Endothelin in cardiovascular control: the role of endothelin antagonists

Avosentan for overt diabetic nephropathy

In the short term, the endothelin antagonist avosentan reduces proteinuria, but whether this translates to protection from progressive loss of renal function is unknown. We examined the effects of avosentan on progression of overt diabetic nephropathy in a multicenter, multinational, double-blind, placebo-controlled trial. We randomly assigned 1392 participants with type 2 diabetes to oral avosentan (25 or 50 mg) or placebo in addition to continued angiotensin-converting enzyme inhibition and/or angiotensin receptor blockade. The composite primary outcome was the time to doubling of serum creatinine, ESRD, or death. Secondary outcomes included changes in albumin-to-creatinine ratio (ACR) and cardiovascular outcomes. We terminated the trial prematurely after a median follow-up of 4 months (maximum 16 months) because of an excess of cardiovascular events with avosentan. We did not detect a difference in the frequency of the primary outcome between groups. Avosentan significantly reduced ACR: In patients who were treated with avosentan 25 mg/d, 50 mg/d, and placebo, the median reduction in ACR was 44.3, 49.3, and 9.7%, respectively. Adverse events led to discontinuation of trial medication significantly more often for avosentan than for placebo (19.6 and 18.2 versus 11.5% for placebo), dominated by fluid overload and congestive heart failure; death occurred in 21 (4.6%; P = 0.225), 17 (3.6%; P = 0.194), and 12 (2.6%), respectively. In conclusion, avosentan reduces albuminuria when added to standard treatment in people with type 2 diabetes and overt nephropathy but induces significant fluid overload and congestive heart failure.

Avosentan reduces albumin excretion in diabetics with macroalbuminuria

Despite the first-line use of angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs), there is still a large need to improve the prevention and progression of diabetic nephropathy and its associated cardiovascular events. Endothelin antagonists have shown anti-inflammatory, antifibrotic, and antiproteinuric effects in experimental studies. This study was a randomized, placebo-controlled, double-blind, parallel-design, dosage-range study of the effect of the endothelin-A antagonist avosentan (SPP301) on urinary albumin excretion rate (UAER) in patients with diabetic nephropathy. We randomly assigned 286 patients with diabetic nephropathy, macroalbuminuria (UAER 0.2 to 5.6 mg/min), and BP <180/110 mmHg to 12 wk of avosentan (5, 10, 25, and 50 mg) or placebo, in addition to standard ACEI/ARB therapy. Relative to baseline, all avosentan dosages decreased mean relative UAER (-16.3 to -29.9%) compared with placebo (35.5%). Median relative UAER decreased with all avosentan dosages (-28.7 to -44.8%) compared with placebo (12.1%). Creatinine clearance and BP were unchanged at 12 wk. The main adverse events were peripheral edema (12%), mainly with high (>/=25 mg) dosages of avosentan; significant increases in liver enzymes did not occur. Twenty-one (7.3%) patients experienced adverse events that led to withdrawal from study medication. In summary, the endothelin-A antagonist avosentan given in addition to standard ACEI/ARB treatment decreases UAER in patients with diabetic nephropathy and macroalbuminuria.

Impaired endothelium-dependent vasodilation in overweight and obese adult humans is not limited to muscarinic receptor agonists

Muscarinic receptor agonists have primarily been used to characterize endothelium-dependent vasodilator dysfunction with overweight/obesity. Reliance on a single class of agonist, however, yields limited, and potentially misleading, information regarding endothelial vasodilator capacity. The aims of this study were to determine 1) whether the overweight/obesity-related reduction in endothelium-dependent vasodilation extends beyond muscarinic receptor agonists and 2) whether the contribution of nitric oxide (NO) to endothelium-dependent vasodilation is reduced in overweight/obese adults. Eighty-six middle-aged and older adults were studied: 42 normal-weight (54 +/- 1 yr, 21 men and 21 women, body mass index = 23.4 +/- 0.3 kg/m(2)) and 44 overweight/obese (54 +/- 1 yr, 28 men and 16 women, body mass index = 30.3 +/- 0.6 kg/m(2)) subjects. Forearm blood flow (FBF) responses to intra-arterial infusions of acetylcholine in the absence and presence of the endothelial NO synthase inhibitor N(G)-monomethyl-l-arginine, methacholine, bradykinin, substance P, isoproterenol, and sodium nitroprusside were measured by strain-gauge plethysmography. FBF responses to each endothelial agonist were significantly blunted in the overweight/obese adults. Total FBF (area under the curve) to acetylcholine (50 +/- 5 vs. 79 +/- 4 ml/100 ml tissue), methacholine (55 +/- 4 vs. 86 +/- 5 ml/100 ml tissue), bradykinin (62 +/- 5 vs. 85 +/- 4 ml/100 ml tissue), substance P (37 +/- 4 vs. 57 +/- 5 ml/100 ml tissue), and isoproterenol (62 +/- 4 vs. 82 +/- 6 ml/100 ml tissue) were 30%-40% lower in the overweight/obese than normal-weight adults. N(G)-monomethyl-l-arginine significantly reduced the FBF response to acetylcholine to the same extent in both groups. There were no differences between the groups in the FBF responses to sodium nitroprusside. These results indicate that agonist-stimulated endothelium-dependent vasodilation is universally impaired with overweight/obesity. Moreover, this impairment appears to be independent of NO.

Laser Doppler imager (LDI) scanner and intradermal injection for in vivo pharmacology in human skin microcirculation: responses to acetylcholine, endothelin-1 and their repeatability

AIMS

The purpose of this study was to evaluate the repeatability of forearm skin blood flow responses to intradermal injections of acetylcholine (ACh) and endothelin-1 (ET-1) using a double injection technique (DIT) and a laser Doppler imager (LDI) scanner in the human skin microcirculation.

METHODS

We used a laser Doppler imager (Moor LDI V3.01) to continuously monitor the change in skin blood flow during intradermal administration of physiological saline (0.9% NaCl), acetylcholine (ACh 10(-7), 10(-8), 10(-9) M) and endothelin-1 (ET-1 10(-14), 10(-16), 10(-18) M) in 10 healthy male subjects. Subjects were examined on 3 different days for assessment of interday and interobserver repeatability. Injections of either drug were randomly placed on different sites of the forearm. Laser Doppler images were collected before and after injection at 2.5 min intervals for 30 min. Data were analysed after the completion of each experiment using Moor Software V.3.01. Results are expressed as changes from baseline in arbitrary perfusion units (PU).

RESULTS

ACh caused a significant vasodilation (P < 0.0001 anova, mean +/- SE: 766 +/- 152 PU, ACh 10(-9) M; 1868 +/- 360 PU, ACh 10(-8) M; 4188 +/- 848 PU, ACh 10(-7) M; mean of days 1 and 2, n = 10), and ET-1 induced a significant vasoconstrictive response (P < 0.0001 anova, -421 +/- 83 PU, ET-1 10(-18) M; -553 +/- 66 PU, ET-1 10(-16) M; -936 +/- 90 PU, ET-1 10(-14) M; mean of days 1 and 2, n = 10). There was no difference on the response to either drug on repeated days. Bland-Altman analyses showed a close agreement of responses between days with repeatability coefficients of 1625.4 PU for ACh, and 386.0 PU for ET-1 (95% CI: ACh, -1438 to 1747 PU, ET-1, -399 to 358 PU) and between observers with repeatability coefficients of 1057.2 PU for ACh and 255.8 PU for ET-1 (95% CI: ACh, -1024 to 1048 PU, ET-1, -252 to 249 PU). The variability between these responses was independent of average flux values for both ACh and ET-1. There was a significant correlation between responses measured in the same site, in the same individual on two different days by the same observer (ACh, r = 0.94, P < 0.0001; ET-1, r = 0.90, P < 0.0006), and between responses measured by two different observers (ACh, r = 0.94, P < 0.0001; ET-1, r = 0.91, P < 0.0003).

CONCLUSION

We have shown that interday and intraobserver responses to intradermal injections of ET-1 and ACh, assessed using the DIT in combination with an LDI scanner, exhibited good reproducibility and may be a useful tool for studying the skin microcirculation in vivo.

INHIBITION OF CYCLIC STRAIN-INDUCED ENDOTHELIN-1 SECRETION BY TETRAMETHYLPYRAZINE

1. Chuanxiong is a Chinese herb that has been used widely in China to treat vascular disorders. 2,3,5,6-Tetramethylpyrazine (TMP) is one of the major components purified from chuanxiong. Many studies have demonstrated that TMP is effective in the treatment of cardiovascular diseases. However, the mechanism of action by which TMP exerts relaxation in vascular vessels remains unclear. 2. Endothelin (ET)-1 is a potent vasopressor synthesised by endothelial cells both in culture and in vivo. The aims of the present study were to test the hypothesis that TMP may alter strain-induced ET-1 secretion and to identify the putative underlying signalling pathways in endothelial cells. 3. We showed that TMP inhibits strain-induced ET-1 secretion. 2,3,5,6-Tetramethylpyrazine also inhibits the strain-induced formation of reactive oxygen species (ROS) and phosphorylation of extracellular signal-regulated kinases (ERK) 1/2. Furthermore, pretreating cells with TMP or the anti-oxidant N-acetyl-cysteine decreased strain-induced increases in ET-1 secretion and ERK1/2 phosphorylation. Using a reporter gene assay, TMP and N-acetyl-cysteine were demonstrated to also attenuate the strain-induced activity of the activator protein-1 reporter. 4. In summary, we have demonstrated, for the first time, that TMP inhibits strain-induced ET-1 gene expression, in part by interfering with the ERK1/2 pathway via attenuation of ROS formation. Thus, the present study provides important new insights into the molecular pathways that may contribute to the proposed beneficial effects of TMP in the vascular system.

Inhibition of Cyclic Strain-Induced Endothelin-1 Gene Expression by Resveratrol

Resveratrol is a phytoestrogen naturally found in grapes and is among the major constituents of wine thought to have a cardioprotective effect. Endothelin-1 (ET-1) is a potent vasopressor synthesized by endothelial cells both in culture and in vivo. The aims of this study were to test the hypothesis that resveratrol may alter strain-induced ET-1 gene expression and to identify the putative underlying signaling pathways in endothelial cells. We show that resveratrol indeed potently inhibits strain-induced ET-1 secretion, ET-1 mRNA level, and ET-1 promoter activity. Resveratrol also inhibits strain-increased NADPH oxidase activity, reactive oxygen species formation, and extracellular signal–regulated kinases1/2 (ERK1/2) phosphorylation. Furthermore, pretreating cells with resveratrol or antioxidant N -acetyl-cysteine decreases strain-increased or hydrogen peroxide–increased ET-1 secretion, ET-1 promoter activity, and ET-1 mRNA and ERK1/2 phosphorylation. Using both the electrophoretic mobility shift assay and a reporter gene assay, resveratrol and N -acetyl-cysteine also attenuated the strain-stimulated activator protein-1 binding activity and activator protein-1 reporter activity. In summary, we demonstrate for the first time that resveratrol inhibits strain-induced ET-1 gene expression, partially by interfering with the ERK1/2 pathway through attenuation of reactive oxygen species formation. Thus, this study provides important new insights in the molecular pathways that may contribute to the proposed beneficial effects of resveratrol in the cardiovascular system.

Enhanced vasoconstriction to endothelin-1, angiotensin II and noradrenaline in carriers of the GNB3 825T allele in the skin microcirculation

Hypertension is associated with enhanced peripheral vascular resistance, which may be mediated by enhanced vasoconstriction. The impact of the recently detected G-protein beta3-subunit gene C825T polymorphism on the response to the major pressor mediators has been studied in vivo in the human microcirculation. We assessed the effects of endothelin-1 (ET-1), angiotensin II (AT), endothelin-antagonists (BQ-123 and BQ-788) and noradrenaline (NA, each 10-16-10-8 mol) on vasoconstriction in the human skin microcirculation in vivo in 25 healthy male volunteers (13 with CC genotype, 12 TC/TT genotype) using laser Doppler flowmetry. The effects of endothelium-derived vasodilation on NA-induced effects were studied using the NO-synthase inhibitor l-nitro-monomethyl-arginine (L-NMMA) and the alpha2-adrenoceptor-antagonist yohimbine (YO). ET-1, AT and NA caused a dose-dependent vasoconstriction (P < 0.001). In carriers of the 825T allele the response to ET-1, AT and NA was significantly enhanced leading to a shift to the left of the dose-response curve of up to two log units (ET-1: P < 0.001 vs. CC; AT: P < 0.01 vs. CC; NA: P < 0.05 vs. CC). After pretreatment with L-NMMA or YO, NA induced vasoconstriction was no longer different between subjects with the CC- and CT/TT genotypes. However, following combined pretreatment with both L-NMMA and YO, vasoconstriction to NA was significantly potentiated in carriers of the T-allele. Vasodilatation to an ETA-antagonist (BQ-123) was more pronounced in the CT/TT genotype, while ETB-antagonism (BQ-788) led to a more pronounced vasoconstriction in the CT/TT genotype (not significant vs. CC). Healthy, normotensive carriers of the 825T-allele have enhanced vasoconstriction to ET-1, AT and NA in the skin microcirculation. This enhanced vasoconstriction appears to be partially antagonized by an enhanced release of endothelium derived vasodilators mediated by the stimulation of endothelial alpha2-adrenoceptors. The GNB3 C825T polymorphism is potentially an attractive pharmacogenetic marker to predict hormone-mediated responses in humans.

Endothelin-A receptor antagonist inhibits angiotensin II and noradrenaline in man

AIMS

Endothelin-1 (ET-1) is a potent vasoconstrictor produced by the vascular endothelium. The interactions of ET with the mediators of the sympathetic nervous system and the renin-angiotensin-system in humans are unclear.

METHODS

We studied the effects of the ETA-selective antagonist BQ-123 and the ETB-selective antagonist BQ-788 (both 10(-10)-10(-8) M) on ET-1 (10(-16)-10(-10) M), angiotensin II (AT, 10(-16)-10(-10) M) and noradrenaline (NA, 10(-16)-10(-10) M) induced vasoconstriction in the human skin microcirculation in vivo in 25 healthy male volunteers using laser Doppler flowmetry and double injection technique.

RESULTS

BQ-123 caused a dose-dependent vasodilatation (maximum effect: + 949 +/- 84 AUC-PU, P < 0.001), whereas BQ-788 induced mild vasoconstriction (maximum effect: -388 +/- 96 AUC-PU, P < 0.01). In the presence of BQ-123, but not BQ-788, ET-1, AT and NA caused markedly less vasoconstriction at any tested agonist dose; the effect was most pronounced on ET-1 (maximum effect at 10(-14) M: + 814 +/- 93 AUC-PU vs ET alone, P < 0.001), followed by noradrenaline (maximum effect at 10(-16) M: +580 +/- 107 AUC-PU vs NA alone, P < 0.01) and angiotensin II (maximum effect at 10(-14) M: + 493 +/- 111 AUC-PU vs AT alone, P < 0.001).

CONCLUSIONS

ETA-selective antagonism inhibits vasoconstriction to AT and NA in vivo in healthy subjects. This beneficial effect may be useful for the treatment of patients with cardiovascular disease including hypertension especially in combination therapy with sympatholytic agents and inhibitors of the renin-angiotensin system.

Antihypertensive drugs and the sympathetic nervous system

The sympathetic nervous system (SNS) plays an important role in the regulation of blood pressure homeostasis and cardiac function. Furthermore, the increased SNS activity is a predictor of mortality in patients with hypertension, coronary artery disease and congestive heart failure. Experimental data and a few clinical trials suggest that there are important interactions between the main pressor systems, i.e. the SNS, the renin-angiotensin system and the vascular endothelium with the strongest vasoconstrictor, endothelin. The main methods for the assessment of SNS activity are described. Cardiovascular drugs of different classes interfere differently with the SNS and the other pressor systems. Pure vasodilators including nitrates, alpha-blockers and dihydropyridine (DHP)-calcium channel blockers increase SNS activity. Finally, central sympatholytics and possibly phenylalkylamine-type calcium channel blockers reduce SNS activity. The effects of angiotensin-II receptor antagonists on SNS activity in humans is not clear; experimental data are discussed in this review. There are important interactions between the pressor systems under experimental conditions. Recent studies in humans suggest that an activation of the SNS with pure vasodilators in parallel increases plasma endothelin. It can be assumed that, in cardiovascular diseases with already enhanced SNS activity, drugs which do not increase SNS activity or even lower it are preferable. Whether this reflects in lower mortality needs to be investigated in intervention trials.