Endothelin-1 Generation Research Articles

Effects of phosphoramidon, BQ 788, and BQ 123 on coronary and cardiac dysfunctions of the failing hamster heart.

Coronary dysfunctions identified in the presence of chronic heart failure are an important pathophysiologic abnormality that influences the prognosis of the disease. Because the endothelin pathway plays a significant role in the increased peripheral vascular tone associated with heart failure, we hypothesized that the endothelin pathway may be involved in the abnormal coronary vasomotion associated with this pathologic condition. Experiments were carried out in failing hearts (UM-X7.1 cardiomyopathic hamsters, aged 225-250 days) and normal hearts (Syrian LVG hamsters, also aged 225-250 days). Isolated hearts were perfused at constant flow and exposed to the blocker of the generation of endothelin-1 (ET-1), phosphoramidon (10 microM infusion), as well as to the selective ET(A)-receptor antagonist BQ 123 (10 microM infusion) and to a selective ET(B)-receptor antagonist BQ 788 (1 microM infusion). Coronary and cardiac effects of exogenous ET-1 (0.01-100 pmol) were also studied. Phosphoramidon, BQ 788, and BQ 123 did not altered coronary perfusion pressure either in normal or in failing hearts, whereas cardiac contractility was significantly impaired in the presence of phosphoramidon and BQ 123. Coronary sensitivity to exogenous ET-1 did not demonstrate a significant difference between normal and failing hearts [median effective concentration (EC50), 7 pmol in failing hearts vs. 12 pmol in normal hearts; p = NS]. In the presence of exogenous ET-1, cardiac contractility was significantly increased in both groups. In normal hearts, the exogenous ET-1-induced increase in coronary perfusion pressure was completely antagonized by BQ 123, whereas combined administration of BQ 788 and BQ 123 was necessary to induce complete inhibition in failing hearts. The positive inotropic effect elicited by exogenous ET-1 (EC50) was completely abolished in the presence of BQ 123, whereas BQ 788 had no significant effect. Results indicate that the endothelin pathway does not play a significant role in the altered coronary vasomotion observed in this model of chronic heart failure. On the contrary, the endothelin pathway appears to participate in the maintenance of myocardial contractility. According to these observations, administration of an inhibitor of ET-1 synthesis, as well as the use of an ET(A)-receptor antagonist, may be contraindicated in the presence of poor left ventricular function because the endothelin pathway contributes significantly to the maintenance of cardiac contractility.

Enhanced endothelin-converting enzyme immunoreactivity in early atherosclerosis.

Endothelin-1 (ET-1) is a 21-amino-acid local and circulating factor whose plasma concentrations are increased in advanced atherosclerosis. ET-1 is cleaved from a prohormone (big ET-1) by endothelin-converting enzymes (ECEs) into the biologically active mature form which mediates vasoconstriction and cell proliferation. This study was designed to test by immunohistochemistry the hypothesis that ECE is present locally in the neointima of atherosclerotic vessels. Two groups of rabbits, control (n = 6) and cholesterol-fed (1% cholesterol diet for 8 weeks; n = 6) were sacrificed. Aortas were excised and divided for determination of tissue ET-1 concentration by RIA and immunohistochemical analysis of ECE. Vascular wall ET-1 was increased in the atherosclerotic aorta (6.1 +/- 0.8 vs. 9.8 +/- 0.9 pg/mg protein; p < 0.05), whereas circulating ET-1 concentrations were similar in the two groups (3.8 +/- 0.4 vs. 2.4 +/- 1.4 pg/ml). Immunostaining revealed the presence of ECE in endothelial and vascular smooth-muscle cells of the control group. Enhanced ECE immunoreactivity was present in atherosclerotic aortas, particularly in the neointimal macrophages and smooth-muscle cells. We conclude that local vascular wall, but not circulating ET-1, is increased in early atherosclerosis. In addition, ECE immunoreactivity is increased in early atherosclerosis and may therefore contribute to the generation of local ET-1 in early experimental atherosclerosis. These studies provide important insights into the regulation of ET-1 in early atherosclerosis, which may contribute to the elucidation of factors involved in the progression of atherosclerosis.

Oxidant stress regulates basal endothelin-1 production by cultured rat pulmonary endothelial cells.

Endothelin-1 (ET-1) is a pluripotent mediator that modulates vascular tone and influences the inflammatory response. Patients with inflammatory lung disorders frequently have elevated circulating ET-1 levels. Because these pathophysiological conditions generate reactive oxygen species that can regulate gene expression, we investigated whether the level of oxidant stress influences ET-1 production in cultured rat pulmonary arterial endothelial cells (RPAEC). Treatment with the antioxidant 1,3-dimethyl-2-thiourea (10 mM) or the iron chelator deferoxamine (1.8 microM) doubles basal ET-1 release. Conversely, exposing cells to H2O2 generated by glucose and glucose oxidase (0.1-10 mU/ml) for 4 h causes a concentration-dependent decrease in ET-1 release. This effect occurs at concentrations of glucose oxidase that do not affect [3H]leucine incorporation or specific 51Cr release from RPAEC. Catalase prevents the decrease in ET-1 synthesis caused by glucose and glucose oxidase. Glucose and glucose oxidase decrease not only ET-1 generation but also ET-1 mRNA as assessed by semiquantitative polymerase chain reaction. Our results indicate that changes in oxidative stress can either up- or downregulate basal ET-1 generation by cultured pulmonary endothelial cells.

Systemic Endothelin Receptor Blockade Decreases Peripheral Vascular Resistance and Blood Pressure in Humans

Although local inhibition of the generation or actions of endothelin-1 has been shown to cause forearm vasodilatation, the systemic effects of endothelin receptor blockade in healthy humans are unknown. We therefore investigated the cardiovascular effects of a potent peptide endothelin ETA/B receptor antagonist, TAK-044, in healthy men. Two randomized, placebo-controlled, crossover studies were performed. In nine subjects, TAK-044 (10 to 1000 mg IV over a 15-minute period) caused sustained dose-dependent peripheral vasodilatation and hypotension. Four hours after infusion of the highest dose (1000 mg), there were decreases in mean arterial pressure of 18 mm Hg and total peripheral resistance of 665 AU and increases in heart rate of 8 bpm and cardiac index of 0.9 L x min(-1) x m(-2) compared with placebo. TAK-044 caused a rapid, dose-dependent increase in plasma immunoreactive endothelin (from 3.3 to 35.7 pg/mL within 30 minutes after 1000 mg). In a second study in eight subjects, intravenous administration of TAK-044 at doses of 30, 250, and 750 mg also caused peripheral vasodilatation, and all three doses abolished local forearm vasoconstriction to brachial artery infusion of endothelin-1. Brachial artery infusion of TAK-044 caused local forearm vasodilation. The endothelin ETA/B receptor antagonist TAK-044 decreases peripheral vascular resistance and, to a lesser extent, blood pressure; increases circulating endothelin concentrations; and blocks forearm vasoconstriction to exogenous endothelin-1. These results suggest that endogenous generation of endothelin-1 plays a fundamental physiological role in maintenance of peripheral vascular tone and blood pressure. The vasodilator properties of endothelin receptor antagonists may prove valuable therapeutically.

Phosphoramidon inhibition of the in vivo conversion of big endothelin-1 to endothelin-1 in the human forearm.

1. The vasoconstrictor peptide, endothelin-1 (ET-1) and a biologically inactive C-terminal fragment (CTF) are generated from an intermediate big ET-1 by a putative ET converting enzyme, sensitive to phosphoramidon. We have developed a procedure using selective solid-phase extraction and specific radioimmunoassays to measure the levels of immunoreactive (IR) big ET-1 and the products of conversion (ET-1 and CTF) in human plasma. These techniques have been used to determine the levels of the three peptides in venous plasma following local infusions of ET-1 and big ET-1, both alone and together with phosphoramidon. 2. Infusion of ET-1 into the brachial artery (5 pmol min-1) significantly increased (P < 0.05) IR ET levels from a basal level of 2.3 pM to 55.2 pM in plasma from the infused arm after 60 min of infusion. This corresponded with a marked decrease in forearm blood flow from a basal level of 2.6 ml dl-1 min-1 to 1.7 ml dl-1 min-1. The levels of IR big ET-1 and CTF were unchanged. Co-infusion of phosphoramidon (30 nmol min-1) with ET-1 had no significant effect on the plasma IR levels of ET, big ET-1, CTF, or blood flow. 3. Big ET-1 (50 pmol min-1) significantly increased (P < 0.05) venous concentrations of all three IR peptides after 60 min compared to basal (ET: from 2.2 to 7.7 pM, big ET-1; from 0 to 386.0 pM, CTF: from 0.2 to 37.0 pM). Forearm blood flow decreased significantly (P<0.05) from a basal level of 3.0 ml dl-1 min-1 to 1.6 ml dl-1 min-1.4. When phosphoramidon was co-infused with big ET-1, both the rise in IR ET and associated vasoconstriction were abolished. However, IR CTF was still detected, suggesting that either some conversion by phosphoramidon-insensitive enzyme(s) was occurring, and/or that CTF was being protected from further degradation by phosphoramidon.5. These data show that in the human forearm the activity of a phosphoramidon-sensitive ET converting enzyme is at least in part responsible for the vasoconstrictor properties of exogenous big ET-1. Furthermore, because measurable levels of newly synthesized ET-1 are likely to be rapidly reduced in the blood/plasma through receptor binding, assay of IR big ET-1 and CTF may be a more sensitive measure of ET-1 generation in disease.

Endothelins as Regulators of Vascular Tone in Man

1. The vascular pharmacology, physiological relevance and pathophysiological roles of the endothelium-derived vasoconstrictor peptide endothelin-1 have been unclear. These issues were investigated, in vivo in man, using infusion of drugs into the brachial artery or dorsal hand vein, with responses measured by forearm plethysmography and the hand vein displacement technique respectively. 2. Endothelin-1 is a potent and sustained constrictor of resistance and capacitance vessels in vivo in man, acting through both subtypes (ETA and ETB) of endothelin receptors. Endothelin-1 stimulates generation of vasodilator prostaglandins, but not of nitric oxide, that act to oppose its direct constrictor actions. Venoconstriction to endothelin-1 is blocked more effectively by K(+)-channel openers than by Ca(2+)-channel antagonists, suggesting a novel cellular mechanism of action for this peptide. 3. The forearm vasculature is able to convert the precursor big endothelin-1 to the mature peptide, endothelin-1, thus demonstrating the local presence of 'endothelin-converting enzyme' in man. Local inhibition of this enzyme, or blockade of ETA receptors, causes slow-onset forearm vasodilatation, suggesting that endogenously generated endothelin-1 contributes to basal resistance vessel tone in man. 4. Venoconstriction to endothelin-1 is selectively enhanced in patients with untreated essential hypertension. Endothelin-1 also potentiates sympathetically mediated vasoconstriction, but only in hypertensive subjects. 5. Endogenous generation of endothelin-1 plays a fundamental physiological role in the maintenance of basal vascular tone. Endothlin-converting enzyme inhibitors and endothelin receptor antagonists possess novel vasodilator properties and should represent a major therapeutic advance in cardiovascular disease.

Physiologic Role of Endothelin in Maintenance of Vascular Tone in Humans

Summary: The physiologic significance of endothelin-1 (ET-1) generation in human resistance vessels is unknown. We therefore investigated whether endothelin-converting enzyme (ECE) activity could be demonstrated in human vessels, and the effects of inhibition of the generation or actions of ET-1 on vascular tone in healthy men. Brachial artery infusion of local doses of big ET-1 caused a slow-onset, dose-dependent forearm vasoconstriction that was abolished by co-infusion of the ECE inhibitor phosphoramidon. Phosphoramidon did not affect responses to ET-1. Phosphoramidon caused slow-onset vasodilatation when infused alone, with blood flow increasing by 37% (p = 0.03). Vasoconstriction to ET-1 was completely abolished by co-infusion of the ETA receptor antagonist BQ-123 (p = 0.006), with forearm blood flow tending to increase. Infusion of BQ-123 alone resulted in progressive vasodilatation, with blood flow increasing by 64% (p = 0.007). These results suggest that endogenous generation of ET-1 contributes to the maintenance of vascular tone in states of normal and elevated blood pressure. ECE inhibitors and ETA receptor antagonists may have potential as vasodilators in the treatment of diseases associated with vasoconstriction, such as hypertension and chronic heart failure.

Physiologic Role of Endothelin in Maintenance of Vascular Tone in Humans

The physiologic significance of endothelin-1 (ET-1) generation in human resistance vessels is unknown. We therefore investigated whether endothelin-converting enzyme (ECE) activity could be demonstrated in human vessels, and the effects of inhibition of the generation or actions of ET-1 on vascular tone in healthy men. Brachial artery infusion of local doses of big ET-1 caused a slow-onset, dose-dependent forearm vasoconstriction that was abolished by co-infusion of the ECE inhibitor phosphoramidon. Phosphoramidon did not affect responses to ET-1. Phosphoramidon caused slow-onset vasodilatation when infused alone, with blood flow increasing by 37% (p = 0.03). Vasoconstriction to ET-1 was completely abolished by co-infusion of the ETA receptor antagonist BQ-123 (p = 0.006), with forearm blood flow tending to increase. Infusion of BQ-123 alone resulted in progressive vasodilatation, with blood flow increasing by 64% (p = 0.007). These results suggest that endogenous generation of ET-1 contributes to the maintenance of vascular tone in states of normal and elevated blood pressure. ECE inhibitors and ETA receptor antagonists may have potential as vasodilators in the treatment of diseases associated with vasoconstriction, such as hypertension and chronic heart failure.

Urinary and plasma endothelin 1 in essential hypertension and in hypertension secondary to renoparenchymal disease

An alteration in renal metabolism of endothelin may contribute to hypertension in the SHR and it has been shown that the excretion rate of endothelin is reduced in patients with essential hypertension. We measured plasma and urinary endothelin 1 (ET-1) in 20 untreated essential hypertensives with normal renal function, in eight normotensive healthy subjects, and in 13 hypertensive patients with primary renoparenchymal disease. Plasma ET-1 was higher (P < 0.01) in essential hypertensives (median 1.69, interquartile range 1.2-3.3 pg/ml) than in normal subjects (0.84, 0.37-1.10 pg/ml) but significantly less (P < 0.01) than in hypertensives with renoparenchymal disease (3.57, 1.45-9.52 pg/ml). ET-1 levels slightly correlated with diastolic pressure in essential hypertensives (r = 0.43, P < 0.05) and tended to be correlated with systolic pressure in hypertensives with renal disease (r = 0.47, P = 0.08). ET-1 excretion in essential hypertensives (137, 99-154 ng/24 h) and in normal subjects (120, 62-150 ng/24 h) was significantly lower than in renal hypertensives (191, 123-241 ng/24 h). The ET clearance/GFR ratio (ClET/GFR) was markedly reduced (30%, 21-67%) in essential hypertensives and substantially raised in renal hypertensives (164%, 86-314%) in comparison with normal subjects (83%, 35-94%). Since the ClET/GFR ratio should be 100% if all filtered ET-1 is excreted, the data indicate that ET-1 is synthesized at a reduced rate and/or broken down at an enhanced rate by the kidney in essential hypertension and confirm that there is a high ET-1 generation rate in remnant nephrons in hypertension secondary to renal disease.

Contribution of endogenous generation of endothelin-1 to basal vascular tone

Summary Endothelin-1 is an endothelium-derived vasoconstrictor peptide, possibly involved in the pathophysiology of cardiovascular disease. We examined the contribution of endogenously generated endothelin-1 to maintenance of peripheral vascular tone in healthy subjects by local intra-arterial administration of an inhibitor of endothelin converting enzyme, phosphoramidon, and of a selective endothelin receptor A antagonist, BQ-123. Brachial artery infusion of local doses of proendothelin-1, the precursor to endothelin-1, caused a slow-onset dose-dependent forearm vasoconstriction which was abolished by co-infusion of phosphoramidon. Phosphoramidon did not affect responses to endothelin-1. Phosphoramidon caused slow-onset vasodilatation when infused alone, with blood flow increasing by 37% at 90 min (p=0·03). Vasoconstriction to endothelin-1 was abolished by co-infusion of BQ-123 (p=0·006), with forearm blood flow tending to increase. Infusion of BQ-123 alone caused progressive vasodilatation, with blood flow increasing by 64% after 60 min (p=0·007). These results show that endogenous production of endothelin-1 contributes to the maintenance of vascular tone. Endothelin converting enzyme inhibitors and receptor antagonists may have therapeutic potential as vasodilators.

ACE Inhibitor-Mediated Attenuation of Mesangial Cell Growth: A Role for Endothelin

Endothelin modulates human mesangial cell (HMC) proliferation in response to angiotensin II (Ang II). Angiotensin converting enzyme inhibitors (ACEIs) have variable effects on HMC growth depending on culture conditions. No studies, however, have investigated the effects of ACEIs on HMC production of endothelin-1 in either actively proliferating or quiescent HMCs. The present study was designed to evaluate the effects of ACEIs on HMC-associated mitogenesis, cell counts, and endothelin-1 production in the presence and absence of insulin in both quiescent and proliferating HMCs. It tests the hypothesis that ACEIs attenuate HMC growth through a reduction in HMC-associated endothelin-1 generation. The effects of four different ACEIs, an Ang II receptor antagonist, losartan, and a monoclonal antibody to endothelin-1 were evaluated. ACEIs inhibited HMC mitogenesis and cell counts in proliferative but not quiescent cells. This was due to the absence of ACE activity in HMCs and its presence in 10% fetal calf serum. Both ACEIs and losartan reduced endothelin-1 production per cell. Compared to vehicle, losartan reduced the amount of endothelin-1 in conditioned media to a greater extent than any ACEI (2.2 +/- 0.3, captopril v 1.9 +/- 0.5, quinaprilat v 3.8 +/- 0.3 delta pg/cell x 10(-3) endothelin-1, losartan; P < .05). Moreover, insulin potentiated the antimitogenic effects of both ACEIs and losartan on HMCs. Lastly, the attenuated increase of endothelin-1 in conditioned media and associated antimitogenic effect on HMCs with losartan alone was not potentiated by the addition of any ACEI to losartan. These data provide indirect evidence that Ang II production may occur in culture media when both its precursors and a sufficient amount of converting enzyme activity are present. This is predicated on the observation that HMCs lack ACE activity and that ACEIs blunt mitogenesis of proliferating HMCs. The kinetics of this reaction, as well as the mechanism of how insulin potentiates the antimitogenic effects of ACEIs, were not studied.

Vasoconstrictors and Renal Protection Induced by β-Selective Adrenoceptor Antagonist Bisoprolol

We investigated the role of the vasoconstrictors endothelin-1 (ET-1) and thromboxane in renal protection by the beta 1-selective adrenoceptor antagonist, bisoprolol, in Dahl salt-sensitive rats (Dahl S) and salt-resistant rats (Dahl R). Six-week bisoprolol treatment (20 mg/kg chow) reduced systolic blood pressure (SBP) by 14% in Dahl S rats fed a high-salt (4% NaCl) diet. This BP reduction was accompanied by a decrease in aortic wall thickness. ET-1 and thromboxane released from renal cortex was significantly decreased by 17 and 30% with bisoprolol, respectively. Other prostaglandin synthesis was unaffected. Renal function such as proteinuria, N-acetyl-beta-D-glucosaminidase (NAG) excretion, and glomerular filtration rate (GFR) was not influenced by bisoprolol. Morphologic investigation showed that bisoprolol significantly improved glomerular sclerosis by 29% and attenuated arterial damage by 71%, although tubular injury was not affected. The more severe the glomerulosclerotic lesions, the greater the generation of thromboxane and ET. The arterial lesions were positively correlated to thromboxane generation. These data indicate that long-term bisoprolol treatment reduces vasoconstrictive ET-1 and thromboxane generation and that these alterations may be partly responsible for the amelioration of glomerular and arterial injury in Dahl S rats.

Effects of phosphoramidon in endothelial cell cultures on the endogenous synthesis of endothelin-1 and on conversion of exogenous big endothelin-1 to endothelin-1.

Several studies have shown that phosphoramidon (PHOS) reduces the release of endothelin-1 (ET-1) from cultured endothelial cells. Moreover, the main endothelin-converting enzyme (ECE) activity in these cells is a membrane-bound metallopeptidase that is also inhibited by PHOS. We have investigated further the role of the PHOS-sensitive ECE in the conversion of big ET-1 to ET-1. ET-1 was measured using a radioimmunoassay specific for the C-terminal ET[16-21] sequence. The effect of PHOS on the production of ET-1 from endogenous precursors was determined using cultured bovine aortic endothelial cells (BAECs) and the human endothelial cell line (EA.hy 926). The concentrations of ET-1 accumulating in the medium over 24 h from BAECs were lowered by PHOS (-27% 10 microM, -76% 100 microM). In contrast, with EA.hy 926 cells, the same concentrations of PHOS increased by five- to sixfold the amount of ET-1 present in the medium after 24-h incubation. In other experiments, incubation of big ET-1 (1 microM) with intact BAECs or EA.hy 926 cells resulted in the generation of ET-1, and with both cell types this was inhibited by PHOS (IC50: BAECs = 6.4 microM; EA.hy 926 = 0.26 microM). These results are consistent with both cell types having a PHOS-sensitive ECE that is readily accessible to exogenous big ET-1 and is therefore probably located on the plasma membrane. Furthermore, another intracellular ECE may play a part in the endogenous intracellular formation of ET-1 in EA.hy 926 cells.

The two-step conversion of big endothelin 1 to endothelin 1 and degradation of endothelin 1 by subcellular fractions from human polymorphonuclear leukocytes.

The metabolism of big endothelin 1 (bET) and endothelin 1 (ET-1) by subcellular fractions from human polymorphonuclear leukocytes (PMNs) was investigated by bioassay and reversed-phase high-performance liquid chromatography. More than 80% of endothelin-converting activity was recovered from the cytosolic fraction, which in addition to ET-1 generated other peptides from bET. The processing of bET to all its metabolites including ET-1 was prevented by the serine protease inhibitor 3,4-dichloroisocoumarin (DCI; 50 microM) or the elastase inhibitor ONO-5046 (100 microM) but not by phenylmethylsulfonyl fluoride (PMSF; 143 microM), another serine protease inhibitor. Paradoxically, human leukocyte elastase, despite generating a bET fragmentation pattern similar to that of PMN cytosol, produced very little ET-1. However, subsequent treatment of the elastase-derived metabolites of bET with PMN cytosol in the presence of ONO-5046 dramatically increased the amount of ET-1 formed. The generation of ET-1 following this intervention was inhibited by DCI. The PMN membrane preparation degraded ET-1 to a major metabolite, similar to that produced from ET-1 by elastase, and several minor products, paralleled by a loss of its smooth muscle contracting activity. The degradation of ET-1 by PMN microsomes was prevented by DCI, PMSF, or ONO-5046. Our results suggest that an elastase-initiated serine protease cascade is responsible for the sequential conversion of bET to ET-1 by the PMN cytosol. Elastase also partly accounts for the ET-metabolizing properties of PMN microsomes.

Phosphoramidon inhibits the generation of endothelin-1 from exogenously applied big endothelin-1 in cultured vascular endothelial cells and smooth muscle cells

When cultured porcine aortic endothelial cells (ECs) were incubated with porcine big endothelin-1 (bit ET-I 1–39), there was a time-dependent increase in immunoreactive (IR)-ET in the culture supernatant, in addition to an endogenous IR-ET release fron the cells. Reverse-phase HPLC of the culture supernatant revealed one major IR-ET component corresponding to the elution position of synthetic ET-1, thereby indicating that the additional increase in IR-ET was due to the conversion of big ET-1 to mature ET-1 1–21. Phosphoramidon, a metalloproteinase inhibitor, strongly suppressed this increase in IR-ET as well as the endogenous IR-ET release. Cultured vascular smooth muscle cells (VSMCs) also released IR-ET. The apparent conversion of exogenously applied big ET-1 to ET-1 and its inhibition by phosphoramidon were observed using cultured VSMCs, although the enzyme inhibitor did not influence the basal secretion of IR-ET from VSMCs. These results suggest that both cultured ECs and VSMCs can generate ET-1 from exogenously applied big ET-1 via action of the same type of phosphoramidon-sensitive metalloproteinase, which is also involved in the endogenous ET-1 generation in ECs.