Neil R. Johnston

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.

Systemic Blockade of the Endothelin-B Receptor Increases Peripheral Vascular Resistance in Healthy Men

Endothelin-1 (ET-1) is an important mediator of vascular tone in humans, and a number of endothelin receptor antagonists are currently in clinical development as vasodilator agents. While the vasoconstrictor role of the ETA receptor is undisputed, the role of the ETB receptor remains unclear. Hemodynamic effects of systemic doses of the ETB-selective antagonist BQ-788 were investigated in 5 healthy male volunteers (age range, 33 to 48 years) in a placebo-controlled, four-way crossover study. After a 15-minute infusion of BQ-788 (3, 30, or 300 nmol/min) or placebo, plasma ET-1 and big ET-1, blood pressure, heart rate, cardiac index, and stroke index were measured. Total peripheral vascular resistance was calculated from cardiac index and mean arterial pressure. Hemodynamic data are expressed as maximum, placebo-corrected, percentage change from baseline following BQ-788 (300 nmol/min) and were examined by ANOVA. Plasma ET-1 increased by 3.7+/-1.2 pg/mL (maximum at 15 minutes, P=0.02), whereas there was no significant change in plasma big ET-1. Although BQ-788 had no effect on mean arterial pressure, there was a reduction in heart rate (13+/-7% at 50 minutes; P=0.002), cardiac index (17+/-5% at 40 minutes; P<0. 0001), and stroke index (8+/-4% at 40 minutes; P=0.002) and an increase in total peripheral vascular resistance (24+/-5% at 40 minutes; P<0.0001). The selective ETB receptor antagonist BQ-788 causes peripheral vasoconstriction in healthy volunteers, suggesting that the overall balance of effects of endogenous ET-1 at the vascular ETB receptor favors vasodilatation. Further investigation is now clearly required to address whether selective ETA or combined ETA/ETB receptor blockade will be more effective in the clinical setting.

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.

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.

Potentiation of bradykinin-induced tissue plasminogen activator release by angiotensin-converting enzyme inhibition.

OBJECTIVES The aim of the present study was to determine the effect of angiotensin-converting enzyme (ACE) inhibition on the local stimulated release of tissue plasminogen activator (t-PA) from the endothelium. BACKGROUND Angiotensin-converting enzyme inhibitor therapy may exert a beneficial effect on the endogenous fibrinolytic balance. METHODS Blood flow and plasma fibrinolytic factors were measured in both forearms of eight healthy males who received unilateral brachial artery infusions of the endothelium-dependent vasodilators substance P (2 to 8 pmol/min) and bradykinin (100 to 1,000 pmol/min), and the endothelium-independent vasodilator sodium nitroprusside (2 to 8 microg/min). These measurements were performed on each of three occasions following one week of matched placebo, quinapril 40 mg or losartan 50 mg daily administered in a double-blind randomized crossover design. RESULTS Sodium nitroprusside, substance P and bradykinin produced dose-dependent increases in the blood flow of infused forearm (analysis of variance [ANOVA], p < 0.001 for all). Although sodium nitroprusside did not affect plasma t-PA concentrations, they were increased dose-dependently in the infused forearm by substance P and bradykinin infusion (ANOVA, p < 0.001 for both). Bradykinin-induced release of active t-PA was more than doubled during treatment with quinapril in comparison to placebo or losartan (two-way ANOVA: p < 0.003 for treatment group, p < 0.001 for t-PA response and p = ns for interaction), whereas the substance P response was unaffected. CONCLUSIONS We have shown a selective and marked augmentation of bradykinin-induced t-PA release during ACE inhibition. These findings suggest that the beneficial clinical and vascular effects of ACE inhibition may, in part, be mediated through local augmentation of bradykinin-induced t-PA release.

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.

Fibrinolytic actions of intra-arterial angiotensin II and bradykinin in vivo in man.

OBJECTIVES Angiotensin II and bradykinin are potent endogenous vasoactive peptides which may play a role in the regulation of endogenous fibrinolysis and, thereby, contribute to the beneficial actions of ACE inhibitors. The aims of the study were to determine the acute effect of angiotensin II and bradykinin on the local vascular release of tissue plasminogen activator (t-PA) and its inhibitor, plasminogen activator inhibitor type 1 (PAI-1), and the endothelium-derived haemostatic factor, von Willebrand factor (vWf) from the forearm. METHODS Blood flow, and plasma haemostatic and fibrinolytic factors, were measured in both forearms of sixteen healthy men: eight subjects received intra-arterial angiotensin II (5, 50 and 500 pmol/min) which was coinfused with sodium nitroprusside (SNP; 0.3, 1.5 and 7.5 microg/min, respectively), and eight received intra-arterial bradykinin at 10-3000 pmol/min. RESULTS Despite substantial rises in plasma angiotensin II concentrations (P<0.001) which caused pressor effects (P<0.003) at the highest dose, angiotensin II infusion did not affect local plasma t-PA, PAI-1 or vWf concentrations. In contrast, bradykinin caused substantial dose-dependent increases in blood flow and t-PA release (>100 ng/100 ml of tissue/min) in the infused forearm (P<0. 001 for both) without affecting plasma PAI-1 or vWf concentrations. CONCLUSIONS Despite high local concentrations with breakthrough of significant systemic effects, angiotensin II did not affect acute endothelial cell t-PA, PAI-1 or vWf release in healthy men. In contrast, bradykinin is a potent vasodilator and selective stimulus for acute local t-PA release. This may, at least in part, explain the fibrinolytic actions of ACE inhibitors in heart failure and ischaemic heart disease.

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 &mgr;g/m3). 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 ETA receptor antagonist, BQ-123 (10 nmol/min) alone and in combination with the ETB 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 ETA 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.

Endogenous angiotensin II contributes to basal peripheral vascular tone in sodium deplete but not sodium replete man

OBJECTIVE Both endothelin-1 and nitric oxide make important contributions to the maintenance of basal peripheral arteriolar tone. However, the role of angiotensin II, a key hormone regulating cardiovascular and renal function, in the regulation of peripheral vascular tone has not been fully characterised. METHODS Using local intra-arterial administration of losartan, a selective angiotensin II type 1 (AT1) receptor antagonist, we examined the contribution of endogenous angiotensin II to the maintenance of basal and sympathetically stimulated vascular tone in the forearm of healthy man under conditions of sodium repletion and depletion. The effects of losartan on responses to exogenous angiotensin I, angiotensin II, bradykinin and noradrenaline were also determined. RESULTS Losartan, in keeping with its actions as a selective AT1 receptor antagonist, inhibited responses to angiotensin I and II, but had no effect on responses to bradykinin or noradrenaline. The dose of angiotensin II required to cause a 20% vasoconstriction was 40- and 250-fold greater with 30 and 300 micrograms/min of losartan, respectively. However, in sodium replete subjects, losartan alone caused no significant changes in basal forearm blood flow (95% confidence interval of -7.2 to +8.0%), forearm vascular resistance or sympathetically stimulated forearm vasoconstriction. Sodium depletion elevated plasma renin activity and angiotensin II concentrations (p < or = 0.002) after which acute local administration of losartan increased forearm blood flow in a dose dependent manner (maximum of 69 +/- 17%; p < 0.001). CONCLUSIONS Endogenous angiotensin II does not contribute to the acute local maintenance of basal peripheral vascular tone in healthy man except under conditions of renin-angiotensin system activation such as sodium depletion.

Chronic administration of losartan, an angiotensin II receptor antagonist, is not effective in reducing portal pressure in patients with preascitic cirrhosis

OBJECTIVES:Plasma angiotensin II (ANG II) concentrations are elevated in cirrhosis and have been implicated as a cause of portal hypertension. We aimed to study both the systemic and portal hemodynamics, and tolerability after chronic administration of losartan, an ANG II receptor antagonist.METHODS:Twelve patients with preascitic cirrhosis were studied: mean age of 53.8 ± 3.3 yr; average Child-Pugh score of 5.8 ± 0.3; alcohol etiology (5), hepatitis B/C (1/3), primary biliary cirrhosis (3). No patients were on diuretics or vasoactive medication. Hemodynamic measurements were performed at baseline and 4 weeks after daily administration of 25 mg losartan.RESULTS:There was no significant change in the hepatic venous pressure gradient (15.4 ± 1.5 to 13.6 ± 1.6 mmHg, −11.7%, p= NS), despite a significant reduction in the wedge hepatic venous pressure (20.3 ± 1.8 to 17.3 ± 1.8 mmHg, −14.8%, p< 0.05). Cardiac output, hepatic blood flow, systemic vascular resistance, creatinine clearance, and natriuresis were unaffected. The plasma renin activity increased significantly from 2.7 ± 0.4 to 5.2 ± 1.1 ng/ml/h (p< 0.05). There was a significant reduction in the mean arterial pressure from 96.9 ± 3.3 to 89.3 ± 3.5 mmHg, −7.8 ± 3.0% (p= 0.02), with 1 patient experiencing symptomatic hypotension.CONCLUSIONS:Chronic administration of low-dose losartan does not lead to a significant reduction in the portal pressure gradient. Losartan is unlikely to be useful in the management of patients with early cirrhosis, who are at risk of variceal bleeding.