7.26±0.09; 4.8-fold concentration shift at EC50), whereas concomitant bosentan administration prevented this effect (pD2, 7.29±0.25) (Fig 2B). In contrast, the maximal response of mesenteric arteries to sodium nitroprusside was similar in all groups.
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In basilar arteries, the sensitivity to serotonin was enhanced in L-NAME–treated rats, whereas the maximal response was not statistically different (Table 2). Bosentan prevented these changes, as it did for the decreased maximal contraction to ET-1 in L-NAME–treated rats. In mesenteric arteries, neither norepinephrine nor ET-1 contractions were modified by the chronic treatments (Table 2). Ang II–mediated contractions were not significantly altered by the chronic treatments in basilar and mesenteric arteries (Table 2).
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Reactivity of the Aorta
As in small mesenteric arteries, the relaxations to acetylcholine were blunted in rats treated with L-NAME as well as those treated with L-NAME plus bosentan (Fig 3A). This decreased responsiveness was normalized when the aortic rings were pretreated with SQ30741, a TxA2-endoperoxide receptor antagonist (Fig 3B). In contrast, the maximal relaxation and the sensitivity to sodium nitroprusside did not change among the groups (Table 3).
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P<.05 vs without SQ30741.
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Contractions to 100 mmol/L KCl were not different and were used to express the contractions of other vasoconstrictors (Table 3
). Endothelium-dependent contractions to acetylcholine in L-NAME–preincubated vessels tended to be enhanced in rats chronically treated with L-NAME (28±6% versus 15±3% in control rats, Fig 4A). The concomitant administration of bosentan markedly enhanced these contractions, which reached 67±10% of contractions to 100 mmol/L KCl. In control rats, acute preincubation of aortic rings with L-NAME and bosentan (10-5 mol/L) almost totally prevented acetylcholine-induced contractions. The TxA2 receptor antagonist SQ30741 had a similar effect at 10-7 mol/L but was weaker than bosentan at 10-8 mol/L (Fig 4B).
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), SQ30741 (10-8 mol/L, 
), or SQ30741 (10-7 mol/L, 
). All curves (A and B) were obtained in quiescent aortic rings preincubated with L-NAME (10-4 mol/L) for 30 minutes. n=7 per curve. *P<.05 vs control curves at 10-4 mol/L acetylcholine.
Norepinephrine concentration-response curves had a similar maximum and sensitivity in the three treatment groups (Table 3). When the vessels were preincubated with L-NAME acutely in the organ chamber, the vessels displayed a greater responsiveness and enhanced sensitivity to norepinephrine in control and L-NAME–treated rats but not in the L-NAME plus bosentan group (Table 3). Contractions to ET-1 were markedly blunted in rats treated with L-NAME and L-NAME plus bosentan (Table 3). This decreased responsiveness was totally (L-NAME) or partially (L-NAME plus bosentan) restored by acute preincubation of the vessels with L-NAME (Table 3).
In rats treated for 2 weeks with bosentan alone, the maximal response of aortic rings to U46619 was enhanced compared with the response in rings obtained from age-matched controls (142±4 versus 128±9, P<.01), whereas the sensitivity to this TxA2 receptor agonist was similar (pD2, 7.79±0.09 versus 7.65±0.12, respectively).
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Discussion |
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In the present study, chronic administration of bosentan, an antagonist of both ETA and ETB receptors, had no significant effect on the systolic BP level reached after 6 weeks of L-NAME treatment, confirming the observation that acute (60 minutes) administration of the selective ETA receptor antagonist BQ-123 could not lower BP in rats treated for 3 weeks with an L-arginine analogue.21 However, the delay in the BP elevation occurring when bosentan was added to L-NAME would suggest that ET is involved in the early hypertensive response to the L-arginine analogue. This observation is in line with studies showing a blunted hypertensive effect of acute L-NAME administrations by ET receptor antagonists12 13 14 and provides experimental evidence to explain the discrepancy in the efficacy of ET receptor antagonists in acute and chronic L-NAME–induced hypertension.
Chronic administration of L-NAME leads to eutrophic remodeling of the basilar artery, as we previously reported.15 The structural alterations of the mesenteric arteries were less striking than in the basilar arteries and confirm those results obtained for arteries mounted on a wire myograph.4 22 One study using pressurized mesenteric arteries failed to show any alteration of the vascular structure in this model of hypertension.23 Besides the strain difference and short duration of treatment (2 weeks), which may explain the discrepancy with our study, some methodological considerations were also discussed in an editorial.24 The new observation provided by the present study is that bosentan was ineffective in preventing the eutrophic remodeling of basilar and mesenteric small arteries submitted to a chronic increase in BP. In previous studies by Li et al,4 5 bosentan could modify vascular structure only in the hypertension models in which it was able to lower BP. Furthermore, its effect was mainly a reduction of vascular hypertrophy,4 a process that is absent in L-NAME–induced hypertension, as suggested by normal cross-sectional areas and a growth index20 equal to or less than 10%. Since both a calcium antagonist and angiotensin-converting enzyme inhibitor were able to prevent L-NAME–induced hypertension as well as remodeling of the basilar artery,15 the lack of effect of bosentan is in line with the hypothesis that eutrophic remodeling is an adaptive mechanism secondary to the increase in BP.
Chronic L-NAME administration blunted acetylcholine-induced relaxations in the aorta and mesenteric arteries, confirming previous experiments in these vascular beds,16 25 in interlobar renal arteries,16 and in the basilar artery.15 It must be remembered, however, that there may be heterogeneity in NO synthase blockade and that the renal circulation, as a whole (studied in isolated perfused kidneys), may be particularly insensitive to chronic L-NAME administration.26 In contrast to what has been observed with other antihypertensive treatments,16 25 bosentan did not improve the endothelium-dependent relaxation of the aorta and small mesenteric arteries. Furthermore, acetylcholine-induced contractions were markedly enhanced in the aorta of rats treated with L-NAME and bosentan. These contractions, performed in the presence of L-NAME in the organ chamber, have been previously shown to be endothelium dependent and mediated through the release of a prostanoid endothelium-derived contracting factor.17 However, there seems to be a discrepancy between the slight enhancement of the contractile portion of the acetylcholine relaxation curve and the marked increase in acetylcholine-induced contractions in aortic rings from rats treated with L-NAME plus bosentan. This may be explained by the acute blockade of NO production with L-NAME necessary for the contraction experiments, whereas this pharmacological agent was obviously not applied in the relaxation protocol, allowing for the vasodilator properties of NO to mask endothelium-dependent contractions.
In order to propose a mechanism for such an effect of bosentan, it must be remembered that we discontinued the chronic treatment 2 days before the in vitro experiments in an attempt to differentiate between acute and chronic effects of the drug. The persistent (maintained after 2 days without bosentan) enhanced endothelium-dependent contraction to acetylcholine observed in the aorta during chronic therapy was opposite to the acute effect of the drug applied in the organ chamber (inhibition of contraction). We have recently shown that bosentan inhibits the receptors involved in acetylcholine-induced endothelium-dependent contractions (TxA2 receptors17 ),27 and this may reconcile the apparently conflicting observations. Indeed, by blocking the TxA2 receptors, chronic bosentan therapy may upregulate these receptors, which would then be unmasked when the drug is discontinued. Since this explanation was limited by our initial study design, which did not include a group treated only with bosentan, we tested this hypothesis further by treating rats for 2 weeks with bosentan alone. In the aorta from these rats we found an increased effectiveness of U46619, an exogenous agonist of TxA2 receptors, thus lending additional support for an interaction of bosentan with these receptors. Bosentan may therefore inhibit the effect of endothelium-derived contracting factor in vivo, but this does not seem to be sufficient for preventing the L-NAME–induced elevation of BP. However, this ancillary property may be important in preventing some vascular complications of hypertension.
The enhanced responsiveness to endothelium-derived contracting factor in the aorta of rats treated with L-NAME and bosentan seemed specific, since potassium chloride, norepinephrine, and Ang II contractions were not different among the groups. L-NAME alone had very little effect on contractions, except on ET-1–induced contractions, which were markedly impaired in the aorta, as previously described.25 Such a decreased maximal response would suggest that ET receptors were downregulated, but the chronic administration of an ET receptor antagonist should have prevented this from occurring. Obviously, the results do not confirm this hypothesis. It was therefore interesting to observe that acute NO blockade with L-NAME nearly restored the contractions to ET-1 in the two L-NAME–treated groups. This would therefore suggest that the decreased contractions by exogenous ET-1 are secondary to an important stimulation of NO (or NO synthase–derived mediator) release in rats chronically treated with L-NAME.
In mesenteric resistance arteries, chronic treatments did not modify the contractions to norepinephrine or ET-1. These results are at variance with results from another group that described an increased sensitivity and responsiveness to norepinephrine but normal contractions to ET-1 and vasopressin in myograph preparations of mesenteric arteries.22 28 These previous studies used a wire myograph, and the different technique may explain the discrepancy. In the basilar artery, ET-1–induced contractions were blunted in L-NAME–treated rats, as previously reported.15 In contrast to the aorta, bosentan administration prevented this alteration. Although this hypothesis deserves direct confirmation, the results suggest that the ET antagonist may have prevented receptor downregulation from occurring in this vascular bed during chronic L-NAME treatment. However, it is not clear how bosentan could restore the sensitivity of the basilar artery to serotonin.
In conclusion, our results do not support a role for ET in the maintenance of L-NAME–induced hypertension, although it delays BP elevation in the early stage of treatment. Taken together with results from previous studies in other models of hypertension (see above), the present results suggest that ET receptor antagonists may not be very effective antihypertensive agents in these animal models. Obviously, this will have to be confirmed by clinical studies in individuals with essential hypertension. In the present study, we could demonstrate an effect of bosentan on endothelium-dependent contractions that may be of clinical relevance in conditions of endothelial dysfunction associated with enhanced endothelium-derived contracting factor release or increased TxA2 release by aggregating platelets.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This work was supported by grants from the Swiss National Research Foundation (No. 32-32541.91 and 3100-039395.93-1). P.M. is the recipient of a fellowship from the Medical Research Council of Canada. C.F.K. holds a stipend of the Senglet Foundation, Basel, Switzerland.
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Footnotes |
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Reprint requests to Thomas F. Luscher, MD, University Hospital, CH-8091 Zurich, Switzerland. E-mail 100771.1237@compuserve.com
Received May 30, 1996;
first decision June 21, 1996;
first decision September 13, 1996;
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