Aging differentially affects direct and indirect actions of endothelin-1 in perfused mesenteric arteries of the rat

The Association of Endothelin-1 with Markers of Arterial Stiffness in Black South African Women: The SABPA Study

Background. Limited data exist regarding endothelin-1 (ET-1), a vasoactive contributor in vascular tone, in a population subjected to early vascular deterioration. We compared ET-1 levels and explored its association with markers of arterial stiffness in black and white South Africans. Methodology. This cross-sectional substudy included 195 black (men: n = 99; women: n = 95) and 197 white (men: n = 99; women: n = 98) South Africans. Serum ET-1 levels were measured as well as markers of arterial stiffness (blood pressure, pulse wave velocity, and arterial compliance). ET-1 levels were higher in black men and white women compared to their counterparts after adjusting for C-reactive protein. In both single and partial (adjusting for body mass index and gamma glutamyl transferase) regression analyses ET-1 correlated with age, interleukin-6, high density lipoprotein cholesterol, systolic blood pressure, pulse pressure, and pulse wave velocity in black women. In multivariate regression analyses the independent association of ET-1 with systolic blood pressure (Adj. R² = 0.13; β = 0.28, p < 0.01) and pulse pressure (Adj. R² = 0.11; β = 0.27, p < 0.01) was confirmed in black women only. ET-1 additionally associated with interleukin-6 in black women (p < 0.01). Conclusion. Our result suggests that ET-1 and its link with subclinical arteriosclerosis are potentially driven by low-grade inflammation as depicted by the association with interleukin-6 in the black female cohort.

Divergent effects of aging and sex on vasoconstriction to endothelin in coronary arterioles

OBJECTIVE

The risk for cardiovascular disease increases with advancing age; however, the chronological development of heart disease differs in males and females. The purpose of this study was to determine whether age-induced alterations in responses of coronary arterioles to the endogenous vasoconstrictor, endothelin, are sex-specific.

METHODS

Coronary arterioles were isolated from young and old male and female rats to assess vasoconstrictor responses to endothelin (ET), and ETa and ETb receptor inhibitors were used to assess receptor-specific signaling.

RESULTS

In intact arterioles from males, ET-induced vasoconstriction was reduced with age, whereas age increased vasoconstrictor responses to ET in intact arterioles from female rats. In intact arterioles from both sexes, blockade of either ETa or ETb eliminated age-related differences in responses to ET; however, denudation of arterioles from both sexes revealed age-related differences in ETa-mediated vasoconstriction. In arterioles from male rats, ETa receptor protein decreased, whereas ETb receptor protein increased with age. In coronary arterioles from females, neither ETa nor ETb receptor protein changed with age, suggesting age-related changes in ET signaling occur downstream of ET receptors.

CONCLUSIONS

Thus, aging-induced alterations in responsiveness of the coronary resistance vasculature to endothelin are sex-specific, possibly contributing to sexual dimorphism in the risk of cardiovascular disease with advancing age.

Age-related changes in endothelin-1 receptor subtypes in rat heart

Density, affinity, and subtype distribution of endothelin-1 (ET-1) binding sites were determined in rat cardiac tissue as a function of age in order to evaluate the association of alterations in the endothelin receptor system and aging in the heart. A significant decrease in the receptor subtype ET-A, which represents 70% to 80% of the total receptor population in cardiac tissue of 3- and 12-month-old rats, was observed in 24-month-old rats with respect to the younger groups. These findings indicate an alteration in ET-1 cardiac receptors associated with aging, mainly due to a variation in the receptor subtype distribution.

Aging reduces skeletal blood flow, endothelium-dependent vasodilation, and NO bioavailability in rats

We determined whether aging diminishes bone blood flow and impairs endothelium-dependent vasodilation. Femoral perfusion was lower in old animals, as well as endothelium-dependent vasodilation and NO bioavailability. These effects could contribute to old age-related bone loss and the increased risk of fracture.

INTRODUCTION

Aging has been shown to diminish bone blood flow in rats and humans. The purpose of this study was to determine whether blood flow to regions of the femur perfused primarily through the principal nutrient artery (PNA) are diminished with aging and whether this putative reduction in flow is associated with impaired endothelium-dependent vasodilation.

MATERIALS AND METHODS

Blood flow was measured in conscious young adult (4-6 mo old) and aged (24-26 mo old) male Fischer-344 rats using radiolabeled microspheres. Endothelium-dependent vasodilation of the PNA was assessed in vitro using acetylcholine (ACh), whereas the contribution of the NO synthase (NOS) and cyclooxygenase (COX) signaling pathways to endothelium-dependent vasodilation was determined using the NOS and COX inhibitors L-NAME and indomethacin, respectively.

RESULTS

Femoral blood flow in the aged rats was 21% and 28% lower in the proximal and distal metaphyses, respectively, and 45% lower in the diaphyseal marrow. Endothelium-dependent vasodilation was reduced with old age (young: 83 +/- 6% maximal relaxation; aged: 62 +/- 5% maximal relaxation), whereas endothelium-independent vasodilation (sodium nitroprusside) was unaffected by age. The reduction in endothelium-dependent vasodilation was mediated through impairment of the NOS signaling pathway, which resulted in lower NO bioavailability (young: 168 +/- 56 nM; aged: 50 +/- 7 nM).

CONCLUSIONS

These data show that reductions in metaphyseal bone and diaphyseal marrow perfusion with old age are associated with diminished endothelium-dependent vasodilation through an impairment of the NOS mechanism. Such age-related changes in bone perfusion and vascular NO signaling could impact clinical bone loss, increase risk of fracture, and impair fracture healing in the elderly.

Enhanced vasoconstrictor responses in eNOS deficient mice

Previous studies suggest that vasoconstriction is modulated by nitric oxide (NO). Contractions to ET-1 and/or thromboxane may be enhanced during chronic deficiency in expression or activity of NO synthase (NOS). Multiple isoforms of NOS are expressed within the vessel wall and purely pharmacological approaches cannot define the role of each. We tested the hypothesis that vasoconstriction to endothelin-1 (ET-1) and/or the thromboxane mimetic, U46619, is enhanced under conditions of chronic, selective deficiency in endothelial NOS (eNOS-/-) by examining responses in aorta from eNOS-/- mice compared to wild type (eNOS+/+). ET-1 produced dose-dependent contraction of aorta from eNOS+/+ mice that was increased twofold following acute inhibition of all NOS isoforms with N(G)-nitro-L-arginine (L-NNA). In eNOS-/- mice, contractions to ET-1 were increased twofold compared to eNOS+/+. L-NNA had no effect. Although contraction of the aorta to thromboxane mimetic U46619 was increased at lower concentrations, maximal contractions to U46619 were not increased following acute inhibition of NOS or in eNOS-/- mice. These studies provide direct evidence that vasoconstriction to ET-1 and thromboxane is augmented in the face of eNOS deficiency, demonstrating that eNOS normally inhibits vascular contractile responses.

Developmental changes in the functional, biochemical and molecular properties of rat bladder endothelin receptors

The effect of aging on functional, biochemical, anatomical and molecular properties of endothelin (ET) receptors in bladder smooth muscle of the 3-week-, 3-month- and 22-month-old rats was examined using isolated muscle bath techniques, radioligand binding on membrane particulates and slide mounted tissue sections, and real-time reverse transcription polymerase chain reaction (RT-PCR). ET-1 induced significantly larger contractile responses in bladder dome muscle strips from 3-week- than from 3-month- and 22-month-old rats. The expression level of total ET receptors, determined by saturation binding experiments with [(125)I]ET-1, was higher in detrusor from 3-week- than 22-month-old rats. Inhibition studies with BQ123, a selective ET(A) receptor antagonist, indicated the predominance of the ET(A) receptor subtype and a similar proportion of ET(A) to ET(B) receptor subtypes in the rat detrusor at all ages studied. Autoradiographic data support the age-dependent decrease in the density of ET receptors and also indicate that the ET(A) receptor subtype is primarily located in the smooth muscle layer, whereas the ET(B) receptor subtype is located in both the urothelial and smooth muscle layers. Determined by real-time RT-PCR, ET 1, ET-3, ECE-1 and ET receptor subtype (ET(A) and ET(B)) mRNAs were shown to be higher in bladders of 3-week- compared to 3-month- or 22-month-old rats. This study indicates age-dependent alterations in the ET receptor system at both gene transcript and protein levels in the Fischer rat detrusor.

Decreases in bone blood flow and bone material properties in aging Fischer-344 rats

The purpose of this study was to quantify precisely aging-induced changes in skeletal perfusion and bone mechanical properties in a small rodent model. Blood flow was measured in conscious juvenile (2 months old), adult (6 months old), and aged (24 months old) male Fischer-344 rats using radiolabeled microspheres. There were no significant differences in bone perfusion rate or vascular resistance between juvenile and adult rats. However, blood flow was lower in aged versus adult rats in the forelimb bones, scapulas, and femurs. To test for functional effects of this decline in blood flow, bone mineral density and mechanical properties were measured in rats from these two age groups. Bone mineral density and cross-sectional moment of inertia in femoral and tibial shafts and the femoral neck were significantly larger in the aged versus adult rats, resulting in increased (+14%-53%) breaking strength and stiffness. However, intrinsic material properties at midshaft of the long bones were 12% to 25% lower in the aged rats. Although these data are consistent with a potential link between decreased perfusion and focal alterations in bone remodeling activity related to clinically relevant bone loss, additional studies are required to establish the mechanisms for this putative relationship.

Vasopeptidase inhibition and endothelial function in hypertension

Vasopeptidase inhibitors are a new class of drugs capable of inhibiting both angiotensin-converting enzyme and neutral endopeptidase 24.11. This involves simultaneous inhibition with a single molecule of two key enzymes, ACE and NEP, which are both involved in the regulation of cardiovascular homeostasis in many ways. This includes metabolism of several vasoactive peptides and their clearance from the circulation, therefore contributing to neurohumoral modulation, which might have therapeutic advantages in the prevention of endothelial dysfunction in hypertension.

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.

Effects of aging on cerebral vasospasm after subarachnoid hemorrhage in rabbits

BACKGROUND AND PURPOSE

The effects of aging on cerebral vasospasm after subarachnoid hemorrhage (SAH) remain to be elucidated. The aim of this study was to clarify age-related differences of vasospasm and of papaverine reactivity in the responses of basilar arteries after SAH in rabbits.

METHODS

Rabbits receiving a single injection of arterial blood into the cisterna magna were divided into 3 groups: young (2 to 3 months old), adult (6 to 9 months old), and old (20 to 40 months old). Vertebrobasilar angiograms were obtained before SAH and 1, 2, 4, and 7 days after SAH. Papaverine was administrated selectively via the vertebral artery on day 2, and serial angiography was performed for up to 2 hours. Vessel structures were assessed with light microscopy on days 1, 2, 4, and 7 after SAH and at 10, 30, and 60 minutes after papaverine infusion.

RESULTS

Mortality from SAH in old rabbits was 40%, whereas that of young and adult rabbits was 0%. Angiograms revealed that SAH induced maximal constriction of the basilar arteries on day 2 in all age groups, and the constrictions were significantly increased with age at all time points investigated. The degree of dilatation of spastic basilar arteries after intra-arterial papaverine administration significantly decreased with age. Duration of the efficacy of papaverine became significantly shorter with age. Vessel diameter returned to the preinfusion value approximately 120, 60, and 30 minutes after infusion in young, adult, and old rabbits, respectively. Light microscopy in old rabbits showed luminal narrowing and corrugation of the internal elastic lamina not only in the basilar arteries but also in small arteries and intraparenchymal arterioles.

CONCLUSIONS

This study suggests that aging increases the degree of vasospasm in rabbits. The impaired reactivity to papaverine with aging might imply the early transition of the aged vessel to the papaverine-resistant chronic stage.

Vascular bed heterogeneity in age-related endothelial dysfunction with respect to NO and eicosanoids

1. Endothelial dysfunction has been described with ageing but the mechanisms responsible have not been clearly elucidated and might be different from one vessel to the other. This study assesses the relative contribution of endothelial nitric oxide (NO) and cyclo-oxygenase (COX) metabolites in relaxation to acetylcholine with ageing in the aorta and the small mesenteric artery of the rat. 2. In the aorta and branch II or III of superior mesenteric artery (SMA), endothelium-dependent relaxation to acetylcholine was not different between 12 - 14 (adult) and 32-week-old rats whereas it was reduced at 70 - 100 (old) weeks of age. 3. Despite an increased endothelial NO-synthase protein expression, the NO-synthase inhibitor, N(G)-nitro-L-arginine-sensitive component of relaxation decreased with ageing. 4. In old rats, exposure to the COX inhibitor, indomethacin, but not the selective COX-2 inhibitor, NS-398, potentiated response to acetylcholine. The thromboxane A(2)/prostaglandin H(2) receptor antagonist, GR 32191B enhanced relaxation to acetylcholine in aorta but it had no effect in SMA. Furthermore, acetylcholine increased thromboxane B(2) production (enzymeimmunoassay) in aorta but not in SMA. Finally, Western blot analysis showed enhanced expression of COX-1 and 2 in the two arteries with ageing. 5. These results suggest that the decrease in acetylcholine-induced relaxation with ageing involves reduced NO-mediated dilatation and increased generation of vasoconstrictor prostanoids most likely from COX-1. They also point out vascular bed heterogeneity related to the nature of prostanoids involved between the aorta (i.e., thromboxane A(2)) and the SMA (unidentified) arteries even though increased expression of COX occurs in both vessels.

Developmental aspects of endothelin receptors in rabbit lower urinary tract

Since evidence of development/age-related alterations of endothelin receptors in circulation and respiration systems has been increasing, we attempted to investigate the pharmacological characterization of endothelin receptors in neonatal, premature, and mature male rabbit lower urinary tract. The biochemical properties of ET receptors were examined in the lower urinary tracts of 1-day (neonatal)-, 6-week (premature)-, and 1-year(mature)-old male rabbits with binding technique utilizing [(125)I]ET-1. The rank orders of the densities (B(max) values) of endothelin receptors in the bladder dome, bladder base, and urethra of different aged rabbits were bladder dome, 1 day > 6 week &vbar;Ls 1 year, bladder base, 1 day > 6 week &vbar;Ls 1 year, and urethra, 1 day > 6 week > 1 year. The pharmacological profiles of these binding sites inhibited by various kinds of endothelin receptor compounds showed similar K(i) values and similar proportions of endothelin receptor subtypes in the same regions of 1-day-, 6-week-, and 1-year-old animals. Our data clearly demonstrated the presence of regional difference and development-related changes in the density and subtype specificity of endothelin receptors in the lower urinary tract of the male rabbit. Neurourol. Urodynam. 19:71-85, 2000.

Age-related changes in vascular responses

Aging causes changes in the structure and function of the vessels that leads to an increase in the incidence of certain cardiovascular diseases, such as hypertension, coronary artery disease, heart failure, and postural hypotension with enhancement of both morbidity and mortality. When aging is associated with hypertension, these changes are increased. Aging alters endothelial cells, and so the vascular tone regulation, reducing the endothelium-dependent relaxations, probably by a decrease in endothelial synthesis or release of nitric oxide. In addition, endothelium-independent relaxations are essentially unaltered, those elicited by beta-adrenoceptor agonists being usually reduced. Aging scarcely modifies the contractions induced by different agents, such as 5-hydroxytryptamine, histamine, high potassium, and angiotensin, whereas reduces those elicited by noradrenaline or endothelin. Vascular Ca(2+) homeostasis appears to be altered in aging. The extracellular Ca(2+) dependence of contractile responses elicited by agonists is enhanced, which explains the increased sensitivity to Ca(2+) antagonists in elderly. Finally, Na(+) pump activity, that controls cellular ionic homeostasis, seems to be reduced in aging. The contractions elicited by Na(+) pump inhibition with ouabain are negatively modulated by the release of a diffusible endothelial factor, an effect lost in aging, being replaced by an endothelium-dependent contracting factor that facilitates ouabain responses.

Endothelin in cardiovascular control: the role of endothelin antagonists

Endothelin (ET) is a potent vasoconstrictor associated with various cardiovascular diseases. ET mediates its effects through ET receptors on vascular smooth muscle cells as well as on the vascular endothelium. Furthermore, a neurotransmitter role for ET has been suggested on the basis of experimental and human in vivo studies. ET antagonists are potent tools for studying the effects of ET and its receptors. They have been widely used in vitro and in experimental models of cardiovascular disease, where ET levels are elevated and reactivity to ET is altered. Promising clinical trials in hypertension, coronary artery disease, and congestive heart failure are discussed in this review. Different forms of renal failure are associated with markedly increased ET levels, and ET antagonists experimentally improve renal function in these models. Extrapolating from experimental and first clinical experience, ET antagonists could be useful in the treatment of hypertension, coronary artery disease, congestive heart failure, and renal failure, especially in combination with other drugs, ie, angiotensin converting enzyme inhibitors. The inhibition of ET-induced stimulation of nociception allows for speculation that ET antagonists might even have analgesic properties.

Changes in the regulation by endothelium of norepinephrine response in isolated, perfused mesenteric vascular bed of rats at different ages

Age-related changes of norepinephrine response were studied in isolated perfused mesenteric arteries and veins of rats at 4, 32 and 75 weeks of age. Norepinephrine (0.1-100 nmole) significantly and dose-dependently increased perfusion pressure of the arteries, whereas it only slightly changed that of the veins. The arterial response increased, but the venous response decreased with age. Indomethacin at 5 x 10(-6) M did not change the arterial response at any of the ages. The combination of indomethacin and NG-nitro-L-arginine methyl ester (L-NAME) at 5 x 10(-6) M, and endothelium denudation similarly augmented the response in 4-week-old rats, but neither of these affected the responses in 32- and 75-week-old rats. At raised tone by 100 mM potassium chloride, acetylcholine-induced decrease in perfusion pressure and inhibition of the response by L-NAME were attenuated gradually with age. The raised tone in 75-week-old rats was higher than that in either 4- or 32-week-old rats, but the difference disappeared in the presence of L-NAME. These results demonstrate that the arterial portion is predominant in norepinephrine response and the arterial and venous responses change oppositely with age. It is also suggested that neither prostanoids nor endothelial nitric oxide regulate the arterial response in mature animals, while only in 4-week-old rats, nitric oxide may counteractively regulate the response. Additionally, the roles of nitric oxide in the regulation of transient norepinephrine response, in the regulation of the potassium-induced tonic contracture, and in the vasodilation through muscarinic receptor stimulation seem to change somewhat differently with age.

Effects of aging on cardiac output, regional blood flow, and body composition in Fischer-344 rats

The purpose of this study was to determine the effects of maturation and aging on cardiac output, the distribution of cardiac output, tissue blood flow (determined by using the radioactive-microsphere technique), and body composition in conscious juvenile (2-mo-old), adult (6-mo-old), and aged (24-mo-old) male Fischer-344 rats. Cardiac output was lower in juvenile rats (51 +/- 4 ml/min) than in adult (106 +/- 5 ml/min) or aged (119 +/- 10 ml/min) rats, but cardiac index was not different among groups. The proportion of cardiac output going to most tissues did not change with increasing age. However, the fraction of cardiac output to brain and spinal cord tissue and to skeletal muscle was greater in juvenile rats than that in the two adult groups. In addition, aged rats had a greater percent cardiac output to adipose tissue and a lower percent cardiac output to cutaneous and reproductive tissues than that in juvenile and adult rats. Differences in age also had little effect on mass-specific perfusion rates in most tissues. However, juvenile rats had lower flows to the pancreas, gastrointestinal tract, thyroid and parathyroid glands, and kidneys than did adult rats, and aged rats had lower flows to the white portion of rectus femoris muscle, spleen, thyroid and parathyroid glands, and prostate gland than did adult rats. Body mass of juvenile rats was composed of a lower percent adipose mass and a greater fraction of brain and spinal cord, heart, kidney, liver, and skeletal muscle than that of the adult and aged animals. Relative to the young adult rats, the body mass of aged animals had a greater percent adipose tissue mass and a lower percent skeletal muscle and skin mass. These data demonstrate that maturation and aging have a significant effect on the distribution of cardiac output but relatively little influence on mass-specific tissue perfusion rates in conscious rats. The old-age-related alterations in cardiac output distribution to adipose and cutaneous tissues appear to be associated with the increases in percent body fat and the decreases in the fraction of skin mass, respectively, whereas the decrease in the portion of cardiac output directed to reproductive tissue of aged rats appears to be related to a decrease in mass-specific blood flow to the prostate gland.

Ocular and regional cerebral blood flow in aging Fischer-344 rats

Vascular remodeling and changes in vascular responsiveness occur in the rat cerebrum with old age. This includes reductions in cerebral arteriolar numerical density, cross-sectional area, distensibility, the relative proportion of distensible elements in the cerebral arteriolar wall, and reduced endothelium-dependent relaxation. The purpose of this study was to test the hypothesis that old age results in an increase in vascular resistance and, correspondingly, a decrease in blood flow to ocular, regional cerebral, and spinal tissue in the rat. Blood flow was measured in the eye, olfactory bulb, left and right cerebrum, pituitary gland, midbrain, pons, cerebellum, medulla, and spinal cord of juvenile (2-mo-old, n = 6), adult (6-mo-old, n = 7), and aged (24-mo-old, n = 7) male Fischer-344 rats. Arterial pressure and blood flow were used to calculate vascular resistance. Vascular resistance in the eye of aged rats (6.03 +/- 1.08 mmHg . ml-1 . min . 100 g) was higher than that in juvenile (3.83 +/- 0.38 mmHg . ml-1 . min . 100 g) and adult rats (3.12 +/- 0.24 mmHg . ml-1 . min . 100 g). Similarly, resistance in the pons of older rats (2.24 +/- 0.55 mmHg . ml-1 . min . 100 g) was greater than in juvenile (0.66 +/- 0.06 mmHg .ml-1 . min . 100 g) and adult rats (0.80 +/- 0.11 mmHg . ml-1 . min . 100 g). In contrast, vascular resistance in the pituitary gland was lower in the aged rats (juvenile, 3.09 +/- 0.22; adult, 2.79 +/- 0.42; aged, 1.73 +/- 0.32 mmHg . ml-1 . min . 100 g, respectively). Vascular resistance was not different in other cerebral tissues or in the spinal cord in the aged rats. These data suggest that regional cerebral and spinal blood flow and vascular resistance remain largely unchanged in conscious aged rats at rest but that elevations in ocular vascular resistance and, correspondingly, decreases in ocular perfusion with advanced age could have serious adverse effects on visual function.

Chronic treatment with losartan ameliorates vascular dysfunction induced by aging in spontaneously hypertensive rats

OBJECTIVE

To evaluate the effects of prolonged treatment with losartan on endothelium-dependent and endothelium-independent relaxations of aortic rings from adult and senescent spontaneously hypertensive rats, and to clarify whether these effects were due to specific mechanisms of the drug or a consequence of its blood-pressure-lowering action.

MATERIALS AND METHODS

Adult (aged 5 months) and senescent (aged 20 months) male spontaneously hypertensive rats were treated for 12 consecutive weeks with 10 mg/kg per day losartan. Systolic blood pressure and plasma concentration of nitrates were evaluated. We studied endothelium-dependent and endothelium-independent relaxations and response to angiotensin II of aortic rings from rats of each group. The direct effects of angiotensin II type 1 receptor antagonism on vascular reactivity of aortic rings from untreated adult and senescent rats that had been incubated beforehand with losartan were also studied.

RESULTS

Losartan treatment comparably reduced blood pressure and increased plasma concentration of nitrates in rats of both age groups. Responses to acetylcholine and sodium nitroprusside were lower for rings from senescent than they were for rings from adult rats. Constrictor responses to angiotensin II were higher for rings from senescent than they were for rings from adult rats. Treatment with losartan increased the magnitude of relaxations in response to acetylcholine for rings from rats in both groups, but increased the magnitude of relaxations in response to nitroprusside only for rings from senescent spontaneously hypertensive rats. Incubation beforehand of aortic rings from untreated rats with losartan enhanced magnitude of relaxations in response both to acetylcholine and to nitroprusside only for rings from senescent spontaneously hypertensive rats.

CONCLUSIONS

The consequences of aging for endothelium-dependent and endothelium-independent relaxations of rings from spontaneously hypertensive rats are ameliorated by losartan treatment, suggesting that angiotensin II plays a role via type 1 receptors. The effects of losartan on senescent spontaneously hypertensive rats were due not only to its blood-pressure-lowering action but also to the blockade of specific mechanisms derived from angiotensin II type 1 receptor antagonism, which might involve an increase in availability of NO.

The role of endothelin in the age dependent increase in renal vascular resistance in the rat kidney

Endothelin (ET) is a powerful vasopressor agent that is activated in a number of pathophysiologic states where renal perfusion is reduced. Since renal vasoconstriction occurs as part of renal aging, we investigated the possibility that ET may be activated in the old kidney. These experiments involved acutely blocking endogenous ET with Bosentan (a non-peptide mixed antagonist to both ET receptor types ETA and ETB), in Sprague-Dawley male rats of various ages: young (4 5 months), middle-aged (12-13 months) and old (19-20 months). Experiments were performed in chronically catheterized, conscious rats, studied under unstressed conditions. Renal hemodynamics and sodium excretion were measured before and during acute ET receptor blockade. In all three age groups, Bosentan had no effect on glomerular filtration rate (GFR), renal plasma flow (RPF), renal vascular resistance (RVR), blood pressure (BP) or urine flow. Sodium excretion increased significantly with Bosentan but the natriuresis was similar in rats of all ages. These results suggest that ET does not contribute to the renal vasoconstriction of the old rat kidney.

Anatomic heterogeneity of vascular aging: role of nitric oxide and endothelin

We investigated the effects of aging, a cardiovascular risk factor, on vascular function with regard to endothelial nitric oxide synthase (eNOS), superoxide dismutase (SOD), and endothelin (ET-1) in aorta and femoral artery of the rat. Concentration-response curves to acetylcholine, calcium ionophore A23187, norepinephrine, ET-1, big endothelin, sodium nitroprusside, and exogenous SOD were obtained. Expression of eNOS mRNA was analyzed by reverse-transcription polymerase chain reaction, SOD activity was assessed using a chemiluminescence-based cytochrome c assay, and ET-1 plasma concentrations were measured by radioimmunoassay. In aorta of old rats, relaxations to acetylcholine and calcium ionophore A23187, basal NO release, and expression of eNOS mRNA in aortic endothelial cells were reduced (P<.05). In femoral arteries, relaxations to acetylcholine were preserved, whereas basal release of NO was attenuated (P<.05). Aging selectively increased contractions to norepinephrine and functional endothelin converting enzyme activity and attenuated contractions to ET-1 in aortas but not femoral arteries. Vascular SOD activity was higher in the femoral artery (P<.05) and unaffected by aging. Plasma ET-1 levels increased and plasma SOD activity decreased with age (P<.05). Aging was associated with an anatomic heterogeneity of endothelial dysfunction, functional endothelin converting enzyme activity, and vascular SOD activity. Vascular function was impaired in the aorta but not the femoral artery, which may be related to lower eNOS mRNA expression and SOD activity. These data suggest differential regulation of the vascular aging process that may contribute to the anatomic heterogeneity of atherosclerosis.

Mechanisms underlying the vascular activity of beta-amyloid protein fragment (beta A(4)25-35) at the level of skin microvasculature

Deposition of beta-amyloid protein (beta A4) in extracellular senile plaques is a pathologic hallmark of Alzheimer's disease (AD). The neurotoxic effect of beta A4 has been ascribed to a discrete 11-amino acid internal sequence (beta A(4)25-35). Substance P (SP) has been found to be depleted in the brain of AD patients while its presence was found to protect against the neurodegenerative effect of beta A(4)25-35. Our previous studies, in vivo, in aged rats showed that beta A(4)25-35 exhibits a potent vasoconstrictor (VC) effect in rat skin microvasculature and can prevent SP but not calcitonin gene-related peptide (CGRP) from inducing a vasodilator (VD) response. It was postulated that beta A(4)25-35 might be interacting with SP at the level of the second messenger system via the phosphoinositide pathway. Using a blister model of inflammation in the rat hind footpad, we examined the ability of beta A(4)25-35 to modulate the vascular activity of bradykinin (BK) and serotonin (5-HT) which also activate the phosphoinositide pathway. In addition, the role of nitric oxide (NO), endothelin (ET, an endothelium-derived constrictor factor) and protein kinase C (PKC) in the vascular effects of beta A(4)25-35 were examined using the NO synthase inhibitor, NG-nitro-L-arginine (L-NOARG), the ET-receptor antagonist, BQ-123, and the PKC inhibitor, bisindolylmaleimide (BIM) respectively. Changes in microvascular blood flow were monitored using laser Doppler flowmetry and the area within the response curve measured. The results showed that beta A(4)25-35 (10 microM) induced a VC effect and inhibited the subsequent VD response to BK (10 microM) and 5-HT (1 microM) in a similar fashion to its effect on SP (1 microM). In the presence of L-NOARG (100 microM), the VD effect of SP was reduced and further attenuated after perfusion of beta A(4)25-35. Superfusion of the blister base with BQ-123 (10 microM) or BIM (1 microM) prior to and during perfusion with beta A(4)25-35 abolished its VC effect and allowed SP to induce a normal VD response in both young and old rats. Based on these results, we suggest that the vascular activity of the active fragment, beta A(4)25-35, is mediated by ET via activation of PKC. This study provides new findings which may help to elucidate the signal transduction mechanisms involved in the vascular activity of beta A(4)25-35. The relevance of these mechanisms to those underlying the pathological effects of beta A4 and their significance in AD remains to be determined.

Age-related changes in vascular smooth muscle and endothelium

Aging is associated with structural and functional changes in the blood vessel wall. In vascular smooth muscle, the effects of aging on the response mediated by beta-adrenoceptors have been most intensively studied. beta-adrenoceptor-mediated relaxation decreases in most arteries, but not veins, with increasing age. In contrast, studies on contractile responses to alpha-adrenergic drugs are conflicting. The response to alpha-adrenoceptor agonists appears to be unchanged for decreased by aging. The endothelium takes part in the local regulation of vascular tone as a source of several vasoactive factors. Basal release of endothelium-derived nitric oxide decreases with age in vitro studies. Aging is also associated with reduced endothelium-dependent relaxations in response to vasoactive substances such as acetylcholine, histamine or adenosine. The impairment of the relaxation is, in most cases, achieved by a decreased release and/or decreased production of endothelium-derived relaxing factors (endothelium-derived nitric oxide, hyperpolarising factor and prostacyclin). An increased release of endothelium-derived, cyclo-oxygenase-dependent contracting factor is also responsible for reduced relaxation in some tissues. On the other hand, the release of endothelin-1 from the endothelium increases with age, while the response to the peptide decreases under the same conditions, especially in small resistance arteries. The alterations of vascular smooth muscle and endothelial cells occurring with age may have important clinical implications for the pathogenesis of cardiovascular disease.

Age and hypertension differently affect coronary contractions to endothelin-1, serotonin, and angiotensins

BACKGROUND

Endothelium-derived substances and the renin-angiotensin system are important regulators of vascular tone. This study was designed to evaluate the effects of age and hypertension on vascular function of rat coronary arteries.

METHODS AND RESULTS

Rings of the left anterior descending coronary artery were isolated from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) at 12 (younger) and 72 (older) weeks of age and suspended in myographs (37 degrees C, 95% O2/5% CO2) for isometric tension recording. Systolic blood pressure was higher in SHR than in WKY rats (P < .05) but was unaffected by age in both strains. Active wall tension to KCl 100 mmol/L (mN/mm) was decreased in younger (0.28 +/- 0.03, n = 9) and older SHR (0.49 +/- 0.06, n = 13) compared with age-matched WKY rats (0.87 +/- 0.05, n = 9 and 1.51 +/- 0.11, n = 11, respectively, P < .05). In both strains, active wall tension to endothelin-1 and serotonin increased with age (n = 6 to 10, P < .05) but was decreased in younger and older SHR compared with WKY rats (P < .05). Active wall tension induced by angiotensin I 10(-7) mol/L was increased in older SHR (0.19 +/- 0.04, n = 7) compared with younger SHR (0.04 +/- 0.01, n = 9) but was similar in younger and older WKY rats (0.10 +/- 0.02 versus 0.15 +/- 0.03, n = 6 to 9) and younger SHR. In younger WKY rats and SHR, pretreatment of coronary arteries with benazeprilat 10(-5) mol/L (n = 5 for each) almost completely abolished the contractions to angiotensin I 10(-7) mol/L. Active wall tension to angiotensin II 10(-7) mol/L was comparable in all four groups, but compared with the contraction to KCl 100 mmol/L, the response was already increased in younger SHR (29 +/- 3%, n = 9) compared with the younger WKY rats (14 +/- 3%, n = 9, P < .05), but it was unaffected by age in both strains. In vitro treatment of younger WKY rat and SHR coronary arteries with the nonpeptide angiotensin II (AT1) receptor antagonist valsartan 10(-5) mol/L (n = 3 for each) fully suppressed contractions to angiotensin II 10(-7) mol/L. In contrast, endothelium-independent relaxations to the nitrovasodilator sodium nitroprusside, endothelium-dependent relaxations to acetylcholine, and endothelium-dependent contractions to N omega-nitro-L-arginine methyl ester were comparable in all four groups of rats.

CONCLUSIONS

In summary, in rat coronary arteries, contractile responses to endothelin-1, serotonin, and KCl increase with age but are decreased by hypertension. In contrast, the L-arginine/nitric oxide pathway remains unaffected. The contractions to angiotensin I markedly increased with increasing duration of hypertension in the SHR only. Despite overall reduced contractile responses of SHR coronary arteries, contractions to angiotensin II were maintained. Hence, aging and hypertension affect contractile responses of rat coronary arteries to vasoconstrictor agonists differently.

Age-related changes in vascular responses: a review

Normal aging is associated with different changes in the cardiovascular system that lead to an increase in pathological processes, such as hypertension, coronary artery disease, heart failure, and postural hypotension with enhancement of both morbidity and mortality. The vascular alterations consist of changes in the function and structure of the arteries, and increasing vascular stiffness, mainly when atherosclerosis is present, whose incidence is increased with age. The arteries accumulate lipids, collagen, and minerals. Cerebral perfusion may be reduced in the elderly, mainly regional cerebral blood flow, which leads to a deterioration of mental and physical functions. The degree of deterioration is increased when aging is associated with hypertension. Aging alters endothelial cells, which play an important role in vascular tone regulation. Such a process tends to reduce endothelium-dependent relaxations, and clearly reduces the vasodilation elicited by beta-adrenoceptor agonists. The contractions induced by different agents, such as 5-hydroxytryptamine, histamine, high potassium and angiotensin are barely affected with aging, whereas those elicited by noradrenaline or endothelin are usually reduced. However, plasma noradrenaline levels are increased with age, mainly due to a reduction in the sensitivity of presynaptic alpha 2-adrenoceptors and also of noradrenaline uptake. Sodium pump activity, that controls cellular ionic homeostasis, may be altered depending on animal species. Finally, vascular Ca2+ regulation appears to be altered and the extracellular Ca2+ dependence of contractile responses elicited by agonists is increased, which justifies the enhanced sensitivity to Ca2+ antagonists in senescence.

Endothelin receptor subtypes in small arteries. Studies with FR139317 and bosentan

We studied the effects of the selective endothelin A receptor antagonist FR139317 and the combined endothelin A/endothelin B receptor antagonist bosentan in rat mesenteric arteries by using a video dimension analyzer. In endothelium-denuded arteries, increasing concentrations of endothelin-1 evoked a biphasic vasoconstriction. The first phase was observed at low concentrations (10(-16) to 10(-11) mol/L) of endothelin-1 and was relatively weak. However, the contractions characterizing the second phase, which occurred at higher concentrations (10(-10) to 3 x 10(-8) mol/L) of endothelin-1, were much stronger. FR139317 concentration-dependently shifted the second phase of the endothelin-1-induced contraction curve to the right without affecting the first phase. In contrast, bosentan inhibited both the first and the second phase. Even after the blockade of endothelin A receptor, increasing concentrations of the endothelin B receptor agonists endothelin-3 and sarafotoxin S6c still induced small contractions. Maximal contractions induced by single-bolus extraluminal application of endothelin-3 (10(-9) mol/L) or sarafotoxin S6c (3 x 10(-8) mol/L) were markedly more pronounced than responses induced by cumulative concentrations, suggesting endothelin B receptor downregulation upon repeated and sustained activation. The response induced by a single bolus of endothelin-3 (10(-9) mol/L) was antagonized by bosentan but not by FR139317, confirming that endothelin B receptors were involved. In endothelium-intact arteries half-maximally precontracted with norepinephrine, bosentan but not FR139317 inhibited the relaxations induced by intraluminally applied endothelin-3. (ABSTRACT TRUNCATED AT 250 WORDS)

ETA and ETB receptors mediate contraction to endothelin-1 in renal artery of aging SHR. Effects of FR139317 and bosentan

We characterized vascular endothelin receptors of the renal artery from adult (12 to 16 weeks of age) and old (72 to 76 weeks) spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY). Vessels were suspended in organ chambers (37 degrees C, aerated with 95% O2/5% CO2), and isometric tension was recorded. The endothelin-A (ETA) receptor antagonist FR139317, the combined ETA/ETB receptor antagonist bosentan, and the ETB-selective agonist sarafotoxin S6c were used. In old (and less so in adult) SHR, cumulative concentration-contraction curves to endothelin-1 showed a small contraction resistant to FR139317 (10(-5) mol/L) at 3 x 10(-9) to 10(-8) mol/L endothelin-1, which was completely inhibited by bosentan (10(-5) mol/L). This FR139317-resistant contraction to endothelin-1 was not present in WKY. Furthermore, in the presence of FR139317 (10(-5) mol/L), sarafotoxin S6c induced a stronger contraction in old SHR than in WKY (P < .05). In rings contracted with norepinephrine, sarafotoxin S6c caused endothelium-dependent relaxations in both strains; these relaxations were blocked by N omega-nitro-L-arginine methyl ester, indicating that nitric oxide is the mediator. In WKY but not SHR, release of nitric oxide by sarafotoxin S6c increased with age (P < .05). Thus, both ETA and ETB receptors mediate contraction to endothelin-1 in the renal artery from SHR but not WKY. ETB receptors on vascular smooth muscle seem to be unmasked with age in SHR, whereas those on endothelium (mediating nitric oxide release) exhibit more efficient responses with age in WKY.

Different mechanisms of endothelial dysfunction with aging and hypertension in rat aorta

We analyzed the effects and mechanisms of aging in aortic endothelium and vascular smooth muscle of 12-week-old (adult) and 72-week-old (senescent) normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Aortas were suspended in organ chambers filled with physiological salt solution (95% O2/5% CO2; 37 degrees C), and isometric tension was measured. In WKY, endothelium-dependent relaxations to acetylcholine were diminished with aging (P < .05); in SHR, they were reduced compared with WKY (P < .05) but unchanged with aging. The thromboxane/endoperoxide receptor antagonist SQ 30741 increased relaxations only in adult SHR. Relaxations to sodium nitroprusside were slightly enhanced with age in WKY and SHR (P < .05). Endothelium-dependent contractions to acetylcholine were unmasked by NG-nitro-L-arginine methyl ester (P < .05) and prevented by SQ 30741 or endothelium removal. In WKY, contractions increased with age. In adult SHR, marked endothelium-dependent contractions occurred (P < .05 versus WKY), which diminished with age (P = NS versus senescent WKY). The thromboxane analogue U46619 elicited similar contractions in adult and senescent WKY and adult SHR, whereas responses in senescent SHR were weaker (P < .05). In WKY and SHR, contractions to norepinephrine were similar and unaltered by aging. In WKY, contractions to endothelin-1 remained unaffected by aging. Adult SHR exhibited contractions to endothelin-1 comparable to those in WKY, whereas senescent SHR contracted less (P < .05). Bosentan, a combined endothelin-A/endothelin-B receptor antagonist, inhibited endothelin-1 markedly, especially in SHR (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the responsiveness to endothelin-1 in isolated atria from diabetic rats

This study investigates the influence of diabetes on the cardiac responsiveness to endothelin-1. The effects of endothelin-1 on rate and force of contraction were examined in isolated right and left atria, respectively, obtained from either streptozotocin (65 mg/kg)-treated rats (diabetic) or vehicle (0.02 M citric acid)-treated rats (control). The positive chronotropic and inotropic effects of endothelin-1 did not change in atria from diabetic rats at 2 and 4 weeks, but were reduced at 8 and 12 weeks. The positive chronotropic response to noradrenaline, but not to sympathetic nerve stimulation, was also reduced in 12-week diabetic rats. Endothelin-1 caused a decrease in the positive chronotropic and inotropic responses to sympathetic nerve stimulation and to noradrenaline; these inhibitory effects of endothelin-1 were not altered in 2-, 4-, 8- or 12-week diabetic rats. The study demonstrates that atrial responses to endothelin-1 and to noradrenaline are reduced by streptozotocin-induced diabetes, but the alteration depends on the duration of diabetes.

Aging and vasoconstrictor responses mediated by alpha 2-adrenoceptors and 5-HT1 and 5-HT2 receptors

The potencies of noradrenaline (NA) at producing alpha 2-adrenoceptor mediated contractions, and of 5-hydroxytryptamine (5-HT) at producing 5-HT1 and 5-HT2 receptor mediated contractions were investigated in rings of human saphenous vein obtained from varicose vein surgery. There was a significant negative correlation between agonist potency and age for NA at alpha 2-adrenoceptors (r = 0.52, n = 21, p < 0.05) and for 5-HT at both 5-HT1 (r = 0.47, n = 19, p < 0.05) and 5-HT2 (r = 0.54, n = 19, p < 0.05) receptors, so that both agonists were less potent with increasing age. This demonstrates an age-related decrease in alpha 2-adrenoceptor and 5-HT1 and 5HT2 mediated contractile responsiveness in the human saphenous vein.

Differences in sensitivity of rat mesenteric small arteries to agonists when studied as ring preparations or as cannulated preparations

1. Pharmacological experiments on vascular tissue are normally performed on isometric ring or strip preparations. The aim of this study was to compare the isometric characteristics with the characteristics obtained if vessels were examined under the more physiologically appropriate isobaric condition. 2. Rat mesenteric small arteries were mounted either on two steel wires for isometric force measurement (wire-myograph) or cannulated for measurement of the internal diameter under isobaric conditions (pressure-myograph). 3. The passive pressure-diameter characteristics of the small arteries were similar on the wire- and pressure-myograph (using the Laplace relation to convert wall tension-internal circumference data from the wire-myograph to effective pressure-diameter characteristics). 4. In cumulative concentration-response experiments with noradrenaline and phenylephrine, the threshold concentration was 8-10 times lower, and the EC50-concentration was 4-5 times lower, in the pressure myograph compared to the wire-myograph. Thus vessels were not only more sensitive on the pressure myograph, but the slopes of the concentration-response curves were less steep. Similar experiments with vasopressin also showed this difference in the threshold-concentration and slope, but EC50 concentrations were similar. 5. Cumulative concentration-response experiments with K+ showed no difference either in EC50 or in slope on the wire- and pressure-myographs. 6. On the wire-myograph, some vessels were stretched longitudinally (to mimic the longitudinal stretch which had to be used in the pressure-myograph to avoid buckling). Such stretch did not affect the passive characteristics. 7. The differences between the EC50 determined on the wire- and pressure-myographs as regards noradrenaline and phenylephrine were eliminated when neuronal noradrenaline uptake was inhibited by denervation. However, the slope of the concentration-response curves on the wire-myograph was not affected by denervation.8. When vessels were exposed to cocaine (3 MicroM) the noradrenaline concentration-response curves were the same on the wire- and pressure-myographs as regards both EC50 and slope.9. On the wire-myograph, the calcium antagonist, methoxyverapamil, (D600) reduced the maximal contractile effect of noradrenaline by 50%, but on the pressure-myograph D600 did not affect the maximal response.10. The present results show that results obtained from vascular tissue under isometric conditions may differ substantially from the characteristics which would be obtained under isobaric conditions.

Changes in the adrenergic mechanisms of cerebral arteries after subarachnoid hemorrhage in goats

We have examined the effects of experimental subarachnoid hemorrhage (SAH), induced by delivering autologous blood into the subarachnoid space, on the adrenergic mechanisms of the goat cerebrovascular bed. To achieve this, the response to noradrenaline was recorded both in vivo, by measuring cerebral blood flow in unanesthetized animals, and in vitro, by recording isometric tension in isolated cerebral arteries. In addition, we checked the function of adrenergic innervation by measuring the tritium efflux evoked by electrical stimulation in cerebral arteries preloaded with [3H]-noradrenaline, and we examined this innervation by using both fluorescent and electron transmission microscopy. All studies were performed before and 3, 7, and 14 days after SAH. Injections of noradrenaline (0.1-10 micrograms) directly into the cerebro-arterial supply produced reductions in cerebral blood flow, with no concomitant changes in mean arterial blood pressure and heart rate, which were significantly enhanced (P < 0.01) 3 and 7 days after SAH and returned to control values 14 days after hemorrhage induction. In isolated cerebral arteries, noradrenaline (10(-8)-10(-4) mol/L) produced concentration-dependent contractions, which were also significantly enhanced (P < 0.05) 3 and 7 days after SAH and returned to control values in cerebral arteries obtained 14 days after SAH. On the other hand, increases in the release of tritium induced by electrical stimulation in cerebral arteries preloaded with [3H]-noradrenaline were significantly lower (P < 0.01) after SAH. Moreover, microscopical studies showed a reduction in catecholamine fluorescence and signs of sympathetic degeneration in some perivascular axons after SAH.(ABSTRACT TRUNCATED AT 250 WORDS)

Both ETA and ETB receptors mediate contraction to endothelin-1 in human blood vessels

BACKGROUND

Endothelin (ET)-1 has potent vascular effects. Two endothelin receptors have been cloned, namely, the ETA receptor, which preferentially binds ET-1, and the ETB receptor, which equally binds ET-1 and ET-3 and preferentially sarafotoxin S6c. We characterized endothelin receptor subtypes on vascular smooth muscle and endothelium of isolated human internal mammary artery (IMA) and vein (IMV) and porcine coronary artery (PCA) using the ETA antagonists FR139317 and BQ-123, the ETB ligand sarafotoxin S6c, and the ETA/ETB antagonist Ro 47-0203 (bosentan).

METHODS AND RESULTS

In endothelium-denuded IMA and PCA and less so in IMV, FR139317 and BQ-123 (in PCA only) shifted the concentration-contraction curves to ET-1 parallel to the right. However, even at 10(-5) mol/L, FR139317 did not inhibit a high-sensitivity portion of the concentration-contraction curve. Moreover, the ETB receptor agonist sarafotoxin S6c induced contraction in vessels preincubated with FR139317. IMV was significantly more sensitive to the contractile effect of ET-1 and sarafotoxin S6c than was IMA (P < .05). Prolonged incubation with sarafotoxin S6c (to downregulate ETB receptors) and FR139317 eliminated the contraction resistant to FR139317. The ETA/ETB receptor antagonist bosentan caused a parallel shift of the concentration-contraction curve to the right at all concentrations of endothelin. ETB receptor mRNA was detected by Northern blot analysis in IMA and aortic smooth muscle cells. In precontracted IMA and PCA with endothelium, sarafotoxin S6c did not cause endothelium-dependent relaxations, whereas transient responses occurred in IMV.

CONCLUSIONS

Vascular smooth muscle cells of human IMA, IMV, and PCA contain both ETA and ETB receptors, whereas the endothelium of IMA and PCA does not express functional ETB receptors linked to nitric oxide and/or prostacyclin production. Hence, inhibition of endothelin-induced contraction in patients requires the use of combined ETA/ETB antagonists.

Aging differentially modifies arterial sensitivity to endothelin-1 and 5-hydroxytryptamine: studies in dog coronary arteries and rat arterial mesenteric bed

The influence of age on vascular reactivity to endothelin-1 (ET-1) and 5-hydroxytryptamine (5-HT) was studied in coronary artery rings from dogs of 9 years of age or younger, and dogs older than 9 years. ET-1 caused concentration-dependent contractions that developed about 100% of the 70 mM KCl-induced tension in the younger dogs; those from older dogs did not generate more than 20%. In contrast, 5-HT developed only about 20% of the KCl-induced tension in rings from young dogs, whereas in the older animals, it developed up to 120% of the KCl tension. No significant difference in the tension developed by 70 mM KCl was noted between both groups of dogs. Mechanical denudation of the endothelial cell layer caused a modest, yet significant, leftward shift of the ET-1 and 5-HT concentration-response curves only in the younger dogs. N omega-Nitro-L-arginine (15 microM) shifted the ET-1 concentration-response curves to the left in rings from both groups of dogs. Rings precontracted with 20 mM KCl relaxed in a concentration-dependent fashion with acetylcholine; its sensitivity was about threefold less in the older group of dogs. To validate the changes in vascular reactivity with age, a parallel study was performed perfusing the arterial mesenteric bed of rats of 3, 7, and 30 weeks of age. In this experimental model, the efficacy of ET-1 significantly decreased with age and that of 5-HT was significantly increased. The vasomotor reactivity of noradrenaline was modestly affected by aging, whereas the acetylcholine-induced vasorelaxation was significantly reduced with age.

Modulation by aging of the coronary vascular response to endothelin-1 in the rat isolated perfused heart

Changes with age in the coronary vascular response to endothelin-1 were investigated in perfused hearts isolated from 2-, 6- and 24-month-old (mo) male Fisher-344 rats. Endothelin-1 injected as a single bolus (0.3, 3 and 30 nmol) into the coronary artery supply caused dose-dependent vasoconstriction in all three age groups. While there was no age-related change in the vasoconstriction induced by the lower doses (0.3 and 3 nmol), the higher dose (30 nmol) elicited a more pronounced vasoconstriction in 6- and 24-mo rats than that in 2-mo rats. NG-nitro-L-arginine (L-NNA), an inhibitor of nitric oxide formation, markedly enhanced the vasoconstriction induced by 30 nmol endothelin-1 in 2- and 6-mo rats but only slightly and non-significantly enhanced that vasoconstriction in 24-mo rats. Haemoglobin, which inhibits activation of guanylate cyclase by nitric oxide, enhanced the endothelin-1-induced vasoconstriction in 2-mo rats, but not in 6- and 24-mo rats. The acetylcholine-induced coronary vasodilation was more pronounced in 2- and 6-mo rats than in 24-mo rats and was attenuated by L-NNA in 2- and 6-mo rats. The coronary vasodilation induced by nitroprusside (0.1 mmol), a pharmacological precursor of nitric oxide, did not change with age. Endothelin-1 (30 nmol) markedly increased the release of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) in all three age groups. The prostaglandin synthesis inhibitor indomethacin enhanced the endothelin-1-induced vasoconstriction in 2- and 6-mo rats to a similar extent.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium antagonists and ACE inhibitors. Effect on endothelium and vascular smooth muscle

The effects of cardiovascular drugs on endothelium and vascular smooth muscle function are important for the prevention of cardiovascular disease. Changes in endothelial function are an early event in most forms of cardiovascular disease and, later in the disease process, vascular smooth muscle cells are functionally altered and begin to migrate to and proliferate in the intima. Calcium antagonists and angiotensin converting enzyme (ACE) inhibitors are widely used in patients with cardiovascular disease and are thought to have vascular protective effects. ACE, an enzyme located in the endothelial cell membrane, activates angiotensin I and angiotensin II, and deactivates bradykinin. Bradykinin activates endothelial bradykinin (B2) receptors, which results in the formation of nitric oxide and prostacyclin. Hence, ACE inhibitors not only prevent the formation of angiotensin II, but also increase the local levels of bradykinin and in turn nitric oxide and prostacyclin. These compounds are vasodilators and potent inhibitors of platelet function, and therefore may mediate important protective effects of ACE inhibitors. Furthermore, nitric oxide may have antiproliferative effects in vascular smooth muscle cells. Calcium antagonists do not appear to affect the release of endothelium-derived relaxing factors or any other endothelial product. However, they facilitate endothelium-dependent relaxation and reduce the contracting effects of endothelin-1 at the level of smooth muscle. Indeed, in some blood vessels, e.g. the large coronary arteries and the human forearm circulation, verapamil and nifedipine antagonise endothelin-induced contractions. In addition, calcium antagonists inhibit the effects of platelet-derived growth factor and may have antiproliferative effects in vascular smooth muscle cells. In conditions involving progressive dysfunction of the endothelium, vascular deposition of platelets increases the local levels of platelet-derived growth factor, and the antiproliferative effects of calcium antagonists may thus be particularly important.

Effects of endothelin-1 on postjunctionally-mediated purinergic and adrenergic components of rat vas deferens contractile responses

The effects of endothelin-1 (ET-1) on electrically- or drug-induced contractile responses mediated by purinergic or adrenergic receptors were studied in isolated prostatic portion of rat vas deferens. ET-1 (0.01 nM to 30 nM) concentration-dependently increased the contractions evoked by electrical field stimulation (EFS, 0.3 msec, 30 V, 8 Hz, 300 pulses). In the presence of prazosin, ET-1 (3 nM) strongly enhanced the prazosin-resistant responses to EFS, while after desensitization of purinergic receptors induced by alpha,beta-methylene adenosine 5'-triphosphate (mATP) the peptide only tended to enhance the mATP-resistant component of the electrically-evoked contractions. ET-1 failed to change the nonpurinergic nonadrenergic responses to electrical stimulation revealing after simultaneous administration of prazosin and mATP. ET-1 concentration-dependently increased the contractile effects of exogenous ATP (30 microM). The effect of ET-1 (3 nM) was not changed after tetrodotoxin (TTX, 0.3 microM) and guanethidine (10 microM). In the presence of TTX and guanethidine ET-1 potentiated the contractile effects of low (0.01-1 microM) concentrations of noradrenaline (NA) and did not change the contractions induced by NA at concentrations higher than 3 microM. Therefore, ET-1 exerted a potentiating effect on the contractility of the prostatic portion of rat vas deferens via postjunctional mechanisms underlying mainly the purinergic and partly the adrenergic smooth muscle contractile responses.

Characteristics of endothelin receptors in the cerebral cortex and spinal cord of aged rats

Characteristics of endothelin receptors were studied in male Fischer 344 rats at 4-, 15- and 24-months of age and [125I]ET-1 binding showed a single high affinity binding site in their cerebral cortex and spinal cord membranes. The density and affinity of ET binding sites were found to be similar in rats of various age groups. To determine the affinity of ET isotypes to ET-1 binding sites in cerebral cortex and spinal cord, competition studies were performed and K(i) values of ET-1, ET-2 and ET-3 for [125I] ET-1 binding sites were determined. It was found that ET-1 had 100 and ET-2 had 25-100 times lower K(i) values as compared to ET-3, indicating that ET receptors in cerebral cortex and spinal cord are of ETA type. In spinal cord, the K(i) values of ET-1 and ET-2 for ET receptor were found to be similar. However, in cerebral cortex the K(i) values of ET-1 were found to be at least 6 times lower than ET-2. It is inferred that there are two subtypes of ET(A) receptors, ET(A1) which have higher affinity for ET-1 in comparison to ET-2, as found in cerebral cortex and ET(A2) which have higher affinity for ET-2, as found in spinal cord. The K(i) and IC50 values of ET-1, ET-2, and ET-3 for [125I]ET-1 binding sites in cerebral cortex and spinal cord were found to be similar in 4-, 15- and 24-month-old rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Age, hypertension and hypercholesterolaemia alter endothelium-dependent vascular regulation

As a source of several vasoactive factors, the endothelium takes part in the regulation of vascular tone. The most important endothelium-derived vasoactive substances are nitric oxide, prostacyclin, endothelin-1 and contracting factors requiring the activity of cyclooxygenase. The endothelium is an obvious target organ of cardiovascular risk factors. Accordingly, functional alterations do occur with aging, hypertension and hypercholesterolaemia. All three conditions are associated with a decreased basal and simulated release of endothelium-derived nitric oxide. On the other hand, the release of endothelin-1 appears to increase with age, while the sensitivity to the peptide markedly decreases under the same conditions. In the spontaneously hypertensive rat, acetylcholine and stretch evoke the release of a cyclooxygenase-dependent endothelium-derived contracting factor, most likely prostaglandin H2. The circulating levels of endothelin-1 on the other hand are not increased in experimental and human hypertension. In the porcine coronary circulation, oxidized low-density lipoproteins selectively reduced endothelium-dependent relaxations to aggregating platelets, serotonin and thrombin which are mediated by nitric oxide. The alterations of endothelial function occurring with aging, hypertension and hypercholesterolaemia may have important clinical implications for the pathogenesis of cardiovascular disease.

Endothelium-derived contracting factors

The endothelium not only mediates relaxation but is a source of contracting factors. Endothelium-dependent contractions are elicited by physical and chemical stimuli (i.e., hypoxia, pressure, and stretch) and autacoids, local and circulating hormones. The mechanism of endothelium-dependent contractions to hypoxia involves withdrawal of nitric oxide. The endothelial cyclooxygenase pathway can produce thromboxane A2, prostaglandin H2, and superoxide anions. The peptide endothelin is a potent contracting factor; its production is stimulated by vasopressor hormones, platelet-derived factors, coagulation products, and cytokines, whereas endothelium-derived nitric oxide, prostacyclin, and a smooth muscle cell-derived inhibitory factor reduce endothelin production. In hypertension, the release of cyclooxygenase-dependent endothelium-derived contracting factors to stretch, acetylcholine, and platelet-derived products is augmented. Vascular endothelin production in hypertension remains controversial but appears mostly normal; it is augmented in the presence of vascular disease or renal insufficiency. The endothelium-dependent inhibition of endothelin-induced contractions is reduced in hypertension while the reactivity of vascular smooth muscle may be normal, increased, or reduced. The potentiating effects of low concentrations of endothelin on contractions to norepinephrine are augmented with aging and hypertension. In atherosclerosis, the production of the cyclooxygenase-dependent endothelium-derived contracting factors and endothelin is enhanced. Thus, endothelium-derived contracting factors can profoundly affect vascular tone and counteract relaxing factors produced within the endothelium. In hypertension and atherosclerosis, the role of contracting factors appears to become more dominant, leading to an imbalance of endothelium-dependent vascular regulation.

Endothelin stimulated by angiotensin II augments contractility of spontaneously hypertensive rat resistance arteries

In cultured endothelial cells, endothelin is produced after stimulation with angiotensin II. The effects of angiotensin II and endothelin-1 on vascular sensitivity to norepinephrine were studied in perfused rat mesenteric resistance arteries. Expression of endothelin messenger RNA (mRNA) was determined in endothelial cells obtained from the mesenteric circulation. Perfusion (5 hours) of the arteries with angiotensin II (10(-7) M) potentiated contractions in arteries with endothelium induced by norepinephrine in spontaneously hypertensive rats but not Wistar-Kyoto rats. The potentiation was inhibited by phosphoramidon and an endothelin antibody. Short-term stimulation (1 hour) with angiotensin II did not cause the potentiation. Stimulation with angiotensin I (10(-7) M; 5 hours) caused a potentiation prevented by captopril. In endothelial cells collected from the mesenteric arterial bed of spontaneously hypertensive rats, endothelin-specific mRNA was constitutively expressed, and the level of endothelin transcripts was increased by angiotensin II (10(-7) M). Threshold concentrations of exogenous endothelin-1 potentiated contractions induced by norepinephrine in arteries with and without endothelium of spontaneously hypertensive rats but not Wistar-Kyoto rats. Thus, angiotensin II stimulates the endothelial production of endothelin in situ and therapy potentiates contractions to norepinephrine in mesenteric resistance arteries of spontaneously hypertensive rats. This suggests that vascular endothelin production acts as an amplifier of the pressor effects of the renin-angiotensin system that may play an important role in hypertension.

Effects of endothelin-1 on endothelial cells in the porcine coronary artery

The effects of endothelin-1 (ET-1) on endothelial cells from the porcine coronary artery were investigated using cascade bioassay and radioimmunoassay methods. For comparison, endothelial cells from the rabbit abdominal aorta were also used. Freshly isolated endothelial cells were perfused with Krebs' solution. Liberation of vasodilatory substances was detected under bioassay conditions by measuring the relaxing activity of the perfusate on endothelium-denuded strips of the porcine coronary artery contracted with ET-1. The perfusate in the absence of ET-1 slightly inhibited the contraction, suggesting that endothelial cells spontaneously release vasorelaxing substances. After application of ET-1, the perfusate-induced relaxation drastically increased. Oxyhemoglobin completely abolished the relaxation induced by perfusate from endothelial cells of the rabbit abdominal aorta but only marginally attenuated the relaxation induced by perfusate from endothelial cells of the porcine coronary artery. In contrast, indomethacin significantly attenuated the relaxation induced by perfusate from the endothelial cells of the porcine coronary artery. In endothelium-intact strips of the porcine coronary artery, ET-1 significantly increased the concentration of 6-ketoprostaglandin F1 alpha but did not modify the cellular concentration of either cAMP or cGMP. It is concluded that ET-1 augments the release of vasorelaxing factors from endothelial cells both in the porcine coronary artery and in the rabbit abdominal aorta and that the major vasorelaxing substance derived from endothelial cells may be different in these two blood vessels. Prostaglandin I2 in the former and nitric oxide in the latter are suggested as possible candidates. The vasorelaxation induced by prostaglandin I2 may not be mediated by cAMP-dependent mechanisms in the porcine coronary artery.