Role of endothelin-1 in the hyper-responsiveness to nitrovasodilators following acute NOS inhibition

Perinatal iron deficiency combined with a high salt diet in adulthood causes sex-dependent vascular dysfunction in rats

KEY POINTS

Perinatal iron deficiency causes changes in offspring mesenteric artery function in adulthood, particularly in males, which can be exacerbated by chronic intake of a high salt diet. Perinatal iron deficient male offspring exhibit enhanced conversion of big endothelin-1 to active endothelin-1, coinciding with decreased nitric oxide levels. Perinatal iron deficient male offspring have reduced nitric oxide-mediated endothelial-dependent vasodilatation coincident with increased vascular superoxide levels following consumption of a high salt diet. Perinatal iron deficiency has no apparent effects on vascular function in female offspring, even when fed a high salt diet. These results help us better understand underlying vascular mechanisms contributing to increased cardiovascular risk from perinatal stressors such as iron deficiency.

ABSTRACT

Pre- and immediate postnatal stressors, such as iron deficiency, can alter developmental trajectories and predispose offspring to long-term cardiovascular dysfunction. Here, we investigated the impact of perinatal iron deficiency on vascular function in the adult offspring, and whether these long-term effects were exacerbated by prolonged consumption of a high salt diet in adulthood. Female Sprague Dawley rats were fed either an iron-restricted or -replete diet prior to and throughout pregnancy. Six weeks prior to experimentation at 6 months of age, adult offspring were fed either a normal or high salt diet. Mesenteric artery responses to vasodilators and vasoconstrictors were assessed ex vivo by wire myography. Male perinatal iron deficient offspring exhibited decreased reliance on nitric oxide with methacholine-induced vasodilatation (interaction P = 0.03), coincident with increased superoxide levels when fed the high salt diet (P = 0.01). Male perinatal iron deficient offspring exhibit enhanced big endothelin-1 conversion to active endothelin-1 (P = 0.02) concomitant with decreased nitric oxide levels (P = 0.005). Female offspring vascular function was unaffected by perinatal iron deficiency, albeit the high salt diet was associated with impaired vasodilation and decreased nitric oxide production (P = 0.02), particularly in the perinatal iron deficient offspring. These findings implicate vascular dysfunction in the sex-specific programming of cardiovascular dysfunction in the offspring by perinatal iron deficiency.

Effects of living at moderate altitude on pulmonary vascular function and exercise capacity in mice with sickle cell anaemia

KEY POINTS

Sickle cell disease (SCD) results in cardiopulmonary dysfunction, which may be exacerbated by prolonged exposure to environmental hypoxia. It is currently unknown whether exposure to mild and moderate altitude exacerbates SCD associated cardiopulmonary and systemic complications. Three months of exposure to mild (1609 m) and moderate (2438 m) altitude increased rates of haemolysis and right ventricular systolic pressures in mice with SCD compared to healthy wild-type cohorts and SCD mice at sea level. The haemodynamic changes in SCD mice that had lived at mild and moderate altitude were accompanied by changes in the balance between pulmonary vascular endothelial nitric oxide synthase and endothelin receptor expression and impaired exercise tolerance. These data demonstrate that chronic altitude exposure exacerbates the complications associated with SCD and provides pertinent information for the clinical counselling of SCD patients.

ABSTRACT

Exposure to high altitude worsens symptoms and crises in patients with sickle cell disease (SCD). However, it remains unclear whether prolonged exposure to low barometric pressures exacerbates SCD aetiologies or impairs quality of life. We tested the hypothesis that, relative to wild-type (WT) mice, Berkley sickle cell mice (BERK-SS) residing at sea level, mild (1609 m) and moderate (2438 m) altitude would have a higher rate of haemolysis, impaired cardiac function and reduced exercise tolerance, and that the level of altitude would worsen these decrements. Following 3 months of altitude exposure, right ventricular systolic pressure was measured (solid-state transducer). In addition, the adaptive balance between pulmonary vascular endothelial nitric oxide synthase and endothelin was assessed in lung tissue to determine differences in pulmonary vascular adaptation and the speed/duration relationship (critical speed) was used to evaluate treadmill exercise tolerance. At all altitudes, BERK-SS mice had a significantly lower percentage haemocrit and higher total bilirubin and free haemoglobin concentration (P < 0.05 for all). right ventricular systolic pressures in BERK-SS were higher than WT at moderate altitude and also compared to BERK-SS at sea level (P < 0.05, for both). Critical speed was significantly lower in BERK-SS at mild and moderate altitude (P < 0.05). BERK-SS demonstrated exacerbated SCD complications and reduced exercise capacity associated with an increase in altitude. These results suggest that exposure to mild and moderate altitude enhances the progression of SCD in BERK-SS mice compared to healthy WT cohorts and BERK-SS mice at sea level and provides crucial information for the clinical counselling of SCD patients.

Endothelin-1 Regulation of Exercise-Induced Changes in Flow: Dynamic Regulation of Vascular Tone

Although endothelin (ET)-1 is a highly potent vasoconstrictor with considerable efficacy in numerous vascular beds, the role of endogenous ET-1 in the regulation of vascular tone remains unclear. The perspective that ET-1 plays little role in the on-going regulation of vascular tone at least under physiologic conditions is supported by findings that potential ET-1 constriction is minimized by the release of the vasodilator and ET-1 synthesis inhibitor, nitric oxide (NO). Indeed, ET-1 release and constriction is self-limited by ET-1-induced, endothelial ET_B receptor-mediated release of NO. Moreover, even if the balance between ET-1 and NO were reversed as the result of lowered NO activity, as occurs in a number of pathophysiologies associated with endothelial dysfunction, the well-known resistance of ET-1 constriction to reversal (as determined with exogenous ET-1) precludes ET-1 in the dynamic, i.e., moment-to-moment, regulation of vascular tone. On the other hand, and as presently reviewed, findings of ET-1-dependent modulation of organ blood flow with exercise under physiologic conditions demonstrate the dynamic regulation of vascular tone by ET-1. We speculate that this regulation is mediated at least in part through changes in ET-1 synthesis/release caused by pulsatile flow-induced shear stress and NO.

Perinatal Resveratrol Supplementation to Spontaneously Hypertensive Rat Dams Mitigates the Development of Hypertension in Adult Offspring

This study was undertaken to determine whether perinatal maternal resveratrol (Resv)--a phytoalexin known to confer cardiovascular protection--could prevent the development of hypertension and improve vascular function in adult spontaneously hypertensive rat offspring. Dams were fed either a control or Resv-supplemented diet (4 g/kg diet) from gestational day 0.5 until postnatal day 21. Indwelling catheters were used to assess blood pressure and vascular function in vivo; wire myography was used to assess vascular reactivity ex vivo. Perinatal Resv supplementation in dams had no effect on fetal body weights, albeit continued maternal treatment postnatally resulted in growth restriction in offspring by postnatal day 21; growth restriction was no longer evident after 5 weeks of age. Maternal perinatal Resv supplementation prevented the onset of hypertension in adult offspring (-18 mm Hg; P=0.007), and nitric oxide synthase inhibition (with L-NG-nitroarginine methyl ester) normalized these blood pressure differences, suggesting improved nitric oxide bioavailability underlies the hemodynamic alterations in the Resv-treated offspring. In vivo and ex vivo, vascular responses to methylcholine were not different between treatment groups, but prior treatment with L-NG-nitroarginine methyl ester attenuated the vasodilation in untreated, but not Resv-treated adult offspring, suggesting a shift toward nitric oxide-independent vascular control mechanisms in the treated group. Finally, bioconversion of the inactive precursor big endothelin-1 to active endothelin-1 in isolated mesenteric arteries was reduced in Resv-treated offspring (-28%; P<0.05), and this difference could be normalized by L-NG-nitroarginine methyl ester treatment. In conclusion, perinatal maternal Resv supplementation mitigated the development of hypertension and causes persistent alterations in vascular responsiveness in spontaneously hypertensive rats.

No redistribution of lung blood flow by inhaled nitric oxide in endotoxemic piglets pretreated with an endothelin receptor antagonist

Inhaled nitric oxide (INO) improves ventilation-perfusion matching and alleviates pulmonary hypertension in patients with acute respiratory distress syndrome. However, outcome has not yet been shown to improve, and nonresponse is common. A better understanding of the mechanisms by which INO acts may guide in improving treatment with INO in patients with severe respiratory failure. We hypothesized that INO may act not only by vasodilation in ventilated lung regions, but also by causing vasoconstriction via endothelin (ET-1) in atelectatic, nonventilated lung regions. This was studied in 30 anesthetized, mechanically ventilated piglets. The fall in oxygenation and rise in pulmonary artery pressure during a sepsislike condition (infusion of endotoxin) were blunted by INO 40 ppm. Endotoxin infusion increased serum ET-1, and INO almost doubled the ratio between mRNA expression of endothelin receptor A (mediating vasoconstriction) and B (mediating vasodilation and clearance of ET-1) (ET-A/ET-B) in atelectatic lung regions. INO caused a shift in blood flow away from atelectatic lung regions in the endotoxemic piglets, but not during ET receptor antagonism. We conclude that INO in short-term experiments, in addition to causing selective pulmonary vasodilation in ventilated lung regions, increases the ET-A/ET-B mRNA expression ratio in lung tissue. This might augment the vasoconstriction in atelectatic lung regions, enhancing the redistribution of pulmonary blood flow to ventilated lung regions which are reached by INO. Such vasoconstriction may be an important additional factor explaining the effect of INO.

Acute nitric oxide synthase inhibition and endothelin-1-dependent arterial pressure elevation

Key evidence that endogenous nitric oxide (NO) inhibits the continuous, endothelin (ET)-1-mediated drive to elevate arterial pressure includes demonstrations that ET-1 mediates a significant component of the pressure elevated by acute exposure to NO synthase (NOS) inhibitors. This review examines the characteristics of this pressure elevation in order to elucidate potential mechanisms associated with the negative regulation of ET-1 by NO and, thereby, provide potential insight into the vascular pathophysiology underlying NO dysregulation. We surmise that the magnitude of the ET-1-dependent component of the NOS inhibitor-elevated pressure is (1) independent of underlying arterial pressure and other pressor pathways activated by the NOS inhibitors and (2) dependent on relatively higher NOS inhibitor dose, release of stored and de novo synthesized ET-1, and ET_A receptor-mediated increased vascular resistance. Major implications of these conclusions include: (1) the marked variation of the ET-1-dependent component, i.e., from 0 to 100% of the pressure elevation, reflects the NO-ET-1 regulatory pathway. Thus, NOS inhibitor-mediated, ET-1-dependent pressure elevation in vascular pathophysiologies is an indicator of the level of compromised/enhanced function of this pathway; (2) NO is a more potent inhibitor of ET-1-mediated elevated arterial pressure than other pressor pathways, due in part to inhibition of intravascular pressure-independent release of ET-1. Thus, the ET-1-dependent component of pressure elevation in vascular pathophysiologies associated with NO dysregulation is of greater magnitude at higher levels of compromised NO.

Inflammation in rat pregnancy inhibits spiral artery remodeling leading to fetal growth restriction and features of preeclampsia

Abnormal maternal inflammation leads to TNF-mediated fetal growth restriction and some features of preeclampsia that can be ameliorated with the nitric oxide mimetic nitroglycerin.

Fetal growth restriction (FGR) and preeclampsia (PE) are often associated with abnormal maternal inflammation, deficient spiral artery (SA) remodeling, and altered uteroplacental perfusion. Here, we provide evidence of a novel mechanistic link between abnormal maternal inflammation and the development of FGR with features of PE. Using a model in which pregnant rats are administered low-dose lipopolysaccharide (LPS) on gestational days 13.5–16.5, we show that abnormal inflammation resulted in FGR mediated by tumor necrosis factor-α (TNF). Inflammation was also associated with deficient trophoblast invasion and SA remodeling, as well as with altered uteroplacental hemodynamics and placental nitrosative stress. Moreover, inflammation increased maternal mean arterial pressure (MAP) and was associated with renal structural alterations and proteinuria characteristic of PE. Finally, transdermal administration of the nitric oxide (NO) mimetic glyceryl trinitrate prevented altered uteroplacental perfusion, LPS-induced inflammation, placental nitrosative stress, renal structural and functional alterations, increase in MAP, and FGR. These findings demonstrate that maternal inflammation can lead to severe pregnancy complications via a mechanism that involves increased maternal levels of TNF. Our study provides a rationale for the use of antiinflammatory agents or NO-mimetics in the treatment and/or prevention of inflammation-associated pregnancy complications.

Direct sGC activation bypasses NO scavenging reactions of intravascular free oxy-hemoglobin and limits vasoconstriction

AIMS

Hemoglobin-based oxygen carriers (HBOC) provide a potential alternative to red blood cell (RBC) transfusion. Their clinical application has been limited by adverse effects, in large part thought to be mediated by the intravascular scavenging of the vasodilator nitric oxide (NO) by cell-free plasma oxy-hemoglobin. Free hemoglobin may also cause endothelial dysfunction and platelet activation in hemolytic diseases and after transfusion of aged stored RBCs. The new soluble guanylate cyclase (sGC) stimulator Bay 41-8543 and sGC activator Bay 60-2770 directly modulate sGC, independent of NO bioavailability, providing a potential therapeutic mechanism to bypass hemoglobin-mediated NO inactivation.

RESULTS

Infusions of human hemoglobin solutions and the HBOC Oxyglobin into rats produced a severe hypertensive response, even at low plasma heme concentrations approaching 10 μM. These reactions were only observed for ferrous oxy-hemoglobin and not analogs that do not rapidly scavenge NO. Infusions of L-NG-Nitroarginine methyl ester (L-NAME), a competitive NO synthase inhibitor, after hemoglobin infusion did not produce additive vasoconstriction, suggesting that vasoconstriction is related to scavenging of vascular NO. Open-chest hemodynamic studies confirmed that hypertension occurred secondary to direct effects on increasing vascular resistance, with limited negative cardiac inotropic effects. Intravascular hemoglobin reduced the vasodilatory potency of sodium nitroprusside (SNP) and sildenafil, but had no effect on vasodilatation by direct NO-independent activation of sGC by BAY 41-8543 and BAY 60-2770.

INNOVATION AND CONCLUSION

These data suggest that both sGC stimulators and sGC activators could be used to restore cyclic guanosine monophosphate-dependent vasodilation in conditions where cell-free plasma hemoglobin is sufficient to inhibit endogenous NO signaling.

Prenatal Hypoxia Causes Long-Term Alterations in Vascular Endothelin-1 Function in Aged Male, but Not Female, Offspring

Prenatal hypoxia can alter the growth trajectory of the fetus and cause lasting health complications including vascular dysfunction. We hypothesized that offspring that were intrauterine growth restricted (IUGR) because of prenatal hypoxia would exhibit altered vascular endothelin-1 (ET-1) signaling in later life. Isolated mesenteric artery responses to big ET-1 (bET-1) and ET-1 were assessed by using wire myography. Male IUGR offspring had 3-fold greater bET-1–induced vasoconstriction compared with controls (n=7 per group; P <0.001); NO synthase inhibition with L-N G -nitro-arginine-methyl ester potentiated bET-1–induced vasoconstriction, albeit this effect was 2-fold greater ( P <0.05) in male control compared with IUGR offspring. Vascular responses to bET-1 were similar between female IUGR and control offspring (n=9–11 per group). In the presence of L-N G -nitro-arginine-methyl ester, pretreatment with the chymase inhibitor chymostatin, the gelatinase inhibitor GM6001, or the neutral endopeptidase inhibitor thiorphan did not alter responses to bET-1; however, the ET-converting enzyme inhibitor CGS35066 almost completely abolished vascular responses to bET-1 in control and IUGR groups. Systolic blood pressure in IUGR male offspring was more responsive to ET-1 antagonism in vivo compared with controls (−9 versus −4 mm Hg; n=5 per group; P =0.02); no such differences were observed in female offspring (n=5–6 per group). These results demonstrate that vascular ET-1 function is programmed by prenatal hypoxia and provide further insights into the sex differences in the long-term vascular effects of developmental stressors.