Acetylcysteine has No Mechanistic Effect in Patients at Risk of Contrast-Induced Nephropathy: A Failure of Academic Clinical Science.

Contrast-induced nephropathy (CIN) is a major complication of imaging in patients with chronic kidney disease (CKD). The publication of an academic randomized controlled trial (RCT; n = 83) reporting oral (N)-acetylcysteine (NAC) to reduce CIN led to > 70 clinical trials, 23 systematic reviews, and 2 large RCTs showing no benefit. However, no mechanistic studies were conducted to determine how NAC might work; proposed mechanisms included renal artery vasodilatation and antioxidant boosting. We evaluated the proposed mechanisms of NAC action in participants with healthy and diseased kidneys. Four substudies were performed. Two randomized, double-blind, placebo-controlled, three-period crossover studies (n = 8) assessed the effect of oral and intravenous (i.v.) NAC in healthy kidneys in the presence/absence of iso-osmolar contrast (iodixanol). A third crossover study in patients with CKD stage III (CKD3) (n = 8) assessed the effect of oral and i.v. NAC without contrast. A three-arm randomized, double-blind, placebo-controlled parallel-group study, recruiting patients with CKD3 (n = 66) undergoing coronary angiography, assessed the effect of oral and i.v. NAC in the presence of contrast. We recorded systemic (blood pressure and heart rate) and renal (renal blood flow (RBF) and glomerular filtration rate (GFR)) hemodynamics, and antioxidant status, plus biomarkers of renal injury in patients with CKD3 undergoing angiography. Primary outcome for all studies was RBF over 8 hours after the start of i.v. NAC/placebo. NAC at doses used in previous trials of renal prophylaxis was essentially undetectable in plasma after oral administration. In healthy volunteers, i.v. NAC, but not oral NAC, increased blood pressure (mean area under the curve (AUC) mean arterial pressure (MAP): mean difference 29 h⋅mmHg, P = 0.019 vs. placebo), heart rate (28 h⋅bpm, P < 0.001), and RBF (714 h⋅mL/min, 8.0% increase, P = 0.006). Renal vasodilatation also occurred in the presence of contrast (RBF 917 h⋅mL/min, 12% increase, P = 0.005). In patients with CKD3 without contrast, only a rise in heart rate (34 h⋅bpm, P = 0.010) and RBF (288 h⋅mL/min, 6.0% increase, P = 0.001) occurred with i.v. NAC, with no significant effect on blood pressure (MAP rise 26 h⋅mmHg, P = 0.156). Oral NAC showed no effect. In patients with CKD3 receiving contrast, i.v. NAC increased blood pressure (MAP rise 52 h⋅mmHg, P = 0.008) but had no effect on RBF (151 h⋅mL/min, 3.0% increase, P = 0.470), GFR (29 h⋅mL/min/1.73m², P = 0.122), or markers of renal injury. Neither i.v. nor oral NAC affected plasma antioxidant status. We found oral NAC to be poorly absorbed and have no reno-protective effects. Intravenous, not oral, NAC caused renal artery vasodilatation in healthy volunteers but offered no protection to patients with CKD3 at risk of CIN. These findings emphasize the importance of mechanistic clinical studies before progressing to RCTs for novel interventions. Thousands were recruited to academic clinical trials without the necessary mechanistic studies being performed to confirm the approach had any chance of working.

Effect of polyphenol-rich grape seed extract on ambulatory blood pressure in subjects with pre- and stage I hypertension.

Dietary polyphenols, such as those from grape products, may exert beneficial effects on cardiovascular health, including anti-hypertensive effects. We investigated the effect of a specific grape seed extract (GSE) rich in low-molecular-weight polyphenolic compounds on ambulatory blood pressure (ABP) in untreated subjects with pre- and stage I hypertension. In addition, potential mechanisms that could underlie the hypothesised effect of GSE on blood pressure (BP), and platelet aggregation, were explored. The study was designed as a double-blind, placebo-controlled, randomised, parallel-group intervention study including seventy healthy subjects with systolic BP between 120 and 159 mmHg. A 1-week run-in period was followed by an 8-week intervention period, during which subjects consumed capsules containing either 300 mg/d of GSE or a placebo (microcrystalline cellulose). Before and after the intervention, daytime ABP readings, 24 h urine samples and fasting and non-fasting blood samples were taken. The mean baseline systolic BP was 135.8 (SE 1.3) mmHg and diastolic BP was 81.5 (SE 0.9) mmHg. BP values were modestly, but not significantly, affected by the polyphenol-rich GSE treatment v. placebo with an effect of - 3.0 mmHg for systolic BP (95% CI - 6.5, 0.5) and - 1.4 mmHg for diastolic BP (95% CI - 3.5, 0.6). Vasoactive markers including endothelin-1, NO metabolites and asymmetric dimethylarginine, plasma renin activity and platelet aggregation were not affected by the GSE intervention. Our findings show that consumption of polyphenol-rich GSE does not significantly lower ABP in untreated subjects with pre- and stage I hypertension.

Histamine-induced vasodilatation in the human forearm vasculature.

AIM: To investigate the mechanism of action of intra-arterial histamine in the human forearm vasculature. METHODS: Three studies were conducted to assess changes in forearm blood flow (FBF) using venous occlusion plethysmography in response to intra-brachial histamine. First, the dose-response was investigated by assessing FBF throughout a dose-escalating histamine infusion. Next, histamine was infused at a constant dose to assess acute tolerance. Finally, a four way, double-blind, randomized, placebo-controlled crossover study was conducted to assess FBF response to histamine in the presence of H1 - and H2 -receptor antagonists. Flare and itch were assessed in all studies. RESULTS: Histamine caused a dose-dependent increase in FBF, greatest with the highest dose (30 nmol min(-1) ) infused [mean (SEM) infused arm vs. control: 26.8 (5.3) vs. 2.6 ml min(-1) 100 ml(-1) ; P < 0.0001]. Dose-dependent flare and itch were demonstrated. Acute tolerance was not observed, with an increased FBF persisting throughout the infusion period. H2 -receptor antagonism significantly reduced FBF (mean (95% CI) difference from placebo at 30 nmol min(-1) histamine: -11.9 ml min(-1) 100 ml(-1) (-4.0, -19.8), P < 0.0001) and flare (mean (95% CI) difference from placebo: -403.7 cm(2) (-231.4, 576.0), P < 0.0001). No reduction in FBF or flare was observed in response to the H1 -receptor antagonist. Itch was unaffected by the treatments. Histamine did not stimulate vascular release of tissue plasminogen activator or von Willebrand factor. CONCLUSION: Histamine causes dose-dependent vasodilatation, flare and itch in the human forearm. H2 -receptors are important in this process. Our results support further exploration of combined H1 - and H2 -receptor antagonist therapy in acute allergic syndromes.

Endothelin-A receptor antagonism modifies cardiovascular risk factors in CKD.

Arterial stiffness and impaired nitric oxide (NO) bioavailability contribute to the high risk for cardiovascular disease in CKD. Both asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO production, and endothelin-1 (ET-1) oppose the actions of NO, suggesting that ET-1 receptor antagonists may have a role in cardiovascular protection in CKD. We conducted a randomized, double-blind, three-way crossover study in 27 patients with proteinuric CKD to compare the effects of the ET(A) receptor antagonist sitaxentan, nifedipine, and placebo on proteinuria, BP, arterial stiffness, and various cardiovascular biomarkers. After 6 weeks of treatment, placebo and nifedipine did not affect plasma urate, ADMA, or urine ET-1/creatinine, which reflects renal ET-1 production; in contrast, sitaxentan led to statistically significant reductions in all three of these biomarkers. No treatment affected plasma ET-1. Reductions in proteinuria and BP after sitaxentan treatment was associated with increases in urine ET-1/creatinine, whereas reduction in pulse-wave velocity, a measure of arterial stiffness, was associated with a decrease in ADMA. Taken together, these data suggest that ET(A) receptor antagonism may modify risk factors for cardiovascular disease in CKD.

Mechanisms for an effect of acetylcysteine on renal function after exposure to radio-graphic contrast material: study protocol.

BACKGROUND: Contrast-induced nephropathy is a common complication of contrast administration in patients with chronic kidney disease and diabetes. Its pathophysiology is not well understood; similarly the role of intravenous or oral acetylcysteine is unclear. Randomized controlled trials to date have been conducted without detailed knowledge of the effect of acetylcysteine on renal function. We are conducting a detailed mechanistic study of acetylcysteine on normal and impaired kidneys, both with and without contrast. This information would guide the choice of dose, route, and appropriate outcome measure for future clinical trials in patients with chronic kidney disease. METHODS/DESIGN: We designed a 4-part study. We have set up randomised controlled cross-over studies to assess the effect of intravenous (50 mg/kg/hr for 2 hrs before contrast exposure, then 20 mg/kg/hr for 5 hrs) or oral acetylcysteine (1200 mg twice daily for 2 days, starting the day before contrast exposure) on renal function in normal and diseased kidneys, and normal kidneys exposed to contrast. We have also set up a parallel-group randomized controlled trial to assess the effect of intravenous or oral acetylcysteine on patients with chronic kidney disease stage III undergoing elective coronary angiography. The primary outcome is change in renal blood flow; secondary outcomes include change in glomerular filtration rate, tubular function, urinary proteins, and oxidative balance. DISCUSSION: Contrast-induced nephropathy represents a significant source of hospital morbidity and mortality. Over the last ten years, acetylcysteine has been administered prior to contrast to reduce the risk of contrast-induced nephropathy. Randomized controlled trials, however, have not reliably demonstrated renoprotection; a recent large randomized controlled trial assessing a dose of oral acetylcysteine selected without mechanistic insight did not reduce the incidence of contrast-induced nephropathy. Our study should reveal the mechanism of effect of acetylcysteine on renal function and identify an appropriate route for future dose response studies and in time randomized controlled trials. TRIAL REGISTRATION: Clinical Trials.gov: NCT00558142; EudraCT: 2006-003509-18.

Selective endothelin-A receptor antagonism reduces proteinuria, blood pressure, and arterial stiffness in chronic proteinuric kidney disease.

Proteinuria is associated with adverse cardiovascular and renal outcomes that are not prevented by current treatments. Endothelin 1 promotes the development and progression of chronic kidney disease and associated cardiovascular disease. We, therefore, studied the effects of selective endothelin-A receptor antagonism in proteinuric chronic kidney disease patients, assessing proteinuria, blood pressure (BP), and arterial stiffness, key independent, surrogate markers of chronic kidney disease progression and cardiovascular disease risk. In a randomized, double-blind, 3-way crossover study, 27 subjects on recommended renoprotective treatment received 6 weeks of placebo, 100 mg once daily of sitaxsentan, and 30 mg once daily of nifedipine long acting. Twenty-four-hour proteinuria, protein:creatinine ratio, 24-hour ambulatory BP, and pulse wave velocity (as a measure of arterial stiffness) were measured at baseline and week 6 of each treatment. In 13 subjects, renal blood flow and glomerular filtration rate were assessed at baseline and week 6 of each period. Compared with placebo, sitaxsentan reduced 24-hour proteinuria (-0.56±0.20 g/d; P=0.0069), protein:creatinine ratio (-38±15 mg/mmol; P=0.0102), BP (-3.4±1.2 mm Hg; P=0.0069), and pulse wave velocity (-0.64±0.24 m/s; P=0.0052). Nifedipine matched the BP and pulse wave velocity reductions seen with sitaxsentan but did not reduce proteinuria. Sitaxsentan alone reduced both glomerular filtration rate and filtration fraction. It caused no clinically significant adverse effects. Endothelin-A receptor antagonism may provide additional cardiovascular and renal protection by reducing proteinuria, BP, and arterial stiffness in optimally treated chronic kidney disease subjects. The antiproteinuric effects of sitaxsentan likely relate to changes in BP and renal hemodynamics.

Contribution of endothelin 1 to the vascular effects of diesel exhaust inhalation in humans.

Diesel exhaust inhalation impairs vascular function, and, although the underlying mechanism remains unclear, endothelin (ET) 1 and NO are potential mediators. The aim of this study was to identify whether diesel exhaust inhalation affects the vascular actions of ET-1 in humans. In a randomized, double-blind crossover study, 13 healthy male volunteers were exposed to either filtered air or dilute diesel exhaust (331+/-13 microg/m(3)). Plasma concentrations of ET-1 and big-ET-1 were determined at baseline and throughout the 24-hour study period. Bilateral forearm blood flow was measured 2 hours after the exposure during infusion of either ET-1 (5 pmol/min) or the ET(A) receptor antagonist, BQ-123 (10 nmol/min) alone and in combination with the ET(B) receptor antagonist, BQ-788 (1 nmol/min). Diesel exhaust exposure had no effect on plasma ET-1 and big-ET-1 concentrations (P>0.05 for both) or 24-hour mean blood pressure or heart rate (P>0.05 for all). ET-1 infusion increased plasma ET-1 concentrations by 58% (P<0.01) but caused vasoconstriction only after diesel exhaust exposure (-17% versus 2% after air; P<0.001). In contrast, diesel exhaust exposure reduced vasodilatation to isolated BQ-123 infusion (20% versus 59% after air; P<0.001) but had no effect on vasodilatation to combined BQ-123 and BQ-788 administration (P>0.05). Diesel exhaust inhalation increases vascular sensitivity to ET-1 and reduces vasodilatation to ET(A) receptor antagonism despite unchanged plasma ET-1 concentrations. Given the tonic interaction between the ET and NO systems, we conclude that diesel exhaust inhalation alters vascular reactivity to ET-1 probably through its effects on NO bioavailability.

Blood pressure-independent reduction in proteinuria and arterial stiffness after acute endothelin-a receptor antagonism in chronic kidney disease.

Endothelin 1 is implicated in the development and progression of chronic kidney disease and associated cardiovascular disease. We, therefore, studied the effects of selective endothelin-A receptor antagonism with BQ-123 on key independent surrogate markers of cardiovascular risk (blood pressure, proteinuria and renal hemodynamics, arterial stiffness, and endothelial function) in patients with nondiabetic chronic kidney disease. In a double-blind, randomized crossover study, 22 subjects with proteinuric chronic kidney disease received, on 2 separate occasions, placebo or BQ-123. Ten of these subjects also received nifedipine (10 mg) as an active control for the antihypertensive effect of BQ-123. Blood pressure, pulse wave velocity, flow-mediated dilation, renal blood flow, and glomerular filtration rate were monitored after drug dosing. BQ-123 reduced blood pressure (mean arterial pressure: -7+/-1%; P<0.001 versus placebo) and increased renal blood flow (17+/-4%; P<0.01 versus placebo). Glomerular filtration rate remained unchanged. Proteinuria (-26+/-4%; P<0.01 versus placebo) and pulse wave velocity (-5+/-1%; P<0.001 versus placebo) fell after BQ-123, but flow-mediated dilation did not change. Nifedipine matched the blood pressure and renal blood flow changes seen with BQ-123. Nevertheless, BQ-123 reduced proteinuria (-38+/-3% versus 26+/-11%; P<0.001) and pulse wave velocity (-9+/-1% versus -3+/-1%; P<0.001) to a greater extent than nifedipine. Selective endothelin-A receptor antagonism reduced blood pressure, proteinuria, and arterial stiffness on top of standard treatment in renal patients. Furthermore, these studies suggest that the reduction in proteinuria and arterial stiffness is partly independent of blood pressure. If maintained longer term, selective endothelin-A receptor antagonism may confer cardiovascular and renal benefits in patients with chronic kidney disease.

Urinary endothelin-1 in chronic kidney disease and as a marker of disease activity in lupus nephritis.

Chronic inflammation contributes to the development and progression of chronic kidney disease (CKD). Identifying renal inflammation early is important. There are currently no specific markers of renal inflammation. Endothelin-1 (ET-1) is implicated in the pathogenesis of CKD. Thus, we investigated the impact of progressive renal dysfunction and renal inflammation on plasma and urinary ET-1 concentrations. In a prospective study, plasma and urinary ET-1 were measured in 132 subjects with CKD stages 1 to 5, and fractional excretion of ET-1 (FeET-1) was calculated. FeET-1, serum C-reactive protein (CRP), urinary ET-1:creatinine ratio, and urinary albumin:creatinine ratio were also measured in 29 healthy volunteers, 85 subjects with different degrees of inflammatory renal disease but normal renal function, and in 10 subjects with rheumatoid arthritis without renal involvement (RA). In subjects with nephritis associated with systemic lupus erythematosus (SLE), measurements were done before and after 6 mo of treatment. In subjects with CKD, plasma ET-1 increased linearly as renal function declined, whereas FeET-1 rose exponentially. In subjects with normal renal function, FeET-1 and urinary ET-1:creatinine ratio were higher in SLE subjects than in other groups (7.7 +/- 2.7%, 10.0 +/- 3.0 pg/mumol, both P < 0.001), and correlated with CRP, and significantly higher than in RA subjects (both P < 0.01) with similar CRP concentrations. In SLE patients, following treatment, FeET-1 fell to 3.6 +/- 1.4% (P < 0.01). Renal ET-1 production increases as renal function declines. In subjects with SLE, urinary ET-1 may be a useful measure of renal inflammatory disease activity while measured renal function is still normal.

Vascular effects of apelin in vivo in man.

OBJECTIVES: This study was designed to establish the direct vascular effects of apelin in vivo in man. BACKGROUND: Apelin is the endogenous ligand for the previously orphaned G-protein-coupled receptor, APJ. This novel pathway is widely expressed in the cardiovascular system and is emerging as an important mediator of cardiovascular homeostasis. In pre-clinical models, apelin causes venous and arterial vasodilation. METHODS: Vascular effects of apelin were assessed in 24 healthy volunteers. Dorsal hand vein diameter was measured by the Aellig technique during local intravenous infusions (0.1 to 3 nmol/min) of apelin-36, (Pyr(1))apelin-13, and sodium nitroprusside (0.6 nmol/min). Forearm blood flow was measured by venous occlusion plethysmography during intrabrachial infusions of apelin-36 and (Pyr(1))apelin-13 (0.1 to 30 nmol/min) and subsequently in the presence or absence of a "nitric oxide clamp" (nitric oxide synthase inhibitor, L-N(G)-monomethylarginine [8 mumol/min], coinfused with nitric oxide donor, sodium nitroprusside [90 to 900 ng/min]), or a single oral dose of aspirin (600 mg) or matched placebo. RESULTS: Although sodium nitroprusside caused venodilation (p < 0.0001), apelin-36 and (Pyr(1))apelin-13 had no effect on dorsal hand vein diameter (p = 0.2). Both apelin isoforms caused reproducible vasodilation in forearm resistance vessels (p < 0.0001). (Pyr(1))apelin-13-mediated vasodilation was attenuated by the nitric oxide clamp (p = 0.004) but unaffected by aspirin (p = 0.7). CONCLUSIONS: Although having no apparent effect on venous tone, apelin causes nitric oxide-dependent arterial vasodilation in vivo in man. The apelin-APJ system merits further clinical investigation to determine its role in cardiovascular homeostasis.

Fractional urinary excretion of endothelin-1 is reduced by acute ETB receptor blockade.

Evidence suggests that urinary excretion of endothelin-1 (ET-1) reflects renal ET-1 production and is independent of systemic ET-1 activity. The influence of ET receptors on urinary ET-1 excretion has not been studied in humans, yet peritubular ETB receptors are abundant within the kidney. We have studied the effects of acute ETA and ETB receptor blockade with BQ-123 and BQ-788, respectively, on urinary ET-1 excretion in a randomized, placebo-controlled, double-blind study in 16 subjects with a wide range of GFRs (15-152 ml/min). Plasma ET-1 concentrations (pET-1) and urinary ET-1 excretion rate (uET-1) at baseline correlated inversely with GFR (R2 = 0.18 and 0.36, respectively, P < 0.01). However, changes in pET-1 after ET receptor antagonism were not related to changes in uET-1 (R2 = 0.007, P = 0.18). pET-1 increased only after BQ-788, alone or in combination with BQ-123, consistent with ETB receptor-mediated clearance of ET-1 from the circulation. uET-1 was reduced only after BQ-788 alone [-4.7 pg/min (SD 5.5), P < 0.01]. Because BQ-788 also reduced GFR, fractional excretion of ET-1 (FeET-1) was calculated. FeET-1 fell after BQ-788 alone [-41% (SD 26%), P < 0.01] or in combination with BQ-123 [-40% (SD 29%), P < 0.01]. FeET-1 was not altered by placebo or BQ-123 alone. In conclusion, urinary ET-1 excretion does not appear to relate to the pool of plasma ET-1. Because of the short duration of this study, it is unlikely that ET receptor blockade had significant effects on renal ET-1 production. Therefore, the reduction in FeET-1 after ETB blockade appears to indicate that renal excretion of ET-1 is at least partly facilitated by ETB receptor activation.

Vasodilator effects of the endothelin ET receptor selective antagonist BMS-193884 in healthy men.

AIMS: A number of endothelin receptor antagonists (ERAs) are currently in clinical development as potential therapies in states characterized by vasoconstriction, such as systemic hypertension. We investigated the haemodynamic effects of locally and systemically active doses of BMS-193884, an endothelin A (ET(A)) receptor selective ERA, and its influence on vasoconstriction to endothelin-1 (ET-1) in healthy men. METHODS: In three separate randomized, placebo-controlled studies, the forearm blood flow (FBF) response to intra-arterial (i.a.) infusion of ET-1 (5 pmol min(-1)) was assessed during i.a. co-infusion of BMS-193884 (5 and 50 nmol min(-1)), and at 12 and at 24 h after oral administration of BMS-193884 (50, 100 and 200 mg at 12 h; and 200 mg at 24 h). Data were examined by repeated-measures analysis of variance (anova) with treatment and subject as factors. RESULTS: ET-1 caused significant forearm vasoconstriction, attenuated after oral dosing with BMS-193884 (200 mg) at 12 (P < 0.01) and 24 h (P < 0.0001). BMS-193884 (50 nmol min(-1), i.a.) caused local vasodilatation (25 +/- 11%) when infused alone (P = 0.02) and abolished forearm vasoconstriction to ET-1 (P < 0.0001 vs. ET-1 alone). Orally, BMS-193884 (200 mg) caused a reduction in total systemic vascular resistance at 12 (-14 +/- 9%, P = 0.03) and 24 h (-12 +/- 7%, P < 0.0001). There was no rise in plasma ET-1 levels. CONCLUSION: BMS-193884 causes local and systemic vasodilatation and attenuation of local vasoconstriction to ET-1. The absence of a rise in plasma endothelin levels suggests BMS-193884 is selective for the ET(A) receptor. This form of pharmacodynamic modelling may be useful in the development of ERAs in cardiovascular disease.

Neutral endopeptidase inhibition augments vascular actions of bradykinin in patients treated with angiotensin-converting enzyme inhibition.

Angiotensin-converting enzyme and neutral endopeptidase (EC 3.4.24.11; neprilysin) are metallopeptidases present on the endothelium that metabolize bradykinin. Inhibitors of angiotensin-converting enzyme potentiate bradykinin-mediated vasodilatation and endothelial tissue plasminogen activator release. Combined angiotensin-converting enzyme and neutral endopeptidase inhibition may have additional beneficial cardiovascular effects mediated through bradykinin potentiation. We investigated the effects of local neutral endopeptidase inhibition on the vascular actions of bradykinin in heart failure patients maintained on chronic angiotensin-converting enzyme inhibition. Ten patients received intrabrachial infusion of thiorphan (30 nmol/min), a neutral endopeptidase inhibitor, in a randomized double-blind placebo-controlled crossover trial. Thiorphan was coinfused with Lys-des-Arg9-bradykinin (1 to 10 nmol/min), bradykinin (30 to 300 pmol/min), atrial natriuretic peptide (10 to 100 pmol/min), and sodium nitroprusside (2 to 8 mug/min). Bradykinin, atrial natriuretic peptide, and sodium nitroprusside caused dose-dependent vasodilatation (peak blood flow 14.4+/-2.2, 3.6+/-0.6, and 8.6+/-1.3 mL per 100 mL/min, respectively; P<0.0001). Bradykinin caused dose-dependent increases in tissue plasminogen activator antigen and activity (peak concentration 31.8+/-3.4 ng/mL and 21.9+/-7.6 IU/mL, respectively; P<0.001) and estimated antigen and activity release (peak release 152+/-46 ng per 100 mL/min and 154+/-22 IU/100 mL/min, respectively; P<0.005). Compared with placebo, thiorphan augmented bradykinin-mediated vasodilatation (1.4-fold; P<0.0001) and net tissue plasminogen activator release (1.5-fold; P<0.005). Neutral endopeptidase contributes to bradykinin metabolism in heart failure patients maintained on angiotensin-converting enzyme inhibitor therapy. Our findings may explain some of the clinical effects of combined angiotensin-converting enzyme and neutral endopeptidase inhibition, including the greater vasodepressor effect observed with combined therapy when compared with angiotensin-converting enzyme inhibition alone.

Endothelin A receptor antagonism and angiotensin-converting enzyme inhibition are synergistic via an endothelin B receptor-mediated and nitric oxide-dependent mechanism.

Animal studies suggest that endothelin A (ETA) receptor antagonism and angiotensin-converting enzyme (ACE) inhibition may be synergistic. This interaction and the role of ETB receptors and endothelial mediators were investigated in terms of systemic and renal effects in humans in two studies. In one study, six subjects received placebo, the ETA receptor antagonist BQ-123 alone, and BQ-123 in combination with the ETB receptor antagonist BQ-788 after pretreatment with the ACE inhibitor enalapril (E) or placebo. In the other, six subjects who were pretreated with E received placebo, BQ-123, and BQ-123 with concomitant inhibition of nitric oxide (NO) synthase or cyclo-oxygenase (COX). Both were randomized, double-blind, crossover studies. Mean arterial pressure was reduced by BQ-123, an effect that was doubled during ACE inhibition (mean area under curve +/- SEM; BQ-123, -2.3 +/- 1.8%; BQ-123+E, -5.1 +/- 1.1%; P < 0.05 versus placebo). BQ-123 increased effective renal blood flow (BQ-123, -0.1 +/- 2.4%; BQ-123+E, 10.9 +/- 4.2%; P < 0.01 versus BQ-123), reduced effective renal vascular resistance (BQ-123, -1.2 +/- 3.1%; BQ-123+E, -12.8 +/- 3.0%; P < 0.01 versus placebo and versus BQ-123), and increased urinary sodium excretion markedly (BQ-123, 2.6 +/- 12.8%; BQ-123+E, 25.2 +/- 12.6%; P < 0.05 versus BQ-123, P < 0.01 versus placebo and versus E) only during ACE inhibition. These effects were abolished by both ETB receptor blockade and NO synthase inhibition, whereas COX inhibition had no effect. In conclusion, the combination of ETA receptor antagonism and ACE inhibition is synergistic via an ETB receptor-mediated, NO-dependent, COX-independent mechanism. The reduction of BP and renal vascular resistance and associated substantial natriuresis make this a potentially attractive therapeutic combination in renal disease.

Vitamin C has no effect on endothelium-dependent vasomotion and acute endogenous fibrinolysis in healthy smokers.

Blood flow and plasma fibrinolytic factors were measured on five occasions in both forearms of eight otherwise healthy male smokers during unilateral brachial artery infusion of the endothelium-dependent vasodilator, substance P (2 to 8 pmol/min), and the endothelium-independent vasodilator, sodium nitroprusside (2 to 8 microg/min). On the first occasion, intra-arterial vitamin C was co-infused at 25 mg/min. On subsequent occasions, subjects attended after 28 and 35 days treatment with oral vitamin C (1 g daily) or placebo in a double-blind randomized crossover design still smoking but with and without acute smoke inhalation (3 cigarettes over 30 minutes). Basal plasma ascorbate concentrations increased from 37 +/- 6 micromol/L to 105 +/- 11 micromol/L following oral vitamin C supplementation (P = 0.002). Substance P caused dose-dependent increases in forearm blood flow (P < 0.001, ANOVA) and t-PA release (P < 0.05, ANOVA) that was unaffected by acute recent smoke inhalation, intra-arterial vitamin C, or oral vitamin C administration (p = ns). Likewise there were no effects on sodium nitroprusside-induced vasodilatation (p = ns). Neither acute local intra-arterial nor prolonged oral vitamin C supplementation reverses smoking-related endothelial dysfunction and impaired endogenous t-PA release. We conclude that the adverse vascular actions of smoking are not principally mediated through oxidative stress.

Endothelin-A receptor antagonism reduces blood pressure and increases renal blood flow in hypertensive patients with chronic renal failure: a comparison of selective and combined endothelin receptor blockade.

BACKGROUND: Endothelin (ET) is implicated in the pathophysiology of chronic renal failure (CRF). We therefore studied the systemic and renal hemodynamic effects of ET receptor antagonists in CRF and examined differences between selective ETA, selective ETB, and combined ETA/B receptor blockade. METHODS AND RESULTS: We conducted a randomized, placebo-controlled, double-blind, 4-way crossover study comparing selective ET receptor antagonists BQ-123 (ETA) and BQ-788 (ETB), given alone and in combination, in acute studies in 8 hypertensive CRF patients and 8 matched healthy controls. BQ-123, alone and in combination with BQ-788, reduced blood pressure in CRF, particularly with BQ-123 alone (mean arterial pressure: controls -4+/-2%, CRF -13+/-2%, P<0.01 versus placebo). In CRF, in the face of this fall in blood pressure, BQ-123 substantially increased renal blood flow (38.8+/-23.9%, P<0.01 versus placebo) and reduced renal vascular resistance (-44.5+/-11.3%, P<0.01 versus placebo) when given alone but not when combined with BQ-788. These changes were accompanied by a reduction in effective filtration fraction. BQ-123, alone or in combination with BQ-788, had minimal effects on the renal circulation in healthy controls, and BQ-788 alone produced both systemic and renal vasoconstriction in CRF and healthy controls. CONCLUSIONS: ETA receptor antagonism was highly effective in lowering blood pressure in CRF patients currently treated for hypertension. In addition, there were effects consistent with a renoprotective action. However, because the ETB receptor appears to play a key role in the maintenance of tonic renal vasodilation, combined ETA/B receptor antagonism, although it lowered blood pressure, did not confer these renal benefits.

Determinants of metabotropic glutamate receptor-5-mediated Ca2+ and inositol 1,4,5-trisphosphate oscillation frequency. Receptor density versus agonist concentration.

Diverse patterns of Ca(2+)(i) release differentially regulate Ca(2+)-sensitive enzymes and gene transcription, and generally the extent of agonist activation of phospholipase C-linked G protein-coupled receptors determines the type of Ca(2+) signal. We have studied global Ca(2+) oscillations arising through activation of the metabotropic glutamate receptor mGluR5a expressed in Chinese hamster ovary cells and find that these oscillations are largely insensitive to agonist concentration. Using an inducible receptor expression system and a non-competitive antagonist, in conjunction with the translocation of eGFP-PH(PLCdelta) to monitor inositol 1,4,5-trisphosphate (InsP(3)) oscillations in single cells, we show that mGluR5a density determines the frequency of these oscillations. The predominant underlying mechanism resulted from a negative feedback loop whereby protein kinase C (PKC) inhibited InsP(3) generation. Down-regulation of PKC by prolonged exposure to phorbol ester revealed a second form of Ca(2+)(i) oscillation at low agonist concentrations. These Ca(2+)(i) signals showed features typical of classic repetitive Ca(2+)-induced Ca(2+) release and were sensitive to agonist concentration. Therefore, a single receptor can stimulate two types of InsP(3)-mediated Ca(2+) signal dependent upon feedback inhibition, producing two distinct means of controlling the final pattern of Ca(2+)(i) release. Our results have physiological implications for Ca(2+) signaling in general and emphasize the importance of mGluR5 surface expression for modulating synaptic plasticity.