The endothelium plays an important role in control of vascular tone by releasing relaxing and contractile factors. Endothelial dysfunction is a common phenomenon in many cardiovascular diseases such as arterial hypertension, arteriosclerosis, or diabetes (1,2). In chronic heart failure (CHF), impairment of the endothelium-dependent vasodilation may contribute to the elevated peripheral vascular resistance present in patients and in experimental models of cardiac dysfunction. This increase in resistance may be due to enhanced release of contractile factors such as endothelin-1 (ET-1) and/or attenuated synthesis of relaxing factors, most notably nitric oxide (NO) (3). Impairment of endothelium-dependent relaxation has been repeatedly described in skeletal muscle vessels in vivo in patients with CHF (4-6). Similar results have been obtained in CHF after coronary artery ligation in rats for the isolated thoracic aorta (7-9) and femoral and mesenteric circulation (10,11), although contradictory results exist for the mesenteric artery (12). No information is available with regard to changes of vasoreactivity in the cerebral circulation. This is of particular interest in view of an increased incidence of cerebrovascular events in patients with CHF (13-15). Endothelial dysfunction with impaired NO release may predispose to local vasoconstriction and in situ thrombosis and thromboembolism, as pointed out recently by Lip and Gibbs (16). The first goal of our study was, therefore, to characterize endothelium-dependent and -independent vasomotor responses of the basilar artery (BA) in rats with CHF.
The endogenous ET system may constitute another important pathophysiologic factor in the development of CHF. Increase of ET-1 plasma level ([ET-1]PL) is well correlated with severity of heart failure (17) and appears to contribute to peripheral vasoconstriction (18,19). Accordingly, long-term ET-receptor blockade has been shown to exert beneficial effects in CHF. Recently we found restoration of endothelial dysfunction in the aorta from rats with chronic myocardial infarction by prolonged ET-receptor blockade (20). The second goal of our study was, therefore, to investigate the effect of long-term ET-receptor blockade on vasoreactivity of the BA. For comparison, third-order branches of the superior mesenteric artery (MA-A3) approximately size-matched to BA were included, as was the main trunk of the superior MA.
Myocardial infarction and study protocol
The investigation was performed according to the Guide for the Care and Use of Laboratory Animals published by the U.S. National Institutes of Health (NIH Publications no. 85-23, revised 1985). Adult male Wistar rats (250-300 g) were used throughout the study. Under ether anesthesia, the thorax was opened, the heart exteriorized, and a ligature placed around the proximal left coronary artery, as described previously (21). The heart was returned to its normal position, the thorax closed, and the animal allowed to recover. Sham operation was performed in an identical manner; however, the thread placed around the coronary artery was not sutured. On postoperative day 7, rats were randomly assigned to one of the following treatment groups: placebo treatment (5% arabic gum), application of the ETA-receptor antagonist LU 135252 (30 mg/kg body wt) or of the mixed ETA/ETB-receptor antagonist bosentan (100 mg/kg body wt). LU 135252 and bosentan were prepared fresh every day as a microsuspension in 5% arabic gum and administered by gavage daily at the same time for 11 weeks. During treatment, rats had free access to standard rat chow and tap water.
Hemodynamic and cardiac measurements
Thirty-six hours after the last treatment, the rats were anesthetized with pentobarbital (40 mg/kg body wt, i.p.), intubated, and ventilated. Saline-filled catheters (PE50) were advanced from right carotid artery and jugular vein into left ventricle and right atrium, respectively, and connected via three-way stopcock to a Millar micromanometer (Millar Instruments, Inc., Houston, TX, U.S.A.) and Statham transducer (Spektramed Statham P 50; Gould Inc., CA, U.S.A.). After measurements of left ventricular pressure, the catheter was repositioned into the aorta for measurement of mean arterial blood pressure (MAP). After completion of these recordings, an arterial blood sample was collected for later measurement of [ET-1]PL, and the rat was killed. The heart was removed and dissected into atria and right and left ventricle, including septum. The left ventricle was cut into three transverse sections: apex, middle ring (∼2-mm thickness), and base. Slices (5 μm) were obtained from the middle ring and stained by the method of van Gieson. Using a planimeter digital image analyzing system, the length of the infarcted and noninfarcted endocardial and epicardial surfaces were traced, and the infarct size determined as a percentage of length.
Plasma endothelin levels
Arterial blood samples were collected in a prechilled tube containing potassium EDTA (2 mg/ml blood), plasma was separated by centrifugation (3,000 g, 4°C, 10 min) and stored at −80°C. Determination of [ET-1]PL was performed as described recently (22). In brief, plasma samples (500 μl) were eluted by using Sep-Pak C-18 cartridges (Waters Corp., Milford, MA, U.S.A.). The eluate was dried at 30°C under reduced pressure, reconstituted in 250-μl assay buffer, and subjected to a radioimmunoassay analysis by using 125I-labeled ET-1 (DuPont NEN, Brussels, Belgium).
Vascular reactivity studies
Only rats with extensive myocardial infarction (≥44% of left ventricle) were included in the vascular reactivity studies. The main trunk of superior mesenteric artery (MA) was removed and kept in ice-cold Krebs solution, as described previously (23). Thereafter, the brain and upper mesentery were taken out and immediately immersed in ice-cold modified Krebs buffer (24). The BA and MA-A3 were meticulously isolated under an operation microscope. Arteries were cut in ring segments for measurement of isometric force.
Segments obtained from BA and MA-A3 were mounted on two metal prongs of 70-μm diameter horizontally adjusted in a 5-ml organ bath (24). After mounting, all segments underwent a 60-min equilibration period during which the bath temperature was gradually increased to 37°C, and resting tension adjusted to 2.5-3.5 mN. Krebs solution was continuously bubbled with a gas mixture of 93% O2/7% CO2 to maintain pH ∼7.35 (37°C). After equilibration, the bath solution was exchanged for a 124 mM K+-Krebs solution (NaCl replaced by KCl) to check the contractile capacity of the segments (minimal tension required for inclusion, 2.5 mN). After a 45-min wash period, endothelial function was tested by cumulative application of acetylcholine (Ach, 10−8−10−4M) after precontraction with 10−6M 5-hydroxytryptamine (5-HT). Segments that did not achieve relaxation of >25% of precontraction (BA) (24) or >50% (MA-A3) were not included in the study. Concentration-effect curves (CECs) were constructed after cumulative application of ET-1 (10−12−10−7M) in vessels under resting tension. After completing CEC to ET-1, sodium nitroprusside (SNP, 10−9−10−3M) was applied, and relaxation measured. Endothelium-dependent receptor-independent relaxation was tested with the calcium ionophore A23187 (10−8−10−4M) after precontraction with 5-HT (10−6M).
Ring segments of MA were mounted in an upright organ bath (Föhr Medical Instruments, Seeheim, Germany) and equilibrated for 30 min under a resting tension of 20 mN. Buffer solution was continuously bubbled with carbogen (95% O2/5% CO2), yielding a pH value of 7.4 at 37°C, as described previously (23). Rings were repeatedly contracted by immersion in a 50 mM K+-Krebs solution until reproducible responses were obtained. Thereafter, the rings were precontracted with phenylephrine (3 × 10−7 to 10−6M) to ensure comparable levels of force, and the relaxant response to cumulative doses of ACh (10−9−10−5M) and to SNP (10−10−10−4M) was assessed.
ET-1 was purchased from Peninsula (Merseyside, England) or Sigma (Deisenhofen, Germany); A23187 from Calbiochem (Bad Soden, Germany); and ACh, phenylephrine, SNP, NG-nitro-L-arginine, and 5-HT from Sigma (Deisenhofen, Germany). All other reagents were obtained from Merck (Darmstadt, Germany). Test solutions were prepared fresh on each day of the experiment and kept on ice throughout. Stock solutions were made up in distilled water. Bosentan was provided by Acetlion (Allschwil, Switzerland), and LU 135252 by Knoll AG (Ludwigshafen, Germany).
Contraction to ET-1 was measured in mN force and calculated in percentage of reference contraction (force developed in the presence of 124 mM K+-Krebs). Relaxation was calculated in percentage decrease of precontraction. For each individual CEC, the maximal effect (Emax) of relaxation or contraction was determined, and the pD2 value calculated as the -log10EC50 (i.e., that concentration at which the half-maximal effect occurred). For statistical analysis, one-way analysis of variance and post hoc LSD tests for multiple comparisons of means were performed by using the SPSS 8.0 package. A p value of <0.05 was considered significant. All values given in the text and figures are expressed as mean ± SEM, with n indicating the number of observations.
Hemodynamic parameters of sham-operated and CHF rats are shown in Table 1. Infarct sizes in the placebo and treatment groups were matched (Table 1). In placebo-treated rats with heart failure, left ventricular end-diastolic pressure (LVEDP) was significantly elevated. However, this increase was not significant in animals treated with either LU 135252 or bosentan, indicating a slight although consistent hemodynamic improvement by both antagonists. In sham-operated rats, treatment did not significantly affect MAP, as was the case in CHF rats receiving placebo. In contrast, MAP was significantly reduced in LU 135252- and bosentan-treated rats with heart failure (Table 1).
The [ET-1]PL was markedly elevated in rats with heart failure (3.9 ± 1.0 pg/ml, n = 9) as compared with sham-operated animals (1.3 ± 0.3 pg/ml, n = 4; p < 0.05). Neither treatment with LU 135252 (3.8 ± 0.7 pg/ml, n = 5) nor with bosentan (3.1 ± 1.0 pg/ml, n = 5) had a significant effect on plasma ET levels in rats with heart failure.
Endothelium-dependent relaxation by ACh. In all vessel types studied, ACh induced a concentration-related relaxation, as exemplified for the BA in Fig. 1. In BA ring segments, partial relaxation was observed with ACh (Table 2), which was abolished in the presence of NG-nitro-L-arginine (10−5M) in both sham and CHF rats (not shown). In contrast, ACh induced complete relaxation in mesenteric arteries after precontraction with either 5-HT (MA-A3) or phenylephrine (MA). The relaxant effect of ACh in BA as well as in MA-A3 and MA was unaffected by heart failure in placebo-treated rats (Fig. 1, Table 2). Similarly, there was no marked change of effect in ET-receptor antagonist-treated rats, either in sham-operated or in ligated animals (Table 2, Fig. 2).
Endothelium-dependent relaxation by A23187. The effect of A23187, which induces receptor-independent relaxation, was tested in BA and MA-A3. In sham-operated placebo-treated rats, A23187 induced maximal relaxation comparable to that achieved with ACh in BA (A23187, 68.9 ± 7.4% vs. 61.3 ± 4.3% with ACh) and MA-A3 (A23187, 83.2 ± 8.1% vs. 91.9 ± 3.5% with ACh). No differences were detectable in placebo-treated rats with CHF. Treatment with the selective ETA-receptor antagonist LU 135252 significantly augmented maximal relaxation to A23187 in BA of sham-operated animals (91.6 ± 11.6%). A similar, albeit not significant increase of the relaxant action of A23187 by LU 135252 treatment was observed in CHF rats. However, bosentan did not affect A23187-induced relaxation.
SNP-induced endothelium-independent relaxation. Similar to the results found with ACh, there was only incomplete relaxation by SNP in BA, whereas in mesenteric artery ring segments, both MA and MA-A3 displayed complete relaxation to SNP. There was a clear-cut difference of sensitivity of arteries, with the order being MA > MA-A3 > BA. Significant differences of maximal relaxation were not detectable in animals with heart failure independent of treatment (Table 2). However, there appeared to be a decrease in sensitivity toward SNP in MA-A3 from CHF rats, as reflected by a decreased pD2 value (Table 2).
ET-1-induced contraction. Before ET-1 was applied, a reference contraction was induced by immersing the ring segments in a 124 mM K+-Krebs solution. The level of reference contraction did not differ among BA and MA-A3 in sham-operated and ligated animals (Table 3). Cumulative application of ET-1 induced a concentration-related contraction, which reached nearly 100% of the reference contraction in BA and MA-A3 (Table 3). Potency of ET-1 was comparable in both vessel types and apparently unaffected by prolonged ET-receptor blockade (Fig. 3A and B). Furthermore, the ET-1-induced contraction was not affected in any way by heart failure, as shown in Table 3.
Coronary artery ligation is a well-established model for studying morphologic and functional consequences of CHF in rats. In this model, animals carrying large infarcts also display hemodynamic signs of CHF, such as elevation of LVEDP and decrease in MAP, as previously described by Pfeffer et al. (21). To ensure severe alteration of cardiovascular function, a threshold of ≥44% infarct size was set in our study, which is comparable to values used in previous studies. (21,22,25).
Alterations of endothelium-dependent relaxation, most notably mediated by NO, have been suspected to contribute to the development of CHF. Possible mechanisms underlying these alterations are a blunted endothelial NO synthesis or release and/or a decreased sensitivity of vascular smooth muscle cells to NO (3). Furthermore, an increase of ET-1 release may be of major importance, as indicated by the elevated plasma concentration found in animal models of CHF (19,22) and humans with CHF (17).
There is good evidence that the occurrence of endothelial dysfunction in CHF differs among vascular territories. In humans, a diminished blood-flow response to ACh has been described in the forearm circulation (4,6,26). In rats with CHF, an impairment of endothelium-dependent relaxation has been consistently found in the thoracic aorta (7-9) and in the femoral bed (10,27), whereas results obtained in the mesenteric circulation are equivocal (11,12). In contrast, the effect of CHF on cerebrovascular reactivity has not yet been addressed. A decrease of resting cerebral blood flow has been found in a rabbit model of CHF (28,29) and in studies on patients (30,31). Alterations of cerebral blood flow and of cerebrovascular reactivity are well known in systemic disease states such as hypertension, diabetes mellitus, and arteriosclerosis (for review, see 32). A major pathophysiologic mechanism appears to be a diminished endothelium-dependent relaxation, as found in the large and small cerebral arteries in spontaneously hypertensive rats (33,34), in rats with diabetes mellitus (35,36), and in hypercholesterolemic rabbits (37). Therefore in our study, we have characterized vasoreactivity after coronary artery ligation in rats, focusing on the BA as part of the cerebral circulation. Although the BA is actually not an arteriole, it nevertheless contributes substantially to the total cerebrovascular resistance, as reviewed elsewhere (38).
To check endothelium-dependent relaxation, ACh and A23187 were used, which are known to act in a receptor-dependent and -independent manner, respectively. Both stimuli resulted in incomplete relaxation in the BA, which is in good agreement with results obtained in normal arteries in our group (39,40) and by others (41). No impairment of endothelium-dependent relaxation induced by ACh and A23187 was found in the BA from rats with CHF. Furthermore, response to SNP, which acts in an endothelium-independent manner, was unaltered. In rat BA, ACh-induced relaxation has been shown to be mediated by release of NO exclusively, because it is abolished in the presence of NG-nitro-L-arginine in control rats as well as in CHF rats (39 and our results). Therefore the NO/cGMP axis appears to be unaffected by CHF in BA.
For the sake of comparison, distal branches of the superior mesenteric artery (MA-A3), which are approximately size-matched to BA, were included in this study. Furthermore, NO-mediated relaxation was also studied in the MA serving as conduit artery of the mesenteric circulation. In these arteries, relaxation induced by ACh and A23187 (MA-A3 only) was complete (i.e., 100% decrease of precontraction) and unaltered in rats with CHF. This observation is in good agreement with the findings described by Baggia et al. (12). In contrast, pD2 values for SNP were decreased in MA-A3 of coronary artery-ligated animals, indicating a diminished response. This decrease may well be due to an alteration in SNP metabolism. Alternatively, one may speculate about a decrease of soluble guanylate cyclase sensitivity in CHF. However, in these animals, ACh-induced relaxation was unchanged. If the NO/cGMP-mediated response were diminished, one would have to assume an additional mechanism of endothelium-dependent relaxation acting for compensation. Under these circumstances, an endothelium-derived hyperpolarizing factor may be considered a suitable candidate (42-44). It has recently been suggested that the functional importance of endothelium-dependent hyperpolarization increases in the mesenteric circulation with decreasing vessel size (45).
In addition to endothelial dysfunction, activation of sympathetic nervous system and neurohumoral mechanisms are involved in the pathophysiology of CHF after myocardial infarction. These mechanisms include the renin-angiotensin system as well as the endogenous ET system. In accord, increased concentrations of noradrenaline, angiotensin II and vasopressin have been found (46). Moreover, a marked elevation of [ET-1]PL has been described in humans (17) and rats (19,22) with CHF. This elevation of [ET-1]PL may well increase peripheral resistance because application of bosentan resulted in a significant reduction of MAP in patients (18) and rats (19) with CHF. Similarly, in our study, long-term treatment for 11 weeks with both ET-receptor antagonists, LU 135252 and bosentan, reduced MAP in coronary artery-ligated rats without affecting the increased [ET-1]PL. Acting in high concentrations, ET-1 has been shown in rabbit BA to compromise endothelium-dependent relaxation directly (47), and in the aorta of CHF rats, prolonged ET-receptor blockade prevented endothelial dysfunction (20).
In our study contraction induced by ET-1 was not affected in sham-operated animals treated with placebo or ET-receptor antagonists, respectively, in either BA or MA-A3. This result indicates that the 36-h washout period before the organ-chamber studies was sufficient to remove the antagonists. In animals with CHF, sensitivity to ET-1 appeared unaffected, as indicated by the virtually identical pD2 values obtained for BA and MA-A3, pointing to unchanged receptor affinity. This observation is in line with recent results reported by Thorin et al. (48). These authors found, in isolated rat middle cerebral and mesenteric arteries, that ET-1-induced contraction was not significantly altered after a 1-week infusion of ET-1. The long-term treatment with ET-receptor antagonists led to a reduction of total peripheral resistance in CHF, indicating vascular function affected by the treatment. However, endothelium-dependent relaxation was found unchanged in the vessels studied. The lack of impairment of vasoreactivity in placebo-treated animals hampers the chance to detect improvement of endothelium-dependent vascular function due to any form of intervention on the level of individual arteries. With regard to the endothelium-independent relaxation, there was a tendency toward augmented SNP-induced relaxation, particularly in bosentan-treated animals. However, these effects appeared in both sham-operated and CHF rats. A possible mechanism may be a change in SNP metabolism.
In patients with CHF, alterations of cerebral metabolism and increased incidence of cerebrovascular events have been documented (13-15,49). These changes may well be explained by local alterations of endothelial function in cerebral arteries as part of Virchow's triad (16) because NO is not only involved in regulation of arterial tone but also affects platelet adhesion and aggregation. The investigation of cerebrovascular reactivity, most notably of endothelium-dependent relaxation, is thus of particular interest and carries potential clinical implications. The results of our study clearly show a largely maintained NO/cGMP axis in cerebral arteries of rats with CHF. Therefore, the increased incidence of ischemic stroke associated with left ventricular dysfunction and CHF in patients may not be due to local impairment of cerebrovascular endothelial function primarily but rather to systemic events such as thrombembolic complications.
Acknowledgment: This work was supported by the Deutsche Forschungsgemeinschaft (SFB 355, B 10) and the Forschungsfonds der Fakultät für Klinische Medizin Mannheim (Schwerpunktprogramm Endothelin 1998/99, #1).
We thank Ms. Nina Goebel and Ms. Claudia Liebetrau for expert technical assistance.
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