Hemodynamic and inotropic effects of endothelin-1 in vivo

Endothelin-1 promotes hypertrophic remodelling of cardiac myocytes by activating sustained signalling and transcription downstream of endothelin type A receptors

G-protein coupled receptor (GPCR) mediated activation of the MAPK signalling cascade is a key pathway in the induction of hypertrophic remodelling of the heart – a response to pathological cues including hypertension and myocardial infarction. While levels of pro-hypertrophic hormone agonists of GPCRs increase during periods of greater workload to enhance cardiac output, hypertrophy does not necessarily result. Here we investigated the relationship between the duration of exposure to the pro-hypertrophic GPCR agonist endothelin-1 (ET-1) and the induction of hypertrophic remodelling in neonatal rat ventricular myocytes (NRVM) and in the adult rat heart in vivo. Notably, a 15 min pulse of ET-1 was sufficient to induce markers of hypertrophy that were present when measured at 24 h in vivo and 48 h in vitro. The persistence of ET-1 action was insensitive to ET type A receptor (ET_A receptor) antagonism with BQ123. The extended effects of ET-1 were dependent upon sustained MAPK signalling and involved persistent transcription. Inhibitors of endocytosis however conferred sensitivity upon the hypertrophic response to BQ123, suggesting that endocytosis of ET_A receptors following ligand binding preserves their active state by protection against antagonist. Contrastingly, α₁ adrenergic-induced hypertrophic responses required the continued presence of agonist and were sensitive to antagonist. These studies shed new light on strategies to pharmacologically intervene in the action of different pro-hypertrophic mediators.

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Acute ET-1 exposure elicits a long-lasting cardiac myocyte hypertrophic response.

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ET-1 effects depend on persistent MAPK signalling and active transcription.

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ET-1 elicited hypertrophy is insensitive to subsequent ET_A receptor antagonism.

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Endocytosis inhibition potentiates ET-1-induction of hypertrophy markers.

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Endocytosis inhibition sensitises effects of ET-1 to ET_A receptor antagonist.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Significance of the endothelin ETA receptor in the haemodynamic and inotropic effects of endothelin-1 in rats

The main aim of the present study was to investigate the direct inotropic effects of stimulation of the endothelin (ET) receptor ETA under in vivo conditions. It is well known that ETA receptor stimulation causes pronounced vasoconstriction. The ET-1-induced coronary vasoconstriction may lead to myocardial ischaemia and, consequently, to cardiodepressor effects that may mask the direct positive inotropic effect of ETA receptor stimulation. Thus, in the present study, steps were taken to avoid this possibility. In anaesthetized open-chest rats the haemodynamic and inotropic effects of ETA receptor stimulation were studied by monitoring responses evoked by ET-1 (1 nmol/kg of body weight) after ETB receptor blockade with BQ 788 (0.5 μmol/kg of body weight); these responses were compared with saline controls (after ETB receptor blockade). To avoid vasoconstrictor effects induced by ETA receptor stimulation, additional experiments were performed in the presence of the vasodilator adenosine (2.0 mg·kg−1 of body weight·min−1). Myocardial function was also examined during aortic clamping so as to circumvent the effect of changes in afterload. We studied further the effect of ETA receptor stimulation on myocardial energy metabolism. ETA receptor stimulation reduced cardiac output (−49% compared with control), raised total peripheral resistance (+173%) and reduced myocardial ATP content (−23%). Aortic clamping did not reveal a positive inotropic effect of ETA receptor stimulation. Furthermore, even though adenosine attenuated the decrease in cardiac output (−21%), the increase of total peripheral resistance (+48%) and prevented the fall of myocardial ATP content (+6%), this did not unmask a positive inotropic effect of ETA receptor stimulation. Thus we conclude that ETA receptor stimulation causes vasoconstriction and myocardial ischaemia, but has no positive inotropic effects in rats.

Transforming growth factor beta-1 attenuates endothelin-1-induced functions in neonatal cardiac myocytes

In the present study we characterized a "crosstalk" mechanism between transforming growth factor beta-1 (TGF beta-1) and endothelin-1 (ET1) signaling pathways in neonatal cardiac myocytes. A 5 minute pretreatment with 1 ng/ml concentrations of TGF beta-1 attenuated ET1-induced negative chronotropic effects and translocation of the alpha, delta and varepsilonPKC isozymes to the particulate cell fraction. We found no effect of TGF beta-1 on responses induced by the P(2) purinergic agonist ATP or phorbol ester. Treatment of cardiac myocytes with acidic fibroblast growth factor (aFGF) did not alter ET1- or ATP-mediated effects on contraction rate or translocation of PKC isozymes to the particulate fraction. Our studies suggest that TGF beta-1 may act as a negative modulator of ET1- but not ATP- or phorbol ester-induced PKC isozyme signaling events in neonatal cardiac myocytes. A better understanding of the complex ET1 and TGF beta-1 signaling mechanisms in neonatal heart cells should enhance our knowledge regarding the interplay between these pathways.

Endothelin ET(A) receptor antagonism attenuates the pressor effects of nicotine in rats

The increased endothelin-1 levels observed after smoking may result from nicotine-stimulated endothelin-1 production by endothelial cells. In this study, we investigated the effects of selective endothelin ET(A) receptors antagonist Cycle D-a-aspartyl-L-prolyl-D-isoleucyl-D-tryptophyl (JKC 301) and of endothelin ET(B) receptors antagonist N-cis-2, 6-dimethylpiperidino-carbonyl-L-gamma-methyl-leucyl-D-L-m ethoxycarbonyl-tryptophanyl-norleucine (BQ 788) on the changes in mean arterial pressure, heart rate, and plasma thromboxane B(2) (the stable product of thromboxane A(2)) levels caused by increasing doses of nicotine (0.6, 2, 6, and 20 micromol/kg) in anesthetised rats. Nicotine (0.6, 2, and 6 micromol/kg) significantly increased the mean arterial pressure in control and BQ 788-pretreated rats, while only a nicotine dose of 2 micromol/kg) increased the mean arterial pressure in JKC 301-pretreated animals. There were no differences in the nicotine-induced changes in heart rate or in the increases in thromboxane B(2) levels among the groups treated with saline, JKC 301 and BQ 788. These results demonstrate that whereas the antagonism of endothelin ET(A) receptors attenuated the increase in blood pressure after nicotine injections, endothelin ET(B) receptor antagonism had no such effect. In addition, the antagonism of endothelin ET(A) or ET(B) receptors did not affect thromboxane A(2) production after nicotine administration. These findings suggest that endothelin-1 may have a role in the acute effects of nicotine.

Inotropic effects of endothelin-1: interaction with molsidomine and with BQ 610

-In vivo studies could not detect a positive inotropy of endothelin (ET)-1 as described in in vitro experiments. ET-induced direct positive inotropy, which seems to be mediated by ETB receptors, may be antagonized in vivo by an indirect cardiodepressive effect owing to an ET-induced coronary vasoconstriction via ETA receptors. This study compares the effects of a dose of 1 nmol/kg ET-1 alone on myocardial contractility and myocardial energy metabolism with the effects of 1 nmol/kg ET-1 after pretreatment with 5 mg/kg molsidomine or with 100 microg/kg of the ETA receptor antagonist BQ 610. We investigated the effects of ET-1 versus saline controls in open-chest rats. In addition to measurements in the intact circulation, myocardial function was examined by isovolumic registrations independent of peripheral vascular effects. We also studied the effect of ET-1 on myocardial high-energy phosphates. Pretreatment with molsidomine and BQ 610 attenuated the ET-induced reduction of cardiac output (ET-1: -62%; molsidomine+ET-1: -47%; BQ 610+ET-1: -27% different from controls). After a transient initial vasodilation, ET-1 raised total peripheral resistance (ET-1: +190%; molsidomine+ET-1: +171%; BQ 610+ET-1: +89%). BQ 610 was more effective in preventing ET-induced vasoconstriction. The increase of isovolumic peak first derivative of left ventricular pressure (ET-1: -2%; molsidomine+ET-1: +16%; BQ 610+ET-1: +19%) after pretreatment with molsidomine or BQ 610 indicates that these drugs unmask the positive inotropy of ET-1. ET-induced myocardial ischemia was abolished by molsidomine and BQ 610. Pretreatment with molsidomine or blockade of ETA receptors by BQ 610 can unmask the positive inotropy of ET-1 by preventing ET-induced myocardial ischemia. The positive inotropic effect of ET-1 seems to be mediated by ETB receptors.

Endothelin-1 in the superior colliculus of rats induces depressor responses due to peripheral hemodynamic changes

Microinjection of endothelin-1 (ET-1; 10 pmol) into the superficial layer of the superior colliculus caused systemic and regional hemodynamic changes, as measured by injection of radioactive microspheres at the peak of the hypotensive effect of endothelin-1. Endothelin-1 decreased total peripheral resistance by 39 +/- 2% (n=5); the vascular resistances were decreased in the spleen, the mesentery, the large intestine and the small intestine. Moreover, we found that in consequence of the increased fraction of cardiac output received by the above organs, decreases in vascular resistances were associated with increases in blood flows in them. Interestingly, ET-1 also decreased the vascular resistances and increased the total blood flows in the kidneys. The haemodynamic changes induced by injection of endothelin-1 to the superior colliculus were associated with significant decreases in the mean arterial blood pressure (37 +/- 4 mmHg, n=6) and no changes in heart rate. Exogenous ET-1, therefore, within the SC decreases blood pressure due to peripheral hemodynamic changes.

Hemodynamic and inotropic effects of the endothelin A antagonist BQ-610 in vivo

The positive inotropy of endothelin-1 (ET-1) described by in vitro studies is not detectable in vivo because this effect is antagonized by cardiodepressive effects due to ET-induced vasoconstriction with subsequent myocardial ischemia. This vasoconstriction is mainly mediated by ETA receptors. In a previous in vivo study with a selective ETB receptor agonist, we showed that ETB receptors play an important role in the ET-induced positive inotropy. The present in vivo study examined whether selective ETA receptor blockade can unmask the ETB receptor-mediated positive inotropy of endogenous ET-1 by preventing its cardiodepressive effects via ETA receptors. In an open-chest rat model, we compared the acute hemodynamic and inotropic effects of the highly selective ETA receptor antagonist BQ-610 (100 micrograms/kg) with NaCl controls during and after a 7-min infusion. In addition to measurements in the intact circulation, the effects on myocardial contractility were studied by isovolumic registrations (peak LVSP, peak dP/dtmax), which are independent of peripheral vascular effects. Acute blockade of the ETA receptors by BQ-610 had no effect on blood pressure and heart rate. BQ-610 caused vasodilatation (total peripheral resistance -7.5% vs. control at the end of infusion; p < 0.01) with a consecutive increase in stroke volume (+15.3%; p < 0.01), cardiac output (+15.4%; p < 0.001), and ejection fraction (+10.4%; p < 0.01). The isovolumic measurements indicated a significant positive inotropic effect of BQ-610 (peak LVSP + 4.2%, p < 0.01; peak dP/dtmax + 5.5%, p < 0.01). Therefore, selective ETA receptor blockade by BQ-610 improves the hemodynamics in the intact circulation by causing a reduction in afterload and an increase in myocardial contractility. The positive inotropic effect of BQ-610 may be mediated by the positive inotropy of endogenous ET-1 via ETB receptors after selective ETA receptor blockade.

Endothelin-1-induced vasopressor responses in essential hypertension

The potential role of endothelin-1 (ET-1) in essential hypertension in humans is still subject to debate. We recently reported strong sodium retention and renal vasoconstriction during pathophysiological increments in plasma ET-1. Apart from this vasoconstrictor action, ET-1 also has mitogenic properties that play a role in the pathophysiology of hypertension. On the other hand, some data refute an important role of ET-1 in hypertension. We therefore investigated in nine subjects with essential hypertension the constrictor actions of ET-1 by challenging these subjects with a systemic infusion of ET-1 (0.5 ng/kg per minute for 60 minutes, then 1.0 ng/kg per minute for 60 minutes, and finally 2.0 ng/kg per minute for 60 minutes). Furthermore, we studied whether these effects of ET-1 could be modulated by oral use of the angiotensin-converting enzyme inhibitor enalapril (20 mg BID) or the calcium channel blocker nifedipine (60 mg OD). ET-1 infusion increased plasma ET-1 levels from 2.5+/-0.4 to 11.6+/-1.0 pmol/L (P<.05). Blood pressure rose by approximately 10 mm Hg (P<.05). Cardiac index decreased by 21+/-22%, whereas calculated systemic vascular resistance increased by 27+/-6% (P<.05). Renal blood flow decreased from 1051+/-94 to 707+/-60 mL/min at the end of the ET-1 infusion (P<.05), and calculated renal vascular resistance increased from 118+/-19 to 189+/-19 mm Hg x min/L (P<.05). Sodium excretion decreased from 227+/-39 to 111+/-15 micromol/min (P<.05). Both enalapril and nifedipine treatment prevented the systemic effects of ET-1 infusion in these subjects. However, during enalapril treatment, despite renal predilatation, ET-1 reduced renal blood flow (from 1119+/-132 to 701+/-75 mL/min, P<.05) and increased renal vascular resistance (from 111+/-16 to 187+/-28 mm Hg x min/L, P<.05) to the same levels as during ET-1 infusion alone. Nifedipine pretreatment attenuated the ET-1-induced fall in renal blood flow (from 1088+/-93 to 907+/-68 mL/min) and increase in renal vascular resistance (from 105+/-9 to 133+/-10 mm Hg x min/L). Although neither drug modulated the antinatriuretic effect of ET-1, nifedipine increased basal sodium excretion (P<.05), which compensated for the decrease during ET-1 infusion. In conclusion, essential hypertensive subjects are sensitive to the vasoconstrictor effects of ET-1. Both enalapril and nifedipine can prevent the systemic effects of ET-1, but nifedipine seems more effective in attenuating the renal constrictor effects of ET-1.

Endothelin-1 and the periaqueductal gray area of the rat: an autoradiographic and functional pharmacological study

Endothelin-1 (ET-1) injected centrally induces pressor effects and associated haemodynamic changes. Here we have evaluated the effects on systemic and regional cardiovascular parameters of injection of ET-1 into the periaqueductal gray (PAG) area of anaesthetized rats. In addition, we have used the ETA receptor-selective antagonist, FR 139317, the ETB receptor-selective antagonist, BQ-788, and the ETA/ ETB receptor non-selective antagonist, SB 209670, to identify the receptor(s) mediating these effects. We have also used in vitro autoradiography to identify binding sites for ET-1 in the PAG. 2. In vitro autoradiography showed dense binding of [125I]-PD 151242 (for ETA receptors) in the PAG area, with the binding sites being homogeneously distributed within the dorsal, lateral and ventral subregions. Tissues incubated with [125I]-BQ 3020 (for ETB receptors) had little binding. 3. Injection of ET-1 (0.1, 1 and 10 pmol per rat) in the dorsolateral PAG area significantly increased, in a dose-dependent manner the mean arterial blood pressure (MAP). The highest dose of ET-1 (10 pmol) also decreased the heart rate by 18 +/- 1%, n = 6 (P < 0.05). Increases in blood pressure induced by ET-1 (1 pmol; 31 +/- 6.6 mmHg, n = 6) were greatly reduced by pre-administration to the PAG area of FR 139317 (5 nmol per rat) or SB 209670 (3 nmol per rat) (97 and 94%, respectively), but were unaffected by BQ-788 (5 nmol per rat). Similarly, FR 139317 and SB 209670 prevented the decrease in heart rate induced by ET-1 while BQ-788 did not affect it. 4. Injection of ET-1 to the PAG area caused falls in renal blood flow (RBF) as measured by an ultrasonic flow probe, and increased renal vascular resistance (RVR). Pre-treatment of the PAG with FR 139317 or SB 209670, but not with BQ-788, prevented this ET-1-induced effect. 5. Injection of ET-1 (10 pmol) also increased total peripheral resistance (TPR; control, 2.39 +/- 0.2 mmHg ml-1 min 100 g body weight) by 100 +/- 9% (n = 5) and reduced the cardiac output (CO; control, 94.7 +/- 3.1 ml min-1) by 30 +/- 3% (n = 5), as determined by radioactive microspheres. Vascular resistances were increased in other organs, such as skeletal muscle (88 +/- 5%, n = 4), the colon (55 +/- 7%, n = 4) and the stomach (47 +/- 3%, n = 4). Pretreatment of the PAG area with FR 139317 or SB 209670 reduced the increases in TPR and vascular resistance, and the reduction in CO caused by ET-1. BQ-788 did not effect the responses to ET-1. 6. Thus, there are predominantly ETA binding sites within the PAG area and injection of ET-1 into the PAG area causes complex haemodynamic changes which are sensitive to ETA receptor antagonism. ETA receptors are, therefore, the predominant mediators of the actions of ET-1 in the PAG of the rat.

Roles of endothelin receptors in the regional and systemic vascular responses to ET-1 in the anaesthetized ganglion-blocked rat: use of selective antagonists

1. Endothelin-1 (ET-1) produces vasoconstriction, via activation of ETA and ETB receptors on vascular smooth muscle, and vasodilatation via ETB receptors on endothelial cells. Here we have used the ETA receptor-selective antagonist, BQ-123, the ETB receptor-selective antagonist, BQ-788 and the ETA/ETB receptor non-selective antagonist, PD 145065, to study the role of these receptors in mediating the haemodynamic changes induced by an infusion of ET-1 to the anesthetized ganglion-blocked rat. 2. Infusion of ET-1 (10 pmol kg-1 min-1) increased the mean arterial pressure (MAP) by 57.5 +/- 5.1 mmHg over 70 min. This pressor response was reduced by about 50% by coinfusion of BQ-123 (10 mmol kg-1 min-1), but was unaffected by either BQ-788 (10 nmol kg-1 min-1) or PD 145065 (10 nmol kg-1 min-1). 3. After infusion of ET-1 for 70 min the cardiac output had fallen from 102.6 +/- 11.3 to 55.7 +/- 7.6 ml min-1 and the total peripheral resistance had increased from 3.24 +/- 0.6 to 10.0 +/- 0.8 mmHg ml-1 min-1 (per 100g body weight). BQ-123 decreased the magnitudes of these changes whereas BQ-788 potentiated them. PD 145065 was without effect. 4. ET-1 increased the vascular resistances of all the organs studied except the brain and stomach. These changes were attenuated by BQ-123 in the kidneys, skin, adrenal glands and caecum and potentiated by BQ-788 in the kidneys, small intestine, large intestine and mesentery. PD 145065 had little effect on the individual tissues. 5. Thus, BQ-123, a selective ETA receptor antagonist, inhibits the pressor and vascular constrictor effects of ET-1 more actively than PD 145065. As BQ-788 potentiates some of the vasoconstrictor effects of ET-1 and increases the effects of ET-1 on total peripheral resistance, the predominant role of ETB receptors in the rat circulation is to limit the pressor effects of ET-1.