Plasma immunoreactive endothelin in essential hypertension

Evaluation of plasma endothelin-1 levels in patients with cerebral infarction

Endothelin-1 (ET-1) is a vasoconstrictor peptide derived from endothelium. Many authors have shown that ischemic stroke is associated with elevated plasma ET-1 levels. Also, the present findings related to plasma ET-1 levels with clinical status, size of the infarction, location of the infarction, and prognosis of the cerebral infarction were contradictory. In this study, plasma ET-1 levels in 30 patients with cerebral infarction within 72 hours after the onset of focal neurologic deficit and at their seventh day postinfarction were measured by a microplate enzyme immunoassay. Thirty sex- and age-matched healthy subjects were accepted as a control group. The mean plasma ET-1 concentrations in patients on admission, in patients at day 7, and in control subjects were 1.93 +/- 1.79, 1.03 +/- 1.02, and 0.65 +/- 0.32 fmol/mL, respectively. The mean plasma ET-1 level of patients on admission was found to be significantly higher than in patients at day 7 and in control subjects (p < 0.05). No significant difference in ET-1 levels was observed between the patients at day 7 and control subjects. Furthermore, there was no correlation between plasma ET-1 concentration and size of infarction, location of infarction, degree of clinical neurologic deficit, or prognosis of cerebral infarction. It was concluded that plasma ET-1 levels shortly after ischemic stroke were increased, which may be associated with the acute-phase reaction of cerebral infarction and may have deleterious effects on development of neuronal injury.

Endothelin-1 and endothelin receptor antagonists as potential cardiovascular therapeutic agents

Endothelin (ET)-1 is an endothelium-derived peptide with potent vasoconstrictor and proliferative properties. The ET system is activated in several cardiovascular disease states associated with functional and structural vascular changes, including hypertension and heart failure. The two ET receptor subtypes are known as ET(A)R and ET(B)R. The former is located mainly on vascular smooth muscle cells and is responsible for mediating vasoconstriction and proliferation. The latter is present predominantly on endothelial cells and mediates vasorelaxation as well as ET-1 clearance. Activation of smooth muscle ET(B)R causes vasoconstriction. Selective ET(A)R antagonists as well as nonselective ET(A)R-ET(B)R antagonists have been developed. Studies with animal models and early-phase clinical trials provided strong evidence that these agents are effective in the treatment of heart failure, essential hypertension, pulmonary hypertension, and atherosclerosis. However, the complexity of biologic effects mediated by two different receptor subtypes complicates therapy with selective versus nonselective ET receptor antagonists. In addition to subtype selectivity and potency, changes in receptor subtype distribution under different pathologic conditions and different patient populations will play a crucial role in the evaluation of these potentially therapeutic drugs.

Therapeutic role of bosentan in hypertension: lessons from the model of perinephritic hypertension

Since its discovery in 1988, there has been increasing evidence that endothelin-1 (ET-1) plays an important role in the pathophysiology of hypertension and its related end-organ damages. First studies, using ET-1 administration in animals or in humans suspected this role by demonstrating the hypertensive properties of ET-1. The latter, due to stimulation of ET(A) receptors inducing sustained vasoconstriction have been reported to follow transient vasodilation linked with activation of an endothelial ET(B) receptor releasing nitric oxide (NO). In certain instances, ET(B) smooth-muscle receptors might also induce contraction. Cloning of these receptors helped to develop ET-1 receptor antagonists. As soon as one of them became available, bosentan, a dual (ET(A) and ET(B)) ET-1 receptor antagonist, we tested its effects in the canine model of perinephritic hypertension. Bosentan was found to exert striking hypotensive effects, due to peripheral vasodilation but without affecting cardiac function. In further experiments, we observed that effects of bosentan were additional to those of ACE inhibitors or angiotensin II antagonists. This opened new therapeutic perspectives and also suggested a proper role of ET-1 in hypertension, independent of the renin-angiotensin system. To explain this role, we demonstrated a real imbalance characterized by an impairment of the NO system in favor of the ET-1 pathway. Recent studies suggest that such an imbalance may also occur in human hypertension. Furthermore, the contribution of ET-1 to human hypertension appears more convincing since bosentan was shown to decrease blood pressure in hypertensive subjects. Finally, ET-1 receptor antagonists might be of therapeutic interest to prevent hypertension induced end-organ damages. Whether or not these compounds are able to prevent or to reverse target organ injuries in man remains to be investigated.

Role of endothelin in the control of peripheral vascular tone in human hypertension

Endothelins are potent 21 amino acid vasoconstrictor isopeptides produced in different vascular tissues, including vascular endothelium. Endothelin-1 is the main endothelin generated by the endothelium and probably the most important in the cardiovascular system. Endothelin-1 acts through specific receptors termed ET(A), represented only on smooth muscle cells and having the function of growth promotion and mediating contractions, and ET(B), located both on smooth muscle cells, where they evoke contractions, and on endothelial cells, inducing relaxation by production of the endothelium-derived relaxing factor nitric oxide. In physiological conditions endothelin-1 administration causes vasodilation and vasoconstriction at low and high concentrations, respectively. However, administration of mixed ET(A)/(B) receptor antagonists causes slight or absent vasodilation, indicating that the direct vasoconstrictor effect of the peptide is probably masked by ET(B)-induced NO-dependent vasodilation. In essential hypertensive patients, the activity of exogenous endothelin-1 is either increased, similar or decreased as compared to normotensive subjects, depending on which vascular district or scheme of administration is considered. But although available evidence does not indicate increased endothelin-1 plasma levels in patients with essential hypertension, simultaneous antagonism of ET(A)/(B) receptors causes a greater degree of vasodilation in hypertensives than in normotensive subjects. Moreover administration of a selective ET(B) receptor antagonist causes vasoconstriction in normotensive subjects and vasodilation in essential hypertensive patients. Finally, the vasodilating effect of a mixed ET(A)/(B) receptor antagonist is inversely related to NO availability. Taken together these findings suggest that essential hypertension is characterized by increased endothelin-1 vasoconstrictor tone. This alteration seems to be dependent on decreased endothelial ET(B)-mediated NO production attributable to impaired NO availability. In such conditions endothelial ET(B)-induced vasodilation no longer compensates for the direct classical endothelin vasoconstrictor effect mediated by smooth muscle cell ET(A) and ET(B) receptors. Therefore endothelin-1 could potentially be involved in the pathogenesis of essential hypertension or of its complications, and blockade of this system is a fascinating new target for therapeutic intervention in this disease.

Coronary vasodilation and improvement in endothelial dysfunction with endothelin ET(A) receptor blockade

The endothelium-derived peptide endothelin-1 (ET-1) causes vasoconstriction predominantly via smooth muscle ET(A) receptor activation. We hypothesized that ET(A) receptor inhibition would improve human coronary vascular function. We studied unobstructed coronary arteries of 44 patients with atherosclerosis or its risk factors. Epicardial diameter (D) and Doppler flow velocity were measured, and coronary vascular resistance (CVR) was calculated during intracoronary infusions of acetylcholine (ACH) and sodium nitroprusside (SNP), and during cold pressor testing, before and after a 60-minute intracoronary infusion of the ET(A) receptor antagonist BQ-123. BQ-123 dilated the coronary circulation; D increased by 5.6+/-1.0% (P<0.0001), and CVR fell by 12+/-3% (P<0.01). The D response to ACH, corrected for the SNP response, improved in segments that constricted with ACH at baseline (P=0.03), whereas segments that initially dilated with ACH did not change with BQ-123 (P=NS). Improvement in D and CVR responses to ACH with BQ-123 inversely correlated with baseline ACH responses (r=-0.44 [P=0.006] and r=-0.78 [P=0.001], respectively), indicating greater improvement in those with endothelial dysfunction. Similarly, cold pressor testing-mediated epicardial vasoconstriction (-2.0+/-1.1%) was reversed after BQ-123 (+1.0+/-0.7%), especially in dysfunctional segments (from -5.6+/-0.9% to +2.2+/-0.9%, P<0.001). There was no correlation between any risk factor and the response to BQ-123. An arteriovenous difference in ET-1 levels developed after BQ-123, which was consistent with enhanced cardiac clearance of ET-1, probably via ET(B) receptors. Thus, ET-1 acting via the ET(A) receptor contributes to basal human coronary vasoconstrictor tone and endothelial dysfunction. This suggests that ET(A) receptor antagonism may have therapeutic potential in the treatment of endothelial dysfunction and atherosclerosis.

Increased endothelin-1 levels in women with polycystic ovary syndrome and the beneficial effect of metformin therapy

Women with polycystic ovary syndrome who present with hyperandrogenemia, hyperinsulinemia, and insulin resistance appear to be at high risk of cardiovascular disease. Elevated levels of endothelin-1, a marker of vasculopathy, have been reported in insulin-resistant subjects with endothelial dysfunction. Male gender also seems to be an aggravating factor for cardiovascular disease. In this study we investigated endothelin-1 levels in women with polycystic ovary syndrome, and we evaluated the effect of an insulin sensitizer, metformin, on endothelin-1 levels. Plasma endothelin-1 levels were measured in 23 obese (mean age, 24.3 +/- 4.6 yr; body mass index, 35 +/- 5.6 kg/m(2)) and 20 nonobese women with polycystic ovary syndrome (24.1 +/- 3.6 yr; body mass index, 21.8 +/- 2.5 kg/m(2)) as well as in 7 obese and 10 nonobese healthy, normal cycling, age-matched women. Additionally, endothelin-1 levels were evaluated in a subgroup of women with polycystic ovary syndrome (10 obese and 10 nonobese) 6 months postmetformin administration (1700 mg daily). Our results showed that obese and nonobese women with polycystic ovary syndrome had higher levels of endothelin-1 compared with the controls [obese, 2.52 +/- 1.87 vs. 0.44 +/- 0.23 pmol/liter (by analysis of covariance, P < 0.02); nonobese, 1.95 +/- 1.6 vs. 0.43 +/- 0.65 pmol/liter (P < 0.009)]. All of the participating women with polycystic ovary syndrome (n = 43) when compared with the total group of controls (n = 17) demonstrated hyperinsulinemia (polycystic ovary syndrome, 24.5 +/- 19.6; controls, 11.2 +/- 3.4 U/liter; P < 0.03), lower glucose utilization (M40) during the hyperinsulinemic euglycemic clamps (3.4 +/- 2.4 vs. 5.6 +/- 1.75 mg/kg.min; P < 0.045, by one-tailed test), and higher levels of endothelin-1 (polycystic ovary syndrome, 2.52 +/- 1.87; controls, 0.44 +/- 0.23 pmol/liter; P < 0.02, analysis of covariance covariate for body mass index). A positive correlation of endothelin-1 with free T levels was also shown (r = 0.4, P = 0.002) as well as a negative correlation of endothelin-1 with glucose utilization (r = -0.3; P = 0.033) in the total studied population. Finally, after metformin therapy, endothelin-1 levels were significantly reduced in obese (endothelin-1 before, 3.25 +/- 2.2; endothelin-1 after, 1.1 +/- 0.9 pmol/liter; P < 0.003) and nonobese (endothelin-1 before, 2.7 +/- 2; endothelin-1 after, 0.7 +/- 0.4 pmol/liter; P < 0.01) women with polycystic ovary syndrome, with no change in body mass index. Moreover, after metformin therapy, hyperandrogenemia and hyperinsulinemia were normalized, and glucose utilization improved [obese before: total T, 0.9 +/- 0.15 ng/ml; fasting insulin, 22.2 +/- 12.1 U/liter; glucose utilization, 2.15 +/- 0.5 mg/kg.min; obese after: total T, 0.5 +/- 0.2 ng/ml; fasting insulin, 11.6 +/- 6 U/liter; glucose utilization, 4.7 +/- 1.4 mg/kg.min 9P < 0.003, P < 0.006, and P < 0.002, respectively); nonobese before: total T, 1 +/- 0.5 ng/ml; fasting insulin, 15.5 +/- 7.6 U/liter; glucose utilization, 3.4 +/- 0.7 mg/kg.min; nonobese after: total T, 0.8 +/- 0.5 ng/ml; fasting insulin, 9 +/- 3.8 U/liter; glucose utilization, 6 +/- 1.7 mg/kg.min (P < 0.04, P < 0.02, and P < 0.0008, respectively)]. In conclusion, our data clearly demonstrate that women with polycystic ovary syndrome, obese and nonobese, have elevated endothelin-1 levels compared with the age-matched control group. In addition, 6 months of metformin therapy reduces endothelin-1 levels and improves their hormonal and metabolic profile.

The relation between plasma endothelin-1 levels and metabolic control, risk factors, treatment modalities, and diabetic microangiopathy in patients with Type 2 diabetes mellitus

We evaluated the possible relation between plasma endothelin-1 (ET-1) levels and metabolic control, risk factors, treatment modalities, and diabetic microangiopathy, including nephropathy, neuropathy, and retinopathy in patients with Type 2 diabetes and healthy control group. Sixty-eight (39 females and 29 males) patients with Type 2 diabetes and 14 (6 females and 8 males) healthy subjects were included in the study. Plasma ET-1 levels were found to be 10.46+/-1.24 pmol/l in the diabetic group, whereas 7.97+/-0.41 pmol/l in the control group, which was statistically significant (P<.01). We also found elevated plasma ET-1 levels in patients with the least one microvascular complication when compared with the uncomplicated diabetes group (P=.02). Moreover, plasma ET-1 levels of the uncomplicated group was higher than the control group (P<.05). Plasma ET-1 levels were significantly elevated in hypertensive diabetics than normotensive diabetics (t=2.58, P=.012). It was also found to be elevated in diabetic patients with diabetes duration of more than 10 years when compared with patients less than 10 years (P=.02). These findings can be interpreted as the increased damage of microvascular complications in the disease process that may lead to elevated ET-1 levels. Mean plasma ET-1 levels in diabetic patients with a family history of diabetes was found to be higher than patients with no family history of diabetes. Genetical and environmental factors may have an effect on ET-1 level. We also studied the correlations of plasma ET-1 levels on age, sex, fasting blood glucose levels, treatment modalities HbA1c, hyperlipidemia, C-peptide, Body Mass Index, and smoking, but did not find any statistically significant difference. In conclusion, plasma ET-1 levels are well correlated with microangiopathy, hypertension, increased disease duration, and family history of diabetes, but poorly correlated with metabolic control, treatment modalities, age, sex, hyperlipidemia, obesity, and smoking.

Chronic endothelin-1 treatment leads to heterologous desensitization of insulin signaling in 3T3-L1 adipocytes

We recently reported that insulin and endothelin-1 (ET-1) can stimulate GLUT4 translocation via the heterotrimeric G protein G alpha q/11 and through PI3-kinase--mediated pathways in 3T3-L1 adipocytes. Because both hormones stimulate glucose transport through a common downstream pathway, we determined whether chronic ET-1 pretreatment would desensitize these cells to acute insulin signaling. We found that ET-1 pretreatment substantially inhibited insulin-stimulated 2-deoxyglucose uptake and GLUT4 translocation. Cotreatment with the ETA receptor antagonist BQ 610 prevented these effects, whereas inhibitors of G alpha i or G beta gamma were without effect. Chronic ET-1 treatment inhibited insulin-stimulated tyrosine phosphorylation of G alpha q/11 and IRS-1, as well as their association with PI3-kinase and blocked the activation of PI3-kinase activity and phosphorylation of AKT: In addition, chronic ET-1 treatment caused IRS-1 degradation, which could be blocked by inhibitors of PI3-kinase or p70 S6-kinase. Similarly, expression of a constitutively active G alpha q mutant, but not the wild-type G alpha q, led to IRS-1 degradation and inhibited insulin-stimulated phosphorylation of IRS-1, suggesting that the ET-1-induced decrease in IRS-1 depends on G alpha q/11 and PI3-kinase. Insulin-stimulated tyrosine phosphorylation of SHC was also reduced in ET-1 treated cells, resulting in inhibition of the MAPK pathway. In conclusion, chronic ET-1 treatment of 3T3-L1 adipocytes leads to heterologous desensitization of metabolic and mitogenic actions of insulin, most likely through the decreased tyrosine phosphorylation of the insulin receptor substrates IRS-1, SHC, and G alpha q/11.

Plasma endothelin concentrations in hypertension

The endothelins comprise a family of potent vasoconstricting peptides. Endothelin-1 appears to be the predominant isoform produced by the vascular endothelium, acting mainly in a paracrine fashion on vascular smooth muscle cells to cause vasoconstriction. It also has a range of other local actions--in the kidney, in the nervous system and on other hormone systems--that could, potentially, play a part in the genesis of hypertension. The association of raised plasma endothelin concentrations in human hypertension has caused much interest, but the literature is not consistent. Given the generally low plasma concentration of the endothelins, and their mainly paracrine actions, it remains unclear whether plasma endothelin has a functional role in hypertension. Additionally, problems remain with the measurement of plasma endothelin that raise doubts about the validity of conclusions drawn from these measurements.

Regulation of plasma endothelin by salt in salt-sensitive hypertension

BACKGROUND

Salt dependency of blood pressure (BP) characterizes most models of experimental hypertension in which endothelins play a significant vasoconstrictor role. Despite this, there are no data on the regulation of plasma endothelin by salt balance in human hypertension.

METHODS AND RESULTS

Plasma endothelin was measured in 47 patients with essential hypertension. Endothelin, catecholamine, and plasma renin activity (PRA) responses to 24-hour sodium deprivation (decreasing Na) were assessed in 29 of these patients. Endothelin was higher in hypertensive patients (4.6+/-0.2 fmol/mL) than in 20 control subjects (3.3+/-0.3 fmol/mL, P:<0.002), was correlated with BP, and was negatively associated with PRA (P:<0.04). Salt-sensitive, salt-resistant, and indeterminate groups were defined by the tertiles of the t statistic for the difference in BP before and after decreasing Na. Systolic BP falls were -15+/-1, -2+/-2, and -9+/-1 mm Hg, respectively. PRA, its response to decreasing Na, and its level after decreasing Na were lowest (albeit nonsignificant) in salt-sensitive patients. Baseline catecholamine and endothelin levels did not differ among the groups. In response to decreasing Na, catecholamines increased more significantly in salt-sensitive patients (+2.4+/-0.9 nmol/L) than in the other groups (0.4+/-0.2 and 0.7+/-0.2 nmol/L for indeterminate and salt-resistant groups, respectively; P:<0.03), whereas endothelin increased in the salt-sensitive group (0.8+/-0.3 fmol/mL), decreased in the salt-resistant group (-0.4+/-0.3 fmol/mL), and sustained minimal change in the indeterminate group (0.2+/-0.3 fmol/mL) (P:<0.04). Thus, endothelin levels in the salt-depleted state were highest in salt-sensitive patients (5.2+/-0.4 fmol/mL) versus the other groups (3.4+/-0.4 and 4.4+/-0.4 fmol/mL for salt-resistant and indeterminate groups, respectively) (P:<0.02). Changes in endothelin during decreasing Na and levels after decreasing Na were correlated with changes in catecholamines (P:<0.02).

CONCLUSIONS

-Our data suggest that salt-depleted salt-sensitive hypertensives with blunted renin responses exhibit enhanced catecholamine-stimulated endothelin levels and may therefore respond better than unselected patients with essential hypertension to endothelin receptor blockers.

The acute and chronic stimulatory effects of endothelin-1 on glucose transport are mediated by distinct pathways in 3T3-L1 adipocytes

We have recently shown that pretreatment with endothelin-1 (ET-1) for 20 min stimulates GLUT4 translocation in a PI3-kinase-dependent manner in 3T3-L1 adipocytes (Imamura, T. et al., J Biol Chem 274:33691-33695). This study presents another pathway by which ET-1 potentiates glucose transport in 3T3-L1 adipocytes. ET-1 treatment (10 nM) leads to approximately 2.5-fold stimulation of 2-deoxyglucose (2-DOG) uptake within 20 min, reaching a maximal effect of approximately 4-fold at approximately 6 h, and recovering almost to basal levels after 24 h. Insulin treatment (3 ng/ml) results in an approximately 5-fold increase in 2-DOG uptake at 1 h, and recovering to basal levels after 24 h. The ETA receptor antagonist, BQ 610, inhibited ET-1 induced glucose uptake both at 20 min and 6 h, whereas the ETB receptor antagonist, BQ 788, was without effect. Interestingly, ET-1 stimulated 2-DOG uptake at 6 h, not at 20 min, was almost completely blocked by the protein-synthesis inhibitor, cycloheximide and the RNA-synthesis inhibitor, actinomycin D, suggesting that the short-term (20 min) and long-term (6 h) effects of ET-1 involve distinct mechanisms. GLUT4 translocation assay showed that 20 min, but not 6 h, exposure to ET-1 led to GLUT4 translocation to the plasma membrane. In contrast, 6 h, but not 20 min, exposure to ET-1 increased expression of the GLUT1 protein, without affecting expression of GLUT4 protein. ET-1 induced 2-DOG uptake and GLUT1 expression at 6 h were completely inhibited by the MEK inhibitor, PD 98059, and partially inhibited by the PI3-kinase inhibitor, LY 294002, and the G alpha i inhibitor, pertussis toxin. The PLC inhibitor, U 73122, was without effect. These findings suggest that ET-1 induced GLUT1 protein expression is primarily mediated via MAPK, and partially via PI3K in 3T3-L1 adipocytes.

Endothelin: what does the radiologist need to know?

Endothelin (ET) is a potent endogenous vasoconstrictor peptide. It has been implicated in various pathological states since its discovery in 1988. The cardiovascular system and the kidneys are important sites for the action of this peptide. Two types of ET receptor, ETA and ETB, govern the biological effects of ET. Drugs that can prevent the endogenous synthesis of ET or block its binding to receptors may offer important therapeutic impact to patients with congestive heart failure, pulmonary hypertension and acute renal failure. Areas of particular interest to the radiologist include the role of ET in mediating some of the side effects of contrast media, particularly contrast medium nephropathy, and the involvement of ET in the pathogenesis of restenosis following angioplasty. This review outlines the basic biology of this important mediator and its role in health and disease.

Role of nitric oxide in a temperature dependent regulation of systemic vascular resistance in cardiopulmonary bypass

OBJECTIVES

Nitric oxide is the most potent vasodilator among inflammation-mediated vasoactive substances. Tepid cardiopulmonary bypass has been known to maintain low vascular resistance and nitric oxide may also be involved. There has been no previous clinical study elucidating a role of nitric oxide in a temperature dependent regulation of systemic vascular resistance in cardiopulmonary bypass.

METHODS

Thirty-one patients who underwent valvular surgery were randomly divided into two comparable groups; consisting of the hypothermic cardiopulmonary bypass (28 degrees C:14 patients) and the tepid cardiopulmonary bypass group (34 degrees C:17 patients). The serum levels of nitric oxide (NO(2)(-)+NO(3)(-)), prostaglandin E(2), bradykinin, 6-keto PGF1alpha, thromboxane B(2), endothelin-1, systemic vascular resistance index were measured before, 0, 12 and 24 h after cardiopulmonary bypass.

RESULTS

The pattern of change in systemic vascular resistance index and nitric oxide during and after cardiopulmonary bypass were significantly different between the two groups (P=0.0008, P=0.02). The tepid group showed significantly lower levels of systemic vascular resistance index after cardiopulmonary bypass than the hypothermic group (0 h: 2278+/-735 vs. 4387+/-1289, 12 h: 1827+/-817 vs. 2817+/-1146 and 24 h: 1690+/-548 vs. 2761+/-641 dyne s cm(-5) m(2), P=0.0001, P=0.03, P=0. 0006). The nitric oxide levels were significantly higher at 0, 12 and 24 h after cardiopulmonary bypass in the tepid group than those in the hypothermic group (84.7+/-33.3 vs. 46.3+/-18.1, 69.8+/-31.1 vs. 40.1+/-17.5 and 80.1+/-38.5 vs. 39.1+/-15.6 micromol/l, P=0.008, P=0.03, P=0.01). The prostaglandin E(2) levels in the tepid group was significantly higher just after cardiopulmonary bypass than that in the hypothermic group (37.3+/-20.0 vs. 15.8+/-8.6 pg/ml, P=0.02). The bradykinin level in the hypothermic group was significantly higher just after cardiopulmonary bypass than that in the tepid group (2.40+/-0.32 vs. 1.85+/-0.21 log(10) (pg/ml), P=0.005). Only nitric oxide showed a significant negative correlation with the systemic vascular resistance index both during and after cardiopulmonary bypass (r=-0.60, P<0.0001) as compared with prostaglandin E(2) and bradykinin.

CONCLUSIONS

These findings demonstrated that serum nitric oxide levels in tepid cardiopulmonary bypass were significantly higher than those in hypothermic cardiopulmonary bypass. Nitric oxide correlated with systemic vascular resistance. Thus, nitric oxide may play a pivotal role in a temperature dependent regulation of systemic vascular resistance in cardiopulmonary bypass.

Hypertension in black patients: an emerging role of the endothelin system in salt-sensitive hypertension

The prevalence of essential hypertension in blacks is much higher than that in whites. In addition, the pathogenesis of hypertension appears to be different in black patients. For example, black patients present with a salt-sensitive hypertension characterized by low renin levels. Racial differences in renal physiology and socioeconomic factors have been suggested as possible causes of this difference, but reasons for this difference remain unclear. Endothelial cells are important in the regulation of vascular tonus and homeostasis, in part through the secretion of vasoactive substances. One of these factors, endothelin-1 (ET-1), is a 21 amino acid residue peptide with potent vasopressor actions. In addition to its contractile effects, it has been shown to stimulate mitogenesis in a number of cell types. Moreover, ET-1 displays modulatory effects on the endocrine system, including stimulation of angiotensin II and aldosterone production and inhibition of antidiuretic hormone in the kidney. Recent data from several laboratories indicate that ET-1 is overexpressed in the vasculature in several salt-sensitive models of experimental hypertension. Moreover, circulating plasma ET-1 levels are significantly increased in black hypertensives compared with white hypertensives. Thus, the ET system might be particularly important in the development or maintenance of hypertension in this population.

Endothelin-1 attenuates bradykinin-induced hypotension in rats

Endothelin-1 has vasoconstrictor and mitogenic properties and may contribute to the pathogenesis of hypertension by enhancing vasoconstrictor mechanisms. In this study, we investigated the ability of endothelin-1 decrease the hypotensive effects of the vasodilator bradykinin in anesthetized rats. We also studied the effects a two-week oral pre-treatment with losartan (10 mg/kg/day) or enalapril (25 mg/kg/day) on endothelin-1-induced changes in the hypotensive responses to bradykinin. Bradykinin (0.4, 1.6, 6.4, and 25 mcg/kg, i.v.) induced dose-dependent hypotensive responses which were attenuated (P<0.05) by endothelin-1 (2 mcg/kg, i.v.). This effect of endothelin-1 was abolished by the mixed endothelin receptor antagonist N-Acetyl-alpha-[10,11-Dihydro-5H-dibenzo[a, d]cycloheptadien-5-yl]-D-Gly-Leu-Asp-Ile-Ile-Trp (PD145065, 1 mg/kg, i.v.). Endothelin-1 also decreased (P<0.05) the responses to bradykinin in rats pre-treated with losartan, but had no effect in rats pre-treated with enalapril. These results suggest that endothelin-1 may contribute to the development of hypertension by decreasing the responses to bradykinin through a mechanism not involving angiotensin AT(1) receptors, although the inhibition of angiotensin converting enzyme blunted the effect of endothelin-1.

Plasma endothelin is increased in early essential hypertension

Local vascular generation of endothelin-1 (ET-1) may contribute to elevated peripheral resistance in hypertension. We tested the hypothesis that immunoreactive ET production in the forearm circulation is increased in early essential hypertensive subjects. Ten young, previously untreated male patients with mild essential hypertension and no signs of target organ damage were compared with matched normotensive subjects in an outpatient setting. Arterial and venous samples were obtained from indwelling catheters in the brachial artery and the medial cubital vein, respectively. Samples were collected at baseline and after induction of endothelium-dependent (acetylcholine) vasodilation. Immunoreactive ET (ET) was measured after column extraction by a sensitive radioimmunoassay employing a C-terminal ET-1 antibody with negligible cross-reaction to big-ET. Individual recovery rates were determined for each sample. Basal ET was significantly higher in hypertensive than in normotensive subjects, both in venous and arterial samples (P < .01). This difference was also present after correction for recovery (P < .01). There was no significant difference between venous and arterial ET concentrations. Local vasodilation did not change arterial or venous ET levels. In conclusion, plasma ET is increased in young, untreated, essential hypertensive subjects with no signs of target organ damage. The increased circulating immunoreactive ET may point to a role for the peptide early in the development of high blood pressure.

Modulation of plasma endothelin levels by the menstrual cycle

Blood pressure varies during the menstrual cycle, but the reason for this is unclear. Administration of (synthetic) sex hormones can influence the level of vasoactive substances such as endothelin (ET). However, it is not known whether short-term variations in sex hormone levels in physiological situations affect ET levels. We assessed the effects of the menstrual cycle on plasma ET-1 in 8 healthy premenopausal women not using oral contraceptives (OCs) and 8 premenopausal women using OCs. ET-1 levels were measured in all subjects on days 1 to 3 (menstrual phase), 9 to 12 (follicular phase), and 20 to 23 (luteal phase) of the menstrual cycle. ET-1 levels remained constant in OC users (2.4 +/- 0.4, 2.6 +/- 0.4, and 2.4 +/- 0.4 pg/mL on days 1 to 3, 9 to 12, and 20 to 23 of the pill cycle). In contrast, ET-1 levels in non-OC users decreased in all women during the follicular and luteal phase of the menstrual cycle compared with the menstrual (low-estrogenic) phase (3.6 +/- 0.5, 2.8 +/- 0.5, and 2.9 +/- 0.3 pg/mL for the menstrual, follicular, and luteal phase, respectively, P < .01 for menstrual vfollicular and P < .01 for menstrual v luteal). The differences between OC users and nonusers were significant in the menstrual phase of the cycle (P < .01). We conclude that ET levels fluctuate during the menstrual cycle. Previously reported effects of the menstrual cycle on blood pressure may be partly explained by the effects of sex hormones on the level of vasoactive mediators. This fluctuation is not present in OC users. Studies on hemodynamic parameters in premenopausal women should account for hormonal variations in the various phases of the menstrual cycle.

Racial differences in endothelin-1 at rest and in response to acute stress in adolescent males

Blacks exhibit greater vasoconstriction-mediated blood pressure (BP) increases in response to stress than do whites. Endothelin-1 (ET-1), a potent vasoconstrictive peptide, has been proposed as having a role in racial differences in stress reactivity. We evaluated the hemodynamic and plasma ET-1 levels of 41 (23 whites, 18 blacks, mean age 18.6 years) normotensive adolescent males at rest and in response to a video game challenge and forehead cold stimulation. Measurements were performed at catheter insertion and before and immediately after the 2 stressors, which were separated by 20-minute rest periods. Blacks exhibited higher absolute levels of diastolic blood pressure, total peripheral resistance index, or both in response to catheter insertion and to the video game challenge and during recovery from video game challenge and cold stimulation (P<0. 05 for all). Blacks exhibited higher absolute levels of ET-1 at every evaluation point (P<0.05 for all) and greater increases in ET-1 in response to both stressors (ps<0.05). These findings suggest that altered endothelial function may be involved in racial differences in hemodynamic reactivity to stress and possibly in the development of essential hypertension.

Heparin reduces serum levels of endothelin-1 and hepatic ischemia reperfusion injury in rabbits

Since the role of heparin in hepatic ischemia/reperfusion (I/R) injury is still not fully understood, we investigated the effects of heparin on hepatic I/R injury in rabbits. Heparin was injected into rabbits after inducing partial hepatic ischemia for 1 h. Thereafter, the serum levels of endothelin-1 (ET-1) and liver transaminase, and tissue levels of oxidized and deoxidized hemoglobin (oxHb, deoxHb) in the reperfused liver were analyzed. Microscopic examinations were also performed. The increased serum levels of ET-1 and liver transaminase after reperfusion were significantly reduced by heparin (P > 0.01). Hepatic ischemia reduced oxHb and increased deoxHb. Reperfusion with heparin immediately reduced deoxHb and increased oxHb, and thereafter the balance between the two kinds of Hb also recovered. However, reperfusion without heparin did not demonstrate any similar recovery, but instead gradually exacerbated the dissociation. Microscopically, heparin appeared to normalize I/R-induced activation of hepatic stellate cells which are the target cells for ET-1. These results suggest that heparin improves the hepatic I/R injury caused by sinusoidal microscirculatory disturbances partly via an inhibition of the ET-1 increase.

Endothelin and the central and peripheral nervous systems: a decade of endothelin research

1. During the past decade, extensive investigation of the endothelin (ET) system, primarily characterized by its potent vasoactive peptide element ET-1, has suggested a prominent role for this humoral agent and its isopeptides in cardiovascular and neural regulation. 2. Major elements of this system, including its peptide isoforms, converting enzymes involved in their formation and metabolism, as well as multiple receptor subtypes, have been localized within various components of the cardiovascular system and the central and peripheral nervous systems. 3. An understanding of the possible roles for the ET system in neural regulation and development has progressed over the past several years; most notable is the influence of ET on the central control of cardiovascular function and sympathetic tone. 4. The present concentrated review of ET and the central and peripheral nervous systems is presented to introduce the other papers within this symposium by briefly describing the presence and influence of components of this unique peptide system within pertinent physiological structures involved in cardiovascular, adrenal, sympathetic and enteric developmental function.

Effects of doxazosin and atenolol on circulating endothelin-1 and von Willebrand factor in hypertensive middle-aged men

Elevated levels of endothelin-1 (ET-1) and von Willebrand factor (vWF), both markers indicative of endothelial function, are associated with hypertension. In a randomized open study we investigated the effect of antihypertensive treatment with the alpha-blocker doxazosin (n = 23) or the beta-blocker atenolol (n = 22) for 22 weeks on circulating levels of ET-1 and vWF in middle-aged men with essential hypertension. Blood pressure reduction was satisfactorily achieved with both drugs, although the decrease in the atenolol group was larger than that in the doxazosin group. A reduction in the levels of vWF occurred in both groups, being more pronounced in the alpha-blocker group compared with the decrease on beta blockers, p = 0.004 and p = 0.056, respectively. In the alpha-blocker group, there was a significant correlation (r = 0.50, p = 0.022) between the reduction in diastolic blood pressure and the decline in vWF. A highly significant decrease in plasma ET-1 was obtained during beta blockade (p = 0.007), whereas no significant change occurred within the alpha-blocker group. There was, however, no correlation between the decrease in blood pressure and the reduction in ET-1. The different favorable effects of alpha and beta blockers on endothelial function expressed as vWF and ET-1, could indicate that the effects are probably related not only to the blood pressure per se, but also to the different pharmacologic mechanisms of the drugs.

Plasma levels of nitric oxide and related vasoactive factors following long-term treatment with angiotensin-converting enzyme inhibitor in patients with essential hypertension

Several mechanisms other than the inhibition of systemic and local formation of angiotensin II (Ang II) have been proposed to play a role in mediating the hypotensive effects of angiotensin-converting enzyme (ACE) inhibitors. In the present study, we measured plasma levels of nitric oxide (NO) and the related vasoactive factors bradykinin, 6-keto prostaglandin F1alpha (6-keto PGF1alpha) a stable metabolite of prostacyclin, and cyclic guanosine-3',5'-monophosphate (cGMP) before and after a 4-week treatment with the ACE inhibitor lisinopril in 17 patients with essential hypertension. Plasma NO levels were measured by the Griess method after conversion of nitrate to nitrite. Long-term lisinopril treatment significantly reduced blood pressure and increased plasma NO and 6-keto PGF1alpha. The treatment also tended to increase plasma levels of bradykinin and cGMP, but not to a significant extent. The posttreatment NO level was inversely correlated with posttreatment systolic, diastolic, and mean blood pressure (n = 17, r= -.68, P< .01, n = 17, r= -.54, P < .05, and n = 17, r= -.66, P< .01, respectively). The posttreatment bradykinin level was also modestly correlated with posttreatment systolic and mean blood pressure (n = 17, r = -.51, P < .05 and n = 17, r = -.55, P < .05, respectively). In contrast, posttreatment 6-keto PGF1alpha and cGMP levels were not correlated with posttreatment systolic, diastolic, or mean blood pressure. These findings raise the possibility that increased formation of NO and bradykinin, as well as inhibition of the renin-angiotensin system, contribute to the hypotensive effect of the ACE inhibitor observed in our hypertensive patients.

State-of-the-Art lecture. Role of endothelin-1 in hypertension

Endothelin-1 (ET-1) is overexpressed in the vascular wall in certain models of experimental hypertension: deoxycorticosterone acetate salt-treated rats, deoxycorticosterone acetate salt-treated spontaneously hypertensive rats (SHR), stroke-prone SHR, Dahl salt-sensitive rats, angiotensin II-infused rats, and 1-kidney 1 clip Goldblatt rats; it is not overexpressed in SHR, 2-kidney 1-clip hypertensive rats, or L-NAME-treated rats. In hypertensive rats without generalized vascular overexpression, however, expression of ET-1 was often enhanced in intramyocardial coronary arteries, suggesting a role of endothelin in myocardial ischemia in hypertension. In rats overexpressing ET-1, ET(A/B) and ET(A)-selective receptor antagonists lowered blood pressure and reduced vascular growth, particularly in small arteries, beyond what could be attributed to blood pressure lowering, suggesting a direct effect of ET-1 on growth. Hypertensive rats treated with endothelin antagonists are protected from stroke and renal injury. The ET(A/B) antagonist bosentan induced blood-pressure reductions in mildly hypertensive patients similar to those achieved with an angiotensin-converting enzyme inhibitor. Moderately to severely hypertensive patients presented with enhanced expression of prepro-ET-1 mRNA in the endothelium of subcutaneous resistance arteries, suggesting that these stages of hypertension may respond particularly well to endothelin antagonism. Hypertensive patients with coronary artery disease have increased arterial expression of ET-1, and increased plasma levels of immunoreactive endothelin have been described in black patients. ET-1 plays an important role in atherosclerosis, for which hypertension is an important risk factor. Thus, ET-1 may be involved in experimental and human hypertension. Endothelin antagonists may prove effective as disease-modifying agents if they are shown clinically, as they are experimentally, to offer target organ protection and reduce long-term complications of hypertension. This remains to be demonstrated in humans.

Endothelin-1–Induced Interleukin-8 Production in Human Brain-Derived Endothelial Cells Is Mediated by the Protein Kinase C and Protein Tyrosine Kinase Pathways

We have previously demonstrated that endothelin-1 (Et-1) induces human central nervous system-derived endothelial cells (CNS-EC) to produce and secrete the chemokine interleukin 8 (IL-8). In the present study, we use specific inhibitors and activators to elucidate the signal transduction pathways involved in this process. Et-1–induced IL-8 production was blocked by ETA receptor antagonist BQ610, but not by ETB receptor antagonist BQ788, demonstrating that CNS-EC activation is initiated by Et-1 binding to the ETA receptor. IL-8 mRNA expression is blocked by the protein kinase C inhibitor bisindolylmaleimide or protein tyrosine kinase inhibitors, genestein and geldanamycin, establishing the involvement of the protein kinase C and protein tyrosine kinase pathways in the activation process. The transcription factor, NF-κB, is involved in Et-1 activation as determined by specific inhibitors of translocation and direct analysis of DNA-binding proteins. Neither inhibition nor activation of cAMP-dependent protein kinase affected IL-8 production in the absence or presence of Et-1. Similarly, no effect was observed upon inhibition of protein phosphatases by okadaic acid. Thus, the signal transduction process induced by Et-1 in CNS-EC, leading to increased mRNA IL-8 expression, is initiated by Et-1 binding to ETA receptor followed by subsequent activation of protein kinase C, protein tyrosine kinase, and NF-κB. Because increased expression of Et-1 is associated with hypertension and stroke and IL-8 is likely to be involved in the accumulation of neutrophils causing tissue damage in ischemic/reperfusion injury, identification of the mechanism involved in the Et-1–induced increase in IL-8 production may have significant therapeutic value.

Endothelin-1–Induced Interleukin-8 Production in Human Brain-Derived Endothelial Cells Is Mediated by the Protein Kinase C and Protein Tyrosine Kinase Pathways

We have previously demonstrated that endothelin-1 (Et-1) induces human central nervous system-derived endothelial cells (CNS-EC) to produce and secrete the chemokine interleukin 8 (IL-8). In the present study, we use specific inhibitors and activators to elucidate the signal transduction pathways involved in this process. Et-1–induced IL-8 production was blocked by ETA receptor antagonist BQ610, but not by ETB receptor antagonist BQ788, demonstrating that CNS-EC activation is initiated by Et-1 binding to the ETA receptor. IL-8 mRNA expression is blocked by the protein kinase C inhibitor bisindolylmaleimide or protein tyrosine kinase inhibitors, genestein and geldanamycin, establishing the involvement of the protein kinase C and protein tyrosine kinase pathways in the activation process. The transcription factor, NF-κB, is involved in Et-1 activation as determined by specific inhibitors of translocation and direct analysis of DNA-binding proteins. Neither inhibition nor activation of cAMP-dependent protein kinase affected IL-8 production in the absence or presence of Et-1. Similarly, no effect was observed upon inhibition of protein phosphatases by okadaic acid. Thus, the signal transduction process induced by Et-1 in CNS-EC, leading to increased mRNA IL-8 expression, is initiated by Et-1 binding to ETA receptor followed by subsequent activation of protein kinase C, protein tyrosine kinase, and NF-κB. Because increased expression of Et-1 is associated with hypertension and stroke and IL-8 is likely to be involved in the accumulation of neutrophils causing tissue damage in ischemic/reperfusion injury, identification of the mechanism involved in the Et-1–induced increase in IL-8 production may have significant therapeutic value.

Effects of low doses of endothelin-1 on basal vascular tone and autoregulatory vasodilation in canine coronary microcirculation in vivo

The plasma level of endothelin-1 (ET-1) increases in several cardiovascular disorders. The present study examined whether threshold doses of ET-1 affect vascular tone and autoregulatory vasodilation during a reduction in perfusion pressure in the coronary microcirculation in vivo. In anesthetized open-chest dogs, arterial microvessels in the epimyocardium were observed through a microscope equipped with a floating objective. In 6 dogs, ET-1 (10(-13) to 10(-8)mol/L) was superfused onto the epimyocardium in a cumulative fashion. In another set of dogs (n= 16), the perfusion pressure of the observed vascular bed was reduced to 60 mmHg (mild stenosis) and to 40 mmHg (severe stenosis) by a hydraulic occluder, and the microvascular responses were observed in the presence (n=9) or absence (n=7) of ET-1 (10(-12) or 10(-11) mol/L). ET-1 > or =10(-11) mol/L constricted coronary arterioles (< or =100 microm in diameter) and small arteries (>100 microm in diameter) in a dose-dependent fashion. ET-1 of 10(-12) mol/L affected neither the basal diameters nor the dilation of vessels during the pressure reduction. ET-1 of 10(-11) mol/L decreased the diameters of arterioles and small arteries before and during the mild and severe stenosis. However, ET-1 did not attenuate the percentage dilation of arterioles from the baseline in response to the mild and severe stenosis. The data indicates the following: (1) ET-1 at doses > or =10(-11) mol/L similarly constricts coronary arterioles and small arteries; (2) ET-1 at 10(-11) mol/L, which is slightly higher than the pathophysiological plasma level, increases the basal vascular tone, but does not attenuate the autoregulatory vasodilation of the coronary microcirculation.

Endothelial dysfunction and hypertensive vasoconstriction

A change in endothelial function is a common phenomenon in patients with essential hypertension and in animals with hypertension, whether primary or induced by a salt-rich diet. In hypertensive subjects, there may be a change in the synthesis, or the effect, of nitric oxide. Nevertheless, hypertensive vasoconstriction is at present associated, above all, with the degradation of this mediator by free radicals, such as the superoxide anion, released in the dysfunctional vascular endothelium. These radicals are also formed when hypoxanthine is turned into xanthine, and when the latter becomes uric acid, both having been catalysed by the enzyme xanthine oxidase. In physiological conditions, the concentration of superoxide radicals remains low within the organism as a result of its reaction with the superoxide dismutase enzyme. However, in pathological situations, such as arterial hypertension, there may be an increase in the production of these radicals or a deficiency of the superoxide dismutase enzyme. In hypertensive patients, the release of vasoconstrictor peroxides derived from the activity of cyclo-oxygenase in the endothelium and the vascular smooth muscle is also important. The excess free radicals released by the dysfunctional endothelium also stimulate the synthesis of these contracting agents. Moreover, it should not be forgotten that endothelin-1, which is similarly synthesized and released in the vascular endothelium, is the most powerful known endogenous vasoconstrictor. This peptide would therefore play a prominent part in some forms of hypertension. Although no changes in endothelin plasma levels have been found in essential hypertension, there may be an increase in its local concentration. It should be borne in mind that endothelin could strengthen the effect of other vasoconstrictors. Moreover, it may also provoke the release of free radicals and of cyclo-oxygenase-derived vasoconstrictor factors. The latest theories therefore indicate that the increase in vasoconstriction, which characterizes arterial hypertension, is associated with a greater production of free radicals. At the present time, antioxidant agents and xanthine oxydase-inhibiting compounds are being used to treat hypertension and other pathologies linked to endothelial dysfunction. In addition, it is thought that the therapeutic benefit of some anti-hypertensive drugs, such as calcium antagonists and angiotensin-converting enzyme inhibitors, could be in part due to the inhibition of the production of free radicals that they provoke.

Endothelin-1 enhances plasminogen activator inhibitor-1 production by human brain endothelial cells via protein kinase C-dependent pathway

The effects of endothelin-1 (ET-1) on the production of plasminogen activator inhibitor 1 (PAI-1) and tissue plasminogen activator (t-PA) by human brain-derived endothelial cells in culture were studied. At 100 nmol/L, ET-1 increased PAI-1 production by 88+/-6% within 72 hours, and increased PAI-1 mRNA expression within 1 hour of stimulation; there was no significant effect on t-PA production. PAI-1 activity was also examined and found to increase with ET-1 treatment. Suboptimal concentrations of ET-1 and tumor necrosis factor-alpha (TNF-alpha) acted synergistically to increase PAI-1 production. ET-1 activated protein kinase C and cAMP-dependent protein kinase pathways within 3 to 5 minutes of treatment, with the peak at 10 minutes. Activation of protein kinase C by phorbol-12-myristate-13-acetate (PMA) resulted in increased PAI-1 production, whereas activation of the cAMP-dependent protein kinase by forskolin or dibutyryl cAMP (dBu-cAMP) significantly decreased PAI-1 production. However, simultaneous activation of protein kinase C by PMA and cAMP-dependent protein kinase by dBu-cAMP only slightly attenuated PMA-induced PAI-1 increase. Inhibition of protein kinase C by GF-109213X abolished the effects of ET-1. These results demonstrate that ET-1 and TNF-alpha function synergistically to induce procoagulant activity of brain endothelial cells in a process that involves a protein kinase C-dependent pathway.

Role of endothelin in the increased vascular tone of patients with essential hypertension

We investigated the possible role of endothelin in the increased vasoconstrictor tone of hypertensive patients using antagonists of endothelin receptors. Forearm blood flow (FBF) responses (strain-gauge plethysmography) to intraarterial infusion of blockers of endothelin-A (ETA) (BQ-123) and endothelin-B (ETB) (BQ-788) receptors, separately and in combination, were measured in hypertensive patients and normotensive control subjects. In healthy subjects, BQ-123 alone or in combination with BQ-788 did not significantly modify FBF (P=0.78 and P=0.63, respectively). In hypertensive patients, in contrast, BQ-123 increased FBF by 33+/-7% (P<0.001 versus baseline), and the combination of BQ-123 and BQ-788 resulted in a greater vasodilator response (63+/-12%; P=0.006 versus BQ-123 alone in the same subjects). BQ-788 produced a divergent vasoactive effect in the two groups, with a decrease of FBF (17+/-5%; P=0.004 versus baseline) in control subjects and transient vasodilation (15+/-7% after 20 minutes) in hypertensive patients (P<0.001, hypertensives versus controls). The vasoconstrictor response to endothelin-1 was slightly higher (P=0.04) in hypertensive patients (46+/-4%) than in control subjects (32+/-4%). Our data indicate that patients with essential hypertension have increased vascular endothelin activity, which may be of pathophysiological relevance to their increased vascular tone. In these patients, nonselective ETA and ETB blockade seems to produce a greater vasodilator effect than selective ETA blockade.

Evidence that endothelin-1 (ET-1) inhibits insulin-stimulated glucose uptake in rat adipocytes mainly through ETA receptors

The specificity of endothelin (ET) receptors involved in the inhibition of insulin-stimulated glucose uptake (ISGU) in rat adipocytes was investigated. Adipocytes were isolated from the epididymal fat pads of Sprague-Dawley rats. To determine receptor subtypes, we used three ET isopeptides, ET-1 and ET-2, both of which are nonselective agonists, and ET-3, a selective agonist for ETC receptors, to displace [125I]ET-1 binding from the fat cells. The efficiency of displacement was ET-1 > ET-2 >> ET-3, indicating that the primary receptors involved belonged to the ETA subtype. At an equal concentration of 1 micromol/L, BQ-610, a selective ETA antagonist, displaced [125I]ET-1 from binding to fat cells, whereas IRL-1038, a selective ETB antagonist, did not. Using [3H]2-deoxy-D-1-glucose ([3H]2-DG) as a tracer in studies of glucose uptake, we found that equimolar BQ-610 completely reversed the inhibitory effect of ET-1 on ISGU, whereas IRL-1038 was ineffective. Northern blot analysis of adipocyte receptors showed abundant mRNA for ETA, but no ETB subtype. These results clearly demonstrate that ETA is the predominant receptor in rat adipocytes.

Overexpression of vascular endothelin-1 and endothelin-A receptors in a fructose-induced hypertensive rat model

OBJECTIVE

To examine the temporal relationship between hyperinsulinemia and hypertension in the fructose-hypertensive rat model and to study the function of endothelin-1 (ET-1) in fructose-induced hypertension.

DESIGN

Since ET-1 induces insulin resistance in conscious rats, we tested the hypothesis that both hyperinsulinemia and hypertension developed in the fructose-hypertensive rat model might be the sequelae of an elevated tissue content of ET-1 and ET(A) receptors.

MATERIALS AND METHODS

Systolic hypertension was induced within 3 weeks in male Sprague-Dawley rats fed on a fructose-rich diet. After continual monitoring of blood pressure and plasma insulin concentrations, the animals were killed at the end of experiment to determine plasma levels of ET-1, the contractile response of aortic rings to ET-1, and ET-1 and ET(A) receptor gene expressions. In a separate experiment, BQ-610 was administered to lower the effect of ET-1 in rats with fructose-induced hypertension.

RESULTS

Compared with control rats given normal chow, the fructose-fed rats developed systolic hypertension after 3 weeks of the diet (127+/-3.7 versus 110+/-5.5 mmHg, P < 0.01) and hyperinsulinemia both before (1 07.1+/-32.5 versus 48.5+/-14.3 pmol/l, P < 0.005) and after (96.6+/-63.7 versus 50.4+/-5.6 pmol/l, P< 0.05) they became hypertensive. Although plasma ET-1 levels did not differ between the rat groups, aortic ring contraction-concentration curves, indicating vessel contractility in response to ET-1, were significantly greater in these rats than in controls (F1,72 = 12.34, P< 0.00077). Messenger RNA extracted from the tail arteries and blotted with both ET-1 and ET(A) probes showed that fructose-fed rats had greater ET-1 and ET(A)-receptor gene expression than control rats. Concomitant administration of BQ-610 to rats fed on a fructose diet significantly reduced the hypertension. Conclusions These findings suggest that elevated vascular expression of ET-1 and ET(A) receptor genes may mediate the development of hypertension and hyperinsulinemia in rats fed a fructose-rich diet

Endothelin and endothelin antagonists in hypertension

The endothelins (ET) are potent 21-amino-acid vasoconstrictor peptides produced in many different tissues, particularly in the endothelium of blood vessels. ET-1 is the main endothelin secreted by the endothelium, and acts in a paracrine or autocrine fashion on blood vessels by interacting with ETA or ETB receptors on smooth muscle to stimulate contraction or on ETB receptors on endothelial cells to induce the release of vasorelaxants (nitric oxide and prostacyclin). Production of ET-1 is enhanced in several experimental models of hypertension in the rat, such as sodium-sensitive forms, e.g. deoxycorticosterone acetate (DOCA)-salt hypertensive, DOCA-salt-treated spontaneously hypertensive rats (SHR) and Dahl salt-sensitive rats, as well as other models such as stroke-prone SHR, angiotensin II-infused rats and fructose-fed rats, and possibly 1-K 1C Goldblatt hypertensive rats. In contrast, SHR, 2-K 1C Goldblatt hypertensive rats and nitric oxide-deficient (L-NAME-treated) hypertensive rats do not exhibit an ET-1 component. Endothelin dependency is manifested by excessive vascular growth, particularly in small arteries, and blood pressure lowering and regression of vascular growth after treatment with endothelin antagonists. The latter may be combined ETA/ETB or selective ETA antagonists, of which several are orally active and already in clinical development. In humans, endothelin-dependent vascular tone has been shown in studies of forearm blood flow. Enhanced expression of ET-1 mRNA has been demonstrated in the endothelium of small arteries of patients with moderate to severe hypertension. In a 4-week trial the combined ETA/ETB antagonist bosentan reduced the blood pressure of essential hypertensive patients equally to enalapril. Bosentan improved hemodynamics in patients with heart failure in acute and 2-week-long studies. Endothelin antagonists also offer promise in a rapidly fatal condition, primary pulmonary hypertension. Thus, the endothelin system appears to be involved in different forms of cardiovascular disease in experimental animals and humans, and its interruption offers great promise as a new therapeutic intervention in hypertension, heart failure and other diseases.

Endothelin-1 Induces Production of the Neutrophil Chemotactic Factor Interleukin-8 by Human Brain-Derived Endothelial Cells

Increased levels of endothelin-1 (Et-1), a potent vasoconstrictor, have been correlated with hypertension and neuronal damage in ischemic/reperfusion injury. The presence of polymorphonuclear cells (PMNs) in the brain has been shown to be directly responsible for this observed pathology. To address the question of whether Et-1 plays a role in this process, human brain-derived endothelial cells (CNS-ECs) were cultured with Et-1. The results demonstrate that Et-1 induces production of the neutrophil chemoattractant interleukin-8 (IL-8) twofold to threefold after 72 hours; mRNA was maximal after 1 hour of stimulation. Conditioned culture medium derived from Et-1–stimulated CNS-ECs induced a chemotactic response in the PMN migration assay. The inflammatory cytokines tumor necrosis factor- (TNF) and IL-1β functioned additively with Et-1 in increasing IL-8 production. In contrast, transforming growth factor-β (TGF-β), but not IL-10, completely abolished the effect of Et-1 on IL-8 production. However, Et-1 did not modulate intercellular adhesion molecule-1 (ICAM-1) expression. These data demonstrate that Et-1 may be a risk factor in ischemic/reperfusion injury by inducing increased levels of the neutrophil chemoattractant IL-8.

© 1998 by The American Society of Hematology.

Endothelin-1 Induces Production of the Neutrophil Chemotactic Factor Interleukin-8 by Human Brain-Derived Endothelial Cells

Increased levels of endothelin-1 (Et-1), a potent vasoconstrictor, have been correlated with hypertension and neuronal damage in ischemic/reperfusion injury. The presence of polymorphonuclear cells (PMNs) in the brain has been shown to be directly responsible for this observed pathology. To address the question of whether Et-1 plays a role in this process, human brain-derived endothelial cells (CNS-ECs) were cultured with Et-1. The results demonstrate that Et-1 induces production of the neutrophil chemoattractant interleukin-8 (IL-8) twofold to threefold after 72 hours; mRNA was maximal after 1 hour of stimulation. Conditioned culture medium derived from Et-1–stimulated CNS-ECs induced a chemotactic response in the PMN migration assay. The inflammatory cytokines tumor necrosis factor- (TNF) and IL-1β functioned additively with Et-1 in increasing IL-8 production. In contrast, transforming growth factor-β (TGF-β), but not IL-10, completely abolished the effect of Et-1 on IL-8 production. However, Et-1 did not modulate intercellular adhesion molecule-1 (ICAM-1) expression. These data demonstrate that Et-1 may be a risk factor in ischemic/reperfusion injury by inducing increased levels of the neutrophil chemoattractant IL-8.

© 1998 by The American Society of Hematology.

Endothelial dysfunction in hypertension

The endothelium modulates the tone of the underlying vascular smooth muscle by releasing relaxing factors, including prostacyclin, nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). In most types of hypertension, endothelium-dependent relaxations are impaired because of a reduced production and/or action of endothelium-derived NO and EDHF. In essential hypertension, endothelium-dependent relaxations are reduced because of a concomitant release of vasoconstrictor prostanoids (endoperoxides and thromboxane A2). These prostanoids may be produced in the vascular smooth muscle rather than in the endothelium. The endothelial dysfunction observed in hypertension is likely to be a consequence rather than a cause of the disease, representing premature aging of the blood vessels due to the chronic exposure to the high blood pressure. The endothelial dysfunction can be improved by antihypertensive therapy, favoring the prevention of the occurrence of vascular complications in hypertension.

Endothelin-1 infusion inhibits plasma insulin responsiveness in normal men

OBJECTIVES

Elevated plasma endothelin (ET)-1 levels have been described in insulin-resistant states such as syndrome X, obesity, non-insulin-dependent diabetes mellitus, and in some studies in essential hypertension. To investigate whether increases in circulating ET-1 to levels observed in insulin-resistant states can modulate insulin levels and/or insulin sensitivity in humans, we assessed these variables during low, non-pressor-dose ET-1 compared with placebo infusion.

DESIGN

In a randomized, single blind, crossover design, 10 lean normotensive male subjects received either an intravenous infusion of subpressor doses of ET-1 dissolved in polygeline or a control infusion of polygeline only (placebo). Using dynamic assessment by the minimal model approach with the modified frequent sampling intravenous glucose tolerance test (FSIGT) the following and other parameters were measured: insulin sensitivity; acute insulin response to glucose (AIR(G)) calculated as the average of the three peak values between 2 and 5 min after injection of glucose from which the basal insulin levels were subtracted; the initial area under the curve (AUC(1-19)) from insulin values between time 0 and 19 min and the first-phase insulin secretion (phi1) from insulin kinetics parameters.

RESULTS

ET-1 infusion reduced AIR(G) (to 34.85 +/- 4.27 compared with 49.3 +/- 6.9 microU/ml during placebo, P=0.017) and the acute C-peptide response to glucose (to 2.33 +/- 0.41 compared with 3.1 +/- 0.44 ng/ml, P=0.018), decreased plasma insulin levels during the FSIGT compared with placebo (analysis of variance P<0.0001) and decreased the AUC(1-19) (to 2.1 +/- 0.2 compared with 2.9 +/- 0.3 U/l per 20 min, P<0.01) while phi1 tended to be lower. S1 measured during ET-1 infusion was unaltered (11.11 +/- 1.91 x 10(-4) versus 10.88 +/- 2.11 10(-4)/min per mU per l, NS).

CONCLUSIONS

These findings demonstrate that an increase in circulating ET-1 to levels observed in insulin-resistant states acutely diminishes the insulin secretory response but does not significantly modify insulin sensitivity.

Endothelin as a regulator of cardiovascular function in health and disease

The endothelins are a family of endothelium-derived peptides that possess characteristically sustained vasoconstrictor properties. Endothelin-1 appears to be the predominant member of the family generated by vascular endothelial cells. In addition to its direct vascular effects, endothelin-1 has inotropic and mitogenic properties, influences homeostasis of salt and water, alters central and peripheral sympathetic activity and stimulates the renin-angiotensin-aldosterone system. Studies with endothelin receptor antagonists have indicated that endothelin-1 probably has complex opposing vascular effects mediated through vascular smooth muscle and endothelial ET(A) and ET(B)receptors. Endogenous generation of endothelin-1 appears to contribute to maintenance of basal vascular tone and blood pressure through activation of vascular smooth muscle ET(A)receptors. At the same time, endogenous endothelin-1 acts through endothelial ET(B) receptors to stimulate formation of nitric oxide tonically and to oppose vasoconstriction. In view of the multiple cardiovascular actions of endothelin-1, there has been much interest in its contribution to the pathophysiology of hypertension. Results of most studies suggest that generation of, or sensitivity to, endothelin-1 is no greater in hypertensive than it is in normotensive subjects. Nonetheless, the deleterious vascular effects of endogenous endothelin-1 may be accentuated by reduced generation of nitric oxide caused by hypertensive endothelial dysfunction. It also appears likely that endothelin participates in the adverse cardiac and vascular remodelling of hypertension, as well as in hypertensive renal damage. Irrespective of whether vascular endothelin activity is increased in hypertension, anti-endothelin agents do produce vasodilatation and lower blood pressure in hypertensive humans. There is more persuasive evidence for increased endothelin-1 activity in secondary forms of hypertension, including pre-eclampsia and renal hypertension. Endothelin-1 also appears to play an important role in pulmonary hypertension, both primary and secondary to diseases such as chronic heart failure. The hypotensive effects of endothelin converting enzyme inhibitors and endothelin receptor antagonists should be useful in the treatment of hypertension and related diseases. Development of such agents will increase knowledge of the physiological and pathological roles of the endothelins, and should generate drugs with novel benefits.

Angiotensin II and endothelin-1 receptor antagonists have cumulative hypotensive effects in canine Page hypertension

OBJECTIVE

To evaluate whether the cumulative hypotensive effect of the endothelin-1 receptor antagonist bosentan, previously demonstrated in the presence of an angiotensin converting enzyme inhibitor, persists under angiotensin II receptor blockade with losartan.

DESIGN

The model of hypertension was canine renovascular hypertension (Page hypertension).

METHODS

Ten conscious dogs, studied on two occasions, were administered losartan (a 0.1 mg/kg bolus plus 90 min infusion at 0.1 mg/kg per min) and then bosentan vehicle (experiment I) or losartan and then two cumulative doses of bosentan (a 0.3 mg/kg bolus plus 30 min infusion at 0.7 mg/kg per min; and a 3 mg/kg bolus plus 30 min infusion at 7 mg/kg per min; experiment II).

RESULTS

At the end of the study, mean aortic pressure in dogs had decreased by 14% in experiment I (from 139 +/- 4.7 to 119 +/- 4.7 mmHg, P<0.05), whereas a 28% reduction occurred in experiment II (from 145 +/- 8.9 to 104 +/- 5.0 mmHg, P<0.005), corresponding to an additional 14% decrease after administration of bosentan (P<0.005 between groups). This cumulative effect of bosentan was related to a decrease in systemic vascular resistance (from 1220 +/- 119 to 847 +/- 189 mmHg/ml per min per kg x 10(3), P<0.05). Plasma angiotensin II level increased similarly in both experiments (in experiment I from 133 +/- 43 to 622 +/- 145 pg/ml, P=0.01; in experiment II from 198 +/- 63 to 771 +/- 134 pg/ml, P<0.005) whereas plasma endothelin-1 level increased only in experiment II (from 3.8 +/- 0.4 to 32.7 +/- 3.2 pg/ml, P<0.001).

CONCLUSION

The cumulative hypotensive effect of bosentan suggests that, besides angiotensin II, endothelin-1 is independently involved in the pathophysiology of hypertension, which presents new therapeutic perspectives.

Immunoreactive endothelin levels in the vitreous fluid are decreased in diabetic patients with proliferative retinopathy

Endothelin (ET) plays an important role in the pathogenesis of various cardiovascular lesions. To elucidate the pathophysiologic significance of ET in the development of diabetic retinopathy, we determined immunoreactive ET levels in the vitreous fluid collected during vitrectomy in seven patients with non-insulin dependent diabetes mellitus with proliferative diabetic retinopathy and in 10 nondiabetic subjects as controls. Immunoreactive vitreous ET levels in diabetic patients with proliferative retinopathy (7.5 +/- 3.4 pg/ml, mean +/- SD) were significantly lower than those in the nondiabetic subjects (55.0 +/- 38.1 pg/ml; p < 0.005). The decreased rather than increased immunoreactive ET levels may simply reflect severe endothelial injury to the retinal vessels caused by diabetic microangiopathy rather than an important role in diabetic retinopathy.

Handgrip increases endothelin-1 secretion in normotensive young male offspring of hypertensive parents

OBJECTIVES

We tested the hypothesis that an abnormal response of plasma endothelin-1 (ET-1) is elicited by handgrip exercise (HG) in young normotensive offspring of hypertensive parents.

BACKGROUND

It has been hypothesized that ET-1 is involved in blood pressure control and plays a pathophysiologic role in the development of clinical hypertension.

METHODS

Two groups of healthy male subjects, 11 with hypertensive parents (group A) and 10 without a family history of hypertension (group B), underwent 4 min of HG at 50% maximal capacity. Heart rate and blood pressure and plasma levels of ET-1, epinephrine and norepinephrine were measured at baseline, peak HG, and after 2 (R2) and 10 (R10) min of recovery.

RESULTS

Group A had higher norepinephrine levels than group B throughout the test (baseline 181+/-32 [SEM] vs. 96+/-12 pg/ml, p < 0.05; peak HG 467+/-45 vs. 158+/-12 pg/ml, p < 0.000001; R2 293+/-46 vs. 134+/-8 pg/ml, p < 0.01; RO1 214+/-27 vs. 129+/-10 pg/ml, p < 0.0005); no significant difference in epinephrine levels was detected. Compared with group B subjects, group A had higher baseline ET-1 levels (1.07+/-0.14 vs. 0.59+/-0.11 pg/ml, p < 0.02), which increased to a greater extent at peak HG (1.88+/-0.31 vs. 0.76+/-0.09 pg/ml, p < 0.005) and R2 (2.46+/-0.57 vs. 1.31+/-0.23 pg/ml, p < 0.05) and remained elevated at R10 (3.16+/-0.78 vs. 0.52+/-0.09 pg/ml, p < 0.002). Multivariate analysis demonstrated that only a family history of hypertension (chi-square=7.59, p=0.0059) and ET-1 changes during HG (chi-square=4.23, p=0.0398) were predictive of blood pressure response to HG and that epinephrine and norepinephrine were not.

CONCLUSIONS

The response to HG in offspring of hypertensive parents produced increased ET-1 plasma levels and resulted in a sustained ET-1 release into the bloodstream during recovery compared with offspring of normotensive parents. This may be an important marker for future clinical hypertension.

The long-term effects of oral and transdermal postmenopausal hormone replacement therapy on nitric oxide, endothelin-1, prostacyclin, and thromboxane

OBJECTIVE

Oral postmenopausal hormone replacement therapy (HRT) decreases the risk of cardiovascular disorders, but the mechanisms of this protection are largely unknown. We compared the long-term effects of sequential oral HRT and transdermal HRT on vasodilatory nitric oxide and prostacyclin as well as vasoconstrictive endothelin- and thromboxane A2, all of which may be factors in the protective effect of HRT against cardiovascular disorders.

DESIGN

Prospective, randomized study.

SETTING

Department of Obstetrics and Gynecology at a university hospital.

PATIENT(S)

Fifty-two healthy postmenopausal female nonsmokers (n = 42) or smokers (n = 10) who had climacteric symptoms.

INTERVENTION(S)

The women received either oral HRT (2 mg of estradiol on days 1-12, 2 mg of estradiol plus 1 mg of norethisterone acetate on days 13-22, and 1 mg of estradiol on days 23-28; n = 21) or transdermal HRT (50 microg/d of estradiol on days 1-28 followed by 250 microg/d of norethisterone acetate on days 14-28; n = 21) for 1 year. Ten female smokers received transdermal HRT for 1 year.

MAIN OUTCOME MEASURE(S)

Plasma levels of nitrate as an index of nitric oxide production, endothelin-1, and urinary output of the prostacyclin metabolite (2,3-dinor-6-keto-PGF1alpha) and that of the thromboxane A2 metabolite (2,3-dinorthromboxane B2) were measured before and during the combined phases of the 2nd, 6th, and 12th treatment months.

RESULT(S)

Both regimens increased plasma estradiol levels and alleviated vasomotor symptoms. Neither regimen caused significant changes in nitrate, endothelin-1, prostacyclin, or thromboxane A2 in nonsmoking women. Female smokers had significantly higher levels of endothelin-1, which were significantly reduced by transdermal HRT at 6 months of treatment.

CONCLUSION(S)

Nitric oxide, endothelin-1, prostacyclin, and thromboxane A2 are not of primary importance in the protective effect of sequential oral HRT against cardiovascular disorders in otherwise healthy nonsmoking postmenopausal women. In this regard, transdermal HRT appears comparable to oral HRT. Postmenopausal female smokers have high levels of endothelin-1 that are reduced by transdermal HRT.

The effect of an endothelin-receptor antagonist, bosentan, on blood pressure in patients with essential hypertension. Bosentan Hypertension Investigators

BACKGROUND

Endothelin is a powerful vasoconstrictor peptide derived from the endothelium. We evaluated the contribution of endothelin to blood-pressure regulation in patients with essential hypertension by studying the effect of an endothelin-receptor antagonist, bosentan.

METHODS

We studied 293 patients with mild-to-moderate essential hypertension. After a placebo run-in period of four to six weeks, patients were randomly assigned to receive one of four oral doses of bosentan (100, 500, or 1000 mg once daily or 1000 mg twice daily), placebo, or the angiotensin-converting-enzyme inhibitor enalapril (20 mg once daily) for four weeks. Blood pressure was measured before and after treatment.

RESULTS

As compared with placebo, bosentan resulted in a significant reduction in diastolic pressure with a daily dose of 500 or 2000 mg (an absolute reduction of 5.7 mm Hg at each dose), which was similar to the reduction with enalapril (5.8 mm Hg). There were no significant changes in heart rate. Bosentan did not result in activation of the sympathetic nervous system (as determined by measurement of the plasma norepinephrine level) or the renin-angiotensin system (as determined by measurements of plasma renin activity and angiotensin II levels).

CONCLUSIONS

An endothelin-receptor antagonist, bosentan, significantly lowered blood pressure in patients with essential hypertension, suggesting that endothelin may contribute to elevated blood pressure in such patients. The favorable effect of treatment with bosentan on blood pressure occurred without reflexive neurohormonal activation.

Circulating levels of endothelin-1 during acute hyperinsulinemia in patients with essential hypertension treated with type 1 angiotensin receptor antagonist or placebo

Insulin and angiotensin II (Ang II) are involved in the regulation of endothelin-1 (ET-1). This study investigates their possible influence on plasma levels of ET-1 in humans. Twenty patients with essential hypertension were included in a randomized, double-blind, placebo-controlled crossover study of 4 weeks' treatment with losartan, a selective type 1 angiotensin (AT1) receptor antagonist. The effect was evaluated in the fasting state and during acute hyperinsulinemia physiologically induced by oral glucose ingestion (OGTT) and by euglycemic glucose clamp. Losartan lowered blood pressure significantly, but did not influence plasma levels of ET-1 in the fasting condition (5.2 +/- 0.2 fmol/mL on placebo and 5.6 +/- 0.3 fmol/mL after losartan treatment). During both models of acute hyperinsulinemia, there was a significant decrease in plasma ET-1. In the OGTT the mean values after placebo treatment decreased from 5.2 +/- 0.2 fmol/mL at time 0 to 4.7 +/- 0.4 (P = .001) and 4.0 +/- 0.5 (P = .001) at 60 and 120 minutes, respectively. During the clamp the mean ET-1 values decreased from 5.7 +/- 0.4 fmol/mL at time 0 to 4.6 +/- 0.2 (P < .001) and 4.3 +/- 0.3 (P = .006) at 60 and 120 minutes, respectively. No differences in these profiles occurred after losartan treatment. Significant inverse correlation between fasting levels of ET-1 and insulin sensitivity index was found, r = -.51, P = .003. In conclusion, losartan did not influence the circulating levels of ET-1 in basal condition or during acute hyperinsulinemia, whereas a significant decrease in plasma ET-1 occurred during acute hyperinsulinemia. A significant inverse correlation demonstrated between basal levels of plasma ET-1 and the insulin-stimulated glucose uptake could point to a possible regulatory influence of ET-1 production on glucose metabolism or vice versa.

Endothelin converting enzyme-1 gene expression in the kidney of spontaneously hypertensive rats

Abnormal renal handling of water and sodium is implicated in the pathogenesis of hypertension in spontaneously hypertensive rats (SHR). Alteration of renal endothelin-1 synthesis is also reported in SHR. Endothelin-1, a potent vasoconstrictor and regulator of sodium reabsorption in the nephron, has a pathophysiological potential in the development of hypertension. Because synthesis of bioactive endothelin-1 requires endothelin converting enzyme-1 (ECE-1), we investigated whether renal ECE-1 gene expression is altered in the kidney of SHR. Kidneys from both 4- and 12-week-old SHR and age-matched Wistar-Kyoto rats (WKY) were studied. ECE-1 mRNA in microdissected nephron segments was assessed by reverse transcription-competitive polymerase chain reaction, and ECE-1 protein level by Western blot. In 4-week-old SHR, ECE-1 mRNA was significantly increased in the proximal straight tubule, medullary thick ascending limb, cortical thick ascending limb, and inner medullary collecting duct. ECE-1 protein level was increased in both the outer and inner medulla. In 12-week-old SHR, ECE-1 gene expression was significantly increased in the proximal straight tubule, medullary thick ascending limb, and also in the glomeruli. Glomerular preproendothelin-1 mRNA expression was not different between the two strains at both 4 and 12 weeks. We conclude that high ECE-1 gene expression in the nephron, via increase of endothelin-1 synthesis, may promote sodium retention that contributes to the development and/or maintenance of hypertension in SHR.

Renoprotective effects of a combined endothelin type A/type B receptor antagonist in experimental malignant hypertension

We previously showed that plasma endothelin-1 (ET-1) concentration was increased in deoxycorticosterone acetate (DOCA)-salt-induced malignant hypertension in spontaneously hypertensive rats (SHR). In contrast, in normal SHR, this value is similar to that seen in Wistar-Kyoto (WKY) rats. The purpose of this study was to examine the effects of the new combined ET type A/type B (ETA/B) receptor antagonist, TAK-044, on the development of hypertension in this model of malignant hypertension. TAK-044 10 mg/kg, which effectively blocks both ETA and ETB receptors, was administered intraperitoneally once per day for 4 weeks in DOCA-salt SHR, and the effects on ET-1 and other parameters were compared with the same values in untreated WKY rats, untreated DOCA-salt SHR, and hydralazine-treated DOCA-salt SHR. DOCA-salt caused marked increases in blood pressure, blood urea nitrogen (BUN), serum creatinine, and plasma ET-1 concentrations in SHR. Both TAK-044 and hydralazine significantly suppressed the increase in blood pressure in DOCA-salt SHR to the same extent. Both treatments also suppressed the increase in BUN and serum creatinine, but this attenuation was less marked with hydralazine than with TAK-044. Neither TAK-044 nor hydralazine affected plasma ET-1 concentration in this model. TAK-044 significantly reduced kidney weight in DOCA-salt SHR, whereas the decrease seen with hydralazine was less marked. Prevention of DOCA-salt-induced renal structural injury (mesangial hypercellularity, glomerular sclerotic changes, and tubulointerstitial damage) in this model was clearly greater with TAK-044 treatment than with hydralazine treatment. These results suggest that endogenous ET-1 may, at least in part, contribute to renal functional and structural damage in malignant DOCA-salt SHR. Our results raise the possibility of renoprotective effects of ETA/B receptor blockers in certain forms of malignant hypertension.

Reduced plasma concentrations of nitrogen oxide in individuals with essential hypertension

Patients with essential hypertension exhibit blunted endothelium-dependent vasodilator responses, which may be largely attributable to reduced bioactivity of nitric oxide (NO). Therefore, we measured the end product of NO, nitrate plus nitrite (nitrogen oxide), and examined the relationship between the degree of hypertension and plasma nitrate plus nitrite levels in patients with essential hypertension. The combined plasma concentration of nitrate plus nitrite, end products of NO metabolism, was reduced in individuals with essential hypertension relative to that in control subjects (15.7+/-1.1 versus 22.8+/-1.4 mmol x L(-1), P<.001); individuals with borderline hypertension showed values that were intermediate between those of the other two groups (18.2+/-1.2 mmol x L(-1), P<.001). The plasma nitrogen oxide concentration showed significant inverse correlations with both systolic and diastolic blood pressures. The basal concentration of nitrogen oxide in the plasma was reduced, at least in the peripheral circulation, in individuals with essential hypertension.

Additional hypotensive effect of endothelin-1 receptor antagonism in hypertensive dogs under angiotensin-converting enzyme inhibition

BACKGROUND

Endothelin-1 (ET-1) may play a role in hypertension. ET-1 receptor antagonism by bosentan lowers blood pressure in hypertension. We evaluated whether the effect of bosentan is still observed under ACE inhibitors (ACEI).

METHODS AND RESULTS

Thirty anesthetized and 18 conscious hypertensive dogs were studied randomly. Anesthetized dogs were divided into 4 groups: group 1 received cumulative doses of bosentan (bolus+30-minute infusion: 0.1 mg/kg+/-0.23 mg/kg per hour to 3 mg/kg+/-7 mg/kg per hour); group 2, the same dose-responses after 1 mg/kg enalaprilat; group 3, the vehicle after enalaprilat; and group 4, the dose responses to bosentan followed by enalaprilat. The conscious dogs were divided into 3 groups: group 5 received 2 cumulative doses of bosentan; group 6, the vehicle; and group 7, enalaprilat alone. In groups 1 and 2, bosentan produced dose-related decreases (P=.0001) in left ventricular systolic pressure and mean aortic pressure (AOP). In group 1, bosentan decreased mean AOP by 22%. In group 2, enalaprilat decreased mean AOP by 25% (from 173+/-26 to 130+/-25 mm Hg; P<.005); an additional 18% decrease was obtained with bosentan, the mean AOP reaching 98+/-21 mm Hg (P<.01). In group 3, the effect of enalaprilat alone was a 22% decrease in mean AOP (P<.005). The additive effect of the bosentan-ACEI association was also observed in group 4. In group 5, bosentan reduced mean AOP by 20% (P<.005), whereas mean AOP remained unchanged in group 6. The effect of ACEI alone (group 7) was similar to that of bosentan.

CONCLUSIONS

Bosentan produces an additional hypotensive effect to that of ACEI, which opens new therapeutic perspectives.