Pivotal role of angiotensin II receptor subtype 1A in the development of two-kidney, one-clip hypertension: study in angiotensin II receptor subtype 1A knockout mice

OBJECTIVE: The present study was performed to examine in two-kidney, one-clip (2K1C) Goldblatt hypertensive mice: first, the relative contribution of angiotensin II receptor subtypes 1A (AT(1A)) and 1B (AT(1B)); second, the role of angiotensin II type 2 (AT(2)) receptors in the development of hypertension in wild-type (AT(1A)+/+) and AT(1A) receptor knockout (AT(1A)-/-) mice; and third, the role of increased nitric oxide synthase activity in counteracting the hypertensinogenic action of angiotensin II in this model. METHODS: AT(1A)+/+ and AT(1A)-/- mice underwent clipping of one renal artery and were infused with either saline vehicle or selective AT(2) receptor agonist CGP-42112A (CGP). Blood pressure was monitored by radiotelemetry. Blood pressure responses to the nitric oxide synthase inhibitor nitro-L-arginine-methyl-ester were evaluated. RESULTS: AT(1A)+/+ mice responded to clipping by a rise in blood pressure that was not modified by CGP infusion. Clip placement caused a slight increase in blood pressure in AT(1A)-/- mice that remained significantly lower than in AT(1A)+/+ mice. Acute nitric oxide synthase inhibition caused greater increase in blood pressure in 2K1C/AT(1A)+/+ than in AT(1A)+/+ mice. CONCLUSION: The present data support the critical role of AT(1A) receptors in the development of 2K1C hypertension, whereas AT(1B) receptors play only a minor role in blood pressure regulation in this model of angiotensin II-dependent hypertension. Activation of AT(2) receptors does not play an antagonistic role in the AT(1) receptor-mediated hypertensinogenic actions of angiotensin II in this model. Finally, enhanced nitric oxide synthase activity plays a protective role by counteracting the vasoconstrictor influences of angiotensin II in 2K1C hypertensive mice.     

D3多巴胺受体基因敲除小鼠高血压产生机制的探讨

目的探讨D3多巴胺受体基因敲除小鼠（D3-/-）高血压产生的机制,以期了解D3多巴胺受体对血压的调节作用.方法以第2代D3-/-小鼠为研究对象,分别对其血压、肾脏尿钠排泄功能、D3 受体的蛋白/mRNA、血浆肾素活性、去甲肾上腺素的浓度、血管紧张素Ⅱ1型（AT1）受体的表达进行测定;并通过离体肠系膜动脉孵育,研究刺激D3 受体对血管舒缩功能的影响.结果无论收缩压或舒张压,D3-/-小鼠均明显高于野生型（D3＋/＋）小鼠.给予盐负荷后,2组小鼠血压均无明显变化,然而在盐负荷后最后1个时间段的尿钠排泄量,D3-/-小鼠低于D3＋/＋小鼠.分析肾脏肾素活性和AT1受体的表达发现D3-/-小鼠高于D3＋/＋小鼠.2组小鼠肾脏中去甲肾上腺素的水平无区别.AT1受体拮抗剂的降血压作用D3-/-小鼠明显且持久.离体肠系膜动脉研究发现刺激D3受体可舒张动脉血管.结论 D3-/-小鼠的血压升高除与肾脏的尿钠排泄功能下降有关,还与D3受体介导的舒血管功能障碍有关.     

Late-onset endothelin-A receptor blockade reduces podocyte injury in homozygous Ren-2 rats despite severe hypertension

We have recently found in male homozygous hypertensive Ren-2 transgenic rats (TGRs) fed a high-salt diet that early onset selective endothelin (ET) A (ET(A)) or nonselective ET(A)/ET B (ET(B)) receptor blockade improved survival rate and reduced proteinuria, glomerulosclerosis, and cardiac hypertrophy, whereas selective ET(A) receptor blockade also significantly attenuated the rise in blood pressure. Because antihypertensive therapy in general is known to be more efficient when started at early age, our study was performed to determine whether onset of ET receptor blockade at a later age in animals with established hypertension will have similar protective effects as does early-onset therapy. Male homozygous TGRs and age-matched normotensive Hannover Sprague-Dawley rats were fed a high-salt diet between days 51 and 90 of age. TGRs received vehicle (untreated), the selective ET(A) receptor blocker atrasentan (ABT-627), or the nonselective ET(A)/ET(B) receptor blocker bosentan. Survival rates in untreated and bosentan-treated TGRs were 50% and 64%, respectively, whereas with atrasentan, survival rate of TGR was 96%, thus, similar to 93% in Hannover Sprague-Dawley rats. From day 60 on, systolic blood pressure in atrasentan-treated TGRs was transiently lower (P<0.05) than in untreated or bosentan-treated TGRs. Glomerular podocyte injury was substantially reduced with atrasentan treatment independent of severe hypertension and strongly correlated with survival (P<0.001). Our data indicate that in homozygous TGR ET receptors play an important role also in established hypertension. Selective ET(A) receptor blockade not only reduces podocyte injury and end-organ damage but also improves growth and survival independently of hypertension.     

Role of ET(A) receptors in the vasoconstriction induced by endothelin-1 in subcutaneous small arteries of normotensive subjects and hypertensive patients

OBJECTIVE: The aim of our study was to investigate contractile responses to endothelin-1 in the presence or absence of selective blockers of ET(A) or ET(B) receptors in subcutaneous small resistance arteries of normotensive subjects and of patients with essential hypertension. METHODS: Twenty-four subjects (eight normotensives aged 50 +/- 4 years, and 16 with essential hypertension aged 53 +/- 4 years) were included in the study. All subjects were submitted to a biopsy of the subcutaneous fat. Small resistance arteries (internal diameter 160-280 microm) were dissected and mounted on a micromyograph as a ring preparation (Mulvany's technique). The media-to-lumen ratio was calculated. A concentration-response curve to endothelin-1 was then performed in the presence or absence of FR 139317, (a selective blocker of ET(A) receptors) or of BQ 788, (a selective blocker of ET(B) receptors). RESULTS: The media-to-lumen ratio was lower in normotensives compared with those subjects with essential hypertension (0.08 +/- 0.02 vs. 0.12 +/- 0.05, p < 0.01). The vasoconstriction induced by endothelin-1 was greater in normotensives than in patients with essential hypertension. In normotensives, almost all the vasoconstriction induced by endothelin-1 was blocked by the addition of FR 139317, while in subjects with essential hypertension the effect was smaller. The selective blocker of ET(B) was devoid of effect in both groups. CONCLUSIONS: The vasoconstrictor effect of endothelin-1 in small resistance arteries of normotensive subjects and, in part, also in hypertensive patients is mediated by ET(A) receptors, while ET(B) receptors play a minor role, if any. It is, however, possible that a vasoconstrictor effect mediated by ET(B) receptors located on vascular smooth muscle cells may be masked by the simultaneous stimulation of endothelial ET(B) receptors which may induce a vasodilation mediated by nitric oxide.     

Long-term effects of selective and nonselective endothelin receptor antagonists in mice with heart failure

BACKGROUND: The ET(A) and ET(B) receptors mediate vasoconstriction, aldosterone release, and fibrosis. However, the role of ET(B) receptors is still controversial because those expressed on endothelial cells also stimulate vasodilatation and may oppose the actions of the ET(A) receptor. Plasma levels of endothelin-1 (ET-1) are increased in heart failure (HF) and are associated with myocardial dysfunction. The relative efficacy of selective and nonselective ET antagonists in the treatment of HF is unclear. We hypothesized that blockade of ET(A) receptors may improve cardiac function and prevent left ventricular remodeling in mice with HF, and these effects may be mediated in part by activation of ET(B). METHODS AND RESULTS: A mouse model of chronic HF induced by myocardial infarction (MI) was used. Seven days after MI, mice were divided into vehicle, ET(A)-ant, or ET(A/B)-ant groups and treated for 23 weeks. Cardiac function, LV dimensions, and hemodynamics were evaluated in conscious mice before MI and during treatment. Histologic analysis of the heart and liver was performed at the end of the study. HF significantly decreased EF and increased LV dimensions, interstitial collagen fraction (ICF) and myocyte cross-sectional area (MCSA). Both ET(A)-ant and ET(A/B)-ant slightly increased EF but had no significant effect on LV dimensions, hypertrophy, or ICF. Both treatments decreased MCSA; however, this was only significant in the ET(A/B)-ant group. CONCLUSIONS: Both selective and nonselective ET-ant have similar slight effects on cardiac function and remodeling. This suggests that (1) ET(B) receptors do not mediate the beneficial cardiac effects of ET(A)-ant and (2) blockade of the ET system alone may not provide significant cardioprotection, at least in mice with HF induced by MI.     

Comparison of selective ET(A) and ET(B) receptor antagonists in patients with chronic heart failure

BACKGROUND: The vasoconstrictor action of endothelin-1 (ET-1) is mediated through ET(A) and ET(B) receptor subtypes on vascular smooth muscle. ET(B) receptors are also present on the vascular endothelium where they mediate vasodilation. Animal studies suggest that the ET(B) receptor also acts as a clearance receptor for endothelin. AIMS: To investigate the effects of a selective ET(A) and a selective ET(B) receptor antagonist alone and in combination on haemodynamics and circulating concentrations of ET-1 in patients with chronic heart failure. RESULTS: Infusion of BQ-123 (n=10), a selective ET(A) receptor antagonist, led to systemic vasodilation and did not change plasma ET-1 concentrations (1.38+/-0.82 to 1.38+/-0.91 fmol/ml, ns). Infusion of BQ-788 (n=8) led to systemic vasoconstriction with a rise in plasma ET-1 (1.84+/-1.06 to 2.73+/-0.99 fmol/ml, p<0.01). The addition of BQ-123 to BQ-788 led to systemic and pulmonary vasodilation with no further increase in plasma ET-1 concentrations (2.80+/-1.14 to 2.90+/-1.20 fmol/ml, ns). CONCLUSION: The rise in plasma ET-1 concentrations in response to selective blockade of ET(B) receptors and the associated adverse haemodynamic effects suggest that ET(B) receptors have a role in the clearance of ET-1 in man and that their blockade may not be advantageous for patients with heart failure.     

Genetic inactivation of the B2 receptor in mice worsens two-kidney,one-clip hypertension: role of NO and the AT2 receptor

OBJECTIVE: Previous studies have suggested that activation of angiotensin II (ANG II) type 2 (AT2) receptors results in nitric oxide (NO) release via activation of endothelial bradykinin B2 (B2R) receptors. The present study was performed to examine the interplay of AT2 and B2R in the development and maintenance of two-kidney, one-clip (2K1C) Goldblatt hypertension. METHODS: B2R knockout (B2R-/-) mice and their wild-type controls (B2R+/+) underwent clipping of the right renal artery and were infused with either saline (SAL) or PD 123319, an AT2 receptor antagonist (PD), via an osmotic pump implanted intraperitoneally. Systolic blood pressure (SBP) was measured in conscious mice. On day 27, mean arterial pressure (MAP) responses were measured in response to consecutive blockade of AT(2) receptors and NO synthase (NOS). RESULTS: A significant and sustained rise in SBP was observed in both 2K1C B2R+/+ and B2R-/- versus sham-operated groups from day 10 to day 24 after clipping. After this time, SBP rose to significantly higher levels in 2K1C/B2R-/- than in 2K1C/B2R+/+ mice. MAP on day 27 was also higher in 2K1C/B2R-/- than 2K1C/B2R+/+ mice. Chronic PD infusion did not alter the course of hypertension in 2K1C/B2R+/+ or 2K1C/B2R-/- mice as compared with saline-infused mice. Likewise, acute PD infusion did not affect MAP in any of the groups. However, acute NOS inhibition caused significantly greater increases in MAP in 2K1C/B2R+/+ and PD/2K1C/B2R+/+ than 2K1C/B2R-/- and PD/2K1C/B2R-/- mice. CONCLUSIONS: These results indicate that B2R inactivation selectively worsens the maintenance phase of 2K1C Goldblatt hypertension and support the notion that B2R-deficient mice exhibit an impaired ability to release NO in response to elevations of ANG II levels. Chronic administration of an AT2 receptor blocker did not modify the course of 2K1C Goldblatt hypertension in either B2R-/- or B2R+/+ mice. Therefore, the role of AT2 receptors in B2R-mediated protection against ANG II-dependent hypertension remains uncertain.     

Renal damage is not improved by blockade of endothelin receptors in primary renin-dependent hypertension

OBJECTIVE: Secondary activation of the renin-angiotensin system plays a major role in the progression of chronic nephropathies, and blockade of endothelin (ET) receptors has been shown to confer nephroprotection in experimental models of proteinuric renal disease. We tested the nephroprotective potential of selective endothelin A receptor (ETA) and non-selective ETA and endothelin B (ETA/B) receptor blockade in the TGR(mRen2)27 transgenic rat model with renin-dependent hypertension (Ren2). DESIGN: Ren2 animals were treated between 10 and 30 weeks of age with the selective ETA receptor antagonist darusentan (Ren2-ETA) and the ETA/B receptor antagonist Lu420627 (Ren2-ETA/B), and compared with transgene negative Sprague-Dawley (SD) controls. Since the elevated systolic blood pressure in Ren2 was not affected in either Ren2-ETA or Ren2-ETA/ETB, an additional Ren-2 group was treated with a non-antihypertensive dose of the angiotensin II type 1 receptor blocker eprosartan (Ren2-AT1). RESULTS: During the 20-week observation period 35% of untreated Ren2, 30% of Ren2-ETA/B, 50% of Ren2-ETA, and 83% of Ren2-AT1 animals survived compared with 100% of SD rats. Renal endothelin-1 mRNA expression and proteinuria (4.1-fold) were significantly elevated in Ren2 compared with SD rats (P < 0.05, respectively). Proteinuria was normalized to SD control levels in Ren2-AT1 (P < 0.05) but increased further in Ren2-ETA (7.7-fold) and Ren2-ETA/B (15-fold) (P < 0.05, respectively). Glomerulosclerosis, tubulointerstitial damage and renal osteopontin mRNA expression were reduced in Ren2-AT1 (P < 0.05, respectively) but remained unchanged or increased further in Ren2-ETA and Ren2-ETA/B compared with Ren2. CONCLUSION: ET receptor blockade fails to improve renal damage and mortality in primary renin-dependent hypertension.     

Amlodipine and loop diuretics as the second anti-hypertensive medication for the treatment of hypertension with chronic kidney diseases

Renoprotective effects of renin-angiotensin system inhibitors are well known. However, hypertension with chronic kidney diseases (CKDs) is usually hard to manage with a single agent, and requires the addition of either a calcium antagonist or diuretics to achieve the goal of blood pressure (BP) lowering. Retrospective study was performed among the patients who regularly visited our office, and whose BP had not reached the goal of BP despite of treatment with an angiotensin receptor blocker. Clinical parameters were observed for 6 months. Comparisons of home BP and proteinuria were made between 16 patients prescribed additional calcium antagonists and 15 patients with diuretics. Patient background including age, sex BP, augmentation index, and renal function were similar between the two groups. Both calcium antagonists and diuretics considerably decreased BP. An addition of either agent resulted in similar control of home BP. While both agents reduced augmentation index (AI), calcium antagonist exerted greater improvements in AI (-7 ± 5 vs. -4 ± 3%, p < 0.01). Although urinary protein excretion in both groups was decreased, the degree of these decreases was greater among the patients treated with a calcium antagonist (-28 ± 15 vs. -11 ± 15%, p < 0.01). During observation periods, eGFR in both groups did not show any significant changes from the base line. Under the inhibition of a renin-angiotensin system, calcium antagonists elicited a greater decrease in urinary protein excretion than diuretics when BP similarly controlled. Calcium antagonists also improved AI more strongly than diuretics. Calcium antagonists appear suited for adding on renin angiotensin system inhibitors to treat hypertension with CKDs.     

Combined endothelin receptor blockade evokes enhanced vasodilatation in patients with atherosclerosis

Endothelin (ET)-1 causes vasoconstriction via ET(A) and ET(B) receptors located on vascular smooth muscle cells and vasodilatation via ET(B) receptors on endothelial cells. Studies in vitro indicate an upregulation of ET(B) receptors in atherosclerosis. The present study investigated the vascular effects evoked by endogenous ET-1 in atherosclerotic patients. Forearm blood flow (FBF) was measured with venous occlusion plethysmography in 10 patients with atherosclerosis and in 10 healthy control subjects during intra-arterial infusion of selective ET receptor antagonists. The ET(B) receptor antagonist BQ788 evoked a significant increase in FBF (31+/-13%) in the patients, whereas a 20+/-9% reduction was observed in the control subjects. The ET(A) receptor antagonist BQ123 combined with BQ788 evoked a marked increase in FBF (102+/-25%) in the patients compared with no effect in the control subjects (-3+/-9%, P<0.001 versus patients). The ET(A) receptor antagonist BQ123 increased FBF to a similar degree in patients (39+/-11%) as in control subjects (41+/-11%). The increase in FBF evoked by selective ET(A) receptor blockade was significantly (P<0.05) less than that evoked by combined ET(A)/ET(B) receptor blockade in the atherosclerotic patients. These observations suggest an enhanced ET-1-mediated vascular tone in atherosclerotic patients, which is at least partly due to increased ET(B)-mediated vasoconstriction.     

The bedtime administration of doxazosin controls morning hypertension and albuminuria in patients with type-2 diabetes: evaluation using home-based blood pressure measurements

The control of high blood pressure (BP) after awakening in the morning (morning hypertension) as determined by home BP (HBP), as well as BP control throughout the day, may prevent diabetic vascular complications. We examined the effect of an alpha-adrenergic blocker (doxazosin) on BP measurements taken by HBP after awakening and during clinic visits (CBP) in 50 patients with type-2 diabetes and morning hypertension. We evaluated the urinary albumin excretion rate as an indicator of nephropathy. Doxazosin was taken orally once at bedtime for 1 to 3 months. The mean (+/- SD) dose was 2.9 +/- 2.1 mg/day (1 to 8 mg/day). The BP was measured monthly at the clinic during the day and at home after awakening in the morning. In this short-term trial (2.8 +/- 0.4 months), the systolic HBP decreased significantly from 164 +/- 17 mmHg before treatment to 146 +/- 19 mmHg after treatment, and the diastolic HBP decreased significantly from 85 +/- 14 mmHg before treatment to 80 +/- 9 mmHg after treatment. The systolic, but not the diastolic CBP, decreased significantly after treatment. There was no significant difference in the systolic or diastolic values between the HBP and the CBP after treatment. The percentage change in the systolic HBP after treatment was three times greater than for the systolic CBP. The median (interquartile) urinary albumin excretion rate decreased significantly (P < 0.001) from 62 (25-203) mg/g creatinine before treatment to 19 (9-76) mg/g creatinine after treatment. On multiple regression analysis, the decrease in the systolic HBP with treatment positively correlated with the reduction in urinary excretion of albumin. The control of morning hypertension reduced the albuminuria found in both untreated and treated hypertensive patients with type-2 diabetes. Bedtime administration of doxazosin appears to be safe and effective in reducing morning hypertension as measured by HBP. This finding also demonstrates that HBP taken in the morning has a stronger predictive power for the albuminuria level than does CBP.     

Effect of erythropoietin on blood pressure and on the vascular endothelial ET-1/ETB receptor system

BACKGROUND: Recombinant human erythropoietin (rhEPO) increases blood pressure (BP) and the vascular production of endothelin-1 in renal failure rats. This study was designed to investigate the effect of rhEPO on BP and on the ET-1/ET(B)R system in rats with normal renal function. To further characterize the effect of rhEPO on the ET-1/ET(B)R system, we also studied heterozygous (+/-) ET(B)R knockout (KO) mice. METHODS: The animals received either the vehicle or rhEPO (100 U/kg subcutaneously three times per week). ET(B)R(+/-) mice were compared with ET(A)R(+/-) and wild-type (WT) mice. In rats, the ET(B)R mRNA expression was assessed in blood vessels as well as the vascular ET(B)R density using immunohistochemistry. In mice, ET-1 concentration was measured in the thoracic aorta. RESULTS: RhEPO administration increased hematocrit levels in all treated animals. This therapy had no effect on BP in normal rats, but it did increase vascular and renal cortex ET(B)R mRNA expression. Immunohistochemistry confirmed that the ET(B)R density was increased in blood vessel endothelium in these normal rats. In contrast, rhEPO increased BP in ET(B)R(+/-) mice and this pressor effect was associated with higher ET-1 concentrations in the thoracic aorta. CONCLUSIONS: RhEPO exerts a pleotropic effect on the endothelial ET-1/ET(B)R system. The increase in endothelial ET(B)R expression may contribute to maintaining normal BP during rhEPO administration in normal animals. Conversely, conditions with deficient ET(B)R expression, such as in ET(B)R(+/-) mice, may lead to hypertension while receiving the same therapy.     

ET(A) and ET(B) receptors modulate the proliferation of human pulmonary artery smooth muscle cells

We determined the distribution of ET(A) and ET(B) receptors in pulmonary arteries from pulmonary hypertensive patients and control subjects, using in vitro autoradiography, and investigated their role in mediating the proliferative effects of endothelin-1 (ET-1) on distal human pulmonary artery smooth muscle cells (PASMCs). Distal arteries possessed more medial [(125)I]-ET-1 binding sites (105 +/- 10 versus 45 +/- 6 amol/mm(2); p < 0.001) and a greater proportion of ET(B) receptors than proximal arteries (36 +/- 3% versus 3 +/- 1%; p < 0.001). Receptor density in distal arteries and lung parenchyma was twofold greater (p < 0.05) in pulmonary hypertensive patients than in control subjects. ET-1 (10(-9)-10(-7) mol/L) stimulated DNA synthesis (147 +/- 10% of control subjects; p < 0.05) and attenuated the antiproliferative action of cicaprost and forskolin on PASMCs, these effects being mediated via ET(A) and ET(B) receptors. Serum-stimulated proliferation was attenuated by inhibiting either endogenous ET-1 release with phosphoramidon (10(-5) mol/L) or its action with PD145065 (10(-5) mol/L). Cicaprost (10(-10)-10(-7) mol/L) inhibited ET-1 release from PASMCs (49 +/- 16% of control after 24 h; p < 0.001) and increased intracellular cAMP levels, whereas ET(B) receptor stimulation selectively reduced cAMP levels. In conclusion, ET(A) and ET(B) receptors are differentially distributed in human pulmonary arteries. Both receptors promote the proliferation of PASMCs in vitro and may contribute to vascular remodeling in pulmonary hypertension.     

Endothelin-1 and hypertension: From bench to bedside

Endothelin-1 (ET-1) exerts a wide range of biologic effects that can influence systemic blood pressure. Recent studies indicate that increased activity of the ET system in the vasculature, with resultant activation of primarily ET A receptors, can contribute to hypertension. In contrast, decreased production of ET-1 in the renal medulla, and reduced activation of collecting duct ET B receptors, can also elevate systemic blood pressure. Both ET A and combined A/B receptor blockers reduce blood pressure in hypertensive patients. Several important questions remain with respect to the ET system in hypertension, including how ET receptor antagonists will interact with other antihypertensive agents, which receptor subtypes should be targeted, and what the effect of ET blockade will be on hypertension-related end-organ damage as opposed to blood pressure alone.     

Sex differences in renal medullary endothelin receptor function in angiotensin II hypertensive rats

We hypothesized that angiotensin (Ang) II hypertensive rats have impaired natriuresis after renal medullary endothelin (ET) B receptor stimulation that would be more evident in male versus female rats. Acute intramedullary infusion of the ET(B) agonist sarafotoxin 6c in normotensive male rats increased sodium excretion from 0.51±0.11 μmol/min during baseline to 1.64±0.19 μmol/min (P<0.05) after S6c. After 2 weeks of Ang II infusion (260 ng/kg per minute SC), male rats had an attenuated natriuretic response to S6c of 0.62±0.16 μmol/min during baseline versus 0.95±0.07 μmol/min after S6c. In contrast, ET(B)-dependent natriuresis was similar in female hypertensive rats (0.48±0.07 versus 1.5±0.18 μmol/min; P<0.05) compared with normotensive controls (1.05±0.07 versus 2.14±0.24 μmol/min; P<0.05). Because ET(A) receptors also mediate natriuresis in normotensive female rats, we examined ET(A) receptor function in female Ang II hypertensive rats. Intramedullary infusion of ET-1 increased sodium excretion in both hypertensive and normotensive female rats, which was partially blocked by the ET(A) antagonist BQ-123. Maximum ET(B) receptor binding in inner medullary membrane preparations was comparable between vehicle and Ang II hypertensive females; however, maximum ET(B) binding was significantly lower in male hypertensive rats (1952±251 versus 985±176 fmol/mg; P<0.05). These results indicate that renal ET(B) function is impaired in male Ang II hypertension attributed, at least in part, to a reduced number of ET(B) binding sites. Furthermore, renal ET receptor function is preserved in female rats during chronic Ang II infusion, suggesting that renal ET receptor function could serve to limit hypertension in females compared with males.     

Effect of an ET(B)-selective and a mixed ET(A/B) endothelin receptor antagonist on venomotor tone in deoxycorticosterone-salt hypertension

OBJECTIVES: Because the ET(B) receptor is important in venoconstriction, we examined the effects of a selective ET(B) receptor antagonist (A-1 92621) and a mixed ET(A/B) receptor antagonist (A-182086) on endogenous endothelin-1 (ET-1) contributions to elevated venomotor tone in deoxycorticosterone acetate-salt (DOCA-salt) hypertension. METHODS: Changes in venomotor tone were assessed using repeated measurements of mean circulatory filling pressure (MCFP) in awake, uninephrectomized, DOCA-salt-treated rats and uninephrectomized sham rats following intravenous (i.v.) injections of the ET(B) antagonist (12 mg/kg i.v.) or the ET(A/B) antagonist (12 mg/kg i.v.) alone, or 1 h before ganglion blockade with hexamethonium (30 mg/kg i.v.). RESULTS: DOCA-salt rats were hypertensive and exhibited higher MCFP than sham normotensive rats. The ET(A/B) receptor antagonist lowered mean arterial blood pressure (MABP) in DOCA-salt and sham rats, but MCFP fell in DOCA-salt rats only. The ET(B) antagonist produced no changes in MCFP while MABP increased in both groups. Pre-treatment of DOCA-salt rats, but not sham rats, with either antagonist produced greater declines in MCFP following hexamethonium than after hexamethonium alone. CONCLUSIONS: The present study confirms previous findings of elevated MCFP in DOCA-salt hypertensive rats compared to normotensive rats, but is the first to show that venomotor tone is affected by the actions of endogenous ET-1 acting at ET(B) receptors to modulate sympathetic input to the veins, as well as direct actions of ET-1 on vascular smooth muscle (VSM) ET(A) receptors. We also showed that mixed ET(A/B) receptor antagonism was effective in lowering MCFP and MABP in DOCA-salt hypertensive rats.     

Heterozygous knock-Out of ET(B) receptors induces BQ-123-sensitive hypertension in the mouse

Homozygous knock-out of ET(A) or ET(B) receptor genes results in lethal developmental phenotypes in the mouse. Such deleterious phenotypes do not occur in heterozygous littermates. However, it remains to be determined whether mice partially defective in ET(A) or ET(B) receptors display significant alterations in their responses to exogenous or endogenous endothelin-1 (ET-1). Furthermore, the anesthetized ET(B) (+/-) knock-out mice showed a significantly higher mean arterial blood pressure than the ET(A) (+/-) knock-out or their wild-type littermates. The pressor response to ET-1 but not to a selective ET(B) agonist, IRL-1620, was significantly reduced in the ET(A) (+/-) knock-out mice. In ET(B) (+/-) knock-out mice, the pressor effect of IRL-1620 was more markedly altered than those induced by ET-1. In wild-type mice, both ET(A) and ET(B) receptors were found to be involved in the pressor effect of ET-1, as confirmed by the significant and specific antagonism induced by either BQ-123 (ET(A) antagonist) or BQ-788 (ET(B) antagonist). Also, ET(A)-selective or mixed ET(A)/ET(B)- but not ET(B)-selective antagonists reversed the hypertensive state of the ET(B) (+/-) knock-out mice to the level of wild-type littermates. Finally, radiolabeled ET-1 plasmatic clearance was altered in ET(B) (+/-) but not ET(A) (+/-) knock-out mice when compared with wild-type animals. Thus, heterozygous knock-out of ET(B) receptors results in a hypertensive state, suggesting an important physiological role for that particular receptorial entity in opposing the endogenous ET-1-dependent pressor effects in the mouse.     

Amelioration of genetic hypertension by suppression of renal G protein-coupled receptor kinase type 4 expression

Abnormalities in D1 dopamine receptor function in the kidney are present in some types of human essential and rodent genetic hypertension. We hypothesize that increased activity of G protein-coupled receptor kinase type 4 (GRK4) causes the impaired renal D1 receptor function in hypertension. We measured renal GRK4 and D1 and serine-phosphorylated D1 receptors and determined the effect of decreasing renal GRK4 protein by the chronic renal cortical interstitial infusion (4 weeks) of GRK4 antisense oligodeoxynucleotides (As-Odns) in conscious- uninephrectomized spontaneously hypertensive rats (SHRs) and their normotensive controls, Wistar-Kyoto (WKY) rats. Basal GRK4 expression and serine-phosphorylated D1 receptors were &90% higher in SHRs than in WKY rats and were decreased to a greater extent in SHRs than in WKY rats with GRK4 As-Odns treatment. Basal renal D1 receptor protein was similar in both rat strains. GRK4 As-Odns, but not scrambled oligodeoxynucleotides, increased sodium excretion and urine volume, attenuated the increase in arterial blood pressure with age, and decreased protein excretion in SHRs, effects that were not observed in WKY rats. These studies provide direct evidence of a crucial role of renal GRK4 in the D1 receptor control of sodium excretion and blood pressure in genetic hypertension.     

Chronic activation of endothelin B receptors: new model of experimental hypertension

Endothelin (ET) exerts powerful pressor actions primarily through activation of the ET(A) receptor subtype. The ET(B) receptor (ET(B)R) subtype, on the other hand, is generally thought to initiate physiological actions that decrease arterial pressure. Such actions include clearing ET from the bloodstream, initiating endothelium-mediated vasodilation, and facilitating renal sodium and water excretion. The effect of long-term activation of the ET(B)R on arterial pressure, however, never has been directly tested. In this study we evaluated cardiovascular responses to chronic (5-day) activation of ET(B)R in male rats using continuous intravenous infusion of the selective agonist sarafotoxin 6c. Surprisingly, we found that sarafotoxin 6c caused a sustained increase in arterial pressure that rapidly reversed on termination of infusion. The hypertension was associated with increased renal excretion of sodium and water and decreased plasma volume. Alterations in daily sodium intake did not affect the magnitude of the hypertension. Hemodynamic studies revealed a decreased cardiac output and increased total peripheral resistance during sarafotoxin 6c infusion. Infusion of sarafotoxin 6c caused a small increase in plasma ET levels. Nevertheless, the hypertension was not affected by coadministration of a selective ET(A) receptor antagonist (atrasentan) but was completely prevented by treatment with a combined ET(A) receptor and ET(B)R antagonist (A186280). These experiments reveal for the first time that chronic activation of ET(B)R in rats causes sustained hypertension.