Effects of endothelin-1 in the isolated heart in ischemia/reperfusion and hypoxia/reoxygenation injury

Heart ischaemia-reperfusion induces local up-regulation of vasoconstrictor endothelin ETB receptors in rat coronary arteries downstream of occlusion

BACKGROUND AND PURPOSE

Endothelins act via two receptor subtypes, ETA and ETB . Under physiological conditions in coronary arteries, ETA receptors expressed in smooth muscle cells mediate vasoconstriction whereas ETB receptors mainly found in endothelial cells mediate vasorelaxation. However, under pathophysiological conditions, ETB receptors may also be expressed in vascular smooth muscle cells mediating vasoconstriction. Here, we have investigated whether vasoconstrictor ETB receptors are up-regulated in coronary arteries after experimental myocardial ischaemia in rats.

EXPERIMENTAL APPROACH

Male Sprague-Dawley rats were subjected to either heart ischaemia-reperfusion (15 min ischaemia and 22 h reperfusion), permanent ischaemia (22 h) by ligation of the left anterior descending coronary artery, or sham operation. Using wire myography, the endothelin receptor subtypes mediating vasoconstriction were examined in isolated segments of the left anterior descending and the non-ligated septal coronary arteries. Endothelin receptor-mediated vasoconstriction was examined with cumulative administration of sarafotoxin 6c (ETB receptor agonist) and endothelin-1 (with or without ETA or ETB receptor blockade). The distribution of ETB receptors was localized with immunohistochemistry and quantified by Western blot.

KEY RESULTS

Endothelin ETB receptor-mediated vasoconstriction and receptor protein levels were significantly augmented in coronary arteries situated downstream of the occlusion after ischaemia-reperfusion compared with non-ischaemic arteries. In contrast, the ETA receptor-mediated vasoconstriction was unaltered in all groups.

CONCLUSIONS AND IMPLICATIONS

Ischaemia-reperfusion induced local up-regulation of ETB receptors in the smooth muscle cells of coronary arteries in the post-ischaemic area. In contrast, in non-ischaemic areas, ETB receptor function was unaltered.

Alterations in atrial perfusion during atrial fibrillation

Left atrial (LA) perfusion during disease states has been a topic of much interest, because the clinical implications and detrimental effects of lack of blood flow to the atria are numerous. In the chronic setting, changes in perfusion may lead to LA ischaemia and structural remodelling, a factor implicated in the self-perpetuation of chronic atrial fibrillation (AF). The association between AF and altered LA perfusion has been studied, but a direct causal association between perfusion changes and AF has not been established. A comprehensive literature search of Medline, Embase and Google Scholar databases was conducted from 1960 to February 2014. We systematically analysed reference lists of physiological articles and reviews for other possibly relevant studies. The aim of this review is to provide a comprehensive discussion of the AF-mediated changes in LA perfusion and the potential mechanisms underlying the alterations in coronary flow to the LA in this setting. In addition, we discuss the clinical contexts in which changes in LA perfusion may be relevant. Finally, this article highlights the need for longitudinal studies of AF that would elucidate the changes in LA perfusion resulting from chronic AF and lead to advancements in effective treatments to prevent progression of this disease.

Oxidative Stress and Microcirculatory Flow Abnormalities in the Ventricles during Atrial Fibrillation

Patients with atrial fibrillation (AF) often present with typical angina pectoris and mildly elevated levels of cardiac troponin (non-ST-segment elevation myocardial infarction) during an acute episode of AF. However, in a large proportion of these patients, significant coronary artery disease is excluded by coronary angiography, which suggests that AF itself influences myocardial blood flow. The present review summarizes the effect of AF on the occurrence of ventricular oxidative stress, redox-sensitive signaling pathways and gene expression, and microcirculatory flow abnormalities in the left ventricle.

Efficacy of an endothelin-A receptor antagonist in heart transplantation from asphyxiated canine non-heart-beating donors

OBJECTIVE

Hypoxic perfusion before arrest, an indeterminate period of warm ischemia, and subsequent reperfusion are major causes of cardiac allograft dysfunction in non-heart-beating donors (NHBDs). The present study was undertaken to elucidate the cardioprotective effects of ET(A) receptor antagonist FR139317 for hearts obtained from asphyxiated NHBDs in a canine transplantation model.

METHODS

Hypoxic cardiac arrest was induced in 17 donor dogs. FR139317 (10 mg/kg) was given to 7 of the dogs over a period of 10 min before disconnecting the ventilator. The hearts were preserved with FR 139317-supplemented cardioplegic solution (FR group). The remaining 10 did not receive FR 139317 at any time during the experiment (control group). Orthotopic transplantation was performed after a mean myocardial ischemic time of 4h.

RESULTS

During the agonal period, the highest systolic pulmonary artery pressure in the FR group was lower than that in the control group (47 +/- 14 vs. 58 +/- 27 mmHg). All animals in the FR group were weaned from cardiopulmonary bypass, whereas only five of the controls were weaned, two of which were identified to have dominant right ventricular failure. After transplantation, recovery rates of the left ventricular end-systolic pressure-volume ratio (E(max)) and the maximum first derivative of pressure measured over time (max dP/dt) were not significantly different between the groups, but recovery rates of the cardiac index, left ventricular minimum dP/dt and exponential time constant of LV relaxation (tau) in the FR group were higher than those in the control group.

CONCLUSIONS

The ET(A) receptor antagonist FR 139317 reduced pressure overload on the right ventricle by decreasing the peak pulmonary artery pressure before donor arrest. Cardioprotective effects of this agent for heart transplantation from NHBDs are manifested by preserved diastolic properties of the left ventricle.

Enhanced response of pig coronary arteries to endothelin-1 after ischemia–reperfusion. Role of endothelin receptors, nitric oxide and prostanoids

To analyse the coronary effects of endothelin-1 after ischemia-reperfusion, the left anterior descending coronary artery of anesthetized pigs was subjected to 30-min occlusion followed by 60-min reperfusion. Then, rings distal (ischemic arteries) and proximal (control arteries) to the occlusion were taken from this artery and prepared for isometric tension recording. The sensitivity of the contraction in response to endothelin-1 (3 x 10(-10)-3 x 10(-7) M) and the endothelin ET(B) receptor agonist IRL-1620 (3 x 10(-10)-3 x 10(-7) M) was greater in ischemic vessels. The endothelin ET(A) receptor antagonist BQ-123 (10(-7)-3 x 10(-6) M) decreased the sensitivity of the response to endothelin-1 similarly in ischemic and control arteries. The endothelin ET(B) receptor antagonist BQ-788 (10(-6) M), endothelium removal or the inhibitor of nitric oxide synthesis N(omega)-nitro-L-arginine methyl ester (L-NAME 10(-4) M) potentiated the response to endothelin-1 and IRL-1620 in control arteries only. The cyclooxygenase inhibitor meclofenamate (10(-5) M) augmented the maximal response to endothelin-1 in control arteries, and reduced it in ischemic arteries. In precontracted arteries, IRL-1620 (3 x 10(-11)-3 x 10(-10) M) relaxed control but not ischemic arteries, and L-NAME or meclofenamate abolished this relaxation. Therefore, ischemia-reperfusion increases the coronary vasoconstriction in response to endothelin-1 probably due to impairment of endothelin ET(B) receptor-induced release of nitric oxide and prostacyclin, augmentation of the contractile response to activation of endothelin ET(B) receptors, and increased release of vasoconstrictor prostanoids.

15-F2t-isoprostane exacerbates myocardial ischemia-reperfusion injury of isolated rat hearts

Reactive oxygen species induce formation of 15-F2t-isoprostane (15-F2t-IsoP), a specific marker of in vivo lipid peroxidation, which is increased after myocardial ischemia and during the subsequent reperfusion. 15-F2t-IsoP possesses potent bioactivity under pathophysiological conditions. However, it remains unknown whether 15-F2t-IsoP, by itself, can influence myocardial ischemia-reperfusion injury (IRI). Adult rat hearts were perfused by the Langendorff technique with Krebs-Henseleit (KH) solution at a constant flow rate of 10 ml/min. 15-F2t-IsoP (100 nM), SQ-29548 (1 μM, SQ), a thromboxane receptor antagonist that can abolish the vasoconstrictor effect of 15-F2t-IsoP, 15-F2t-IsoP + SQ in KH, or KH alone (vehicle control) was applied for 10 min before induction of 40 min of global ischemia followed by 60 min of reperfusion. During ischemia, saline (control), 15-F2t-IsoP, 15-F2t-IsoP + SQ, or SQ in saline was perfused through the aorta at 60 μl/min. 15-F2t-IsoP, 15-F2t-IsoP + SQ, or SQ in KH was infused during the first 15 min of reperfusion. Coronary effluent endothelin-1 concentrations were significantly higher in the group treated with 15-F2t-IsoP than in the control group during ischemia and also in the later phase of reperfusion ( P < 0.05). Infusion of 15-F2t-IsoP increased release of cardiac-specific creatine kinase, reduced cardiac contractility during reperfusion, and increased myocardial infarct size relative to the control group. SQ abolished the deleterious effects of 15-F2t-IsoP. 15-F2t-IsoP exacerbates myocardial IRI and may, therefore, act as a mediator of IRI. 15-F2t-IsoP-induced endothelin-1 production during cardiac reperfusion may represent a mechanism underlying the deleterious actions of 15-F2t-IsoP.

Endothelin A and B receptor antagonist bosentan reduces postischemic myocardial injury in the rat: critical timing of administration

The purpose of this study was to investigate the effects of bosentan, a mixed endothelin receptor A and B subtype antagonist, on myocardial ischemia-reperfusion injury and to explore the influence of the timing of bosentan administration on its cardioprotective effects. Adult rat hearts were perfused by the Langendorff technique with Krebs-Henseleit solution (KH) at a constant flow rate at 10 mL/min. Global myocardial ischemia was induced by stopping KH perfusion for 40 min, and this was followed by 60 min of reperfusion. Hearts were randomized to 1 of 3 experimental groups (n = 7 each): untreated control; treatment with bosentan 1 micromol/L 10 min prior to, during 40 min global ischemia, and for 15 min of reperfusion (BOS); or treatment with bosentan 1 micromol/L after 15 min of reperfusion (BOS-R). We observed that BOS-R, but not the BOS treatment regimen, significantly reduced the release of cardiac-specific creatine kinase and postischemic myocardial infarct size (P < 0.05 vs. control) without affecting myocardial contractility. Left ventricular developed pressure in the BOS group was significantly (P < 0.01) lower than that in the control group throughout reperfusion. It is concluded that pharmacologically delayed antagonism of endothelin-1 during reperfusion attenuates postischemic myocardial injury. Endothelin-1 antagonist application during early reperfusion may exacerbate postischemic myocardial dysfunction.

Effect of ischemia duration and nitric oxide on coronary vasoconstriction after ischemia–reperfusion

The effects of the duration of ischemia on coronary vasoconstriction after ischemia-reperfusion were analysed in rat hearts. After 15, 30 or 45 min of global zero-flow ischemia and 15 min reperfusion, the coronary response to endothelin-1 (10(-10)-10(-7) M) and the thromboxane A2 analogue 9,11-dideoxy-1a,9a-epoxymethanoprostaglandin F2alpha (U46691, 10(-8)-10(-6) M) was recorded. Vasoconstriction induced by endothelin-1 only increased after short 15 min periods of ischemia. In contrast, the vasoconstriction induced by U46619 remained unmodified by short ischemias but was reduced after longer periods of ischemia (30 and 45 min). Inhibition of nitric oxide synthesis with the Nw-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) augmented the vasoconstriction induced by endothelin-1 in non-ischemic hearts, but not following ischemia. Similarly, L-NAME increased the vasoconstriction induced by U46619 to a greater extent in non-ischemic hearts than following ischemia. These results suggest that ischemia-reperfusion inhibits nitric oxide production, causing an increased coronary response to endothelin-1 after brief ischemias. Longer ischemias may non-specifically inhibit coronary vasoconstriction and reduce nitric oxide production.

Endothelin receptor antagonists in heart failure: current status and future directions

Experimental evidence suggests that endothelin substantially contributes to left ventricular remodelling and progression of heart failure. Plasma endothelin (ET)-1 levels are increased in patients with heart failure, independent of the aetiology, and correlate with the severity of the disease. Furthermore, tissue endothelin levels and endothelin receptors are upregulated in myocardium from animals and humans with heart failure. In several experimental models of left ventricular remodelling and/or heart failure, treatment with nonselective ET-A and -B as well as selective ET-A antagonists exerted beneficial cardiovascular effects. In patients with heart failure, short-term studies of treatment with endothelin antagonists demonstrated an improvement of haemodynamic parameters; however, long-term treatment with these drugs did not significantly improve combined morbidity/mortality endpoints. Furthermore, in the recently completed Endothelin-A Receptor Antagonist Trial in Heart Failure (EARTH) trial in patients with chronic heart failure, the selective ET-A receptor antagonist darusentan did not significantly affect left ventricular remodelling as assessed by cardiac magnetic resonance imaging. Potential reasons for the lack of beneficial effects of long-term treatment with ET antagonists in patients with heart failure include the following. Firstly, adverse effects on left ventricular healing have been observed when endothelin antagonist therapy was introduced early after myocardial infarction in rats. Secondly, the role of the ET-B receptor in the pathophysiology of heart failure and remodelling processes has not been clearly defined. Finally, for the detection of improvement in left ventricular remodelling, a study needs to be conducted in patients with recent myocardial infarction and signs of heart failure.

Microcirculatory dysfunction during intestinal ischemia-reperfusion

Oxido-reductive stress is a crucial factor of the tissue response during ischemia-reoxygenation injuries. Reperfusion affects primarily the microvasculature in a manner consistent with an acute inflammatory reaction. In this respect, the salient data suggest an important connection between endothelial cell-derived humoral mediators and the perivascular mast cell system. Increased endothelin-1 and decreased nitric oxide formation, mast cell degranulation and leukocyte accumulation coexist in gastrointestinal ischemia-reperfusion syndromes too. Constitutively produced nitric oxide inhibits, while increasingly formed endothelin-1 significantly enhances the degranulation of the intestinal mast cells. The endothelin-A receptor-dependent mast cell degranulation per se plays a secondary role in reperfusion-induced structural injury, but contributes significantly to leukocyte recruitment into the reperfused intestinal mucosa. It is conceivable therefore, that the nitric oxide--endothelin-1--mast cell cycle is involved in the mechanism of ischemia-reperfusion-induced endothelial cell-leukocyte interactions, where mast cells act to amplify the process of leukocyte sequestration. The alteration in the balance between endothelial cell-derived proadhesive vasoconstrictor and antiadhesive vasodilator factors exerts a significant influence on the mucosal integrity, and the antagonism of endothelin-A receptor activation in this setting tips the equilibrium toward tissue salvage.

Coronary reactivity to endothelin-1 during partial ischemia and reperfusion in anesthetized goats. Role of nitric oxide and prostanoids

To examine the coronary reactivity to endothelin-1 and its interaction with nitric oxide or prostanoids during partial coronary ischemia and reperfusion, left circumflex coronary artery flow was electromagnetically measured, and partial occlusion of this artery was induced for 60 min, followed by reperfusion in anesthetized goats (eight non-treated, six treated with N(w)-nitro-L-arginine methyl esther (L-NAME) and five treated with meclofenamate). During partial occlusion, coronary vascular conductance was reduced by 24-37% (P<0.01), and the coronary vasodilatation in response to acetylcholine (3-100 ng) and sodium nitroprusside (1-10 microg) was much decreased in every case; the vasoconstriction in response to endothelin-1 (1-10 microg) was depressed in non-treated animals, and this depression was reversed by L-NAME and was accentuated by meclofenamate. At 30 min of reperfusion coronary vascular conductance remained decreased by 22-27% (P<0.01), and the vasodilatation in response to acetylcholine (3-100 ng) and sodium nitroprusside (1-10 microg), as well as the vasoconstriction with endothelin-1 (1-10 microg), were as in the control and comparable in the three groups of animals. These results suggest: (a) that during ischemia, the coronary vasodilator reserve is greatly reduced, and the vasoconstriction with endothelin-1 is blunted, with preservation of the modulatory role of nitric oxide and involvement of vasoconstrictor prostanoids in this vasoconstriction, and (2) that during reperfusion, the coronary vasodilator reserve and the coronary reactivity to acetylcholine and endothelin-1 recover, but the modulatory role of nitric oxide in this reactivity may be attenuated.

Coronary and aortic vasoreactivity protection with endothelin receptor antagonist, bosentan, after ischemia and hypoxia in aged rats

This study investigated the effects of bosentan, a dual endothelin ET(A) and ET(B) receptor antagonist, during hypoxia-reoxygenation of senescent aorta and during ischemia-reperfusion of senescent heart. Isolated aortic rings and isolated hearts from adult and senescent rats were submitted, respectively, to hypoxia/reoxygenation (20/30 min) and to low-flow ischemia/reperfusion (45/30 min), without or with bosentan (10(-5) M). In the aorta, bosentan treatment prevented the impairment of relaxation in response to acetylcholine after hypoxia-reoxygenation at both ages. In the heart, coronary flow recovery during reperfusion, which is lower in senescents than in adults (48% vs. 76% of baseline value, respectively; P<0.05) was fully prevented by bosentan. Prevention of endothelial dysfunction during reoxygenation of hypoxic aorta and of coronary vasoconstriction during reperfusion of ischemic heart with a dual endothelin ET(A) and ET(B) receptor antagonist suggests a role of endothelin in the vulnerability of aorta to hypoxia-reoxygenation, and of coronary arteries to ischemia-reperfusion, especially during aging.

ET(B) receptor-deficient rats exhibit reduced contraction to ET-1 despite an increase in ET(A) receptors

Several disease states, including hypertension, are associated with elevations in plasma endothelin-1 (ET-1) and variable changes in vascular contraction to ET-1. The spotting lethal (sl) rat carries a deletion of the endothelin-B (ET(B)) receptor gene that prevents expression of functional ET(B) receptors, resulting in elevated plasma ET-1. On a normal diet, these rats are normotensive and thus provide an opportunity to study the vascular effects of chronically elevated ET-1 in the absence of hypertension. Studies were performed in rats homozygous for the ET(B) deficiency (sl/sl; n = 8) and in transgenic rats heterozygous for the ET(B) deficiency (sl/+; n = 8). Plasma ET-1 was elevated in sl/sl rats (3.85 +/- 0.55 pg/ml) compared with sl/+ rats (0.31 +/- 0.11 pg/ml). Mean arterial blood pressure in conscious unrestrained sl/sl and sl/+ rats was 101 +/- 5 and 107 +/- 6 mmHg, respectively. Concentration-dependent contractions to ET-1 (10(-11)-10(-8) M) were reduced in mesenteric small arteries (150-250 microm) from sl/sl rats, as indicated by an approximately 10-fold increase in EC(50). A selective ET(A) antagonist, A-127722 (30 nM), abolished contraction to ET-1 in both groups, whereas a selective ET(B) antagonist had no effect. Also, ET(B) agonists (IRL-1620 and sarafatoxin 6c) produced neither contraction nor relaxation in either group, indicating that contraction to ET-1 in this vascular segment was exclusively ET(A) dependent. Despite increased plasma ET-1, protein expression of ET(A) receptors in membrane protein isolated from mesenteric small arteries was increased in sl/sl compared with sl/+ rats, as shown by Western blotting. These results indicate that, in ET(B)-deficient rats, ET(A)-induced contraction is reduced in vessels normally lacking ET(B)-mediated effects. Reduced contraction may be related to elevated plasma ET-1 and occurs in the presence of increased ET(A) receptor protein expression, suggesting an uncoupling of ET(A) receptor expression from functional activity.

Role of endogenous endothelin on coronary flow in health and disease

The role of the endothelium in the control of coronary flow has been demonstrated. Results of recent studies, both on animals and on humans, suggest that endogenous endothelin also plays an important role in basal coronary tone. Disease processes such as ischaemia-reperfusion injury, congestive heart failure, hypertension and atherosclerosis may be contributed to by an imbalance in, or excess of, release of endothelin. With the discovery of newer endothelin antagonists and endothelin converting enzyme inhibitors, especially with fewer hepatic side effects, there is the potential for much future research into novel therapeutic management of these common cardiovascular disorders.

Combined blockade of endothelin-1 and thromboxane A(2) receptors against postischaemic contractile dysfunction in rat hearts

1. Endothelin-1 (ET-1) may play a role in myocardial ischaemia/reperfusion injury because both the release and vasoconstrictor effect of ET-1 are increased after ischaemia. Since the increased vasoconstrictor effect of ET-1 can be mediated by ET-1-induced release of thromboxane A(2) (TXA(2)), the aim of this study was to test whether combined blockade of ET and TXA(2) receptors protects the coronary flow, contractile performance, and cardiac energy metabolism during ischaemia and reperfusion. 2. Bosentan (antagonist for ET(A) and ET(B) receptors, 1 microM based on concentration-response curves of ET-1), SQ 30,741 (antagonist of TXA(2) receptors, 0.1 microM), or the combination thereof was administered to isolated perfused rat hearts undergoing 15 min of global ischaemia and 60 min of reperfusion. 3. Neither bosentan or SQ 30,741 alone, nor the combination thereof, improved the incomplete postischaemic recovery of coronary flow, left ventricular developed pressure, phosphocreatine, or ATP. However, they attenuated ischaemia-induced acidosis but this did not translate into a measurable effect on haemodynamic or metabolic variables. 4. Thus, combined blockade of ET and TXA(2) receptors does not protect the coronary flow, contractile performance, and cardiac energy metabolism during ischaemia and reperfusion in isolated perfused rat hearts. This finding suggests that neither ET-1 nor ET-1-induced release of TXA(2) play a major role in the postischaemic recovery of the cardiac contractile function and energy metabolism.

Multiple mechanisms are involved in the acute vasodilatory effect of 17beta-estradiol in the isolated perfused rat heart

The purpose of this study was to define the dose-dependent effects of 17beta-estradiol on coronary flow and cardiac function in isolated rat hearts and to identify the mechanisms involved in its vasodilator action. Hearts from female and male Wistar rats were perfused at constant pressure (100 mm Hg). Stereoisomer specificity and the mechanism of vasodilation by 17beta-estradiol were examined in female rat hearts. Function was measured by a left ventricular (LV) balloon and coronary flow (CF) with an ultrasonic flowmeter. 17Beta-estradiol at 10(-6), 5 x 10(-6), and 10(-5) M increased CF in female hearts by 5 +/- 2, 27 +/- 4 (p < 0.05 vs. baseline), and 40 +/- 4% (p < 0.05 vs. baseline), respectively. The effect of 17beta-estradiol in hearts from male rats was similar but less pronounced compared with females [deltaCF 8 +/- 3, 19 +/- 3 (p < 0.05 vs. baseline)] and 25 +/- 7% (p < 0.05 vs. baseline; p < 0.05 vs. female 17beta-estradiol). Maximum vasodilation by the stereoisomer 17alpha-estradiol was significantly smaller [deltaCF 5 +/- 3, 4 +/- 3 (p < 0.05 vs. female 17beta-estradiol) and 14 +/- 1% (p < 0.05 vs. baseline; p < 0.05 vs. female 17beta-estradiol)] for 10(-6), 5 x 10(-6), and 10(-5) M. Pretreatment with the NO-synthesis inhibitor Nomega-methyl-L-arginine (10(-4) M) had no effect on the maximal vasodilator response to 17beta-estradiol (10(-5) M) [deltaCF 36 +/- 6% (p < 0.05 vs. baseline)]. When hearts were pretreated with the prostaglandin-synthesis inhibitor diclofenac (10(-6) M), the maximal vasodilator effect of 17beta-estradiol was partially attenuated [deltaCF 12 +/- 7% (p < 0.05 vs. female 17beta-estradiol)]. Similarly, pretreatment with the K+ATP-blocker glibenclamide (10(-6) M) partially inhibited the maximal vasodilator effect of 17beta-estradiol [deltaCF 22 +/- 6% (p < 0.05 vs. baseline; p < 0.05 vs. female 17beta-estradiol)]. Pretreatment with the Ca2+ channel antagonist nifedipine (7.2 x 10(-8) M) completely blocked the vasodilator effect. In isolated perfused rat hearts, 17beta-estradiol induced marked acute coronary vasodilation; this effect is in part gender specific, and in female hearts, largely stereoisomer specific. The dilator effect is mediated predominantly by calcium channel blockade, but prostaglandin release and K+ATP channel activation also are involved. In the isolated perfused rat heart, NO production does not contribute to the acute vasodilator effect of 17beta-estradiol.

The many aspects of endothelins in ischemia-reperfusion injury: emergence of a key mediator

The endothelins (ETs) are regulatory peptides, distributed in many organ systems and producing potent physiological effects. They are the most powerful vasoconstrictive substances known today. They also act as promitogens. Many data supporting pathophysiological roles for ETs are reported, especially regarding diseases related to the vascular system, such as hypertension, pulmonary hypertension, preeclampsia, ischemic heart diseases, renal failure, subarachnoidal hemorrhage, and cerebral ischemia. The development of drugs blocking ET binding to its receptors (antagonists) and the biosynthesis of ETs (ECE inhibitors) presently attracts great interest in terms of establishing new treatments for diseases in which ETs are believed to be involved. Here we review the evidence supporting a role for ETs in the various etiologies related to ischemia-reperfusion injury, such as is found in heart disease, cerebral ischemia, and organ transplantation.

Endothelin-1: a scientist's curiosity, or a real player in ischemic heart disease?

Endothelin-1, the most potent endothelium-derived vasoconstrictor peptide identified so far, exerts multiple biologic effects that are potentially relevant for the pathogenesis of coronary atherosclerosis and ischemic heart disease. Since the discovery of the peptide, a good deal of experimental and clinical data have been accumulated to support an important role of endothelin-1 in ischemic heart disease. In experimental animals, exogenous endothelin-1 was found to cause coronary vasoconstriction and, at higher doses, ventricular fibrillation and death. Endothelin receptor subtypes have been demonstrated and pharmacologically characterized in the coronary vascular bed. The plasma levels of immunoreactive endothelin-1 were found to be increased in patients with coronary atherosclerosis, acute myocardial infarction, and angina. Given its growth-promoting and mitogenic action, endothelin-1 has also been suspected to participate in the mechanism of restenosis after PTCA. The purpose of this study was to critically review the experimental and clinical data supporting the involvement of endothelin-1 in ischemic heart disease and the results of more recent studies on the effects of endothelin-1 blockade on experimental myocardial necrosis and restenosis after PTCA.

Endothelin-1-induced release of thromboxane A2 increases the vasoconstrictor effect of endothelin-1 in postischemic reperfused rat hearts

BACKGROUND

The release and vasoconstrictor effect of endothelin-1 (ET-1) are increased after myocardial ischemia, suggesting a role for ET-1 in ischemia/reperfusion injury. However, the mechanisms of the increased vasoconstriction by ET-1 are unknown. The aim of this study was to test whether ET-1-induced release of thromboxane A2 (TXA2) contributes to the vasoconstrictor effect of ET-1 in nonischemic hearts and whether such release can increase the vasoconstrictor effect of ET-1 in postischemic reperfused hearts.

METHODS AND RESULTS

ET-1-induced release of TXA2 was assessed by measurement of the concentrations of its stable metabolite thromboxane B2 (TXB2) in the coronary effluent of nonischemic and reperfused isolated rat hearts before and after administration of 0.01 nmol ET-1 using an enzyme immunoassay. The contribution of ET-1-induced release of TXA2 to the vasoconstrictor effect of ET-1 was assessed by measurement of the effects of ET-1 with and without the cyclooxygenase inhibitor indomethacin or the TXA2/endoperoxide receptor antagonist SQ 30,741 using 31P magnetic resonance spectroscopy. In nonischemic hearts, ET-1 led to a small increase in TXB2 in the coronary effluent (3.9 +/- 1.5 pg/mL; n = 3), but neither indomethacin nor SQ 30,741 significantly diminished the vasoconstrictor effects of ET-1 (reduction of coronary flow, 4.0 +/- 0.4 and 4.5 +/- 0.3 mL/min, respectively, versus 4.9 +/- 0.5 mL/min for ET-1 alone; n = 8, 6, and 9, respectively). In postischemic reperfused hearts, however, ET-1 led to a greater increase in TXB2 (13.7 +/- 1.5 pg/mL; P < .05 versus nonischemic hearts; n = 3), and both indomethacin and SQ 30,741 diminished the vasoconstrictor effects of ET-1 (reduction of coronary flow, 2.6 +/- 0.3 and 2.2 +/- 0.3 mL/min, respectively, versus 4.0 +/- 0.1 mL/min for ET-1 alone; n = 8, 8, and 6, respectively; P < .05). Furthermore, indomethacin and SQ 30,741 prevented the detrimental effects of ET-1 on left ventricular developed pressure, intracellular pH, and phosphocreatine during reperfusion.

CONCLUSIONS

ET-1-induced release of TXA2 does not significantly contribute to the vasoconstrictor effect of ET-1 in nonischemic hearts but can increase the vasoconstrictor effect of ET-1 in postischemic reperfused hearts.

Effects of a new endothelin receptor antagonist, TAK-044, on myocardial stunning in dogs

The effects of a new endothelin receptor antagonist, TAK-044, (cyclo[D-alpha-aspartyl-3-[(4-phenylpiperazin-1-yl)carbonyl]L-alan yl-L-alpha aspartyl-D-2-(2-thienyl)glycyl-L-leucyl-D-tryptophyl]disodium salt, on ischemic and post-ischemic myocardial dysfunction (stunned myocardium) were studied in anesthetized open-chest dogs. A short (15 min) occlusion of the left anterior descending coronary artery followed by 5-h reperfusion significantly reduced myocardial segment shortening during and after the ischemic period in the ischemic region. Regional myocardial blood flow was also decreased significantly 10 min after the occlusion, whereas it returned almost completely to its pre-ischemic value 5 h after reperfusion TAK-044 (3 mg/kg,i.v.) administered 10 min before occlusion significantly improved the reduced myocardial segment shortening in the ischemic region during and after occlusion. Cardiovascular hemodynamics and regional myocardial blood flow in a TAK-044-treated group were identical to those in the control group. These results indicate that endogenous endothelin contributes to the cause of ischemic and post-ischemic myocardial dysfunction without changing either hemodynamics or regional myocardial blood flow.

Coronary artery endothelial function after myocardial ischemia and reperfusion

BACKGROUND

The consequences of ischemia-reperfusion injury on myocytes has been studied intensely, and previous investigations of methods of myocardial protection during global and regional ischemia have focused on resultant alterations in myocardial function. However, the coronary artery endothelium is also vulnerable to damage, and only recently have investigators been able to assess coronary endothelial function.

METHODS

This review examines some aspects of coronary flow abnormalities that occur after ischemia and reperfusion. In addition, we summarize recent data that address the hypothesis that injury to the coronary artery endothelium may contribute to the pathophysiology of global (and regional) cardiac ischemia and reperfusion.

RESULTS

It appears that ischemia and reperfusion selectively injure a component in the receptor/G-protein complex linking receptor-stimulus coupling to the activation of nitric oxide synthase. Further, oxygen radicals may contribute to this injury. Recent investigations demonstrate that oxygen radicals impair the receptor/G-protein complex specific to the nitric oxide signal transduction pathway rather than causing global receptor/G-protein dysfunction.

CONCLUSIONS

The understanding of endothelial cell function and the elucidation of the nitric oxide pathway should further clarify our understanding of the pathogenesis of endothelial reperfusion injury and coronary vasospasm and contribute to the development of effective therapeutic interventions.

Investigation of coronary vessels in microscopic dimensions by two- and three-dimensional NMR microscopic imaging in the isolated rat heart. Visualization of vasoactive effects of endothelin 1

BACKGROUND

Nuclear magnetic resonance (NMR) imaging of macroscopic coronary vessels is rapidly advancing, whereas little attention has focused on development of NMR techniques for investigation of coronary microvessels. Such techniques would be of particular importance, since conventional methods to visualize coronary microvessels have specific limitations. The aim of our study was to develop two- and three-dimensional (2D and 3D) high-resolution imaging of coronary microvessels. Quantitative analysis of vessel size was performed in tomograms and applied to evaluate the vasoconstrictor effect of endothelin 1.

METHODS AND RESULTS

Angiographic imaging was performed on an 11.75-T magnet by 2D and 3D gradient-echo pulse sequences. In tomograms, the validity of this method in providing correct vessel size was tested by phantom experiments. Experiments were carried out in the isolated constant-pressure-perfused rat heart with continuous registration of coronary flow and left ventricular pressure. NMR pulse sequences were pressure-triggered in mid diastole. Four groups of hearts were studied. In group 1 (n = 20), 2D imaging perpendicular and parallel to the long axis of the heart was performed. Cross sections of vessels with diameter > 140 microns were clearly detectable. In group 2 (control, n = 5) and group 3 (n = 13), tomograms perpendicular to the long axis were obtained before and after administration of vehicle (group 2) and 200 pmol endothelin 1 bolus (group 3). Vehicle had no effect on vessel cross section. Endothelin 1, which decreased global coronary flow by 47%, reduced vessel cross section by 38 +/- 19%. A weak but, on average, significant inverse correlation between area of cross section and vessel size was found. In group 4 (n = 10), 3D imaging was performed in 7 normal hearts and 3 hearts with anterior myocardial infarction. A 3D image of the entire coronary artery tree was obtained, revealing excellent agreement with anatomic studies. In infarcted rat hearts, occlusion of the left coronary artery was demonstrated.

CONCLUSIONS

Visualization and quantification of coronary microvessels are feasible by NMR microscopy. NMR microscopy bears the potential of becoming a powerful tool for the investigation of the coronary microcirculation.

A low-affinity, low-molecular-mass endothelin-A receptor in neonatal rat heart

Endothelin receptors with endothelin-A (ETa) specificity were present in neonatal rat ventricle. However, in both receptor-binding studies and studies of inositol phosphate accumulation, these receptors had lower affinity for endothelin-1 than ETa receptors on isolated neonatal cardiomyocytes or adult left atria. Receptors in the three myocardial preparations were cross-linked to 125I-endothelin-1 and their molecular masses measured using SDS/PAGE. Receptors on left atria and neonatal cardiomyocytes had the expected molecular mass of 48 kDa, whereas the receptors in neonatal ventricle were smaller (38 kDa). Despite this, neonatal ventricles contained ETa receptor mRNA which was not different in size from that in the isolated cells (4.5 kb). Thus the 38 kDa ETa receptor present in neonatal ventricle appears to be transcribed from full-length ETa receptor mRNA and is possibly formed by processing of the 48 kDa receptor.

Endothelin-1 levels in ischaemia, reperfusion, and haemorrhagic shock in the canine infrarenal aortic revascularisation model

Endothelin-1 (ET-1) is a potent vasoconstrictive polypeptide produced from vascular endothelial cells. The effects of ischaemia, reperfusion, and exsanguination on plasma ET-1 levels were studied and compared in the mongrel dog after infrarenal aortic cross clamping. Ischaemia produced a trend toward increased ET-1 serum levels (p < 0.07 with Bonferroni correction) that did not reach significance. Plasma ET-1 levels were significantly increased during reperfusion and even further elevations were found following exsanguination. We found a 2-3 fold increase in ET-1 levels following reperfusion (Initial 3.19 +/- 0.27 pg/ml vs. Reperfusion maximum 6.32 +/- 0.72 pg/ml, Bonferroni p < 0.01). Haemorrhagic shock was associated with a 3-4 fold increase in ET-1 levels (Initial 3.19 +/- 0.27 pg/ml vs. Exsanguination maximum 8.37 +/- 0.97 pg/ml Bonferroni p < 0.001). These data reveal that ET-1 is released during reperfusion and exsanguination and may mediate remote vascular events associated with infrarenal aortic cross clamping and acute blood loss.

Role of endogenous endothelin in myocardial and coronary endothelial injury after ischaemia and reperfusion in rats: studies with bosentan, a mixed ETA-ETB antagonist

1. Previous studies suggested that endothelin-1 (ET-1) may play a role in myocardial ischaemia and reperfusion. This study was designed to test the effect of a new nonpeptide antagonist of endothelin ETA and ETB receptors, bosentan, on myocardial infarct size, ventricular arrhythmias, and coronary endothelial dysfunction after ischaemia and reperfusion. 2. Anaesthetized male Wistar rats were subjected to 20 min ischaemia (left coronary artery occlusion) followed by 1 h (for the evaluation of coronary endothelial dysfunction) or 2 h (for the evaluation of infarct size) reperfusion, or 5 min ischaemia followed by 15 min reperfusion (for the evaluation of reperfusion arrhythmias). Vascular studies were performed on 1.5-2 mm coronary segments (internal diameter 250-300 microns) removed distal to the site of occlusion and mounted in wire myographs for isometric tension recording. Area at risk and infarct size were determined by Indian ink injection and triphenyl tetrazolium staining, using computerized analysis of enlarged sections after colour video acquisition. 3. Bosentan, administered at a dose which virtually abolished the pressor response to big ET-1 (3 mg kg-1, i.v. before ischaemia) did not affect heart rate, arterial pressure or the rate pressure product before ischaemia, during ischaemia and during reperfusion. Bosentan did not affect the incidence of reperfusion-induced ventricular fibrillation (controls: 86%, n = 14; bosentan: 93%, n = 15), and did not modify infarct size (% of area at risk: controls: 63 +/- 4, n = 10; bosentan: 60 +/- 6, n = 8). Ischaemia followed by reperfusion markedly reduced the endothelium-dependent relaxations to acetylcholine(maximal response: sham: 59 +/- 4%, n = 9; ischaemia-reperfusion: 26+/- 6%, n = 8; P<0.01), characteristic of reperfusion-induced endothelial dysfunction, and this dysfunction was not prevented by bosentan (maximal response to acetylcholine: 25 +/-5%, n = 9; P<0.01 vs sham; P = NS vs ischaemia/reperfusion).4. These experiments suggest that endogenous endothelin does not contribute to myocyte or coronary endothelial injury in this rat model of ischaemia and reperfusion.

Endothelin and myocardial ischemia

Endothelin is a potent vasoconstrictor with a wide range of effects on the heart. Changes in myocardial and circulating levels of endothelin have been described in various experimental models of myocardial ischemia, and in humans with acute myocardial infarction and different forms of angina pectoris. The role played by endothelin in the different states of myocardial ischemia is unclear. However, myocardial damage has been shown to be reduced in several experimental models of myocardial infarction by administering agents that block the action of endothelin. The aim of this review article is to present the current literature concerning the interaction between endothelin and the various forms of myocardial ischemia, and to explore the significance of such interactions.

Deterioration in myocardial blood flow following relief of sustained ischemia is not associated with release of endothelin into the coronary sinus

Numerous studies have described a progressive deterioration in resting myocardial blood flow following relief of sustained ischemia in both necrotic and salvaged myocardium (termed "no reflow" and "low reflow", respectively). We sought to determine whether release of the potent vasoconstrictor peptide endothelin-1 may play a role in these phenomena. As part of a previous study in our laboratory, 14 anesthetized open-chest dogs underwent 1 h of coronary artery occlusion and 4 h of reperfusion, while 2 dogs served as time-matched sham-operated controls (artery isolated but not occluded). Regional myocardial blood flow was measured by injection of radiolabeled microspheres at 30 min and 4 h post reflow; endothelin-1 concentrations in the coronary sinus were determined by radioimmunoassay at baseline, during coronary occlusion and at 30 min and 4 h after reperfusion; and the extent of myocardial necrosis was delineated by post-mortem tetrazolium staining. As expected, in dogs subjected to ischemia/reperfusion, regional myocardial blood flow deteriorated between 30 min and 4 h post reflow in both the subendocardium (1.40 +/- 0.30 versus 0.48 +/- 0.06 ml/min/g; p = 0.003; reflecting a mixture of no reflow and low reflow) and subepicardium (0.84 +/- 0.08 versus 0.64 +/- 0.07 ml/min/g; p = 0.03; due to low reflow). However, endothelin levels showed only a modest and nonsignificant increase during the protocol (4.1 +/- 0.5, 4.7 +/- 0.2 and 4.9 +/- 0.6 pg/ml plasma at baseline, 30 min and 4 h post reflow; p = NS), and regression analysis revealed no correlation between release of endothelin and deterioration in blood flow in either myocardial layer. Moreover, the sham-operated controls showed a similar modest increase in endothelin levels, with no change in myocardial perfusion during the course of the protocol. We therefore conclude that deterioration in myocardial blood flow following relief of sustained ischemia in the anesthetized open-chest dog is not associated with release of endothelin-1 into the coronary sinus.

The effects of the endothelin ETA receptor antagonist, FR 139317, on infarct size in a rabbit model of acute myocardial ischaemia and reperfusion

1. The effects were investigated of the ETA receptor antagonist, FR 139317, on endothelin-1 (ET-1)-induced coronary vasoconstriction in the isolated perfused heart of the rabbit. In addition, this study examined whether FR 139317 reduced infarct size in a rabbit model of coronary artery occlusion and reperfusion. 2. In the rabbit isolated perfused heart, ET-1 (1-100 pmol) elicited a dose-dependent increase in coronary perfusion pressure (CPP). For example, 30 pmol ET-1 caused CPP to rise by 22 +/- 8 mmHg and 100 pmol ET-1 by 47 +/- 10 mmHg (n = 8). Infusion of FR 139317 (1 microM) significantly attenuated the increase in CPP caused by ET-1 (30 pmol: 3 +/- 1 mmHg, 100 pmol: 8 +/- 2 mmHg; n = 8). 3. In the anaesthetized rabbit, infarct size (expressed as a percentage of the area at risk) after 45 or 60 min of coronary artery occlusion followed by 2 h of reperfusion was 47 +/- 6% (n = 6) and 55 +/- 7% (n = 5), respectively. A continuous infusion of FR 139317 (0.2 mg kg-1 min-1 preceded by a loading dose of 1.0 mg kg-1, i.v.; n = 5-6) had no effect on the extent of the myocardial infarct size (45 min: 47 +/- 6%; 60 min: 49 +/- 7%). Even a three-times higher dose (0.6 mg kg-1 min-1 preceded by a loading dose of 3 mg kg-1, i.v.; n = 4) of FR 139317 had no effect on myocardial infarct size (48 +/- 5%) after 45 min occlusion of the antero-lateral branch of the left coronary artery (LAL) and 2 h reperfusion.4. In a separate group of experiments, the LAL was occluded for 60 min and subsequently reperfused for 6 h. FR 139317 (0.6 mg kg-1 min-1 preceded by a loading dose of 3 mg kg-1, i.v.; n =4) had no significant effect on infarct size even in this long reperfusion model (control: 48 +/- 3%, FR 139317:61 +/- 6%).5. Thus, the vasoconstrictor effects elicited by ET-1 in the coronary vasculature of the rabbit are primarily mediated via the ETA receptor, for they were inhibited by the ETA receptor antagonist, FR 139317. However, an enhanced formation of endogenous ET-1 does not play a major role in ischaemia/reperfusion injury of the rabbit heart, for FR 139317 had no effect on infarct size.

Myocardial ischaemia: what happens to the coronary arteries?

Study of the effects of myocardial ischaemia and reperfusion has largely been confined to the impairment of cardiac contractility. However, emerging recognition of the importance of endothelium-derived vasoactive factors in blood flow regulation has stimulated interest in the effect of pathological states such as ischaemia on coronary vascular function and is discussed in this review by Christopher Sobey and Owen Woodman. Neutrophils may play a key role in impaired reperfusion and endothelial damage, and pharmacological intervention to preserve endothelial function could significantly improve coronary blood flow and cardiac function after an ischaemic attack.

A low affinity, low molecular weight endothelin-A receptor present in neonatal rat heart

1. Addition of endothelin-1 (ET-1) to [3H]-inositol-labelled neonatal rat hearts stimulated the accumulation of [3H]-labelled inositol phosphates (InsP), but only at high concentrations; concentration at half maximum stimulation (EC50) > 0.1 mumol/L). When similar experiments were performed using isolated myocytes, the potency of endothelin-1 was higher and the EC50 value averaged 3.2 +/- 0.5 nmol/L (mean +/- s.e.m., n = 4). 2. The binding affinity of [125I]-endothelin-1 was higher for receptors on isolated cells than for receptors on membranes prepared from intact heart (72 +/- 16 pmol/L compared with 3.9 +/- 0.7 nmol/L, mean +/- s.e.m., n = 4, P < 0.01; Students' t test). 3. Receptors from both sources were cross-linked to [125I]-endothelin-1 and their molecular weights measured using sodium dodecylsulfate gradient polyacrylamide gel electrophoresis (SDS-PAGE). The receptors present on the isolated cells had a higher molecular weight (48 kD) than the receptor on the heart membranes (38 kD).