Endothelin in hypertensive resistance arteries. Intraluminal and extraluminal dysfunction

Estrogen modulates serotonin effects on vasoconstriction through Src inhibition

Estrogen has diverse effects on cardiovascular function, including regulation of the contractile response to vasoactive substances such as serotonin. The serotonin system recently emerged as an important player in the regulation of vascular tone in humans. However, hyperreactivity to serotonin appears to be a critical factor for the pathophysiology of hypertension. In this study, we examined the modulatory mechanisms of estrogen in serotonin-induced vasoconstriction by using a combinatory approach of isometric tension measurements, molecular biology, and patch-clamp techniques. 17β-Estradiol (E2) elicited a significant and concentration-dependent relaxation of serotonin-induced contraction in deendothelialized aortic strips isolated from male rats. E2 triggered a relaxation of serotonin-induced contraction even in the presence of tamoxifen, an estrogen receptor antagonist, suggesting that E2-induced changes are not mediated by estrogen receptor. Patch-clamp studies in rat arterial myocytes showed that E2 prevented Kv channel inhibition induced by serotonin. Serotonin increased Src activation in arterial smooth muscle required for contraction, which was significantly inhibited by E2. The estrogen receptor-independent inhibition of Src by E2 was confirmed in HEK293T cells that do not express estrogen receptor. Taken together, these results suggest that estrogen exerts vasodilatory effects on serotonin-precontracted arteries via Src, implying a critical role for estrogen in the prevention of vascular hyperreactivity to serotonin.

Diabetic retinopathy and endothelin system: microangiopathy versus endothelial dysfunction

In the face of the global epidemic of diabetes, it is critical that we update our knowledge about the pathogenesis of diabetes and the related micro alterations on the vascular network in the body. This may ultimately lead to early diagnosis and novel treatment options for delaying the progression of diabetic complications. Research has recently revealed the pivotal role of endothelin in the pathogenesis of diabetic complications, particularly in the regulation of the capillary flow, which is affected in the course of retinopathy. Although there are several reviews on various approaches to the treatment of diabetes, including normalization of glucose and fat metabolism, no reviews in literature have focused on the endothelin system as a therapeutic target or early indicator of diabetic microangiopathy. In this review, we summarize some of the experimental and clinical evidence suggesting that current therapeutic approaches to diabetes may include the modulation of the blood concentration of compounds of the endothelin system. In addition, we will briefly discuss the beneficial effects produced by the inhibition of the production of high levels of endothelin in vasculopathy, with focus on diabetic retinopathy. The cutting-edge technology currently widely used in opththalmology, such as the OCT angiography, allows us to detect very early retinal morphological changes alongside alterations in choroidal and retinal vascular network. Combination of such changes with highly sensitive measurements of alterations in serum concentrations of endothelin may lead to more efficient early detection and treatment of diabetes and related macro/microvascular complications.

Higher circulatory level of endothelin-1 in hypertensive subjects screened through a cross-sectional study of rural Bangladeshi women

Endothelin-1 (ET-1) is a potential marker of the endothelial dysfunction, which has been shown to be elevated in hypertensive subjects. No previous study has investigated the circulatory level of ET-1 and hypertension in a South Asian country. The present study assessed the circulating levels of ET-1 in subjects with or without hypertension and further examined the association of ET-1 with clinical and metabolic parameters. A total of 2543 rural Bangladeshi women with a mean age of 44.5 years were studied using a cross-sectional survey. Multiple regressions were used to examine the association between the circulatory ET-1 levels and hypertension. The prevalence of hypertension was 29.3%. The ET-1 levels were significantly higher in the hypertensive (mean 3.08 pg ml(-1), s.e. (0.19)) than in the non-hypertensive subjects (mean 2.01 pg ml(-1), s.e. (0.03)) (P = 0.001). After adjusting for age, the ET-1 level had significant positive associations with the diastolic blood pressure (P = 0.002), systolic blood pressure (P = 0.001), mean arterial pressure (P = 0.002) and fasting blood glucose (P = 0.002). In a tertile analysis, we found that hypertension in the subjects was significantly increased as the levels of ET-1 increased (P for the trend = 0.001). In a stepwise multiple regression analysis, after adjusting for age and all other potential variables, we found that the mean arterial pressure and the fasting plasma levels have significant associations with the ET-1 level. The present study demonstrates that there is a higher concentration of ET-1 among the hypertensive subjects in an apparently healthy population of Bangladeshi rural women. The relationship between ET-1 and hypertension requires further investigation to define the clinical utility and predictive value of serum ET-1 levels for hypertension for a South Asian population.

Selective blockade of endothelin-B receptor improves survival of critically perfused musculocutaneous flaps

BACKGROUND AND AIMS

Insufficient perfusion of distal flap areas, which may lead to partial necrosis, still represents a challenge in reconstructive surgery. In the process of microvascular and endothelial dysfunction, endothelins (ETs) and their receptors may play an important role. Therefore, the aim of the study was to investigate in a chronic in vivo model the effect of various ET-receptor antagonists in critically perfused flap tissue.

MATERIALS AND METHODS

A random pattern musculocutaneous flap was elevated in the back of 25 C57BL/6 mice and fixed into a dorsal skinfold chamber. Repetitive intravital fluorescence microscopy was performed over a 10-day observation period, assessing arteriolar diameter, arteriolar blood flow (aBF), functional capillary density (FCD), the area of tissue necrosis, and the development of newly formed blood vessels. ET-receptor blockers were administrated intraperitoneally 30 min before induction of ischemia, as well as daily during the subsequent 4-day period, including (1) BQ-123, a specific ET-A-receptor antagonist (ET-A = 1 mg/kg), (2) BQ-788, a selective ET-B-receptor antagonist (ET-B = 1 mg/kg), and (3) PD-142893, a nonselective ET-AB-receptor antagonist (ET-AB = 0.5 mg/kg). Animals receiving saline only served as controls (n = 7).

RESULTS

Despite an increase in aBF during the 10-day observation period (day 1 = 1.92 +/- 0.29 nl/s; day 10 = 4.70 +/- 1.64 nl/s), the flaps of saline-treated controls showed a distinct decrease in FCD (94 +/- 12 cm/cm(2)). This perfusion failure resulted in flap necrosis of 52 +/- 3%. Selective blockade of the ET-B receptor caused a further increase in aBF already at day 1 (2.97 +/- 0.42 nl/s), which persisted during the following 10-day observation period (day 10 = 5.74 +/- 0.69 nl/s). Accordingly, adequate FCD could be maintained (day 10 = 215 +/- 8 cm/cm(2); p < 0.05 vs control), resulting in a significant reduction in flap necrosis (day 10 = 25 +/- 4%; p < 0,05). In contrast, neither selective blockade of the ET-A receptor nor nonselective ET-A- and ET-B-receptor blockade were able to significantly affect aBF when compared to controls (day 1 = ET-A = 1.39 +/- 0.10 nl/s; ET-AB = 1.53 +/- 0.80 nl/s; n.s.). Accordingly, flap necrosis after ET-A- and ET-AB-receptor inhibition did not differ from that of controls (day 10 = ET-A: 46 +/- 10%; ET-AB = 51 +/- 7%).

CONCLUSION

Our data show that only selective ET-B-receptor inhibition is capable of maintaining nutritive perfusion and, hence, reducing necrosis in critically perfused flap tissue. Accordingly, administration of ET-B-receptor antagonists may be considered in the treatment of critically perfused flaps.

Endothelin and erectile dysfunction: a target for pharmacological intervention?

Although erectile dysfunction (ED) is not life threatening, this common problem can significantly affect the quality of life and psychological and social well-being. The Massachusetts male ageing study (1,290 men aged 40 - 70 years) showed that 52% of men reported some degree of ED (17.1% mild, 25.2% moderate, 9.6% total). In the UK, an estimated 17 - 19% of men are thought to suffer from ED. This problem is more common with advancing age and since this proportion of the population is increasing, the prevalence of ED is expected to rise. Endothelin-1 (ET-1) belongs to a family of potent vasoconstrictor peptides consisting of 21 amino acids. We review the evidence showing that ET-1 plays a role via (ET(A) and ET(B) receptors) in the regulation of cavernosal smooth muscle tone. We also consider the various risk factors that are involved in the pathogenesis of ED and how these relate to the action of ET-1. In particular, the role of diabetes, hypertension, smoking and dyslipidaemia are discussed. The pharmaceutical industry has declared an interest in the development of ET antagonists for use in the treatment of various diseases including ED. We briefly comment on experimental ET-1 antagonists that may be of therapeutic benefit in ED.

Endothelin-B-receptor-selective antagonist inhibits endothelin-1 induced potentiation on the vasoconstriction to noradrenaline and angiotensin II

OBJECTIVE

Endothelin-A-receptor-antagonists inhibit angiotensin II- and noradrenaline-induced vasoconstriction. Whether functional constrictive endothelin-B-receptors play a role in the endothelin-1-mediated potentiation of vasoconstriction to angiotensin II and noradrenaline is thus far unknown.

METHODS

We studied the effects of noradrenaline and angiotensin II (10 mol/l) in the presence of exogenous endothelin-1 (10 mol/l) with and without selective endothelin-B-receptor-blockade by BQ-788 (10 mol/l) and dual receptor blockade with BQ-788 and the endothelin-A-selective antagonist BQ-123 (10 mol/l) in 14 healthy male volunteers (aged 20-28). Studies were performed in the human skin microcirculation under in vivo conditions using laser-Doppler flowmetry and double injection technique. The area under the time-response curve of all doses was calculated.

RESULTS

Endothelin-1 potentiated the effects of angiotensin II and noradrenaline (-944 +/- 139 perfusion units (PU), P < 0.01; -926 +/- 117 PU, P < 0.05, respectively). In the presence of BQ-788, the potentiating effect of endothelin-1 was significantly blunted (-624 +/- 132 PU, P < 0.01; -549 +/- 136 PU, P < 0.01, respectively). In the presence of BQ-123 and BQ-788 the vasoconstriction was fully inhibited (431 +/- 108 PU, P < 0.001 and 421 +/- 86 PU, P < 0.001, respectively).

CONCLUSIONS

These data suggest that functional vasoconstrictive endothelin-B receptors on vascular smooth muscle cells may contribute to the potentiating effects of high local concentrations of endothelin-1 on the vasoconstriction to noradrenaline and angiotensin II in human microcirculation.

Vasoactive Response of Isolated Pulpal Arterioles to Endothelin-1

The vasoactive effect of endothelin-1 applied intraluminally or extraluminally was studied in vitro in isolated perfused porcine pulpal arterioles using a microperfusion system. Pulpal arterioles (outer diameter, 94.2 +/- 2.8 microm, n = 12) were cannulated and perfused at a constant flow rate in an environment-controlled bath on the stage of an inverted microscope. The vessel diameters were measured online. Both intraluminal and extraluminal application of endothelin-1 (10(-16) M to 10(-8) M) induced dose-dependent constrictions, reaching 82.3 +/- 1.7% (n = 12) and 70.5 +/- 1.3% (n = 12) at 10(-8) M, respectively. Nifedipine reversed endothelin-1-induced constriction dose-dependently at 10(-7) M and above. These data demonstrate that endothelin-1 induces calcium-dependent vasoconstriction in porcine pulpal arterioles, with extraluminal application more potent, which seems to reflect the possible modulation of vascular endothelium in the control of vascular tone.

[Endothelium-derived factors in hypertensive blood vessels, especially nitric oxide and hypertension]

Endothelium-dependent relaxation (EDR) in the blood vessels of spontaneously hypertensive rats (SHR) and the role of nitric oxide (NO) in the initiation of hypertension are reviewed. EDR was impaired in blood vessels of SHR depending on age and degree of hypertension when compared with those of normotensive rats. The cause of the impairment varied among the type of blood vessels: a decrease in the production of NO and endothelium-derived relaxing factor (EDRF) and an increase in the production of endothelium-derived contracting factor (EDCF) are the main causes of the impairment in large arteries, while a decrease in endothelium-dependent hyperpolarization and increased release of EDCF are the main causes of the impairment in small arteries. Interactions among these endothelium-derived factors and changes in the interactions are also causes of impairment. Superoxide may be involved in the impairment of EDR by destroying NO. The endothelium depresses smooth muscle contraction, including spontaneous tone developed in vascular smooth muscle, and the depressing effect of the endothelium is impaired in the preparations from SHR. The endothelium of blood vessels of SHR are structurally injured as demonstrated by scanning electron microscopy. Antihypertensive treatment prevented these functional and structural changes. Chronic treatment with inhibitors of NO production in normotensive rats impaired EDR and elevated blood pressure. The impairment of EDR is a secondary change due to continued hypertension, and early initiation of antihypertensive therapy is recommended.

An in vivo and in vitro comparison of the effects of vasoactive mediators on pulpal blood vessels in rat incisors

The effects of endogenous vasoactive substances were evaluated in anaesthetized rats using a laser Doppler flowmeter to monitor changes in pulpal blood flow, as well as directly in isolated pulpal arteriole preparations utilising a microperfusion and monitoring system to observe changes in vessel diameter. In anaesthetized rats, while systemic arterial blood pressure remained relatively stable, intra-arterial delivery of adrenaline (epinephrine) (A), noradrenaline (norepinephrine) (NA), phenylephrine (PHE), dopamine (DOPA), 5-hydroxytryptamine (5-HT), or endothelin-1 (ET-1) produced a dose-dependent reduction in pulpal blood flow (order of potency: ET-1>>A=NA>PHE=DOPA=5-HT); acetylcholine induced a dose-dependent increase in pulpal blood flow; histamine, isoproterenol and adenosine produced no significant changes. In isolated arteriole preparations, intraluminal delivery of A, NA, PHE, DOPA or 5-HT produced dose-dependent vasoconstriction (A=NA>PHE=DOPA=5-HT). Acetylcholine relaxed NA-precontracted vessels dose-dependently. Histamine and isoproterenol produced a small vasodilatation. Intraluminal ET-1 produced a small vasoconstriction at 10(-8)M, whereas extraluminal ET-1 produced a dose-dependent vasoconstriction from 10(-10)M and above. Intraluminal adenosine failed to dilate vessels precontracted with ET-1, whereas extraluminal adenosine caused a complete relaxation. These combined in vivo and in vitro data suggest that, in the rat incisor, the pulpal microcirculation is capable of functional regulation and that pulpal blood flow may be modulated by endothelium-related factors, metabolic (tissue-related) factors, as well as humoral (blood-borne) factors.

Effect of endothelin blockade on basal and stimulated forearm blood flow in patients with essential hypertension

Endothelin receptor antagonism lowers blood pressure in patients with essential hypertension, but the contribution of the endothelin system to increased vascular tone in these patients remains controversial. We used strain-gauge venous plethysmography to measure changes in basal and metabolically stimulated (peak reactive hyperemia [PRH]) forearm blood flow (FBF) induced by continuous intrabrachial infusion of SB 209670, a dual endothelin type A/endothelin type B receptor antagonist (ETRA) in 11 patients with hypertension and 12 healthy age-matched control subjects. In both groups, ETRA caused significant vasodilation (increase in FBF compared with baseline over the last 30 minutes of ETRA infusion: 75+/-12% in control subjects, 40+/-13% in hypertension patients; P<0.01 for both groups). By repeated-measures ANOVA, there was no greater increase in FBF after ETRA in hypertension patients compared with control subjects. In addition, FBF responses to PRH, expressed as percent change from baseline (prePRH), were similar in both groups (control subjects: preETRA, 1381+/-222%; during ETRA, 921+/-178%; and hypertension patients: preETRA, 1232+/-221%: during ETRA, 865+/-285%). We conclude that basal and stimulated vasodilation induced by short-term ETRA infusion in the forearm vasculature of hypertension patients is not increased compared with that of control subjects. An enhanced contribution of the endothelin system to vascular tone in hypertension patients could not be demonstrated using this experimental approach.

Endothelin receptor antagonists and cardiovascular diseases of aging

Our understanding of the role of the endothelin system in human cardiovascular physiology and pathophysiology has evolved very rapidly since the initial description of its constituent parts in 1988. Endothelin-1 (ET-1) is the predominant endothelin isoform in the human cardiovascular system and has potent vasoconstrictor, mitogenic and antinatriuretic properties which have implicated it in the pathophysiology of a number of cardiovascular diseases. The effects of ET-1 have been shown to be mediated by 2 principal endothelin receptor subtypes: ET(A) and ET(B). The development of a range of peptidic and nonpeptidic endothelin receptor antagonists represents an exciting breakthrough in human cardiovascular therapeutics. Two main classes of endothelin receptor antagonist have been developed for possible human therapeutic use: ET(A)-selective and nonselective antagonists. Extensive laboratory and clinical research with these agents has highlighted their promise in various cardiovascular diseases. Randomised, placebo-controlled clinical trials have yielded very encouraging results in patients with hypertension and chronic heart failure with more preliminary data suggesting a possible role in the treatment and prevention of atherosclerosis and stroke. Much more research is needed, however, before endothelin receptor antagonists can be considered for clinical use.

Vascular smooth muscle growth: autocrine growth mechanisms

Vascular smooth muscle cells (VSMC) exhibit several growth responses to agonists that regulate their function including proliferation (hyperplasia with an increase in cell number), hypertrophy (an increase in cell size without change in DNA content), endoreduplication (an increase in DNA content and usually size), and apoptosis. Both autocrine growth mechanisms (in which the individual cell synthesizes and/or secretes a substance that stimulates that same cell type to undergo a growth response) and paracrine growth mechanisms (in which the individual cells responding to the growth factor synthesize and/or secrete a substance that stimulates neighboring cells of another cell type) are important in VSMC growth. In this review I discuss the autocrine and paracrine growth factors important for VSMC growth in culture and in vessels. Four mechanisms by which individual agonists signal are described: direct effects of agonists on their receptors, transactivation of tyrosine kinase-coupled receptors, generation of reactive oxygen species, and induction/secretion of other growth and survival factors. Additional growth effects mediated by changes in cell matrix are discussed. The temporal and spatial coordination of these events are shown to modulate the environment in which other growth factors initiate cell cycle events. Finally, the heterogeneous nature of VSMC developmental origin provides another level of complexity in VSMC growth mechanisms.

Paradoxically Enhanced Endothelin-B Receptor–Mediated Vasoconstriction in Conscious Old Monkeys

Background —We investigated the effects of aging on the responses to endothelin (ET) in conscious old (19.8±0.6 years) and young adult (6.8±0.3 years) monkeys and compared these results with those of other vasoconstrictors, eg, phenylephrine (PE) and angiotensin II (Ang II).

Methods and Results —The monkeys ( Macaca fascicularis ) were chronically instrumented. Baseline total peripheral resistance (TPR) was not different between the 2 groups. As expected, TPR rose less ( P <0.05) with PE (5 μg/kg) in old monkeys (34±3%) than in young monkeys (57±6%); TPR also rose less with Ang II. Surprisingly, TPR rose more ( P <0.05) with endothelin-1 (ET-1, 25 ng · kg −1 · min −1 ) in old monkeys (36±6%) than in young monkeys (10±2%). An ET B receptor agonist, sarafotoxin (S6c, 30 ng · kg −1 · min −1 ) was administered in the presence of an ET A receptor antagonist, BQ-123 (1 mg/kg). Under these conditions, TPR increased more ( P <0.05) in old monkeys (59±10%) than in young monkeys (31±4%). In the presence of nitric oxide synthase (NOS) inhibition with N W -nitro- l -arginine methyl ester (60 mg/kg), vasoconstriction induced by S6c no longer differed with age, because it was enhanced in young monkeys ( P <0.05) (68±9% versus 31±4%) but not in old monkeys (58±6% versus 59±10%). Thus, after NOS inhibition, vasoconstrictor responses to ET were no longer enhanced in old monkeys.

Conclusions —Peripheral vasoconstriction (PE and Ang II) is reduced in old monkeys, as expected. Paradoxically, vasoconstriction induced by ET-1 was actually enhanced in old monkeys, which appears to be a result of impaired endothelium-dependent vasodilation, which with ET-1 should involve the ET B receptor.

Microvascular endothelial dysfunction and its mechanism in a rat model of subarachnoid hemorrhage

After subarachnoid hemorrhage (SAH), large cerebral arteries are prone to vasospasm. Using a rat model of SAH, we examined whether cortical microvessels demonstrate vasomotor changes that may make them prone to spasm and whether endothelial dysfunction may account for any observed changes. Two days after percutaneous catheterization into the cisterna magna, 0.3 mL of autologous blood was injected into the subarachnoid space. The brain tissue was harvested 20 min later, and microvessels were dissected from the parietal cortex. Vasomotor responses to the thromboxane analog U46619, the protein kinase C agonist phorbol acetate, endothelin-1, adenosine diphosphate, nitroprusside, and isoproterenol were examined in vitroin cerebral arterioles from the control, sham-operated, and SAH animals. Endothelial nitric oxide synthase (NOS3) messenger RNA and protein concentration was measured by northern and western blotting, respectively. Arterioles from the SAH animals demonstrated attenuated dilation to the endothelium-dependent dilator adenosine diphosphate and accentuated constriction to endothelin-1, while responses to the other agents tested were unchanged. NOS3 protein concentration was decreased, but NOS3 messenger RNA was increased after SAH. After SAH, cortical arterioles demonstrate endothelial dysfunction, which may be the basis for microvascular spasm. This is in part related to decreased NOS3, which occurs despite an increase in its transcription.

IMPLICATIONS

Acute microvascular endothelial dysfunction may occur after subarachnoid hemorrhage and contribute to microvascular spasm.

Endothelium-dependent vasoactive modulation in the ophthalmic circulation

The vascular endothelium is strategically located between the circulating blood and the vascular smooth muscle cells. Different agonists or stimuli transported by the circulating blood can trigger the endothelium to release potent relaxing (nitric oxide, prostacyclin, endothelium-derived hyperpolarizing factor) or contracting factors (endothelin, cycloxygenase products). These endothelium-derived vasoactive factors can modulate blood flow locally. Heterogeneity exists from one vascular bed to the other, or even between vessels, in the agonists able to stimulate the release of endothelium-derived vasoactive factors. In the ophthalmic circulation, nitric oxide and endothelin are strong vasoactive modulators. In many vascular diseases that are of importance in ophthalmology (hypercholesterolemia, arteriosclerosis, hypertension, diabetes, vasospastic syndrome, ischemia and reperfusion, etc) the function of the endothelium can be impaired. There exist different drugs that can modulate the vasoactive function of the vascular endothelium. In other words, it appears that the vascular endothelium plays an important role in both the physiology and pathophysiology of the regulation of blood flow. The modulation of this regulatory system by different drugs might open new therapeutical approaches to treat vascular disorders in ophthalmology.

Altered contractions to endothelin-1, phenylephrine, potassium chloride and relaxations to acetylcholine at various stages of renal hypertension in the rat

This study is aimed at investigating the contraction and relaxation responses in the thoracic and abdominal aortae at various stages of hypertension. Hypertension in the rats was produced by aortic banding and the responses in the abdominal and thoracic aortic rings were studied 2 and 8 weeks after aortic banding. Contractile responses to phenylephrine ( 10(-6)M), KCl (80 mM) or to endothelin-1 ( 10(-12)to 10(-6)M) and the relaxation responses to acetylcholine ( 10(-7)to 10(-5)M) were similar in the thoracic and abdominal rings of normotensive rats. The intact thoracic rings from 2 week aortic-banded hypertensive rats (ABHR) showed attenuated responses to all the contractile agents used. However, the relaxation to acetylcholine was not altered. In the rings from 8 week ABHR, the responses to contractile agents were not significantly altered but the acetylcholine-induced relaxations were significantly attenuated. The endothelial-derived relaxing factors might act to antagonize the vasoconstrictive responses during the onset of hypertension but might be disabled, as the endothelial dysfunction becomes predominant after 8 weeks of hypertension. The results thus suggest that the contractile and relaxant responses are differentially altered during different stages of hypertension.

Regulation of matrix proteins and impact on vascular structure

The vascular extracellular matrix is responsible for the mechanical properties of the vessel wall and is also involved in biologic processes such as cellular adhesion, regulation, and proliferation. Thus, an adequate balance of its components is necessary for the normal functioning of the vasculature. Vascular disorders affect this balance, and this plays a key role in their pathophysiology. Atherogenesis is accompanied by an increase in matrix deposition in response to low-density lipoprotein accumulation. However, this matrix, mainly collagen, also has a protective role by forming a fibrous cap around the lipid core, avoiding contact with blood. A decrease in the amount of collagen will weaken the cap and make it prone to rupture, leading to thrombosis and acute coronary syndromes. In hypertension, the increase in matrix deposition results in vascular stiffness and cardiac dysfunction. In this paper, we discuss the relevance of matrix regulation in these conditions.

Chronic endothelin-1 improves nitric oxide-dependent flow-induced dilation in resistance arteries from normotensive and hypertensive rats

Endothelin-1 (ET-1) is released on stimulation by shear stress of the vascular wall. In several pathological situations, an involvement of ET-1 is suspected. Nevertheless, the effect of a chronic increase in circulating ET-1 on vascular tone in resistance arteries is not yet fully understood. We investigated the response to tensile stress (pressure-induced myogenic tone) and shear stress (flow-induced dilation, FD) of rat mesenteric resistance arteries cannulated in an arteriograph. Intraluminal diameter was measured continuously. Rats (normotensive Wistar-Kyoto rats [WKYs] and spontaneously hypertensive rats [SHRs]) were treated for 2 weeks with ET-1 (5 pmol. kg(-1). min(-1) SC; n=8 to 16 per group). Systolic arterial blood pressure increased significantly in ET-1-treated rats (171+/-7 versus 196+/-6 mm Hg in WKYs and 216+/-8 versus 245+/-6 mm Hg in SHRs, P<0.05). Passive arterial diameter in isolated resistance arteries ranged from 78+/-9 to 169+/-4 microm in WKYs and from 62+/-6 to 149+/-7 microm in SHRs (pressure from 10 to 150 mm Hg). Myogenic tone was not significantly affected by chronic ET-1. Flow (9 to 150 microL/min) significantly increased the arterial diameter by 2+/-0.5 to 22+/-2 microm in WKYs and by 1.3+/-0. 7 to 8.3+/-0.8 microm in SHRs (P<0.001 versus WKYs). The NO synthesis blocker N(G)-nitro-L-arginine methyl ester (L-NAME; 100 micromol/L) attenuated FD in WKYs (eg, 22+/-2 versus 15+/-3 microm after L-NAME, flow=150 microL/min) and, to a lesser extent, in SHRs (P<0.001 versus WKYs). The cyclooxygenase inhibitor indomethacin (3 micromol/L) attenuated the remaining FD in WKYs (eg, 15+/-3 versus 8+/-3 microm, flow=150 microL/min) and in SHRs (eg, 7.5+/-0.5 versus 5.0+/-0.6 microm). Chronic ET-1 significantly increased FD in SHRs but not in WKYs. In both strains, NO-dependent FD was significantly increased by chronic ET-1. Furthermore, indomethacin-sensitive FD was increased by chronic ET-1 in SHRs only. Thus, chronic ET-1 increased NO-dependent FD in resistance mesenteric arteries from both WKYs and SHRs and increased indomethacin-sensitive FD in SHRs only.

Functional characterization of endothelin receptors in hypertensive resistance vessels

OBJECTIVE

The physiological and pathophysiological functions of endothelin-1 in modulating the regional blood flow of normal and spontaneously hypertensive rats (SHR) were studied in the perfused mesenteric vascular bed, a useful model for investigating resistance vessels.

DESIGN AND METHODS

We used 12-week-old SHR and Wistar-Kyoto (WKY) rats. Endothelin A (ETA) receptor responsiveness was evaluated by endothelin-1 (0.2-2 mumol/l) concentration-response curves, and repeated in the presence of indomethacin and the ETA and endothelin B (ETB) receptor antagonists BQ-485 and BQ-788, respectively. ETB receptor responsiveness was tested by sarafotoxin S6c concentration-response curves, obtained in the noradrenaline-precontracted mesenteric vascular bed, and repeated after treatment with BQ-788 and after endothelial denudation.

RESULTS

In both groups, endothelin-1 induced concentration-dependent contraction; SHR exhibited a markedly increased maximal effect compared with WKY rats (P < 0.01). BQ-485 produced a shift to the right for endothelin-1 concentration-response curves in both groups, with a higher pA2 (negative common logarithm of the antagonist that produces an agonist dose ratio of 2) value in SHR than in WKY rats (P < 0.01). The increase in the maximal effect produced by endothelin-1 in SHR was prevented by indomethacin, which also induced a significant increase in the endothelin-1 concentration producing the half-maximal response (EC50) in SHR (P < 0.05). Sarafotoxin S6c produced an ETB-dependent endothelium-mediated relaxant effect in WKY rats, which was not observed in SHR.

CONCLUSIONS

The higher vasoconstriction induced by endothelin-1 in SHR may be related to a greater number of available ETA receptors, due to the presence of an ETA receptor subtype. This mechanism may be linked to the production of prostanoids that add to the direct endothelin-1-evoked vasoconstriction. These results, together with the lack of relaxation in response to sarafotoxin S6c in SHR, suggest that an imbalance in the endothelin-1 ability to induce both contraction and relaxation is present in SHR with sustained hypertension, manifesting as a greater contractile effect evoked in this strain.

Hemodynamic and coronary effects of the endothelin antagonist bosentan in patients with coronary artery disease

BACKGROUND

Endothelin is a potent endothelium-derived vasoconstrictor peptide with proliferative properties. Elevated levels of the peptide occur in coronary artery disease; however, its pathophysiological role as a regulator of coronary tone and structure is uncertain. Endothelin-receptor antagonists are specific tools to clarify this issue and might be useful in the treatment of coronary artery disease.

METHODS AND RESULTS

In a double-blind, placebo-controlled randomized study, we investigated the effects of the ETA/ETB endothelin-receptor antagonist bosentan or placebo on systemic and coronary hemodynamics in 28 patients with angiographically documented stable coronary artery disease by quantitative coronary angiography and an intracoronary Doppler guidewire. Bosentan 200 mg IV decreased systolic blood pressure (P<0. 05), whereas heart rate increased slightly (P<0.05). Coronary diameter increased, particularly in vessels with no or mild angiographic changes (P<0.01). Glycerol trinitrate did not further dilate these segments, whereas coronary diameter increased significantly after nitrate in the placebo group. The increase in coronary diameter after bosentan correlated inversely with plasma LDL-cholesterol levels (P<0.01) in both stenotic and angiographically normal coronary segments. Coronary flow velocity did not change. Bosentan was well tolerated.

CONCLUSIONS

Endogenous endothelin exerts a vasoconstrictor tone in epicardial coronary arteries of patients with coronary artery disease, as evidenced by the vasodilation exerted by the combined ETA/ETB endothelin-receptor antagonist bosentan under acute conditions. Bosentan can safely be given to these patients. Hence, further long-term studies are necessary to determine the therapeutic potential of endothelin-receptor antagonists in patients with coronary artery disease.

Effects of hypotensive and non-hypotensive doses of manidipine on structure, responses to endothelin-1 and ICAM-1 production in mesenteric small resistance arteries of spontaneously hypertensive rats

OBJECTIVE

We have evaluated the effects of a new calcium channel blocker, manidipine, given at both high, hypotensive and low, non-hypotensive doses, on vascular morphology, response to endothelin-1 and ICAM-1 production in mesenteric small resistance arteries of spontaneously hypertensive rats (SHR).

METHODS

Ten SHR were treated with manidipine 3 mg/kg per day (high dose) and 10 with manidipine 0.3 mg/kg/per day (low dose). The drug was administered by gavage from the 4th to 12th weeks of age. Eighteen Wistar-Kyoto (WKY) rats and 18 SHR were kept untreated as controls. Rats were killed at 13 weeks. Mesenteric small arteries were dissected and mounted on a micromyograph for determination of indexes of vascular structure (media thickness, wall thickness, media/lumen ratio).

RESULTS

Systolic blood pressure was significantly reduced by the high dose of the drug, while no effect was observed with low-dose manidipine. A reduction in the media/lumen ratio was observed only in SHR treated with high-dose manidipine. The response to endothelin-1 in untreated SHR was significantly lower in comparison with WKY; a significant reduction was observed in SHR treated with high-dose manidipine. ICAM-1 vascular concentrations were higher in untreated SHR than in WKY controls. Both high- and low-dose manidipine reduced ICAM-1 concentrations toward normalization.

CONCLUSIONS

Manidipine at high, hypotensive, but not at low, non-hypotensive doses has been proven to reduce structural alterations in mesenteric small resistance arteries, and to normalize vascular responses to endothelin-1. In addition, manidipine, at both low and high doses, may reduce ICAM-1 vascular production, thus suggesting a possible anti-atherogenic effect.

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 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.

Role of endothelin and nitric oxide imbalance in the pathogenesis of hypoxia-induced arterial hypertension

BACKGROUND

We have recently demonstrated that prolonged hypobaric hypoxia can lead to a hematocrit-independent sustained arterial hypertension (HTN) in genetically normotensive Sprague-Dawley rats. The rise in blood pressure in the hypoxic animals was accompanied by a marked but transient increase in plasma endothelin level. In addition, hypoxia has been shown to decrease nitric oxide (NO) production by cultured endothelial cells. This study was designed to test the hypothesis that hypoxia-induced HTN may be mediated by increased endothelin and/or decreased NO production.

METHODS

Blood pressure, plasma endothelin and urinary NO metabolites (NOx)were monitored in rats during a 24-hour exposure to hypobaric hypoxia (air pressure = 390 mm Hg). The results were compared with hypoxia (air pressure = 390 mm Hg). The results were compared with those obtained in animals maintained under normoxic condition (control group). To test the possible role of excess endothelin and depressed NO production, the studies were repeated using subgroups of animals treated with either an endothelin receptor ET-A/B blocker (L-754,142) or L-arginine.

RESULTS

The untreated hypoxic group exhibited a threefold rise in plasma endothelin and a threefold fall in urinary NOx, prior to the onset of HTN. Endothelin receptor blockade led to a further fall in urinary NOx excretion and failed to mitigate HTN. In contrast, L-arginine supplementation improved the urinary NOx excretion and prevented HTN. Neither therapy affected the hypoxia-induced erythrocytosis.

CONCLUSIONS

We conclude that hypoxia-induced HTN is associated with depressed NO production and can be mitigated by L-arginine supplementation.

Endothelin-1-like immunoreactivity in a new rodent model of spontaneous hypertension

The purpose of this study was to determine plasma and tissue endothelin-1 (ET-1)-like immunoreactivity in a new rodent model of spontaneous hypertension. Plasma and tissues were procured from pentobarbital-anesthetized 16- to 18-week-old male hamsters with spontaneous hypertension and genetically/age-matched normotensive hamsters. We found that ET-1-like immunoreactivity in the plasma was similar in both groups. However, renal and cardiac ET-1-like immunoreactivity was 11- and 1.7-fold higher in spontaneously hypertensive hamsters relative to normotensive hamsters, respectively (P < .05). ET-1-like immunoreactivity was slightly, but significantly, lower in the lung and spleen of spontaneously hypertensive hamsters relative to normotensive hamsters (P < .05). ET-1-like immunoreactivity in the liver and brain was similar in both groups. We conclude that ET-1-like immunoreactivity is significantly higher in two target organs for hypertension, kidney and heart, but not in plasma or brain of adult male hamsters with spontaneous hypertension, relative to genetically/age-matched normotensive hamsters. We suggest that renal and cardiac ET-1 could play a role in the natural history of spontaneous hypertension in hamsters.

Combination of ACE inhibitors and calcium antagonists: a logical approach

An increasing body of evidence indicates that impairment of endothelial function is crucially involved in the pathogenesis of cardiovascular disease. Injury to the endothelium precipitates atherosclerosis by causing smooth-muscle cell migration and proliferation, induction of expression of growth factors, and impairment of plasma coagulation and endogenous fibrinolysis. Angiotensin-converting enzyme (ACE) inhibitors and calcium antagonists are widely used in patients with cardiovascular disease and have beneficial vascular effects beyond blood pressure control alone. Both exhibit a synergistic hemodynamic profile. Whereas calcium antagonists dilate large conduit and resistance arteries, ACE inhibitors inhibit the renin-angiotensin system (RAS) and reduce sympathetic outflow. Certain calcium antagonists, such as verapamil and diltiazem, reduce heart rate, whereas dihydropyridines tend to increase it. In the blood vessel wall, the local vascular effects of ACE inhibitors and calcium antagonists are complementary. ACE inhibitors diminish transformation of angiotensin I (Ang I) into angiotensin II (Ang II) and prevent degradation of bradykinin [which stimulates nitric oxide (NO) and prostacyclin formation]. Calcium antagonists inhibit the effects of Ang I and endothelin-1 (ET-1) at the level of vascular smooth muscle by reducing Ca2+ inflow and facilitating the vasodilator effects of NO. The resistance circulation is particularly dependent on extracellular Ca2+, thereby explaining why nifedipine and verapamil effectively inhibit ET-induced vasoconstriction in vitro and in vivo. In hypertension, ACE inhibitors and calcium antagonists markedly improve structural changes and increase the media/lumen ratio in resistance arteries. Long-term combination therapy with verapamil and trandolapril is particularly effective in reversing endothelial dysfunction in hypertensive animals. ACE inhibitors substantially reduce morbidity and mortality in patients with left ventricular dysfunction after myocardial infarction (MI). There is a strong trend indicating benefit with verapamil as well, but this is confined to patients with a normal left ventricular ejection fraction. Clinical studies have confirmed that calcium antagonists exhibit antiatherogenic properties. However, the clinical relevance of these findings has recently been disputed because short-acting dihydropyridines are reported to increase risk for MI. Because ACE inhibitors and calcium antagonists exhibit synergistic hemodynamic, antiproliferative, antithrombotic, and antiatherogenic properties, combination therapy provides a promising concept in patients with cardiovascular and renal disease.

Endothelin-1 induces vasodilation in human skin by nociceptor fibres and release of nitric oxide

AIMS

Endothelin is a peptide produced by endothelial cells with many biological properties. In the human skin microcirculation endothelin induces neurogenic vasodilation associated with burning pruritus. We investigated the mechanisms involved in this response.

METHODS

The effects of prolonged pretreatment with capsaicin, a specific inhibitor of polimodal nociceptor fibres, and of the nitric oxide synthase inhibitor L-NMMA on endothelin-1-induced vasodilation were studied in 15 human subjects. Furthermore, we investigated the effects of the ET(A)-selective antagonist PD147953 on bradykinin-induced vasodilation.

RESULTS

After local injection, endothelin-1 caused vasoconstriction at the injection site and a profound vasodilation in the surrounding area (flare reaction, P<0.01). This response was specific and not induced by saline, albumin, acetylcholine or an ET-antagonist. Prolonged capsaicin pretreatment inhibited endothelin-1 induced vasodilation in the area surrounding the injection site, but not the central vasoconstriction at the injection site. Bradykinin also induced a marked vasodilation in the area surrounding the injection site; this was not inhibited by an ETA-selective antagonist, while the flare reaction was. L-NMMA applied at the site of the flare reaction prevented endothelin-1-induced vasodilation.

CONCLUSIONS

Endothelin-1 in the human skin microcirculation stimulates polimodal nociceptor fibres leading to the release of nitric oxide. This response may play a pathophysiological role in inflammatory processes in the human skin.

Effect of chronic NG-nitro-L-arginine methyl ester (L-NAME) on blood pressure and renal function in conscious uninephrectomized spontaneously hypertensive rats

We have studied during 30 days the effect of a low dose of NG-nitro-L-arginine methyl ester (1 mg ·kg-1 ·day-1 in drinking water) in the presence of D- or L-arginine (1 mg ·kg-1 ·day-1 in drinking water) in comparison with D- or L-arginine alone on blood pressure and renal function in conscious uninephrectomized female spontaneously hypertensive rats. At the end of the study, there was a significant increase in systolic blood pressure in the NG-nitro-L-arginine methyl ester + D-arginine group (307 ± 6 mmHg (1 mmHg = 133.3 Pa), n = 14, p << 0.05) in comparison with NG-nitro-L-arginine methyl ester + L-arginine (281 ± 6 mmHg, n = 14), L-arginine (262 ± 5 mmHg, n = 13), and D-arginine (258 ± 7 mmHg, n = 12) groups. There were no changes in diuresis, proteinuria, or sodium and potassium excretion between differently treated animals during this study. These results suggest that in uninephrectomized female spontaneously hypertensive rats, after 1 month blockade of NO synthesis with a low dose of NG-nitro-L-arginine methyl ester, vasculature is under tonic control by NO and it is not correlated with renal dysfunction.Key words: Key words: NG -nitro-L-arginine methyl ester (L-NAME), kidney, hypertension, spontaneously hypertensive rats, renaldysfunction, uninephrectomy.

Endothelin and active renin levels in essential hypertension and hypertension with renal artery stenosis before and after percutaneous transluminal renal angioplasty

OBJECTIVE

To determine whether active renin and endothelin levels in venous plasma differ between patients with renal artery stenosis and patients with primary hypertension. Among the patients with renal artery stenosis we also compared active renin and endothelin levels between subjects who had been cured or whose blood pressure had improved after treatment of the stenosis and those without a beneficial reaction after such treatment.

METHODS

We measured immunoreactive endothelin and active renin levels in peripheral venous plasma before and 1 h after angiotensin converting enzyme inhibition in 25 patients with primary hypertension and in 27 patients with hypertension and renal artery stenosis. Percutaneous transluminal angioplasty was performed in 21 patients of the latter group. For 11 patients of this group, hypertension was cured or there was an improvement, whereas 10 other patients did not respond to this treatment. Baseline active renin and endothelin levels were compared between these groups, as were the clinical characteristics of the patients.

RESULTS

Baseline endothelin levels were similar in members of the renal artery stenosis [median 3.6 pg/ml (range 1.4-11.7)] and in members of the no stenosis group [5.0 pg/ml (1.5-8.0)]. Also baseline endothelin levels did not differ between members of the successfully treated [3.6 pg/ml (1.8-8.9)] and unsuccessfully treated groups [3.75 pg/ml (1.4-8.3)]. Angiotensin converting enzyme (ACE) inhibition failed to cause a significant change in endothelin level in members of any of the patient groups. Although baseline renin levels differed significantly between members of the renal artery stenosis and no stenosis groups [40.2 microu/ml (0.9-543) versus 13.4 microu/ml (2.5-931), (P< 0.05)], there was no difference in baseline renin levels between the members of successful and unsuccessful groups [25.7 microu/ml (9.2-475.6) versus 65.3 microu/ml (12.3-542.6)]. ACE inhibition caused a significant increase in renin level in members of all groups except the unsuccessfully treated group.

CONCLUSIONS

Circulating endothelin levels did not differ significantly among patients with essential hypertension, hypertension with renal artery stenosis and proven renovascular hypertension and, although the renin-angiotensin system was clearly activated in members of the renovascular hypertension group, ACE inhibition did not affect their endothelin levels. These results suggest that endothelin does not play a direct role in the pathophysiology of renovascular hypertension.

The vasoconstriction induced by endothelin-1 is mediated only by ET(A) receptors in mesenteric small resistance arteries of spontaneously hypertensive rats and Wistar Kyoto rats

OBJECTIVE

To evaluate the functional responses of mesenteric small resistance arteries of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rat controls to endothelin-1 (ET-1), in the presence and absence of an ET(A) receptor antagonist drug as well as to an ET(B) receptor agonist.

METHODS

Twenty rats aged 12 weeks were studied. They were 10 SHR and 10 WKY rats. Mesenteric small resistance arteries (relaxed diameter 100-180 microm) were dissected and mounted on a micromyograph (Mulvany's technique). A dose-response curve for response to ET-1 was plotted for cumulative concentrations (from 10(-11) to 10(-8) mol/l) in the presence and absence of 10(-6) mol/l FR 139317 (a selective antagonist of ET(A) receptors). In addition, the effects of 10(-7) mol/l N-succinyl-[Glu9, Ala11,15]-endothelin 1 fragment 8-21 (IRL 1620, a selective agonist of ET(B) receptors) were evaluated.

RESULTS

The response of ET-1 was greater in WKY rats than it was in SHR. Almost all the vasoconstrictor effect of ET-1 could be prevented by addition of FR 139317, whereas the agonist of ET(B) receptors had no effect (no change in active force).

CONCLUSIONS

The contractile effects of ET-1 on mesenteric small resistance arteries of SHR and WKY rats are mediated mostly by ET(A) receptors, whereas ET(B) receptors play a minor role, if any. It is possible, however, that a vasoconstrictor effect of ET(B) receptors on the smooth muscle could be masked by the concomitant stimulation of endothelial ET(B) vasodilator receptors.

Effect of dietary salt loading and high-calcium diet on vascular smooth muscle responses and endothelium function in rats

1. The present study examined the effects of concurrent manipulation of dietary calcium and salt on contractile responses of vascular smooth muscle (VSM) and endothelial function of aortic rings from Sprague-Dawley rats. 2. Salt loading enhanced the contractile response of the aortic rings to noradrenaline (NA), an effect that was blunted by a high calcium intake. 3. Removal of the endothelium and incubation of aortic rings in physiological salt solution containing methylene blue increased the sensitivity of the rings to NA. 4. The increase in the sensitivity of aortic rings induced by endothelium removal was more pronounced in aortic rings from salt-loaded rats. 5. Acetylcholine caused similar degrees of relaxation in all experimental groups, but the relaxation to histamine was smaller (P < 0.05) in salt-loaded rats than in other groups of rats; however, after removal of the endothelium, the contractile response to histamine was higher in salt-loaded rats. 6. The results indicate that the hypersensitivity of isolated aortic rings to agonists, as observed in salt-loaded rats, is due to altered responses of the VSM and not as a result of changes in the endothelium. In addition, salt loading tends to increase the synthesis of endothelium-dependent relaxing factor. The ability of salt loading to enhance the contractile responses of VSM to agonists can be prevented by supplementing the diet with high calcium.

Endothelial dysfunction and cardiorenal injury in experimental salt-sensitive hypertension: effects of antihypertensive therapy

BACKGROUND

Pharmacological control of hypertension has contributed to a significant decrease in cardiovascular morbidity and mortality, although the beneficial effect on cardiac and renal diseases has been far more modest than the reduction in stroke. The endothelium plays a crucial homeostatic role in the regulation of vascular tone thrombogenesis and vascular remodeling. We studied the relationship between endothelial dysfunction and cardiorenal injury in hypertensive rats and evaluated the effects of two classes of antihypertensive agents commonly used in the clinical setting, a diuretic (DIU) and an ACE inhibitor (CEI).

METHODS AND RESULTS

Dahl salt-sensitive rats (DS) given high dietary salt (4% NaCl) developed hypertension (systolic blood pressure [SBP], 218+/-9 versus 147+/-3 mm Hg in DS given 0.5% NaCl; P<.001), which was associated with impaired endothelium-dependent relaxations (EDRs) in aortic rings (ED50, 5.44+/-.18 versus 7.51+/-.10; P<.05) and mesenteric vessels (area under the curve, 299+/-11 versus 217+/-11 arbitrary units; P<.05). Hypertensive DS also demonstrated depressed nitric oxide synthase activity in the aorta (0.76+/-.15 versus 2.83+/-.17 nmol x min(-1) x g protein(-1); P<.05), left ventricular hypertrophy (0.43+/-.02 versus 0.29+/-.02 g ventricular weight/100 g body weight; P<.05), glomerular injury (histological injury score: 151+/-8 versus 11+/-2; P<.05), and increased urinary protein excretion (95+/-21 versus 25+/-5 mg/24 hours; P<.05). Treatment of DS rats with the CEI perindopril (4.56 mg x kg(-1) x d(-1)) did not affect SBP (225+/-6 mm Hg) but modestly improved EDR (ED50: 6.07+/-.37; P<.05 versus hypertensive DS) as well as proteinuria and glomerular histology. Addition of the DIU indapamide (1.44 mg x kg(-1) x d(-1)) normalized SBP (151+/-2 mm Hg; P<.05), EDR (ED50, 7.33+/-.08; P<.05), left ventricular hypertrophy (0.27+/-.01 g/100 g body weight; P<.05), and proteinuria (31+/-4 mg/24 hours; P<.05) and prevented glomerular injury (injury score: 30+/-2; P<.05). Monotherapy with DIU reduced SBP (175+/-3 mm Hg; P<.05) and normalized EDR and left ventricular hypertrophy but did not provide effective renal protection. In hypertensive DS, impaired EDR and left ventricular hypertrophy were positively correlated with SBP. In addition, we found a significant correlation between cardiac hypertrophy and endothelial dysfunction. Indeed, a hierarchical regression analysis revealed that impaired aortic EDR, and therefore decreased aortic compliance, positively contributed to left ventricular hypertrophy in addition to but independently of SBP [F(2,37)=6.29; P=.004].

CONCLUSIONS

These studies suggest a dissociation of the endothelial, cardiac, and renal effects of antihypertensive therapy in hypertension and may explain the variable success of antihypertensive regimens in treating hypertension while preventing cardiac and renal damage.

Inverse correlation between endothelin-1-induced peripheral microvascular vasoconstriction and blood pressure in glaucoma patients

BACKGROUND

The potent vasoconstrictor peptide endothelin-1 has been shown to participate in the control of peripheral vascular tone and in the regulation of ocular perfusion. In glaucoma patients vasospasms and arterial hypotension have been identified as risk factors for the progression of glaucomatous damage, and the regulation of endothelin-1 release is disturbed in some of these patients. The aim of this study was to assess the relationship between resting blood pressure and cutaneous vascular responsiveness to endothelin-1 and phenylephrine in patients with glaucoma and in matched controls.

METHODS

In 9 patients with primary open-angle glaucoma (POAG), 7 patients with normal tension glaucoma (NTG), and 16 age- and sex-matched controls, endothelin-1 and phenylephrine responses were assessed in the human forearm microcirculation using laser Doppler flowmetry during intra-arterial drug administration. Blood pressure was measured intra-arterially.

RESULTS

In contrast to alpha 1-adrenergic effects, endothelin-1 responses were inversely correlated to both systolic (r2 = 0.27, P = 0.05) and diastolic (r2 = 0.54, P = 0.001) blood pressure in glaucoma patients, whereas there was no such correlation in controls. Patients with lower blood pressure values were more sensitive to the vasoconstrictor effects of endothelin-1. Cutaneous responsiveness to endothelin-1 and phenylephrine was similar in glaucoma patients and in controls.

CONCLUSION

These results reveal that glaucoma patients appear to have peripheral microvascular abnormalities which are exhibited as altered responsiveness to endothelin-1. Thus, this study supports the hypothesis that endothelin-1-related microvascular dysfunction may be involved in the pathogenesis of glaucomatous damage.

Calcium sensitivity and agonist-induced calcium sensitization in small arteries of young and adult spontaneously hypertensive rats

The sensitivity of the myofilaments to Ca2+ is increased during agonist-induced contraction of vascular smooth muscle. Given the important contribution of vascular tone to the elevation of peripheral resistance observed in genetic hypertension, we have investigated whether alterations in myofilament Ca2+ sensitivity occur in small arteries from spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls during the developmental and established phases of hypertension. Segments of mesenteric, renal, and femoral artery with an average lumen diameter <300 microm from 5- or 20-week-old rats were mounted in a wire myograph. Morphological measurements were made and the vessels permeabilized with Staphylococcus aureus alpha-toxin. Dose-response curves to increasing concentrations of Ca2+ were obtained and the ability of 100 nmol/L endothelin-1 (ET-1) or 10 micromol/L norepinephrine (NE) in the presence of 10 micromol/L GTP to enhance tension in response to low Ca2+ (pCa6.7) was determined. Systolic, diastolic, and mean blood pressures were higher in SHR than in WKY at 5 and 20 weeks. The media thickness:lumen diameter ratio was increased in mesenteric and femoral arteries from SHR compared with WKY at 5 and 20 weeks. There was no difference in media thickness:lumen diameter ratio in renal arteries or between 5- and 20-week animals in any vascular bed. The pCa curves were not different in mesenteric, renal, or femoral arteries from hypertensive compared with normotensive rats or between age groups, except in femoral arteries at 20 weeks, which exhibited a greater sensitivity to Ca2+ in SHR. Tension developed in response to maximal Ca2+ (pCa5.0) was greater in permeabilized mesenteric arteries from SHR compared with WKY at 20 weeks of age only; media stress was again similar in both strains but increased in older animals compared with younger animals in mesenteric arteries from WKY. The submaximal contraction induced by pCa6.7 was greater in femoral and renal than mesenteric arteries. GTP (10 micromol/L) augmented the tension developed to pCa6.7 in mesenteric arteries at 5 and 20 weeks and in renal arteries at 20 weeks. Addition of 100 nmol/L ET-1 or 10 micromol/L NE in the continued presence of GTP markedly increased tension in mesenteric arteries at 5 and 20 weeks. In renal arteries, 10 micromol/L NE enhanced Ca2+ sensitivity in the presence of GTP in SHR at 5 and 20 weeks and WKY at 5 weeks. In femoral arteries, there was a tendency for ET-1 and NE to increase Ca2+ sensitivity, but this increase was significant in WKY at 20 weeks (ET-1) and SHR at 5 weeks (NE) only. We have demonstrated that the sensitivity of the myofilaments to Ca2+ and ET-1- or NE-induced Ca2+ sensitization is not different in permeabilized small mesenteric, renal, or femoral arteries from SHR compared with WKY controls. Only in SHR mesenteric arteries at 20 weeks of age was there evidence of increased active tension in response to maximal Ca2+, despite structural differences, consistent with increased muscle mass in femoral arteries from SHR. We conclude that it is unlikely that a ubiquitous abnormality of the sensitivity of the contractile apparatus to Ca2+ or agonist-induced Ca2+ sensitization in vascular smooth muscle underlies the elevated total peripheral resistance associated with hypertension.

Is there a rationale for combining angiotensin-converting enzyme inhibitors and calcium antagonists in cardiovascular disease?

Coronary artery disease and its sequelae remain the most important cause of morbidity and mortality in Western countries. Because the pathophysiologic characteristics of coronary artery disease are multifactorial, impairment of endothelial function featuring enhanced vasoconstriction, increased platelet vessel wall interaction, adherence of monocytes, migration and proliferation of vascular smooth muscle cells are crucially involved. Endothelial cells release numerous vasoactive substances regulating function of vascular smooth muscle and trafficking blood cells such as nitric oxide (NO), which is a potent vasodilator also inhibiting cellular growth and migration. In addition, NO possesses antiatherogenic and thromboresistant properties by preventing platelet aggregation and cell adhesion. These effects are counterbalanced by endothelial vasoconstrictors such as angiotensin II and endothelin-1. In the blood vessel wall, the local vascular effects of angiotensin-converting enzyme (ACE) inhibitors and calcium antagonists are synergistic. ACE inhibitors diminish the conversion of angiotensin I into angiotensin II and the inactivation of bradykinin. Calcium antagonists counteract angiotensin II and endothelin-1 at the level of vascular smooth muscle by reducing Ca2+ inflow and facilitating the vasodilator effects of NO. In hypertensive animals, long-term combination therapy with verapamil and trandolapril is particularly effective in reversing endothelial dysfunction. Further, ACE inhibitors and calcium antagonists exert beneficial vascular and complementary hemodynamic effects. Whereas ACE inhibitors inhibit the renin-angiotensin system and reduce sympathetic outflow, calcium antagonists dilate large conduit and resistance arteries. Because small vessels appear to be more dependent on extracellular Ca2+ than larger vessels, nifedipine and verapamil effectively inhibit endothelin-induced vasoconstriction in vitro and in vivo in the resistance circulation. Long-term treatment with ACE inhibitors substantially reduces morbidity and mortality rates in patients with left ventricular dysfunction after myocardial infarction; beneficial effects of verapamil in secondary prevention are confined to patients with normal left ventricular ejection fraction. In summary, long-term combination therapy of ACE inhibitors and calcium antagonists might provide beneficial effects in cardiovascular disease because they exert synergistic hemodynamic, antiproliferative, antithrombotic, and antiatherogenic properties.

Endothelin-1-induced vasopressor responses in essential hypertension

The potential role of endothelin-1 (ET-1) in essential hypertension in humans is still subject to debate. We recently reported strong sodium retention and renal vasoconstriction during pathophysiological increments in plasma ET-1. Apart from this vasoconstrictor action, ET-1 also has mitogenic properties that play a role in the pathophysiology of hypertension. On the other hand, some data refute an important role of ET-1 in hypertension. We therefore investigated in nine subjects with essential hypertension the constrictor actions of ET-1 by challenging these subjects with a systemic infusion of ET-1 (0.5 ng/kg per minute for 60 minutes, then 1.0 ng/kg per minute for 60 minutes, and finally 2.0 ng/kg per minute for 60 minutes). Furthermore, we studied whether these effects of ET-1 could be modulated by oral use of the angiotensin-converting enzyme inhibitor enalapril (20 mg BID) or the calcium channel blocker nifedipine (60 mg OD). ET-1 infusion increased plasma ET-1 levels from 2.5+/-0.4 to 11.6+/-1.0 pmol/L (P<.05). Blood pressure rose by approximately 10 mm Hg (P<.05). Cardiac index decreased by 21+/-22%, whereas calculated systemic vascular resistance increased by 27+/-6% (P<.05). Renal blood flow decreased from 1051+/-94 to 707+/-60 mL/min at the end of the ET-1 infusion (P<.05), and calculated renal vascular resistance increased from 118+/-19 to 189+/-19 mm Hg x min/L (P<.05). Sodium excretion decreased from 227+/-39 to 111+/-15 micromol/min (P<.05). Both enalapril and nifedipine treatment prevented the systemic effects of ET-1 infusion in these subjects. However, during enalapril treatment, despite renal predilatation, ET-1 reduced renal blood flow (from 1119+/-132 to 701+/-75 mL/min, P<.05) and increased renal vascular resistance (from 111+/-16 to 187+/-28 mm Hg x min/L, P<.05) to the same levels as during ET-1 infusion alone. Nifedipine pretreatment attenuated the ET-1-induced fall in renal blood flow (from 1088+/-93 to 907+/-68 mL/min) and increase in renal vascular resistance (from 105+/-9 to 133+/-10 mm Hg x min/L). Although neither drug modulated the antinatriuretic effect of ET-1, nifedipine increased basal sodium excretion (P<.05), which compensated for the decrease during ET-1 infusion. In conclusion, essential hypertensive subjects are sensitive to the vasoconstrictor effects of ET-1. Both enalapril and nifedipine can prevent the systemic effects of ET-1, but nifedipine seems more effective in attenuating the renal constrictor effects of ET-1.

How to assess vascular remodelling in small and medium-sized muscular arteries in humans

The study of vascular wall changes in humans has generated great interest with the increasing realization that, independently of the potential contribution to mechanisms involved in blood pressure elevation, these structural alterations (remodelling) or functional changes may contribute to the complications of elevated blood pressure. Moreover, some of these changes may be corrected partially or totally by administration of antihypertensive agents and other drugs. This has fuelled interest in the techniques used to evaluate changes in the vascular wall in humans, which are reviewed critically here with a focus on human studies in hypertension. Remodelling of large and small arteries has different characteristics, and is studies with different techniques. In hypertensive patients, small arteries less than 400 microns in diameter exhibit a reduction in lumen diameter, accompanied sometimes but not always by an increase in media width or in media cross-section. The study of capillaries and small arteries of the skin or the eye can be performed non-invasively, but for the sake of obtaining the information of interest in hypertension, at present invasive techniques are required to investigate small arteries. These consist of a biopsy of subcutaneous tissue, usually from the gluteal region, and the study of vessels after they have been mounted on a 'wire myograph' or on a pressurized system. In contrast to small arteries, large arteries from hypertensive humans present increases in media width without a significant reduction in the lumen diameter (when studied under conditions isobaric relative to those in normotensive subjects). Conduit arteries may be studies non-invasively with the use of ultrasound techniques. The study of large elastic arteries is not addressed here. The use of echo-tracking devices to study muscular medium-sized arteries such as the radial artery is described. The relative advantages and disadvantages of these techniques, the questions which may be asked and the relevance of the information obtained using these approaches are discussed.

Increased wall-lumen ratio of mesenteric vessels from the spontaneously hypertensive rat is not associated with increased contractility under isobaric conditions

We investigated the morphological (wall-lumen ratio) and contractile characteristics of distal mesenteric arteries from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) controls at a distending pressure of 63% of the mean aortic pressure of each rat using a pressure arteriograph. The wall-lumen ratios obtained were compared with those obtained at a pressure of 100 mm Hg. Experiments were carried out at 5 and 20 weeks. Mean aortic pressure of SHR was significantly increased at 5 weeks compared with that of WKY and was further increased by 20 weeks. At 63% of mean aortic pressure, no difference in the wall-lumen ratio of the arteries was observed between strains at 5 weeks; at 20 weeks, the wall-lumen ratio of SHR arteries was significantly increased compared that in WKY arteries. The wall-lumen ratio of SHR vessels did not differ at 63% mean aortic pressure compared with 100 mm Hg at either 5 or 20 weeks, whereas this parameter was significantly reduced in WKY vessels at 100 mm Hg compared with 63% mean aortic pressure at 5 and 20 weeks. In the presence of spontaneous myogenic tone, there was a borderline reduction in the lumen diameter of SHR vessels compared with WKY vessels and with increasing norepinephrine concentrations at 5 weeks. At 20 weeks, lumen diameter between strains did not differ in the presence of myogenic tone nor with increasing norepinephrine concentrations. Similar results were obtained when vessels from both rat strains were pressurized to 80 mm Hg. Thus, the increased wall-lumen ratio in the distal mesenteric arteries from adult SHR compared with those from WKY is not associated with an increased contractility under isobaric conditions when studied at physiological distending pressure.

Roles of prostaglandins and nitric oxide in the effect of endothelin-1 on renal hemodynamics

It is known that endothelin-1 stimulates the release of nitric oxide and prostaglandins in various vascular beds. We designed the present study to analyze the roles of prostaglandins and nitric oxide in the effect of endothelin-1 on the regulation of renal hemodynamics and renin release. We used N omega-nitro-L-arginine methyl ester (L-NAME) and meclofenamic acid to inhibit the production of nitric oxide and prostaglandins, respectively. With a nonfiltering kidney model, renal blood flow was reduced 21% in dogs treated with L-NAME and 18% in dogs treated with meclofenamic acid. Inhibition of nitric oxide and prostaglandins, however, produced opposite effects on estimated glomerular hydraulic pressure: L-NAME increased glomerular hydraulic pressure from 63.1 +/- 0.9 to 64.6 +/- 1.3 mm Hg (P < .01), and meclofenamic acid reduced glomerular hydraulic pressure from 63.3 +/- 1.4 to 59.8 +/- 1.6 mm Hg (P < .01). Endothelin-1 infusion produced a dose-dependent reduction in renal blood flow after blockade of nitric oxide and prostaglandins. The responses of glomerular hydraulic pressure were different in the two groups during endothelin-1 infusion. Endothelin-1 progressively reduced glomerular hydraulic pressure in a dose-dependent fashion in the meclofenamic acid group. However, endothelin-1 slightly increased glomerular hydraulic pressure until the infusion rate reached 5.0 ng/kg per minute. At that rate, endothelin-1 reduced glomerular hydraulic pressure from 63.3 +/- 1.4 to 47.0 +/- 1.4 mm Hg in the meclofenamic acid group (P < .01), a more than 25% reduction, whereas at the same dose, endothelin-1 reduced glomerular hydraulic pressure only less than 2% in the L-NAME group. In addition, blockade of nitric oxide and prostaglandins did not alter the inhibitory effect of endothelin-1 on renin release in the non-filtering kidney. Therefore, the present study demonstrates that the release of nitric oxide and prostaglandins might modulate the effects of endothelin-1 on the renal circulation. The present findings suggest that the differential vasoconstrictive effects of endothelin-1 on preglomerular and postglomerular vessels are associated with its stimulation of nitric oxide and prostaglandin production.

Endothelin and calcium antagonists in the skin microcirculation of patients with coronary artery disease

BACKGROUND

Endothelin, a potent endothelium-derived vasoconstrictor peptide, is elevated in coronary artery disease (CAD); however, its pathophysiological role is uncertain. Calcium antagonists are widely used in patients with CAD. Using laser Doppler flowmetry, we investigated the influence of two endothelin antagonists and the calcium antagonist diltiazem on endogenous and exogenous endothelin in the skin microcirculation of CAD patients and healthy control subjects.

METHODS AND RESULTS

Both endothelin antagonists and diltiazem applied intradermally induced vasodilation in CAD patients, which was more pronounced with the ETA/ETB antagonist than with the ETA antagonist or diltiazem. Exogenous endothelin led to profound vasoconstriction in CAD patients and healthy volunteers. Both endothelin antagonists and diltiazem blunted the vasoconstriction to exogenous endothelin in CAD patients and young healthy volunteers and less so in old healthy volunteers. However, compared with both endothelin antagonists, a 10-times-higher dose of diltiazem was required. Systemic diltiazem (240 mg, slow release) attenuated endothelin-induced vasoconstriction in CAD patients. Neurogenic vasodilation to exogenous endothelin was inhibited by both endothelin antagonists.

CONCLUSIONS

This study demonstrates that endogenous endothelin of CAD patients contributes to the regulation of vascular tone in the skin microcirculation not only through ETA receptors but also possibly through ETB receptors. Diltiazem inhibited endothelin-induced vasoconstriction, but endothelin antagonists were slightly more effective. Thus, endothelin antagonists represent potent new tools to interfere with the vascular effects of endothelin in CAD patients. Future studies must confirm these findings in other areas of the circulation.

BQ-123, a specific endothelin (ETA) receptor antagonist, prevents ischemia-reperfusion injury in kidney transplantation

We studied the effects of the specific endothelin (ETA) receptor antagonist, BQ-123, on reperfusion injury in a rat model of kidney transplantation. First, Sprague-Dawley rats were divided into three groups: a sham nephrectomy (SNEPH), an autotransplantation (AUTO-Tx), and an allotransplantation (ALLO-Tx) group. In a fourth group, ALLO-Tx + BQ, allografts were flushed with 20 micrograms BQ-123 containing cold Ringer's lactate before transplantation. For the allograft groups, kidneys from white Wistar albino rats were transplanted into allogeneic Sprague Dawley recipients. Grafts were allowed 120 min of reperfusion after 40 min of cold ischemia. ET-1,2 plasma concentrations in the renal venous blood, and kidney tissue prostaglandin (PG) E2 and leukotriene (LT) B4 levels were studied. Diene conjugates (DC), hydroxyalkanals (HAA), hydroxyalkenals (HAE) and malondialdehyde (MDA) levels, as the products of lipid peroxidation, and protein carbonyls (PC) and protein sulphydryls (PS), as the parameters of protein oxidation, were also analyzed in the kidney tissue. Plasma ET concentrations increased significantly in the AUTO-Tx and ALLO-Tx groups (P < 0.05 and P < 0.01, respectively) but this increase was reversed in the ALLO-Tx + BQ group. None of the lipid peroxidation products except DCs (P < 0.05) increased in the AUTO-Tx group, whereas they all increased in the ALLO-Tx group (P < 0.01). Protein oxidation parameters also changed significantly (P < 0.01) in the ALLO-Tx group but did not in the AUTO-Tx group (P < 0.05). The differences in PGE2 and LTB4 levels were not significant. Histopathologic examination revealed prominent glomerular and tubular injury in the AUTO-Tx and ALLO-Tx groups but less in the ALLO-Tx + BQ group. In the last group, all parameters of lipid peroxidation (P < 0.001 for all) and PCs decreased, and PSs were preserved (P < 0.001 for both) when compared with the AUTO-Tx and ALLO-Tx groups. We conclude that BQ-123, in addition to inhibiting the binding of ET-1,2 to the ETA receptor, may also inhibit the release and/or synthesis of ET-1,2 and prevent reperfusion injury in kidney transplantation.

The endothelium of resistance arteries: physiology and role in hypertension

The endothelium plays a very important role in the regulation of vascular function by way of its barrier role, by interaction with circulating cells such as platelets, which may release vasoactive or growth regulating agents, and through production of substances which modulate vascular tone and smooth muscle cell growth, and which may also exert antithrombotic effects. The endothelium of resistance arteries, vessels critically involved in generating resistance to flow and which play an important role in hypertension, has been studied mainly from the point of view of generation of agents which regulate vascular tone and growth. Endothelium-derived relaxing factors such as nitric oxide, prostacyclin, hyperpolarizing factors (EDHF) and possibly C-type natriuretic peptide (CNP), are counteracted by endothelium-derived contracting factors, which include endothelins and contracting factors (EDCF) which are less well characterized and appear to be cyclooxygenase products. In experimental hypertension in animals, and in human essential hypertension, these mechanisms may be altered. There may be a reduced generation of endothelium-derived nitric oxide and enhanced production of EDCF. Some of the mechanisms involved in the role these agents play in the physiology of resistance arteries and pathologically in hypertension will be reviewed.

An in vivo model of chronic optic nerve ischemia: the dose-dependent effects of endothelin-1 on the optic nerve microvasculature

The purpose of this study was to evaluate the effects induced by chronic microapplication of endothelin-1 on the anterior optic nerve microvasculature and to determine the dose-response characteristics of endothelin-1 on this vascular bed. Daily dosages between 4.69 x 10(-4) and 9.0 x 10(-1) micrograms/day of endothelin-1 were delivered continually over 3 days, and at a constant flow rate, to the perineural region of the anterior optic nerve of 15 albino rabbits via osmotically-driven minipumps. The vasomotor effect of local endothelin-1 on the microvasculature of the optic nerve was examined using intraluminal microvascular corrosion casting technique. The vasomotor effects were quantified by measuring the relative amount of vasoconstriction of the arterioles supplying the anterior optic nerve (primary and secondary branches of the short posterior ciliary arteries). The average constriction was calculated for the endothelin-treated eyes and the untreated, contralateral eyes. The mean vasoconstriction in the endothelin treated eyes ranged from 14.7% to 30.0% and was highly correlated with the logarithmic value of the daily dose of endothelin-1 (R2 = 0.59, p = 0.00083). The interocular difference (between treated and untreated eyes) of the optic nerve vasoconstriction ranged from 0-19% (mean +/- SD: 7.23 +/- 5.7%). This interocular difference also correlated highly with the log of the daily endothelin-1 dosage (R2 = 0.80; p < 0.0001). By additionally accounting for the weight and sex in a multiple linear regression function, the correlation was markedly improved (R2 = 0.92; p < 0.0001). In conclusion, the microvasculature supplying the anterior optic nerve of the rabbit demonstrates a dose-dependent vasoconstriction with chronic local application of endothelin-1. This in vivo, experimental model offers a titratable method with which the effects of chronic vasoconstriction and vascular insufficiency on the optic nerve can be examined.

Endothelin subtype B receptor-mediated calcium and contractile responses in small arteries of hypertensive rats

Endothelin-1 elicits vasoconstrictor responses through endothelin subtype A receptors, which are located on vascular smooth muscle cells, and vasodilator responses through endothelin subtype B receptors, which occur predominantly on endothelial cells. Endothelin subtype B receptors also may be present on vascular smooth muscle cells, in which they may mediate vasoconstriction. The aims of this study were to determine the presence of vascular smooth muscle vasoconstrictor endothelin subtype B receptors in mesenteric resistance arteries and to assess whether endothelin subtype B receptor-mediated responses differ between spontaneously hypertensive rats and Wistar-Kyoto rats. Contractile responses to the endothelin subtype B receptor agonist sarafotoxin S6c and endothelin-1 were measured simultaneously with [Ca2+]i in endothelium-denuded mesenteric resistance arteries from adult spontaneously hypertensive rats and Wistar-Kyoto rats. To simulate in vivo conditions matched as closely as possible to in vitro conditions, vessels were mounted in a vessel flow chamber in which intraluminal pressure was maintained at 60 mm Hg. Contraction was determined by video imaging to record lumen diameter, and [Ca2+]i was measured by the fura 2 method. Basal [Ca2+]i was significantly higher (P < .01) in hypertensive (170 +/- 4 nmol/L) compared with normotensive rats (134 +/- nmol/L). The endothelin subtype B receptor agonist sarafotoxin S6c increased [Ca2+]i in a concentration-dependent manner. Sarafotoxin S6c-induced [Ca2+]i and contractile responses were significantly lower in hypertensive compared with normotensive rats. These data demonstrate that endothelin subtype B receptors are present in vascular smooth muscle of small arteries and that endothelin subtype B receptor-mediated vasoconstriction occurs through intracellular calcium signaling pathways.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac endothelin-1 content and receptor subtype in spontaneously hypertensive rats

Endothelin-1 (ET-1), a newly discovered peptide with potent vasoconstrictor and growth-promoting effects, has been implicated in high blood pressure and cardiac hypertrophy in the spontaneously hypertensive rat (SHR). In the present study, we measured plasma ET-1 levels and tissue ET-1 concentrations in the four cardiac chambers of 17- to 18-week-old SHR and their normotensive controls. Wistar Kyoto (WKY) rats. SHR had slightly but significantly higher plasma ET-1 levels than WKY. The ventricles had the highest ET-1 content and the atria in both strains had the highest ET-1 concentrations. ET receptor subtypes were analysed by radiogand binding with ET-1, BQ-123 and IRL 1620 in crude membrane preparations of the four cardiac chambers. No differences in receptor subtype densities or affinities were apparent between the two strains. ET(A) represented 75 to 85% of both ET receptors. Competition analysis revealed that in both strains left ventricular tissue had lower receptor densities and higher affinities than the atria. These results suggest that ET-1 and its receptor although contributing in the maintenance of high blood pressure may not be an important factor during stable cardiac hypertrophy in adult SHR. The differential distribution of ET-1 content and receptor densities favoring the atria in both strains suggest that this peptide may have a different physiological role in the atria from that in the ventricles.

Structure and function of the rat basilar artery during chronic nitric oxide synthase inhibition

BACKGROUND AND PURPOSE

Nitric oxide is an important regulator of vascular tone and may also be implicated in the modulation of vascular growth and structure. This study presents the effects of chronic nitric oxide inhibition, with or without antihypertensive treatment, on the structure and function of the basilar artery in rats.

METHODS

Rats were treated for 6 weeks with N omega-nitro-L-arginine methyl ester (50 mg/kg per day) alone or in combination with verapamil (100 mg/kg per day) or with trandolapril (1 mg/kg per day). Untreated rats served as controls. The structure and reactivity of perfused and pressurized basilar arteries were analyzed in vitro using a video dimension analyzer.

RESULTS

Systolic arterial pressure increased only in the nitro-arginine-treated group, as did the media-to-lumen ratio of the basilar artery. This structural alteration, which was prevented by verapamil and trandolapril, was mainly due to remodeling and not to growth. Chronic inhibition of the L-arginine pathway increased the response of the basilar artery to serotonin, while the opposite was found for endothelin. Verapamil and trandolapril prevented these functional alterations that seemed related to the changes in the vascular structure.

CONCLUSIONS

The remodeling and functional alterations of the basilar artery seem to depend mainly on the elevation of arterial pressure with little contribution of the L-arginine pathway. Furthermore, nitric oxide does not seem to be implicated in the modulation of normal cerebral vascular growth in vivo. However, hypertension-induced changes in vascular reactivity and structure could alter cerebral blood flow and eventually contribute to the development of stroke in this model of hypertension.