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Cardiovascular Research 1997 33(1):131-138; doi:10.1016/S0008-6363(96)00174-5
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Copyright © 1997, European Society of Cardiology

Regulation of left ventricular relaxation in the isolated guinea-pig heart by endogenous endothelin

Bernard D Prendergasta, Peter B Anningb, Malcolm J Lewisb and Ajay M Shaha,*

aCardiovascular Sciences Research Group, Department of Cardiology, University of Wales College of Medicine, Heath Park, Cardiff CF4 4XN, UK
bCardiovascular Sciences Research Group, Department of Pharmacology, University of Wales College of Medicine, Heath Park, Cardiff CF4 4XN, UK

Received 16 February 1996; accepted 2 August 1996


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: To examine the effects of endogenous endothelin-1 on cardiac contraction in the isolated heart using endothelin receptor antagonists. Methods: Isolated ejecting guinea-pig hearts were perfused with Krebs buffer (1 µM indomethacin) at 37°C, constant loading and heart rate, and high-fidelity left ventricular pressure was monitored by an apical 2F Millar catheter. The effects of the following interventions on left ventricular performance and coronary flow were determined: (a) no treatment (i.e., time controls) (n = 8); (b) the specific ETA receptor antagonist, BQ123 (1 µM, n = 8); (c) the specific ETB receptor antagonist, IRL1038 (0.1 µM, n = 4; 1 µM, n = 6); (d) exogenous endothelin-1 (0.01 nM, n = 6; 0.1 nM, n = 6); (e) the specific ETB receptor agonist, BQ3020 (5 nM, n = 8). Results: All parameters were stable in control (untreated) hearts. BQ123 induced progressive acceleration of early left ventricular pressure decline and a fall in left ventricular end-diastolic pressure with no effect on peak left ventricular pressure, dP/dtmax, stroke volume or coronary flow. IRL1038 had no effect on any of these parameters. In contrast, exogenous endothelin-1 exerted potent vasoconstrictor effects associated with a fall in peak left ventricular pressure, dP/dtmax and stroke volume. Similar changes were observed with BQ3020. Concentrations of endothelin-1 < 0.1 nM, which had no vasoconstrictor effect, produced no change in LV function. Conclusions: These data indicate that basal intracardiac release of endothelin-1 significantly delays LV relaxation in the isolated guinea-pig heart, but has no effect on coronary flow. The contrasting effects of endogenous endothelin-1 (elicited by BQ123) and exogenous endothelin-1 are likely to reflect differences in their site of action and in their effective concentrations at these sites.

KEYWORDS Endothelin-1; Endothelium; Contractile function; Relaxation; Guinea pig; heart


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
Endothelin-1, originally isolated from the supernatant of cultured porcine aortic endothelial cells [1], is now known to be one of a family of three closely related 21-amino-acid peptides. It has powerful vasoconstrictor properties, exerts inotropic and chronotropic effects on myocardium, and can induce vascular smooth muscle cell proliferation and myocardial hypertrophy (for detailed review, see [2]). The vascular actions of the endothelins are mediated via two distinct receptor subtypes: ETA which are highly expressed in vascular smooth muscle cells and mediate vasoconstriction, and ETB which are found on the luminal surface of endothelial cells where they mediate release of vasodilator substances, and also in vascular smooth muscle in certain beds where they contribute to vasoconstriction. Both ETA and ETB receptors are also present on cardiac myocytes[3, 4], where they are coupled to multiple subcellular signalling pathways including the stimulation of phosphoinositide hydrolysis, activation of arachidonic acid metabolism, and inhibition of adenylate cyclase [5].

The potent positive inotropic properties of exogenous endothelin-1 have been confirmed in a variety of isolated superfused myocardial preparations including guinea-pig[6] and rat [7] atria, rat [8], ferret [9], rabbit [10] and human[8] papillary muscles, and rat [11] and human [12] cardiac myocytes (reviewed in [2]). In contrast, studies employing isolated perfused heart preparations have failed to consistently demonstrate positive inotropic effects of exogenous endothelin-1, probably because of the myocardial ischaemia and contractile depression that results from the potent coronary vasoconstriction caused by the peptide. Low (picomolar) concentrations of endothelin-1 induced a transient positive inotropic effect in Langendorff-perfused rat hearts [13, 14], and increased cardiac output in ejecting (working) rat hearts [15], but higher concentrations caused coronary vasoconstriction and were negatively inotropic. A similar negative inotropic effect was reported in Langendorff-perfused rabbit hearts [16]. Intravenous administration of endothelin-1 in anaesthetised dogs caused a transient increase and subsequent fall in cardiac output [17], while intracoronary administration in dogs also resulted in negative inotropic effects [18].

Whether endogenous endothelin-1 has a physiological role in regulating myocardial contractile function remains uncertain. Circulating levels of endothelin-1 are probably too low to have inotropic activity, but polar secretion of the peptide to the abluminal surface of vascular endothelial cells [19] could result in much higher local concentrations. Endothelin-1 is known to be produced basally by coronary microvascular endothelial cells and endocardial endothelial cells, both in culture [20, 21] and in situ [22–24]. It may also be produced by cardiac myocytes themselves during ischaemia [25]. Recently Mebazaa et al. [20] reported that the superfusate of cultured sheep right ventricular endocardial endothelial cells exerted positive inotropic effects on isolated rat cardiac myocytes which were attributable to endothelin-1, consistent with paracrine release of the peptide from endocardial endothelial cells. Subsequent studies in isolated ferret papillary muscle preparations demonstrated that the selective ETA receptor antagonist, BQ123, induced an earlier onset of isometric twitch relaxation in endocardium-intact muscles but not in endocardium-denuded preparations, indicating that intact endocardial endothelium tonically releases endothelin-1 in this preparation[22]. Likewise, McClellan et al. [24] reported that BQ123 produced a fall in maximum isometric force in isolated endocardium-intact rat ventricular trabeculae, while exogenous endothelin-1 exerted the opposite effect. The magnitude of reduction in peak force with BQ123 in that study was correlated to the magnitude of pre-existing endothelin activity. These studies suggest that endogenously released endothelin-1 exerts significant inotropic effects in the isolated superfused papillary muscle preparation. It is not known whether a similar tonic release of endothelin-1 in the whole heart affects cardiac contractile function.

In the present study, we have used endothelin receptor antagonists to investigate whether endogenously produced endothelin-1 has any effects on cardiac function in the isolated, buffer-perfused, ejecting guinea-pig heart.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
All procedures conformed with the Guide for the Care and Use of Laboratory Animals, published by the US National Institutes of Health (NIH Publication No. 85–23, revised 1985).

2.1. Experimental preparation
The isolated ejecting guinea-pig heart preparation used in these studies has been described in detail previously[26, 27]. Guinea-pigs (350–450 g) of either sex were sacrificed by an overdose of intraperitoneal pentobarbitone (60 mg/kg). Hearts were excised and immediately immersed in ice-cold Krebs-Henseleit buffer solution (composition in mM: NaCl 118, KCl 4.7, MgSO4 · 7H2O 1.2, NaHCO3 24, KH2PO4 1.1, glucose 10, CaCl2 · 2H2O 2.5) constantly gassed with 95% O2-5% CO2 and containing acebutolol (0.1 µM) and indomethacin (1 µM) to inhibit β-adrenergic and cyclo-oxygenase effects respectively. After rapid aortic cannulation, hearts were perfused in Langendorff mode with warmed buffer (37°C) of identical composition at a constant pressure of 80 cmH2O. The left atrium was cannulated via the largest pulmonary vein and after a period of stabilisation hearts were switched to recirculating ejecting mode [28], being paced at 10% above their intrinsic rate by an external right atrial electrode. Loading conditions were constant (left atrial filling pressure 10 cmH2O, aortic afterload 70 cmH2O) and coronary effluent drained freely via a pulmonary artery vent. The total recirculating volume was 150 ml.

Aortic flow (AF) was measured with a flotation flowmeter (KDG Mobrey Ltd., Slough, UK) and coronary flow (CF) determined from timed collections of pulmonary arterial effluent. Stroke volume (SV) was calculated as the sum of aortic and coronary flow divided by heart rate. High-fidelity left ventricular (LV) pressure was monitored by a 2-French micromanometer-tipped catheter (Millar Instruments, Houston, TX, USA) inserted directly into the left ventricular apex, taking great care to avoid any leakage of fluid around the catheter [26, 27]. This was calibrated using a transducer control unit (TC-510, Millar Instruments) and zeroed to atmospheric pressure at the level of the LV apex. The pressure signal was sampled at 4 kHz and fed via a bridge amplifier into a Macintosh Maclab recording and analysis system (Analog Digital Instruments, Hastings, UK). LV dP/dtmax was obtained from the first derivative of the LV pressure signal.

Baseline LV pressure, aortic and coronary flow were monitored for an initial 12 min equilibration period and hearts in which these parameters were unstable were excluded from the study. Study drugs (0.15 ml volume) were introduced via the gassing chamber into the recirculating buffer, resulting in a 1000-fold dilution.

2.2. Experimental protocol
We studied the effects of specific ETA and ETB receptor antagonists to inhibit the actions of endogenously released endothelin-1, and compared these with the effects of exogenous endothelin-1 (which activates both ETA and ETB receptors) and with a specific ETB receptor agonist. No specific agonist of ETA receptors was available to us. The following interventions were studied: (a) the specific ETA receptor antagonist, BQ123 [29]; (b) the specific ETB receptor antagonist, IRL1038 [30]; (c) pure exogenous endothelin-1; (d) the specific ETB receptor agonist, BQ3020 [31]; (e) time controls (i.e., no intervention).

Appropriate concentrations of each agent were determined from a series of pilot experiments and from the published literature [2].

At least 4 consecutive left ventricular traces were averaged for each measurement of peak LV pressure (LVP), dP/dtmax, and LV end-diastolic pressure (LVEDP), measured as the pressure at the time of the initial upward deflection of the dP/dt trace. LV pressure fall, which is biphasic in this preparation, was characterised by mono-exponential time constants of early and late relaxation, TE and TL respectively, as previously described [26]. The early slower phase of pressure fall commences immediately after peak LVP, while the later more rapid phase commences around the time of LV dP/dtmin and corresponds approximately to isovolumic relaxation. Briefly, the natural logarithm of LV pressure (lnP) for data points between peak LV pressure and the LV pressure at dP/dtmin was plotted against time and the negative inverse of this relationship taken as TE. The corresponding time constant for LV pressure data points 20 ms after dP/dtmin (i.e., 80 data points) was taken as TL. Under control conditions, strong linear fits (r2 ≥ 0.99) were obtained for both these relationships [26].

All data are expressed as mean ± standard error of the mean. For statistical comparison of each experimental group with the control group, data were normalised by calculating percentage change from baseline at each time point and one-way analysis of variance (ANOVA) was performed, followed by Dunnett's test to isolate differences. Comparison within groups was made by paired t-tests on absolute values, and corrected for multiple testing. A probability (P) value of < 0.05 was considered statistically significant.

2.3. Materials
BQ123 was obtained from Neosystem Laboratoire (Strasbourg, France), BQ3020 and IRL 1038 from Affiniti Research Products Ltd. (Nottingham, UK), and endothelin-1 from Sigma Chemical Company (Dorset, UK). All other reagents were obtained from Sigma and were of the highest grade available. Stock solutions were prepared in distilled water with the exception of endothelin-1 and indomethacin, which were dissolved in 0.1 M hydrochloric acid and 100% ethanol, respectively. Neither of these solvents had any effect on myocardial contractile function in their respective final concentrations of 100 µM and 0.01%. All buffer solutions were freshly prepared each day.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
The baseline characteristics of the hearts studied, prior to any intervention, are shown in Table 1. There were some minor differences between groups that are unlikely to have affected the results since each heart served as its own control in these studies.


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  		Table 1 Baseline characteristics of isolated ejecting guinea-pig hearts (i.e., prior to drug administration)

 
All recorded parameters were stable in the control (untreated) hearts (n = 8) for the duration of these experiments (Fig. GR2).


Figure 2
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  		Fig. GR2 Effects of the ETA receptor antagonist, BQ123 (1 µM {blacksquare}), on left ventricular function. Mean (±s.e) percentage changes from baseline in: (a) coronary flow, CF; (b) peak left ventricular pressure, LVP; (c) peak rate of left ventricular pressure rise, dP/dtmax; (d) left ventricular end-diastolic pressure, LVEDP; (e) time constant of early left ventricular relaxation, TE; (f) time constant of late left ventricular relaxation, TL. * P < 0.05 compared with control group ({square}).

 
3.1. Effects of endothelin receptor antagonists
The specific ETA receptor antagonist, BQ123 (1 µM), induced characteristic changes in left ventricular relaxation without significantly altering systolic pressure development (Fig. GR1 A). The mean changes in cardiac function in the group of 8 hearts treated with BQ123 (1 µM) are shown in Fig. GR2. There was a progressive acceleration of early LV pressure decline (i.e., a reduction in TE) maximal at 16 min (– 11.5 ± 1.9%, P < 0.01 c.f. control) and persisting at 20 min, but no significant change in late pressure fall (TL). Peak LVP, LV dP/dtmax and SV (not shown) were all unaltered. LVEDP fell after BQ123 in most experiments, but this did not reach statistical significance. Coronary flow was unaltered by BQ123. There was no correlation between the magnitude of the fall in TE and the baseline level of LVP (r2 = 0.08), LV dP/dtmax (r2 = 0.23) or TE (r2 = 0.15) (all P = n.s.).


Figure 1
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  		Fig. GR1 Representative traces of left ventricular pressure (LVP) in isolated guinea-pig hearts. The individual panels illustrate typical changes from baseline after exposure to: (A) the ETA receptor antagonist, BQ123 (1 µM, 16 min), (B) endothelin-1 (ET, 1 nM, 4 min), (C) endothelin-1 (ET, 0.1 nM, 16 min), (D) the ETB receptor agonist, BQ3020 (5 nM, 4 min).

 
The reversal of the actions of endogenous endothelin-1 by a receptor antagonist is likely to be a relatively slow process in view of the tight binding between endothelin-1 and its receptors [2, 32]. However in 3 hearts exposed to BQ123 (1 µM) for 28 min or more, the magnitude of change in LV relaxation was similar to that presented inFig. GR2 (data not shown).

A lower concentration of BQ123 (0.1 µM, 28 min exposure) studied in 2 hearts produced no significant effects on LV function (data not shown), consistent with previous studies of BQ123 in isolated ferret papillary muscles [22] in which this dose was without effect.

In contrast to the ETA receptor antagonist, BQ123, the specific ETB receptor antagonist, IRL1038 (0.1 µM, 4 hearts; 1 µM, 6 hearts) produced no significant sustained effects on LV function despite exposure for up to 28 min. The maximum changes from baseline after exposure to IRL1038 (1 µM) were: LVP, –7.9 ± 2.6%; LV dP/dtmax, – 12.7 ± 3.7%; SV, – 5.9 ± 1.6%; CF, – 1.3 ± 1.4%; LVEDP, 7.6 ± 7.8%; TE, – 2.0 ± 1.6%; TL, 11.4 ± 4.6% (all P > 0.05 compared with the control group at an equivalent time-point of 20 min).

3.2. Effects of receptor activation by exogenous endothelin-1
Exogenous endothelin-1 (1 nM; 6 hearts) caused pronounced, rapid and sustained vasoconstriction, accompanied by a significant fall in LVP, dP/dtmax and SV (Fig. GR1 B and Fig. GR3). These effects were maximal at 8–12 min, with a subsequent slow recovery in function.


Figure 3
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  		Fig. GR3 Effects of exogenous endothelin (1 nM {diamondsuit}) on left ventricular function. Mean (± s.e.) percentage changes from baseline in: (a) coronary flow, CF; (b) peak left ventricular pressure, LVP; (c) peak rate of left ventricular pressure rise, dP/dtmax; (d) stroke volume, SV; (e) time constant of early left ventricular relaxation, TE; (f) time constant of late left ventricular relaxation, TL. *P < 0.05 compared with control group ({square}).

 
In most hearts, the depression in LVP and dP/dtmax was associated with an alteration of the characteristic LV pressure waveform usually observed in this preparation (Fig. GR1 B), such that the linear fit for the lnP versus time relation used for the calculation of TE was weakened (r2 values 0.89–0.95 vs. r2 ≥ 0.99). Although there were no significant sustained changes in the relaxation time constant TE (Fig. GR3), it should be recognised that the altered LV pressure waveform could have affected the calculation of TE. The linear fit for the ln P versus time relation used for the calculation of TL was unaffected and there were no statistically significant changes in TL after exposure to endothelin.

A lower dose of endothelin-1 (0.1 nM; 6 hearts) had no immediate effects on coronary flow or left ventricular function, but produced late changes commencing at approximately 12 min similar to those observed with the higher concentration of 1 nM endothelin-1 (Fig. GR1 C). Doses of endothelin-1 of 0.01 nM or lower (6 hearts) had no significant effect on coronary flow, LV contraction or relaxation and doses of 10 nM or higher (3 hearts) caused severe vasoconstriction and rapid death of hearts (data not shown).

The selective ETB receptor agonist, BQ3020 (5 nM; 8 hearts), produced a similar rapid fall in coronary flow as observed with 1 nM endothelin-1, followed by gradual recovery. This coronary vasoconstriction was accompanied by a corresponding fall in LVP, dP/dtmax and SV (Fig. GR1 D). However, the recovery of LV systolic performance appeared to be faster than the recovery in CF. There were no significant effects of BQ3020 on left ventricular relaxation (Fig. GR4).


Figure 4
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  		Fig. GR4 Effects of the ETB receptor agonist, BQ3020 (5nM {blacktriangleup}), on left ventricular function. Mean (± s.e.) percentage changes from baseline in: (a) coronary flow, CF; (b) peak left ventricular pressure, LVP; (c) peak rate of left ventricular pressure rise, dP/dtmax; (d) stroke volume, SV; (e) time constant of early left ventricular relaxation, TE; (f) time constant of late left ventricular relaxation, TL. *P < 0.05 compared with control group ({square}).

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
This study was undertaken to address the hypothesis that endothelin-1 produced endogenously within the heart exerts significant effects on global cardiac contractile performance. Antogonism of endogenous endothelin-1 in the isolated ejecting guinea-pig heart using the selective ETA receptor antagonist, BQ123, caused progressive acceleration of early LV pressure decline and a fall in LV end-diastolic pressure (LVEDP) with no effect on coronary flow or systolic performance. In contrast, the selective ETB receptor antagonist, IRL 1038, had no significant effects on LV function. These results indicate that basal tonic release of endothelin-1 modulates contractile function in the isolated guinea-pig heart, by significantly delaying LV relaxation. These effects appear to be mediated principally by ETA receptors in this preparation.

To our knowledge, the effects of endothelin receptor antagonists on contractile function in the isolated heart or in vivo have not previously been characterised in detail. In isolated Langendorff-perfused rat hearts, a 5 min exposure to BQ123 had no effect on coronary flow or on LVP, but no assessment of relaxation was undertaken [33]. In anaesthetised dogs, an intravenous bolus injection of the mixed ETA/ETB receptor antagonist, bosentan, had no effect on cardiac output or LV dP/dtmax over a 10 min period [34], but again changes in LV relaxation were not assessed. In both these studies, the duration of exposure to the endothelin receptor antagonist may have been insufficient to significantly influence the actions of endogenous endothelin-1. Endothelin-1 binds very tightly to its receptors and it has been suggested that a 100–10 000-fold excess of an antagonist is required for the reversal of its effects [35]. The lack of effect of BQ123 at lower doses is also consistent with this view.

The results obtained in the present study with BQ123 extend our previous observations in isolated superfused ferret papillary muscles in which BQ123 induced an earlier onset of isometric twitch relaxation in endocardial endothelium-intact but not in endocardial-denuded preparations, indicating a basal release of endothelin-1 from endo-cardial endothelium [22]. Similarly, in the study of Mc-Clellan et al. [24], using isolated rat ventricular trabeculae, BQ123 induced a decline in maximum isometric force which was greater in endocardial endothelium-intact than in endocardial endothelium-denuded preparations. The present study indicates an influence of basal endothelin-1 release on myocardial relaxation in the whole heart. The source of this endogenous endothelin-1 was not defined. As discussed earlier, possible sources in the heart include endocardial endothelial cells, coronary microvascular endothelial cells and cardiac myocytes—the latter generally only during ischaemia [25]. Endothelin-1 released by endo-cardial endothelial cells would have to diffuse extensively into the myocardium in order to influence global cardiac function. On the other hand, release from coronary microvascular endothelial cells is likely to have a greater impact on global function, given the much greater access to the overall myocardial mass.

In contrast to the effects of BQ123, exogenous endothelin-1 (1 nM) caused a significant fall in LVP, SV and LV dP/dtmax, associated with (and possibly due to) potent coronary vasoconstriction. These effects were replicated by the selective ETB receptor agonist, BQ3020. In both cases, LV function tended to recover with time. This recovery could be explained either by a late direct positive inotropic action of endothelin-1 on cardiac myocytes or by degradation or binding of the added exogenous endothelin-1 within the recirculating buffer solution. It is possible that binding or degradation of endothelin-1 may have resulted in somewhat lower concentrations in the heart than the administered doses. However, since we studied both low doses, which had no effect, and high doses, which induced rapid death of hearts, the doses for which data are reported here are likely to represent the physiological range of effects of endothelin-1 in this preparation. Following administration of BQ3020, the changes in LV pressure development were less marked and of shorter duration than following endothelin-1, despite a larger fall in coronary flow. This discrepancy could be explained by a more potent positive inotropic effect of BQ3020 compared with exogenous endothelin-1 at the doses studied. Concentrations of endothelin-1 of 0.01 nM or lower had no effect on coronary flow or ventricular function.

There are a number of possible reasons for the disparities between the effects of exogenous endothelin-1 and the reversal of the effects of endogenous endothelin-1 by BQ123: e.g., (i) differences in site of action, (ii) different effective concentrations of the peptide at different sites, and/or (iii) the interaction of endogenous endothelin-1 with other cardioactive factors released by endothelial cells[36]. The lack of effect of BQ123 on coronary flow suggests that levels of endogenous endothelin-1 are subthreshold in the coronary vasculature; the results with exogenous endothelin-1 confirm the presence of receptors in coronary vascular smooth muscle. A plausible explanation is that endothelin-1 is released by capillary endothelial cells rather than the endothelial cells of the more proximal resistance vasculature, and that this occurs towards the abluminal surface rather than the lumen.

The subcellular mechanism underlying the enhancement of LV relaxation by BQ123 was not addressed in this study. Endothelin-1 is known to augment myofilament responsiveness to calcium, at least in part through a protein-kinase-C-mediated stimulation of Na+/H+ exchange and subsequent cytosolic alkalinisation [11, 37]. It can also raise cytosolic calcium concentration [2, 20]. The selective LV relaxant effects induced by inhibition of endogenous endothelin-1 in this study are analogous to the previously reported effects of endogenous and exogenous nitric oxide, both in the isolated ejecting guinea-pig heart [26, 38] and in humans in vivo [39, 40]. The subcellular mechanism underlying the latter effects is thought to be a cGMP-mediated reduction in myofilament responsiveness to calcium [36]. In view of the similar pattern of effect of BQ123, a similar subcellular action is possible. In addition to altering the pattern of LV relaxation, nitric oxide increases LV end-diastolic distensibility in humans [39, 40]. In the present study, we observed a trend towards a fall in LVEDP after administration of BQ123, which would be consistent with an increase in LV distensibility. Confirmation of this possibility would require measurement of LV volumes which is not technically possible at present in the ejecting heart preparation, but could be pursued in isovolumic preparations[41].

The potential physiological significance of these findings is speculative. Although the changes in LV relaxation pattern and in LVEDP induced by inhibition of endogenous endothelin-1 were relatively small, they could influence ventricular filling and subendocardial coronary perfusion (i.e., ‘diastolic function’) as well as ventricular ejection[42]. In vivo, clearly other factors such as haemodynamic load, heart rate and neurohumoral status will be additional important determinants of cardiac function. The inferred role of endothelin-1 in the regulation of cardiac contractile function is diametrically opposed to that of nitric oxide demonstrated in recent studies, analogous to their respective roles in regulating vascular tone. These endothelium-derived factors may have a close reciprocal relationship in the maintenance of cardiovascular homeostasis, the action of nitric oxide being rapid in onset and easily reversible whilst the effects of endothelin-1 are slower and longer lasting. The results of the present study provide further evidence for the concept that prototypic endothelial factors (e.g., endothelin, nitric oxide) released within the heart exert a paracrine regulating influence on myocardial contractile function. In view of recent data that cardiac myocytes may themselves produce and release endothelin-1 under conditions of ischaemia [25], and that the binding of endothelin-1 to its receptors may be enhanced during ischaemia-reperfusion [43], such effects of endothelin-1 could be augmented in pathological situations.


   Acknowledgements
 
This work was supported by the British Heart Foundation (BHF) and the UK Medical Research Council (MRC). B.D.P. is a BHF Junior Research Fellow, P.B.A. a BHF PhD Student, and A.M.S. an MRC Senior Clinical Fellow.


   Notes
 
* Corresponding author. Tel.+44 1222 742338; Fax +44 1222 761442; E-mail: shaham2@cf.ac.uk Back


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

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