Endothelium-derived hyperpolarizing factor and endothelium-dependent relaxations

Is It Possible to Train the Endothelium?-A Narrative Literature Review

This review provides an overview of current knowledge regarding the adaptive effects of physical training on the endothelium. The endothelium plays a crucial role in maintaining the health of vessel walls and regulating vascular tone, structure, and homeostasis. Regular exercise, known for its promotion of cardiovascular health, can enhance endothelial function through various mechanisms. The specific health benefits derived from exercise are contingent upon the type and intensity of physical training. The review examines current clinical evidence supporting exercise's protective effects on the vascular endothelium and identifies potential therapeutic targets for endothelial dysfunction. There is an urgent need to develop preventive strategies and gain a deeper understanding of the distinct impacts of exercise on the endothelium.

Sex differences in the participation of endothelial mediators and signaling pathways involved in the vasodilator effect of a selective GPER agonist in resistance arteries of gonadectomized Wistar rats

AIM

Endothelial mechanisms underlying the vascular effects of estrogen modulated by the G protein-coupled estrogen receptor (GPER) are not well understood, especially in gonadal sex hormone deprivation. Thus, we investigated vascular function and endothelial signaling pathways involved in the selective activation of GPER in resistance arteries of gonadectomized rats.

METHODS

Gonadectomy was performed in Wistar rats of both sexes. After 21 days, the animals were euthanized. Concentration-response curves were obtained by cumulative additions of G-1 in third-order mesenteric arteries. The vasodilatory effects of G-1 were evaluated before and after endothelium removal or incubation with pharmacological inhibitors. Tissue protein expression was measured by western blotting. Assays with 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM) and 2',7' dichlorodihydrofluorescein-diacetate (DCF-DA) were performed in the arteries investigated. Immunolocalization was assessed by immunofluorescence.

RESULTS

G-1 induced partially endothelium-dependent relaxation in both sexes. The three isoforms of the enzyme nitric oxide synthase contributed to the production and release of nitric oxide in both gonadectomized groups, but the role of inducible nitric oxide synthase is more expressive in males. The mechanistic pathway by which endothelial nitric oxide synthase is phosphorylated appears to differ between sexes, with the rapid signaling pathway phosphatidylinositol-3-kinase/protein kinase B/endothelial nitric oxide synthase (PI3k-Akt-eNOS) being identified for males and mitogen-activated protein kinase/extracellular signal-regulated kinase/endothelial nitric oxide synthase (MEK-ERK-eNOS) for females. The contribution of hydrogen peroxide as an endothelial relaxation mediator seems to be greater in females.

CONCLUSION

These results provide new insights into the effects of estrogen-induced responses via GPER on vascular function in gonadal sex hormone deprivation.

The Acute Effect of Exercise on Arterial Stiffness in Healthy Subjects: A Meta-Analysis

Arterial stiffness has been shown to be a subclinical marker associated with cardiovascular disease. Meanwhile, long-term exercise has been demonstrated to reduce arterial stiffness, providing a decrease in cardiovascular risk. However, the acute effect of exercise on arterial stiffness is unclear. This systematic review and meta-analysis aimed to assess the acute effect of exercise interventions on arterial stiffness in healthy adults. We searched the Cochrane Central Register of Controlled Trials, MEDLINE (via Pubmed), Scopus, and Web of Science databases, from their inception to 30 June 2020. A meta-analysis was performed to evaluate the acute effect of exercise on arterial stiffness using random-effects models to calculate pooled effect size estimates and their corresponding 95% CI. Pulse wave velocity was measured as an arterial stiffness index. The 30 studies included in the meta-analysis showed that pulse wave velocity was not modified immediately after exercise (0 min post) (ES: 0.02; 95% CI: −0.22, 0.26), but subsequently decreased 30 min after exercise (ES: −0.27; 95% CI: −0.43, −0.12). Thereafter, pulse wave velocity increased to its initial value 24 h after exercise (ES: −0.07; 95% CI: −0.21, 0.07). Our results show that, although there is a significant reduction in pulse wave velocity 30 min after exercise, the levels of arterial stiffness return to their basal levels after 24 h. These findings could imply that, in order to achieve improvements in pulse wave velocity, exercise should be performed on a daily basis.

Acute effects of different types of aerobic exercise on endothelial function and arterial stiffness

BACKGROUND

Chronic aerobic exercise training is associated with improved endothelial function and arterial stiffness and favourable long-term cardiovascular effects.

DESIGN

We investigated the acute effects of continuous moderate intensity aerobic exercise (CAE) and high intensity interval aerobic exercise (hIAE) on endothelial function and arterial stiffness in healthy participants.

METHODS

Twenty healthy men were recruited to this cross-over study. They participated in two exercise sessions: (a) CAE, volume at 50% of maximum aerobic work for 30 minutes; and (b) hIAE, interval maximum aerobic work for 30 minutes. Endothelial function was evaluated by flow-mediated dilation in the brachial artery. The carotid femoral pulse wave velocity and the femoral dorsalis pedis pulse wave velocity were measured as indices of central aortic and peripheral arterial stiffness. Measurements were carried out before and immediately after each exercise session.

RESULTS

There was no statistically significant difference in the baseline measurements before CAE and hIAE with respect to flow-mediated dilation, the carotid femoral pulse wave velocity and the femoral dorsalis pedis pulse wave velocity (p = NS). Both CAE and hIAE significantly improved the flow-mediated dilation compared with baseline (p < 0.001). Similarly, the femoral dorsalis pedis pulse wave velocity was improved after CAE and hIAE (p < 0.005), whereas the carotid femoral pulse wave velocity was not significantly affected (p = NS).

CONCLUSION

Both CAE and hIAE can favourably affect endothelial function, suggesting another cardioprotective effect of acute exercise. These types of aerobic exercise have a different impact on the central and peripheral arterial stiffness.

Type 2 Diabetes: Endothelial dysfunction and Exercise

PURPOSE

Vascular endothelial dysfunction is an early marker of atherosclerosis characterized by decreased nitric oxide bioavailability in the vascular endothelium and smooth muscle cells. Recently, some animal models and in vitro trials demonstrated that excessive superoxide production from mitochondria within vascular endothelial cells played a role in the pathogenesis of atherosclerosis in type 2 diabetes. This review provides a systematic assessment of the effectiveness of exercise to identify effective approaches to recognize diabetes risk and prevent progression to heart disease.

METHODS

A systematic literature search was conducted to retrieve articles from 1979 to 2013 using the following databases: the MEDLINE, PubMed. Articles had to describe an intervention that physical activity and exercise to identify effective approaches to heart and vascular endothelium.

RESULTS

Currently, physical activity and exercise guidelines aimed to improve cardiovascular health in patients with type 2 diabetes are nonspecific. Benefit of aerobic exercise training on vascular endothelial function in type 2 diabetic patients is still controversial.

CONCLUSION

it is necessary to demonstrate the mechanism of endothelial dysfunction from live human tissues so that we can provide more specific exercise training regimens to enhance cardiovascular health in type 2 diabetic patients.

Influence of regular exercise training on post-exercise hemodynamic regulation to orthostatic challenge

To prevent orthostatic hypotension, arterial blood pressure (BP) is neurally and hormonally regulated via increases in heart rate (HR) and peripheral vascular tone. After dynamic exercise, however, the latter arm is blunted because of the increased vasodilators in exercised muscles. Orthostatic tachycardia is likely a more important compensatory mechanism for post-exercise orthostatic intolerance in individuals who have higher leg vasodilator capacity, such as endurance-trained athletes. To test the hypothesis that regular endurance training was associated with the greater augmentation of tachycardia response to post-exercise orthostasis, we compared hemodynamic responses to 5-min 60° head-up tilt (HUT) before and after 60 min of cycling at 70% of HR reserve in the endurance-trained (n = 8) and sedentary men (n = 9). Calf peak vascular conductance was 62% greater in the endurance-trained than the sedentary (P < 0.001). After the exercise, the HUT-induced reduction of SV was significantly augmented in the endurance-trained (from −27.7 ± 6.9 to −33.7 ± 7.7 ml, P = 0.03) but not in their sedentary peers. Nevertheless, MAP was well maintained during post-exercise HUT even in the endurance-trained (from 81 ± 10 to 80 ± 8 mmHg). Tachycardia responses during sustained orthostasis were significantly increased in the sedentary (1.3-fold vs. pre-exercise) and more in the endurance-trained (2.0-fold). The augmented response of HUT-induced tachycardia was greater in the endurance-trained than the sedentary (P = 0.04). Additionally, cardiovagal baroreflex sensitivity (BRS), evaluated by the HR response to the hypotensive perturbation, was improved after the exercise in the endurance-trained (from −0.56 ± 0.32 to −1.03 ± 0.26 bpm/mmHg, P = 0.007) but not in the sedentary. These results suggest that in the endurance-trained men the increased orthostatic tachycardia and augmented cardiovagal BRS may favorably mitigate accumulated risks for orthostatic intolerance in the early phase of post-exercise.

The role of the vessel wall

The role of the vessel wall is complex and its effects are wide-ranging. The vessel wall, specifically the endothelial monolayer that lines the inner lumen, possesses the ability to influence various physiological states both locally and systemically by controlling vascular tone, basement membrane component synthesis, angiogenesis, haemostatic properties, and immunogenicity. This is an overview of the function and structure of the vessel wall and how disruption and dysfunction in any of these regulatory roles can lead to disease states.

The potent relaxant effect of resveratrol in rat corpus cavernosum and its underlying mechanisms

The aim of this study was to evaluate the relaxant effect of resveratrol (RVT), one of the most commonly employed dietary polyphenols, in rat corpus cavernosum, and to further investigate the contribution of possible underlying mechanisms. Strips of corpus cavernosum were used in organ baths for isometric tension studies. RVT (10(-6)-10(-4) M) produced concentration-dependent relaxation responses in rat corpus cavernosum precontracted by phenylephrine. The relaxant responses to RVT partially, but significantly inhibited by removal of endothelium. Nitric oxide (NO) synthase (NOS) blocker N-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) or soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10(-5) M) caused a significant inhibition on relaxation response to RVT, whereas cyclooxygenase inhibitor indomethacin (10(-5) M) did not significantly alter relaxant responses of corpus cavernosum strips to RVT. Corpus cavernosum contractions induced by stepwise addition to Ca2+ to high KCl solution with no Ca2+ were significantly inhibited by RVT incubation. The treatment of corpus cavernosum tissues with non-specific potassium channel inhibitor tetraethylammonium (TEA, 10(-2) M) did also significantly affect the relaxant activity of RVT. Otherwise, the relaxation response of corpus cavernosum induced by the phosphodiesterase-5 inhibitor sildenafil increased significantly in the group pretreated with 10(-5) M RVT. These results demonstrated that RVT has a potent relaxant effect on rat corpus cavernosum via endothelium-dependent and -independent mechanisms. Endothelium-dependent relaxation of corpus cavernosum to RVT is thought to be mediated primarily through NO/cGMP signaling pathway, and possibly an additional mechanism, endothelium-dependent hyperpolarization factor (EDHF). The residual endothelium-independent corpus cavernosum relaxation induced by RVT is uncertain but seems to depend on the interactions of RVT with Ca2+ entry mechanism from the extracellular space and also other undefined direct effects in this tissue.

The contribution of d-tubocurarine-sensitive and apamin-sensitive K-channels to EDHF-mediated relaxation of mesenteric arteries from eNOS-/- mice

The nature of the potassium channels involved in determining endothelium-derived hyperpolarizing factor-mediated relaxation was investigated in first-order small mesenteric arteries from male endothelial nitric oxide synthase (eNOS-/-)-knockout and control (+/+) mice. Acetylcholine-induced endothelium-dependent relaxation of small mesenteric arteries of eNOS-/- was resistant to N-nitro-L-arginine and indomethacin and the guanylyl cyclase inhibitor, 1H-(1,2,4) oxadiazolo (4,3-a) quinoxalin-1-one. Apamin and the combination of apamin and iberiotoxin or apamin and charybdotoxin induced a transient endothelium-dependent contraction of small mesenteric arteries from both eNOS-/- and +/+ mice. Acetylcholine-induced relaxation in eNOS-/- mice was unaffected by charybdotoxin or apamin alone but significantly inhibited by the combination of these agents. However, the combination of scyllatoxin and iberiotoxin did not mimic the inhibitory effect of the apamin/charybdotoxin combination. Tubocurarine alone completely blocked acetylcholine-induced relaxation in eNOS-/- mice. Single channel analysis of myocytes from small mesenteric arterioles revealed a large conductance calcium-activated potassium channel that was sensitive to iberiotoxin, charybdotoxin, and tetraethylammonium. Tubocurarine blocked this channel from the cytosolic side but not when applied extracellularly. Solutions of nitric oxide (NO) gas also relaxed small mesenteric arteries that had been contracted with cirazoline in a concentration-dependent manner, and the sensitivity to NO was reduced by iberiotoxin and the combination of apamin, scyllatoxin, or tubocurarine with charybdotoxin but not by apamin, charybdotoxin, scyllatoxin, or tubocurarine alone. These data indicate that acetylcholine-induced endothelium-derived hyperpolarizing factor-mediated relaxation in small mesenteric arteries from eNOS-/- involved the activation of tubocurarine and apamin-/charybdotoxin-sensitive K-channels. In eNOS+/+ mice, the acetylcholine-induced response was primarily mediated by NO and was sensitive to iberiotoxin and the combination of apamin and charybdotoxin.

Exercise training enhances flow-mediated dilation in spontaneously hypertensive rats

This study investigated the effect of exercise training on the flow-mediated dilation (FMD) in gastrocnemius muscle arteries from spontaneously hypertensive rats (SHR). SHR and WKY rats were divided into sedentary and exercised groups. After swimming exercise for eight weeks, the isolated arteries were mounted on pressurized myograph and FMD responses examined. The role of nitric oxide (NO), prostaglandins (PGs) and endothelium derived hyperpolarizing factor (EDHF) on FMD were assessed by obtaining dilation responses in the presence and absence of pharmacological antagonists. N(omega)-nitro-L-arginine methyl ester (L-NAME), indomethacin (INDO) and tetraethylamonium (TEA) were used to inhibit nitric oxide synthase, cyclooxygenase and EDHF-mediated responses, respectively. The FMD response was significantly blunted in arteries of SHR compared with WKY rats, and, improved by exercise training in SHR (SHR-ET) group. In SHR arteries, L NAME and TEA did not affect dilation responses to flow, while INDO led to a significant enhancement in this response. Although dilation response was not altered by L-NAME in arteries obtained from trained SHR, TEA caused a significant attenuation and INDO led to significant increases. These results demonstrate that exercise training improves FMD in SHR, and, this enhancement induced by exercise training occurs through EDHF-mediated mechanism(s).

Reactive oxygen-derived free radicals are key to the endothelial dysfunction of diabetes

Vascular complications are an important pathological issue in diabetes that lead to the further functional deterioration of several organs. The balance between endothelium-dependent relaxing factors and endothelium-dependent contracting factors (EDCFs) is crucial in controlling local vascular tone and function under normal conditions. Diabetic endothelial dysfunction is characterized by reduced endothelium-dependent relaxations and/or enhanced endothelium-dependent contractions. Elevated levels of oxygen-derived free radicals are the initial source of endothelial dysfunction in diabetes. Oxygen-derived free radicals not only reduce nitric oxide bioavailability, but also facilitate the production and/or action of EDCFs. Thus, the endothelial balance tips towards vasoconstrictor responses over the course of diabetes.

Hydrogen peroxide acts as both vasodilator and vasoconstrictor in the control of perfused mouse mesenteric resistance arteries

BACKGROUND

Hydrogen peroxide (H2O2) plays a key role in the control of resistance artery (RA) tone and is hypothesized as an endothelial-derived hyperpolarizing factor.

METHODS

In this study, we investigated the effects of the same concentration of exogenous H2O2 on mouse mesenteric RA tone induced by intraluminal pressure, G-protein coupled receptor activation and KCl. RAs were cannulated in an arteriograph in the absence or presence of vasoconstrictors.

RESULTS

RA developed myogenic tone (MT) in response to stepwise pressure increases. Under phenylephrine, H2O2 induced a dose-dependent (1-50 micromol/l) vasodilation with maximum dilation at 50 micromol/l. H2O2 at 50 micromol/l induced a full dilation of RA under MT or contraction by phenylephrine that was independent of nitric oxide synthase, cyclooxygenase, endothelium and potassium channels iberiotoxin and apamin sensitive. The Ca2+ channel inhibitor, nimodipine, significantly blocked MT and also the contraction to phenylephrine and KCl. Under these conditions, H2O2 had no effect on RA diameter. Under KCl, the same concentration of H2O2 induced a potent vasoconstriction. This contraction involved p38 mitogen-activated protein-kinase activation but not ERK1/2.

CONCLUSIONS

These findings provide the first evidence showing that the same and low concentrations of H2O2 can act as a relaxing factor but also as a vasoconstrictor under conditions in which hyperpolarization is compromised.

Unfractioned Heparin Produces Vasodilatory Action on Human Internal Mammary Artery by Endothelium-Dependent Mechanisms

The aim of this study was to investigate whether unfractioned heparin produces a direct vasodilatory effect on the human internal mammary artery (IMA) and the possible underlying mechanisms. Samples of redundant IMA were obtained from 20 patients undergoing coronary artery bypass graft surgery, and concentration-response curves to unfractioned heparin were constructed. Unfractioned heparin (0.5-6 U/mL) caused a concentration-dependent relaxation in the endothelium-intact human IMA rings precontracted with phenylephrine (10(-6) M). Removal of endothelium significantly inhibited the responses of human IMA to unfractioned heparin (P < 0.05). Nomega-Nitro-L-arginine methyl ester (L-NAME, 10(-4) M), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10(-5) M) and L-NAME (10(-4) M) plus ODQ (10(-5) M) partially reduced unfractioned heparin-induced vasodilatory response in endothelium-intact rings, whereas indomethacin alone had no effect. The vasodilatory effect of unfractioned heparin was completely inhibited by 40 mM KCl in the presence of L-NAME, ODQ, and indomethacin. These results clearly demonstrated that unfractioned heparin causes a concentration-dependent vasodilatation in human internal mammary artery, and this action seems to be via endothelium-dependent mechanisms, including generation of nitric oxide and endothelium-derived hyperpolarizing factor.

Effects of nitric oxide synthase inhibitor on decrease in peripheral arterial stiffness with acute low-intensity aerobic exercise

We previously reported that even low-intensity, short-duration acute aerobic exercise decreases arterial stiffness. We aimed to test the hypothesis that the exercise-induced decrease in arterial stiffness is caused by the increased production of NO in vascular endothelium with exercise. Nine healthy men (age: ∼22–28 yr) performed a 5-min single-leg cycling exercise (30 W) in the supine position under an intravenous infusion of NG-monomethyl-l-arginine (l-NMMA; 3 mg/kg during the initial 5 min and subsequent continuous infusion of 50 μg·kg−1·min−1 in saline) or vehicle (saline) in random order on separate days. The pulse wave velocity (PWV) from the femoral to posterior tibial artery was measured on both legs before and after the infusion at rest and 2 min after exercise. Under the control condition, exercised leg PWV significantly decreased after exercise ( P < 0.05), whereas nonexercised leg PWV did not show a significant change throughout the experiment. Under l-NMMA administration, exercised leg PWV was increased significantly by the infusion ( P < 0.05) but decreased significantly after the exercise ( P < 0.05). Nonexercised leg PWV increased with l-NMMA administration and maintained a significantly higher level during the administration compared with baseline (before the infusion, all P < 0.05). The NO synthase blockade × time interaction on exercised leg PWV was not significant ( P = 0.706). These results suggest that increased production of NO is not a major factor in the decrease of regional arterial stiffness with low-intensity, short-duration aerobic exercise.

ATP-induced vasodilation in human skeletal muscle

1. The purine nucleotide adenosine-5'-triphosphate (ATP) exerts pronounced effects on the cardiovascular system. The mechanism of action of the vasodilator response to ATP in humans has not been elucidated yet. The proposed endothelium-derived relaxing factors (EDRFs) were studied in a series of experiments, using the perfused forearm technique. 2. Adenosine 5'-triphosphate (0.2, 0.6, 6 and 20 nmol dl(-1) forearm volume min(-1)) evoked a dose-dependent forearm vasodilator response, which could not be inhibited by separate infusion of the nonselective COX inhibitor indomethacin (5 microg dl(-1) min(-1), n=10), the blocker of Na(+)/K(+)-ATPase ouabain (0.2 microg dl(-1) min(-1), n=8), the blocker of K(Ca) channels tetraethylammonium chloride (TEA, 0.1 microg dl(-1) min(-1), n=10), nor by the K(ATP)-channel blocker glibenclamide (2 microg dl(-1) min(-1), n=10). All blockers, except glibenclamide, caused a significant increase in baseline vascular tone. The obtained results might be due to compensatory actions of unblocked EDRFs. Combined infusion of TEA, indomethacin and l-NMMA (n=6) significantly increased the baseline forearm vascular resistance. The ATP-induced relative decreases in forearm vascular resistance were 48+/-5, 67+/-3, 88+/-2, and 92+/-2% in the absence and 23+/-7, 62+/-4, 89+/-2, and 93+/-1% in the presence of the combination of TEA, indomethacin and l-NMMA (P<0.05, repeated-measures ANOVA, n=6). A similar inhibition was obtained for sodium nitroprusside (SNP, P<0.05 repeated-measures ANOVA, n=6), indicating a nonspecific interaction due to the blocker-induced vasoconstriction. 3. ATP-induced vasodilation in the human forearm cannot be inhibited by separate infusion of indomethacin, ouabain, glibenclamide or TEA, or by a combined infusion of TEA, indomethacin, and l-NMMA. Endothelium-independent mechanisms and involvement of unblocked EDRFs, such as CO, might play a role, and call for further studies.

Dietary protein restriction in pregnancy induces hypertension and vascular defects in rat male offspring

It is established that dietary protein restriction of pregnant rats results in their offspring developing hypertension. However, to date no studies have investigated peripheral vascular function of offspring using the low protein model. Therefore, the aim of the study was to assess isolated resistance artery function from adult male offspring of control (C, 18% casein) and protein-restricted (PR, 9% casein) pregnant dams at two different ages. The birthweight of PR offspring did not significantly differ from that of C offspring. Systolic blood pressure was significantly elevated in PR compared with C (p < 0.05). Maximal vascular contraction to phenylephrine and the thromboxane analog U46619 were similar in C and PR offspring at postnatal d 87 and 164. Relaxation induced by the endothelium-dependent vasodilators acetylcholine or bradykinin was significantly reduced in the PR group (p < 0.05). Relaxation to the endothelium-independent vasodilator sodium nitroprusside and phosphodiesterase type 3 inhibitor cilostamide was less in the PR offspring compared with C (p < 0.01). Dietary protein restriction in pregnancy induces hypertension and vascular dysfunction in male offspring. Abnormalities in the nitric oxide-cGMP pathway may explain the defect in endothelium-dependent and -independent relaxation. Reduced vasodilation may be a potential mechanism underlying the elevated systolic blood pressure observed in this model.

Mechanisms underlying the programming of small artery dysfunction: review of the model using low protein diet in pregnancy in the rat

Human and animal studies have shown that unbalanced maternal nutrition is associated with the development of cardiovascular and metabolic disease in adulthood. In the Southampton maternal low protein model (SMLP), protein deprivation (50%) throughout pregnancy in rats leads to elevated blood pressure in adult offspring. Impaired peripheral arterial function may contribute to the cardiovascular dysfunction observed in these offspring. This review discusses the impact of such a dietary insult on the vascular function of resistance arteries from pregnant rats (pF(o)), their offspring (F(1)), the pregnant offspring (pF(1)) and the second generation (F(2)). At each stage, disturbances in endothelium-dependent relaxation were observed, implicating changes in endothelial nitric oxide (NO)-guanylate cyclase (GC) signalling pathway in the vascular adaptations to pregnancy and the programmed effects on offspring.

Pulmonary endothelial dysfunction after cardiopulmonary bypass in neonatal pigs

BACKGROUND

In neonatal pigs cardiopulmonary bypass (CPB) is associated with endothelial dysfunction in isolated large pulmonary arteries. It is, however, of great importance if this functional change extends to the small pulmonary resistance arteries, which are the key regulators of pulmonary flow and pressure. The aim of this study was to assess changes in pulmonary microvascular function after CPB using a clinically relevant pediatric procedure.

METHODS

From three groups of neonatal pigs (CPB-, sham- and control group) pulmonary resistance arteries and systemic resistance arteries (from skeletal muscle) were isolated and mounted as ring preparations in wire myographs. Vessel diameters were less than 500 microm. Concentration-response curves were constructed for norepinephrine (NA), vasopressin (Vp), and the thromboxane A2-analog U46619, while the endothelium-dependent and -independent vasodilator functions were assessed as responses to acetylcholine and nitric oxide (NO).

RESULTS

Maximum pulmonary vasodilator response to acetylcholine was attenuated after CPB compared with sham-operated and control animals (P=0.04). NO-induced relaxation, and contractile responses to NA, Vp, and U46619 were not influenced by CPB. In systemic arteries no changes in contractile or relaxant responses were seen after CPB.

CONCLUSION

CPB seems to induce pulmonary endothelial dysfunction in pulmonary but not peripheral resistance arteries in neonatal piglets.

Altered guanylyl-cyclase activity in vitro of pulmonary arteries from fetal lambs with congenital diaphragmatic hernia

Nitric oxide (NO) plays a major role in the modulation of perinatal pulmonary vascular tone. Congenital diaphragmatic hernia (CDH), a major cause of severe persistent pulmonary hypertension of the newborn (PPHN), is often refractory to inhaled NO. Alterations in NO/cyclic guanosine 3',5' monophosphate (cGMP)-mediated pulmonary vasodilatation may contribute to PPHN in CDH. We assessed NO/cGMP-mediated pulmonary vasorelaxation in vitro in 140-d gestational lamb fetuses with surgically created left CDH (term = 147 d) to age-matched controls. Relaxation of fourth generation intralobar pulmonary artery rings in response to the endothelium-dependent vasodilator, acetylcholine (ACh), and to the specific inhibitor of cGMP-phosphodiesterase (PDE), zaprinast, did not differ between the two groups. By contrast, relaxation in response to the calcium ionophore A23187 was impaired in CDH as compared with control animals. Relaxation in response to the NO donor sodium nitroprusside (SNP) (a direct activator of soluble guanylyl cyclase [sGC]) was also impaired in CDH animals as compared with controls. Repeating the challenge increased vasorelaxation in response to SNP in CDH as compared with control animals. Immunohistochemistry revealed the presence of endothelial NO-synthase in the endothelium of pulmonary arteries from both control and CDH animals. We conclude that endothelium-dependent vasodilatation in response to ACh and A23187 was differently affected in the fetal surgical CDH-lamb model. Furthermore, activity of sGC but not that of PDE was impaired in CDH animals. PPHN and decreased inhaled NO responsiveness in CDH may involve decreased sGC activity.

Role of endothelium-derived relaxing factors in adrenomedullin-induced vasodilation in the rat kidney

The present study aimed to evaluate the contributions of endothelium-derived hyperpolarizing factor (EDHF), the nitric oxide (NO)-cGMP pathway, and prostaglandins to adrenomedullin-induced vasodilation in isolated rat kidney. Inhibition of the NO-cGMP pathway with N(omega)-nitro-L-arginine methyl ester (L-NAME) or 1H-[1,2,4]oxadiazolo-[4,3a]quinoxalin-1-one (ODQ) reduced the maximal vasodilator response to adrenomedullin by approximately 50%. Pretreatment of the vessels with the potassium channel inhibitor, tetraethylammonium or increased extracellular K(+), also decreased the maximal response to adrenomedullin by approximately 50%. The simultaneous administration of blockers of both endothelium-derived relaxing factors had a combined effect that almost suppressed adrenomedullin-induced vasodilation. The administration of indomethacin did not modify the renal response to adrenomedullin. Our results suggest that the vasodilator response to adrenomedullin in the isolated perfused kidney of rats is mediated by EDHF and NO to a similar extent. Our data also provide evidence that prostaglandins play no role in the vasodilator response to adrenomedullin in the renal vasculature.

Effect of nitric oxide synthase inhibitor N(omega) nitro-L-arginine methyl ester on relaxant responses to calcium channel antagonists in isolated aortic rings from Dahl normotensive and hypertensive rats

Differences exist between the pharmacological actions of calcium channel antagonists in blood vessels from hypertensive versus normotensive animals. In this investigation, we have examined the impact of nitric oxide synthase inhibitor N(omega) nitro-L-arginine methyl ester (L-NAME) on relaxant responses produced by the calcium channel antagonists (nifedipine, diltiazem, and mibefradil) in isolated aortic rings from Dahl salt-resistant normotensive (SRN) and salt-sensitive hypertensive (SSH) rats on a 4% salt diet. Morphological examination of the aortic rings revealed significantly larger lumen area, smooth muscle wall thickness, and perimeter in vessels of SSH rats versus SRN rats. Rank order potency for the antagonists was nifedipine > mibefradil > or = diltiazem in aortic rings from SRN rats, but mibefradil was found to be the most efficacious. The rank order potency for the antagonists in aortic rings from SSH rats was nifedipine > diltiazem > mibefradil, although all three drugs showed similar efficacy. The presence of L-NAME attenuated relaxations elicited by the antagonists in aortic rings from SRN rats. Treatment of tissues with L-NAME significantly reduced maximal response and decreased pIC(50). The presence of L-NAME had no effects on concentration-response curves to nifedipine and diltiazem in aortic rings from SSH rats, but it significantly attenuated relaxant responses of mibefradil. Our current results support the view that these calcium channel antagonists produce relaxations by mechanisms that are sensitive and insensitive to L-NAME. Moreover, the component insensitive to L-NAME was lacking in tissues from SSH rats for nifedipine and diltiazem but not mibefradil.

Estrogen replacement therapy in postmenopausal women augments reactive hyperemia in the forearm by reducing angiotensin converting enzyme activity

The precise mechanism of the vasoprotective effect of estrogen replacement therapy in postmenopausal women is not fully understood. The present study sought to determine the role of nitric oxide (NO) and angiotensin-converting enzyme (ACE) in the vasodilator response of the forearm vessels induced by estrogen administration to postmenopausal women. Subjects were divided into two groups. One group received conjugated equine estrogen (0.625 mg daily) orally for 3 months (n=26), while the other received no treatment (control group, n=10). Forearm blood flow was measured by strain-gauge plethysmography. The concentrations of nitrite/nitrate (metabolites of NO), ACE activity, and lipid parameters were measured. Basal forearm blood flow, body weight, blood pressure, and heart rate were similar at baseline in both groups. After 3 months of estrogen administration, the maximal forearm blood flow response during reactive hyperemia and the serum level of nitrite/nitrate each showed a significant increase over baseline values: from 23.6+/-2.0 to 36.5+/-3.1 ml/min per 100 ml tissue (P<0.01), and from 24.8+/-2.3 to 38.6+/-3.6 micromol/l (P<0.01), respectively. Plasma levels of ACE activity were significantly reduced from baseline after 3 months of estrogen treatment (from 12.2+/-0.6 to 10.9+/-0.6 IU/l, P<0.01). No changes were seen in controls. The change in forearm blood flow after sublingual nitroglycerin was similar at baseline versus after 3 months of estrogen administration. The increase in the serum level of nitrite/nitrate after 3 months of estrogen therapy showed a significant inverse correlation (r=0.52, P<0.01) with the reduction in the plasma level of ACE activity. There was no significant correlation between the increase in serum nitrite/nitrate and any change in serum lipids, blood pressure, or other parameters. The administration of oral estrogen to postmenopausal women for 3 months increased the NO-mediated forearm endothelium-dependent vasodilatation. This was likely due, at least in part, to ACE inhibition. The latter may be one mechanism by which ERT provides its well-known cardiovascular benefit.

Effect of 1-week treatment with erythropoietin on the vascular endothelial function in anaesthetized rabbits

Chronic administration of erythropoietin (EPO) is often associated with hypertension in animals and humans. The aim of this study was to estimate whether 1-week treatment with EPO can affect the vascular endothelial function. Rabbits were given with EPO (400 iu kg(-1) s.c.) or saline each other day for 1 week. Hypotensive responses to intravenously given acetylcholine (ACh), endothelium-independent nitric oxide donors (NOC7, nitroprusside and nitroglycerin) and prostaglandin I2 were tested before and after administration of N(G)-nitro-L-arginine methyl ester (L-NAME), a specific nitric oxide synthase inhibitor, under pentobarbitone anaesthesia. Blood haemoglobin concentration in EPO group was significantly higher than that in control group, whereas baseline values of aortic pressure, heart rate and femoral vascular resistance were similar. The dose of ACh (172 ng kg(-1)) requiring for a 15 mmHg hypotension from the baseline in EPO group was apparently higher than that (55 ng kg(-1)) in control group. On the contrary, hypotensive responses to NOC7, nitroprusside, nitroglycerin and prostaglandin I2 were comparable between two groups. The extent of ACh-induced hypotension did not correlate with haemoglobin concentration. L-NAME significantly inhibited the ACh-induced vasodilating response in control group but did not in EPO group. In another set of rabbits, the same treatment with EPO also decreased vasodilating responses to carbachol, bradykinin and substance P besides ACh as compared with control group. These results indicate that 1-week treatment with EPO selectively attenuates depressor responses to endothelium-dependent vasodilators in anaesthetized rabbits, most likely due to inhibition of endothelial nitric oxide synthase.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

Acetylcholine-mediated vasodilation in the forearm circulation of patients with heart failure: indirect evidence for the role of endothelium-derived hyperpolarizing factor

Vasomotor responses to intraarterial administration of acetylcholine are mediated by release of nitric oxide, prostaglandins, and an unidentified hyperpolarizing factor from vascular endothelial cells. The contribution of endothelium-derived hyperpolarizing factor (EDHF) to the vasodilatory response to acetylcholine in the skeletal muscle circulation of patients with congestive heart failure (CHF) has not been previously characterized. Accordingly, to specifically assess the role of EDHF, the regional vascular effects of sequential administration of acetylcholine and nitroglycerin in the brachial artery were determined in the forearm circulation with strain-gauge venous occlusion plethysmography in patients with CHF and in normal subjects during combined systemic inhibition of cyclooxygenase activity with indomethacin and regional inhibition of nitric oxide synthase activity with l-N(G)-monomethylarginine (l-NMMA). After administration of indomethacin, infusion of l-NMMA significantly decreased the forearm blood flow response to acetylcholine in normal subjects (5.4 +/- 1.2 to 3.5 +/- 0.6 ml/min/100 ml, p < 0.05) but not in patients with CHF (5.7 +/- 1.3 to 5.7 +/- 1.4 ml/min/100 ml). Infusion of l-NMMA did not change forearm blood flow responses to nitroglycerin in either group. The presence of a noncyclooxygenase, non-nitric-oxide relaxing factor indicates that EDHF, rather than nitric oxide, may be the predominant endothelium-derived substance mediating vasodilation in response to acetylcholine in patients with CHF.

Vasodilatory effect of basic fibroblast growth factor in isolated rat cerebral arterioles: mechanisms involving nitric oxide and membrane hyperpolarization

Basic fibroblast growth factor (bFGF), a potent mitogen, acutely dilates cerebral blood vessels and may be effective in reducing cerebral infarction. However, the vasodilatory mechanism, which may involve nitric oxide (NO), is not completely understood. This study investigated whether membrane hyperpolarization is also involved in this mechanism. Membrane potential (MP) of smooth muscle cells and vessel diameter of isolated intracerebral arterioles were simultaneously measured following extraluminal application of bFGF in rats. The involvement of NO and adenosine triphosphate-sensitive potassium (KATP) channels in bFGF-induced vasodilation and membrane hyperpolarization was evaluated using specific inhibitors, NG-monomethyl-L-arginine (L-NMMA, 10(-4) M) and glibenclamide (GB, 10(-5) M), respectively. The resting MP was recorded at a mean value of -31.9 +/- 4.5 mV. bFGF (1 to 1000 ng/ml) produced significant vasodilation and hyperpolarization. Treatment with L-NMMA caused vasoconstriction and significantly attenuated bFGF-induced vasodilation without affecting membrane hyperpolarization. In the presence of GB, the membrane potential was significantly depolarized but the vessel diameter was only marginally reduced, so bFGF-induced membrane hyperpolarization was inhibited while arteriolar dilation was attenuated. These results suggest that bFGF-induced vasodilation is mediated by a mechanism involving both NO and membrane hyperpolarization, and that membrane hyperpolarization is caused by the activation of KATP channels.

Effect of the angiotensin-converting enzyme inhibitor imidapril on reactive hyperemia in patients with essential hypertension: relationship between treatment periods and resistance artery endothelial function

OBJECTIVES

The purpose of this study was to evaluate the effects of the angiotensin-converting enzyme (ACE) inhibitor imidapril and the calcium antagonist amlodipine on endothelial function before and after 2, 4, 8, 12, 24 and 48 weeks of treatment.

BACKGROUND

There are limited data on whether and how long endothelial function is improved after initiation of ACE inhibitor treatment and how the grade of endothelial function further progresses after improvement of endothelial dysfunction in patients with essential hypertension.

METHODS

The forearm blood flow (FBF) was measured in 25 patients with essential hypertension and in 25 normotensive subjects by using strain-gauge plethysmography during reactive hyperemia (RH) (280 mm Hg for 5 min) and after sublingual administration of nitroglycerin (NTG, 0.3 mg).

RESULTS

The FBF of patients with essential hypertension during RH was significantly less than that of normotensive subjects. The increase in FBF after sublingual NTG was similar in both groups. Both imidapril (n = 13) and amlodipine (n = 12) significantly reduced systolic blood pressure and diastolic after eight weeks of treatment from the pretreatment values. Forearm vascular resistance was significantly decreased after two weeks of treatment. Imidapril significantly augmented RH after 12 weeks of treatment from the pretreatment values (31.6 +/- 5.7 to 38.2 +/- 6.0 m/min per 100 ml tissue, p < 0.05), whereas amlodipine did not alter RH for each treatment period. The ability of imidapril to improve RH was maintained throughout the 48-week treatment period. There was no significant difference in RH at 12, 24 and 48 weeks. The increase in FBF after sublingual administration of NTG was similar in all treatment periods for the two groups. The infusion of NG-monomethyl-L-arginine, a nitric oxide (NO) synthase inhibitor, abolished the enhancement of RH in hypertensive patients treated with imidapril.

CONCLUSIONS

These findings suggest that the ACE inhibitor imidapril augments RH after 12 weeks of treatment in patients with essential hypertension and that this ACE inhibitor-induced augmentation of RH may be due to an increase in NO.

Endothelial cell protein kinase G inhibits release of EDHF through a PKG-sensitive cation channel

The release of dilator agents from vascular endothelial cells is modulated by changes in cytosolic Ca(2+) concentration ([Ca(2+)](i)). In this study, we demonstrate the presence of a Ca(2+)-permeable cation channel in inside-out membrane patches of endothelial cells isolated from small mesenteric arteries. The activity of the channel is increased by KT-5823, a highly selective inhibitor of protein kinase G (PKG), while it is decreased by direct application of active PKG. Application of KT-5823 induces Ca(2+) influx in the endothelial cells isolated from small mesenteric arteries, and it also causes endothelium-dependent relaxations in isolated small mesenteric arteries. KT-5823-induced relaxations in small mesenteric arteries are greatly reduced by 35 mM K(+) or 50 nM charybdotoxin + 50 nM apamin, suggesting that endothelium-derived hyperpolarizing factor (EDHF) is the participating dilator. The involvement of EDHF is further supported by experiments in which the relaxations of small mesenteric arteries are shown to be accompanied by membrane repolarization. These data strongly argue for a major role of a PKG-sensitive cation channel in modulating the release of EDHF from endothelial cells in rat small mesenteric arteries.

Mechanisms of the increased pressor response to vasopressors in the mesenteric bed of nitric oxide-deficient hypertensive rats

In the present study we analyzed mesenteric vascular reactivity of chronic nitric oxide (NO)-deficient hypertensive rats (NW-nitro-L-Arginine Methyl Ester, L-NAME, 50 mg/kg/day, oral, 3 weeks). Perfusion pressure changes in response to cumulative additions of methoxamine and KCl were significantly increased in the mesenteric vessels of the L-NAME-treated as compared with vessels of the controls. Verapamil reduced the responses to methoxamine, but those of the hypertensive rats were still enhanced. In contrast, responses to KCl were almost completely abolished by verapamil. In mesenteric vessels perfused with zero calcium and high-potassium Krebs, pressor responses to the re-addition of calcium were also significantly enhanced in the hypertensive rats compared to the controls. Vasodilator responses to acetylcholine in KCl-preconstricted vessels, while still significant, were reduced in the L-NAME-treated rats. In this case, acute inhibition of NO blocked the vasodilator responses to acetylcholine and abolished the differences between the two groups. In methoxamine-preconstricted vessels and in the presence of acute inhibition of NO and prostaglandins, vasodilator responses to acetylcholine were significantly greater in the hypertensive vessels than in controls. In conclusion, the mesenteric vessels of L-NAME hypertensive rats show an enhanced response to vasopressors which is related to calcium entry. These data also reveal the existence of an enhanced role of a NO and prostaglandin-independent vasodilator factor, probably endothelium-derived hyperpolarizing factor that may play a compensatory role in the deficiency of NO.

Role of endothelial Ni(2+)-sensitive Ca(2+) entry pathway in regulation of EDHF in porcine coronary artery

Elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in endothelial cells is proposed to be required for generation of vascular actions of endothelium-derived hyperpolarizing factor (EDHF). This study was designed to determine the endothelial Ca(2+) source that is important in development of EDHF-mediated vascular actions. In porcine coronary artery precontracted with U-46619, bradykinin (BK) and cyclopiazonic acid (CPA) caused endothelium-dependent relaxations in the presence of N(G)-nitro-L-arginine (L-NNA). The L-NNA-resistant relaxant responses were inhibited by high K(+), indicating an involvement of EDHF. In the presence of Ni(2+), which inhibits Ca(2+) influx through nonselective cation channels, the BK-induced EDHF relaxant response was greatly diminished and the CPA-induced response was abolished. BK and CPA elicited membrane hyperpolarization of smooth muscle cells of porcine coronary artery. Ni(2+) suppressed the hyperpolarizing responses in a manner analogous to removal of extracellular Ca(2+). EDHF-mediated relaxations and hyperpolarizations evoked by BK and CPA in porcine coronary artery showed a temporal correlation with the increases in [Ca(2+)](i) in porcine aortic endothelial cells. The extracellular Ca(2+)-dependent rises in [Ca(2+)](i) in endothelial cells stimulated with BK and CPA were completely blocked by Ni(2+). These results suggest that Ca(2+) influx into endothelial cells through nonselective cation channels plays a crucial role in the regulation of EDHF.

Bradykinin attenuates the [Ca(2+)](i) response to angiotensin II of renal juxtamedullary efferent arterioles via an EDHF

1. Bradykinin (BK) effect on the [Ca(2+)](i) response to 1 nM angiotensin II was examined in muscular juxtamedullary efferent arterioles (EA) of rat kidney. 2. BK (10 nM) applied during the angiotensin II-stimulated [Ca(2+)](i) increase, induced a [Ca(2+)](i) drop (73+/-2%). This drop was prevented by de-endothelialization and suppressed by HOE 140, a B2 receptor antagonist. It was neither affected by L-NAME or indomethacin, nor mimicked by sodium nitroprusside, 8-bromo-cyclic GMP or PGI(2). The BK effect did not occur when the [Ca(2+)](i) increase was caused by 100 mM KCl-induced membrane depolarization and was abolished by 0.1 microM charybdotoxin, a K(+) channel blocker. 3. Although proadifen prevented the BK-caused [Ca(2+)](i) fall, more selective cytochrome P450 inhibitors, 17-octadecynoic acid (50 microM) and 7-ethoxyresorufin (10 microM) were without effect. 4. Increasing extracellular potassium from 5 to 15 mM during angiotensin II stimulation caused a [Ca(2+)](i) decrease (26+/-4%) smaller than BK which was charybdotoxin-insensitive. Inhibition of inward rectifying K(+) channels by 30 microM BaCl(2) and/or of Na(+)/K(+) ATPase by 1 mM ouabain abolished the [Ca(2+)](i) decrease elicited by potassium but not by BK. 5. A voltage-operated calcium channel blocker, nifedipine (1 microM) did not prevent the BK effect but reduced the [Ca(2+)](i) drop. 6. These results indicate that the BK-induced [Ca(2+)](i) decrease in angiotensin II-stimulated muscular EA is mediated by an EDHF which activates charybdotoxin-sensitive K(+) channels. In these vessels, EDHF seems to be neither a cytochrome P450-derived arachidonic acid metabolite nor K(+) itself. The closure of voltage-operated calcium channels is not the only cellular mechanism involved in this EDHF-mediated [Ca(2+)](i) decrease.

Involvement of potassium channels in the protective effect of 17beta-estradiol on hypercholesterolemic rabbit carotid artery

The involvement of endothelium-derived hyperpolarizing factor (EDHF) in the protective effect of 17beta-estradiol was investigated on the phenylephrine-precontracted carotid artery from cholesterol fed rabbits. Animals were fed for 8 weeks as follows: control group, standard chow; (control+estradiol) group, standard chow+17beta-estradiol; standard chow+1% cholesterol, cholesterol group; or (cholesterol+estradiol) group, 1% cholesterol chow+17beta-estradiol. Relaxations to acetylcholine (ACh) (3 nM-30 microM) were performed with N(omega) nitro-L-arginine methyl ester (300 microM) and indomethacin (10 microM). Charybdotoxin (50 nM)+apamin (50 nM), glibenclamide (10 microM) or 4-aminopyridine (1 mM) were used to block, respectively, calcium-activated-K(+), adenosine triphosphate (ATP)-sensitive-K(+) and voltage-dependent K(+) channels. In the control group, ACh induced a residual concentration-dependent relaxation. This response was impaired by hypercholesterolemia and restored by 17beta-estradiol. In control and cholesterol groups, 4-aminopyridine or glibenclamide did not affect this relaxation, but in (control+estradiol) and (cholesterol+estradiol) groups, glibenclamide suppressed it. In all groups, this persisting relaxation was completely abolished by charybdotoxin alone or with apamin, by hemoglobin (10 microM), a nitric oxide scavenger, or by LY83183 (10 microM), a guanylate cyclase inhibitor. Thus, in the rabbit carotid artery, the protective effect of 17beta-estradiol against hypercholesterolemia is probably mediated by a nitric oxide/cyclic GMP pathway which activates calcium-targeted and ATP-dependent K(+) channels.

Improvement by phosphoramidon of damaged endothelial function in porcine coronary artery

BACKGROUND

The bradykinin (BK)-induced endothelium-dependent relaxation is impaired in the presence of elevated potassium concentration enhancing the vasospastic tendency of large coronary arteries. Inhibition of the angiotensin-converting enzyme responsible for bradykinin degradation was found to enhance the endothelium-dependent relaxation by BK. The aim of the present study was to investigate the effect of phosphoramidon, known to inhibit a BK-metabolizing neutral endopeptidase enzyme, on relaxation of porcine-isolated coronary artery in depolarizing solution.

METHODS

Endothelium intact porcine coronary artery rings were studied in organ chambers. The rings were isometrically contracted with potassium chloride (30 mmol/L) and the response to BK (1 to 1,000 nmol/L)-induced relaxation was investigated in the presence of nitric oxide synthase inhibitor Nomega-nitro-L-arginine (300 micromol/L) alone and in combination with the cyclooxygenase inhibitor indomethacin (10 micromol/L), and that of the inhibitor of calcium-dependent potassium channels tetraethylammonium (7 mmol/L). Under these conditions, phosphoramidon (10 micromol/L), an inhibitor of a neutral endopeptidase enzyme (EC.3.4.24.11.), which is responsible for the degradation of BK, was used to enhance the endothelium-dependent relaxation.

RESULTS

Phosphoramidon potentiated the maximum vasorelaxant effect of BK in Nomega-nitro-L-arginine (control 26.6%+/-10.86% versus phosphoramidon 49.05%+/-4.52%; n = 6, p < 0.05) or in Nomega-nitro-L-arginine + indomethacin-pretreated rings (control 20.7%+/-9.92% versus phosphoramidon 42.0%+/-12.26%; n = 5, p < 0.05) and this increased vasodilation was not modified by tetraethylammonium.

CONCLUSIONS

In the present study phosphoramidon potentiated the effect of BK in the absence of nitric oxide and prostaglandins in porcine-isolated coronary artery. This effect did not depend on tetraethylammonium-sensitive potassium channels. Phosphoramidon may be a useful pharmacologic tool for preserving the vasorelaxing capacity of coronary arteries after cardioplegia.

Comparison of the pharmacological properties of EDHF-mediated vasorelaxation in guinea-pig cerebral and mesenteric resistance vessels

In the presence of L-NNA (100 microM), indomethacin (10 microM) and ODQ (10 microM), acetylcholine induced a concentration-dependent vasorelaxation of guinea-pig mesenteric and middle cerebral arteries precontracted with cirazoline or histamine, but not with high K(+), indicating the contribution of an endothelium-derived hyperpolarizing factor (EDHF). In cerebral arteries, charybdotoxin (ChTX; 0.1 microM) completely inhibited the indomethacin, L-NNA and ODQ-insensitive relaxation; iberiotoxin (IbTX, 0.1 microM), 4-aminopyridine (4-AP, 1 mM), or barium (30 microM) significantly reduced the response; in the mesenteric artery, ChTX and IbTX also reduced this relaxation. Glibenclamide (10 microM) had no affect in either the mesenteric or cerebral artery. Neither clotrimazole (1 microM) nor 7-ethoxyresorufin (3 microM) affected EDHF-mediated relaxation in the mesenteric artery, but abolished or attenuated EDHF-mediated relaxations in the cerebral artery. AM404 (30 microM), a selective anandamide transport inhibitor, did not affect the vasorelaxation response to acetylcholine in the cerebral artery, but in the mesenteric artery potentiated the vasorelaxation response to acetylcholine in an IbTX, and apamin-sensitive, but SR 141816A-insensitive manner. Ouabain (100 microM) almost abolished EDHF-mediated relaxation in the mesenteric artery, but enhanced the relaxation in the cerebral artery whereas the addition of K(+) (5 - 20 mM) to precontracted guinea-pig cerebral or mesenteric artery induced further vasoconstriction. These data suggest that in the guinea-pig mesenteric and cerebral arteries different EDHFs mediate acetylcholine-induced relaxation, however, EDHF is unlikely to be mediated by K(+).

In eNOS knockout mice skeletal muscle arteriolar dilation to acetylcholine is mediated by EDHF

The mechanisms that account for acetylcholine (ACh)-induced responses of skeletal muscle arterioles of mice lacking endothelial nitric oxide (NO) synthase (eNOS-KO) were investigated. Isolated, cannulated, and pressurized arterioles of gracilis muscle from male eNOS-KO (74.1 +/- 2.3 microm) and wild-type (WT, 87.2 +/- 2.1 microm) mice developed spontaneous tone accounting for 63 and 61% of their passive diameter (116.8 +/- 3.4 vs. 143.2 +/- 2.8 microm, respectively) and dilated dose-dependently to ACh (10(-9)-10(-7) M). These dilations were significantly smaller in vessels of eNOS-KO compared with WT mice (29.2 +/- 2.0 microm vs. 46.3 +/- 2.1 microm, at maximum concentration) but responses to the NO donor, sodium nitrite (NaNO(2), 10(-6)-3 x 10(-5) M), were comparable in the vessels of the two strains. N(G)-nitro-L-arginine (L-NNA, 10(-4) M), an inhibitor of eNOS, inhibited ACh-induced dilations by 60-90% in arterioles of WT mice but did not affect responses in those of eNOS-KO mice. In arterioles of eNOS-KO mice, dilations to ACh were not affected by indomethacin but were essentially abolished by inhibitors of cytochrome P-450, clotrimazole (CTZ, 2 x 10(-6) M) or miconazole (MCZ, 2 x 10(-6) M), as well as by either high K(+) (40 mM) or iberiotoxin [10(-7) M, a blocker of Ca(2+)-dependent K(+) channels (K(Ca) channels)]. On the other hand, in WT arterioles CTZ or MCZ inhibited ACh-induced dilations only by approximately 10% and only in the presence of L-NNA. These results indicate that in arterioles of eNOS-KO mice, endothelium-derived hyperpolarizing factor (EDHF), synthesized via cytochrome P-450, accounts entirely for the mediation of ACh-induced dilation via an increase in K(Ca)-channel activity. In contrast, in arterioles of WT mice, endothelium-derived NO predominantly mediates ACh-induced dilation in which participation of EDHF becomes apparent only after inhibition of NO synthesis.

High K(+)-induced membrane depolarization attenuates endothelium-dependent pulmonary vasodilation

Impairment of endothelium-dependent pulmonary vasodilation has been implicated in the development of pulmonary hypertension. Pulmonary vascular smooth muscle cells and endothelial cells communicate electrically through gap junctions; thus, membrane depolarization in smooth muscle cells would depolarize endothelial cells. In this study, we examined the effect of prolonged membrane depolarization induced by high K(+) on the endothelium-dependent pulmonary vasodilation. Isometric tension was measured in isolated pulmonary arteries (PA) from Sprague-Dawley rats, and membrane potential was measured in single PA smooth muscle cells. Increase in extracellular K(+) concentration from 4.7 to 25 mM significantly depolarized PA smooth muscle cells. The 25 mM K(+)-mediated depolarization was characterized by an initial transient depolarization (5-15 s) followed by a sustained depolarization that could last for up to 3 h. In endothelium-intact PA rings, ACh (2 microM), levcromakalim (10 microM), and nitroprusside (10 microM) reversibly inhibited the 25 mM K(+)-mediated contraction. Functional removal of endothelium abolished the ACh-mediated relaxation but had no effect on the levcromakalim- or the nitroprusside-mediated pulmonary vasodilation. Prolonged ( approximately 3 h) membrane depolarization by 25 mM K(+) significantly inhibited the ACh-mediated PA relaxation (-55 +/- 4 vs. -29 +/- 2%, P < 0.001), negligibly affected the levcromakalim-mediated pulmonary vasodilation (-92 +/- 4 vs. -95 +/- 5%), and slightly but significantly increased the nitroprusside-mediated PA relaxation (-80 +/- 2 vs. 90 +/- 3%, P < 0. 05). These data indicate that membrane depolarization by prolonged exposure to high K(+) concentration selectively inhibited endothelium-dependent pulmonary vasodilation, suggesting that membrane depolarization plays a role in the impairment of pulmonary endothelial function in pulmonary hypertension.

Coronary artery blood flow: physiologic and pathophysiologic regulation

Acute myocardial ischemia, which results from a significant imbalance between myocardial oxygen demands and myocardial oxygen supply, occurs in as many as six million persons with atherosclerotic coronary artery disease in the United States. Accordingly, a clear understanding of the physiologic and pathophysiologic factors that influence coronary artery blood flow is important to the clinician and provides the basis for the judicious use of medications for the treatment of patients with atherosclerotic coronary artery disease. This review discusses the endothelial, metabolic, myogenic, and neurohumoral mechanisms of coronary blood flow regulation and the interaction of the different mechanisms in the regulation of coronary blood flow. The importance of nitric oxide in coronary blood flow regulation is emphasized. We also discuss the common clinical problems of hyperlipidemia and coronary atherosclerosis, coronary artery spasm, and systemic arterial hypertension that result in coronary artery endothelial dysfunction, the impaired production and increased inactivation of nitric oxide, and impairment in coronary blood flow regulation. This information is important to clinicians because more than forty million people in the United States have atherosclerotic or hypertensive heart disease and therefore are at risk for significant myocardial complications due to impairment of coronary blood flow regulation.

Reactive oxygen species: role in the relaxation induced by bradykinin or arachidonic acid via EDHF in isolated porcine coronary arteries

Although endothelium-derived hyperpolarizing factor (EDHF) is thought to be a cytochrome P-450 product (arachidonic acid metabolite) in some tissues, in porcine coronary arteries (PCAs) its nature remains unclear. Because phospholipase A2 and C are involved in the synthesis and/or release of EDHF in the PCA, the arachidonic acid (AA) pathway may be involved. In the presence of the cyclooxygenase inhibitor indomethacin (10(-5) M) and the NOS inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME; 10(-4) M), both bradykinin (BK; 10(-9)-10(-6) M) and AA (10(-7)-10(-4) M) induced dose-dependent relaxation of PGF2alpha-contracted PCA rings, which was blocked by a high extracellular concentration of KCl (30 mM) or pretreatment with ouabain, a Na+/K+-adenosine triphosphatase (ATPase) inhibitor (5 x 10(-7) M). Eicosatetraynoic acid (ETYA; 20 microM), which inhibits all AA pathways, slightly affected the response to BK and AA; however, lipoxygenase or cytochrome P-450 inhibitors had no effect, suggesting that relaxation is independent of these enzymatic pathways. Because endothelial cells can generate reactive oxygen species (ROS) via metabolism of AA and independent of cyclooxygenase activity, we also studied (a) whether ROS can relax the PCA, as well as the mechanism(s) involved, and (b) the role of ROS in BK- and AA-induced relaxation. Xanthine (X; 100 microM) plus xanthine oxidase (XO; 0.02 U/ml) induced time-dependent relaxation of PGF2alpha-contracted PCA rings in the presence of indomethacin and L-NAME. Dilatation was not affected by superoxide dismutase (SOD; 500 U/ml) but was abolished by catalase (300 U/ml), suggesting that hydrogen peroxide (H2O2) is involved. When rings were contracted by depolarizing them with 30 mM KCl, X/XO failed to elicit relaxation. Ouabain abolished the response to X/XO, suggesting that X/XO may induce relaxation by hyperpolarizing vascular smooth muscle cells via stimulation of the Na+/K+-ATPase pump. We therefore questioned whether ROS might be involved in BK- and AA-induced relaxation. Because catalase combined with SOD had little or no effect, we concluded that in the PCA, the relaxation induced by BK via EDHF involves some mechanism independent of NO, AA metabolism, or ROS.

Characterization of nitric oxide- and prostaglandin-independent relaxation in response to acetylcholine in rabbit renal artery

1. We investigated the characterization of acetylcholine (ACh)-induced NG-nitro-L-arginine methyl ester (L-NAME)- and indomethacin (IND)-resistant relaxations, which can be mediated by endothelium-derived hyperpolarizing factor (EDHF), in rabbit renal arterial rings. 2. The relaxations were inhibited by SKF 525A, a cytochrome P450 inhibitor, but were not affected by other inhibitors, namely clotrimazole, 17-octadecynoic acid and alpha-naphthoflavone. Furthermore, 11,12-epoxyeicosatrienoic acid, a cytochrome P450 metabolite, did not relax arterial rings. 3. Arterial relaxations were significantly attenuated by charybdotoxin and iberiotoxin, but not by apamin, all K+ channel blockers. 4. In a sandwich bioassay experiment, ACh-induced L-NAME- and IND-resistant relaxations were not transferred to the detector site. 5. Relaxations were also significantly attenuated by 1-heptanol and 18 alpha-glycyrrhetinic acid, gap junctional coupling inhibitors. 6. These results indicate that, in the rabbit renal artery, L-NAME- and IND-resistant relaxations are mediated by factors other than cytochrome P450-derived arachidonic acid metabolites, which may be able to diffuse into the lumen but be partly transferred via myoendothelial gap junctions to adjacent vascular smooth muscle cells and relax muscles by opening high-conductance Ca(2+)-activated K+ channels.

NO mediates postjunctional inhibitory effect of neurogenic ACh in guinea pig small intestinal microcirculation

The present study was designed to evaluate the role of the endothelium as an effector organ of neurally mediated inhibition of vascular tone. Acetylcholine (ACh), either released by stimulation of the submucosal ganglia or applied exogenously, inhibited phenylephrine (PE)-induced constrictions in arterioles of the guinea pig intestinal submucosa. N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) synthesis, attenuated the response to superfused ACh by 74% compared with 94% attenuation obtained with N(G)-nitro-L-arginine (L-NNA). L-NNA attenuated the response to neurally released ACh by 98% and that to iontophoretically applied ACh by 92%. L-Arginine reversed the effects of both L-NMMA and L-NNA. Functional integrity of the endothelium was essential for the neurally mediated inhibition of PE-induced constrictions. However, neurogenic inhibition of neurally evoked constrictions was preserved despite endothelial disruption. It was concluded that at the postjunctional level, the mechanism of action of neurally released ACh was almost exclusively via a NO-dependent pathway, with the source of NO being the vascular endothelium.

Important role of endothelium-derived hyperpolarizing factor in shear stress--induced endothelium-dependent relaxations in the rat mesenteric artery

Shear stress is one of the most important stimulators for the release of endothelium-derived relaxing factors. Although shear stress-induced release of nitric oxide (NO) has been extensively investigated, it remains to be elucidated whether endothelium-derived hyperpolarizing factor (EDHF) contributes to the endothelium-dependent relaxations to shear stress. This study was designed to address this point in the isolated rat mesenteric artery. Large mesenteric arteries (400-500 microm) and resistance mesenteric arteries (150-250 microm) of the rat were precontracted with phenylephrine (at 80 mm Hg of perfusion pressure), and the changes in vessel diameter in response to variable flow (0-300 microl/min) were continuously examined. The relative contributions of vasodilator prostaglandins, NO, and EDHF were analyzed by the inhibitory effects of indomethacin (10(-5) M), N(G)-nitro-L-arginine (L-NNA, 10(-4) M), and KCl (40 mM), respectively. The shear stress-induced relaxations were totally endothelium dependent in both-sized blood vessels, and the contribution of NO was more prominent in large arteries than in resistance arteries, whereas that of EDHF was noted in both-sized blood vessels. Tetrabutylammonium (a nonselective inhibitor of K channels) almost abolished, whereas the combination of charybdotoxin (an inhibitor of both large- and intermediate-conductance Ca2+ -activated K channels) and apamin (an inhibitor of small-conductance Ca2+ -activated K channels) significantly inhibited the EDHF-mediated component of the shear stress-induced relaxations. These results indicate that EDHF plays an important role in shear stress-induced endothelium-dependent relaxations, where K channels, especially calcium-activated K channels, appear to be involved.

Inhibition of endothelium-dependent vascular relaxation by lysophosphatidylcholine: impact of lysophosphatidylcholine on mechanisms involving endothelium-derived nitric oxide and endothelium derived hyperpolarizing factor

Hyperlipidemia has been associated with an increase in the incidence of atherosclerosis. The oxidation of low density lipoprotein (LDL) plays an important role in the initiation and progression of atherosclerosis, one of its effects being the inhibition of endothelium dependent relaxation (EDR). The elevated level of lysophosphatidylcholine (LPC) in oxidatively modified LDL has been shown to be a biochemical factor responsible for the impairment of EDR in vascular ring preparations. Several endothelium-derived modulators are thought to control vascular responsiveness. The present work examined whether acetylcholine (ACh)-induced EDR in rat aorta (pre-contracted with phenylephrine, PE) involved both endothelium-derived nitric oxide (EDNO) and endothelium-dependent hyperpolarizing factor (EDHF) and whether LPC inhibited either of these selectively. Indomethacin (10(-5) M), had no significant effect on EDR, indicating that products of cyclooxygenase, including prostacyclin, are not involved. Treatment with either N(W)-nitro-L-arginine methyl ester (L-NAME, 6.8 microM) to inhibit the production of EDNO or with elevated K+ (15 mM), to block the hyperpolarizing effect of EDHF impaired EDR considerably (each of these shifting the inhibitory dose-response relationship to ACh by almost one log unit); in muscles treated with both of these agents EDR was completely inhibited. In each of L-NAME- and K-treated muscles, the addition of LPC (20 microM) further impaired EDR. LPC did not independently raise the tone of resting- or PE-contracted aorta. We conclude that the inhibition of EDR of rat aorta by LPC involves the actions of both EDNO and EDHF.