Effects of hypoxia on endothelium-dependent relaxation of rat pulmonary artery

Pulmonary Hypertension in Acute and Chronic High Altitude Maladaptation Disorders

Alveolar hypoxia is the most prominent feature of high altitude environment with well-known consequences for the cardio-pulmonary system, including development of pulmonary hypertension. Pulmonary hypertension due to an exaggerated hypoxic pulmonary vasoconstriction contributes to high altitude pulmonary edema (HAPE), a life-threatening disorder, occurring at high altitudes in non-acclimatized healthy individuals. Despite a strong physiologic rationale for using vasodilators for prevention and treatment of HAPE, no systematic studies of their efficacy have been conducted to date. Calcium-channel blockers are currently recommended for drug prophylaxis in high-risk individuals with a clear history of recurrent HAPE based on the extensive clinical experience with nifedipine in HAPE prevention in susceptible individuals. Chronic exposure to hypoxia induces pulmonary vascular remodeling and development of pulmonary hypertension, which places an increased pressure load on the right ventricle leading to right heart failure. Further, pulmonary hypertension along with excessive erythrocytosis may complicate chronic mountain sickness, another high altitude maladaptation disorder. Importantly, other causes than hypoxia may potentially underlie and/or contribute to pulmonary hypertension at high altitude, such as chronic heart and lung diseases, thrombotic or embolic diseases. Extensive clinical experience with drugs in patients with pulmonary arterial hypertension suggests their potential for treatment of high altitude pulmonary hypertension. Small studies have demonstrated their efficacy in reducing pulmonary artery pressure in high altitude residents. However, no drugs have been approved to date for the therapy of chronic high altitude pulmonary hypertension. This work provides a literature review on the role of pulmonary hypertension in the pathogenesis of acute and chronic high altitude maladaptation disorders and summarizes current knowledge regarding potential treatment options.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Enhancement of myofilament calcium sensitivity by acute hypoxia in rat distal pulmonary arteries

Hypoxic contraction of pulmonary arterial smooth muscle is thought to require increases in both intracellular Ca(2+) concentration ([Ca(2+)](i)) and myofilament Ca(2+) sensitivity, which may or may not be endothelium-dependent. To examine the effects of hypoxia and endothelium on Ca(2+) sensitivity in pulmonary arterial smooth muscle, we measured the relation between [Ca(2+)](i) and isometric force at 37°C during normoxia (21% O(2)-5% CO(2)) and after 30 min of hypoxia (1% O(2)-5% CO(2)) in endothelium-intact (E+) and -denuded (E-) rat distal intrapulmonary arteries (IPA) permeabilized with staphylococcal α-toxin. Endothelial denudation enhanced Ca(2+) sensitivity during normoxia but did not alter the effects of hypoxia, which shifted the [Ca(2+)](i)-force relation to higher force in E+ and E- IPA. Neither hypoxia nor endothelial denudation altered Ca(2+) sensitivity in mesenteric arteries. In E+ and E- IPA, hypoxic enhancement of Ca(2+) sensitivity was abolished by the nitric oxide synthase inhibitor N(ω)-nitro-l-arginine methyl ester (30 μM), which shifted normoxic [Ca(2+)](i)-force relations to higher force. In E- IPA, the Rho kinase antagonist Y-27632 (10 μM) shifted the normoxic [Ca(2+)](i)-force relation to lower force but did not alter the effects of hypoxia. These results suggest that acute hypoxia enhanced myofilament Ca(2+) sensitivity in rat IPA by decreasing nitric oxide production and/or activity in smooth muscle, thereby revealing a high basal level of Ca(2+) sensitivity, due in part to Rho kinase, which otherwise did not contribute to Ca(2+) sensitization by hypoxia.

NADPH oxidase-dependent signaling in endothelial cells: role in physiology and pathophysiology

Reactive oxygen species (ROS) including superoxide (O(2)(.-)) and hydrogen peroxide (H(2)O(2)) are produced endogenously in response to cytokines, growth factors; G-protein coupled receptors, and shear stress in endothelial cells (ECs). ROS function as signaling molecules to mediate various biological responses such as gene expression, cell proliferation, migration, angiogenesis, apoptosis, and senescence in ECs. Signal transduction activated by ROS, "oxidant signaling," has received intense investigation. Excess amount of ROS contribute to various pathophysiologies, including endothelial dysfunction, atherosclerosis, hypertension, diabetes, and acute respiratory distress syndrome (ARDS). The major source of ROS in EC is a NADPH oxidase. The prototype phagaocytic NADPH oxidase is composed of membrane-bound gp91phox and p22hox, as well as cytosolic subunits such as p47(phox), p67(phox) and small GTPase Rac. In ECs, in addition to all the components of phagocytic NADPH oxidases, homologues of gp91(phox) (Nox2) including Nox1, Nox4, and Nox5 are expressed. The aim of this review is to provide an overview of the emerging area of ROS derived from NADPH oxidase and oxidant signaling in ECs linked to physiological and pathophysiological functions. Understanding these mechanisms may provide insight into the NADPH oxidase and oxidant signaling components as potential therapeutic targets.

Oxygen tension modulates the expression of pulmonary vascular BKCa channel alpha- and beta-subunits

At birth, the lung environment changes from low to relatively high O(2) tension. Pulmonary blood flow increases and pulmonary artery pressure decreases. Recent data suggest that pulmonary vascular calcium-sensitive K(+) channel (BK(Ca)) activation mediates perinatal pulmonary vasodilation. Although BK(Ca) channel expression is developmentally regulated, the molecular mechanisms responsible for BK(Ca) expression remain unknown. We tested the hypothesis that the low-O(2) tension environment of the normal fetus modulates BK(Ca) channel expression. We analyzed BK(Ca) expression under conditions of hypoxia and normoxia both in vitro and in vivo. BK(Ca) alpha-subunit mRNA expression increased twofold in ovine pulmonary artery smooth muscle cell (PASMC) primary cultures maintained in hypoxia. In vivo, BK(Ca) expression was similarly affected by hypoxia. When adult Sprague-Dawley rats were placed in hypobaric hypoxic chambers for 3 wk, hypoxic animals showed an increase of threefold in the expression of BK(Ca) alpha- and more than twofold in the expression of BK(Ca) beta(1)-subunit mRNA. Immunochemical staining was consistent with the genetic data. To assess transcriptional activation of the beta-subunit of the BK(Ca), both BK(Ca) beta(1)- and beta(2)-subunit luciferase (K(Ca) beta:luc(+)) reporter genes were constructed. Hypoxia increased PASMC K(Ca) beta(1):luc(+) reporter expression by threefold and K(Ca) beta(2):luc(+) expression by 35%. Fetal PASMC treated with the hypoxia-inducible factor-1 mimetic deferoxamine showed a 63 and 41% increase in BK(Ca) channel alpha- and beta(1)-subunit expression, respectively. Together, these results suggest that oxygen tension modulates BK(Ca) channel subunit mRNA expression, and the regulation is, at least in part, at the transcriptional level.

Hypoxic pulmonary vasoconstriction and pulmonary artery tissue cytokine expression are mediated by protein kinase C

Pulmonary arteries exhibit a marked vasoconstriction when exposed to hypoxic conditions. Although this may be an adaptive response to match lung ventilation with perfusion, the potential consequences of sustained pulmonary vasoconstriction include pulmonary hypertension and right heart failure. Concomitant production of proinflammatory mediators during hypoxia may exacerbate acute increases in pulmonary vascular resistance. We hypothesized that acute hypoxia causes pulmonary arterial contraction and increases the pulmonary artery tissue expression of proinflammatory cytokines via a protein kinase C (PKC)-mediated mechanism. To study this, isometric force displacement was measured in isolated rat pulmonary artery rings during hypoxia in the presence and absence of the PKC inhibitors calphostin C or chelerythrine. In separate experiments, pulmonary artery rings were treated with the PKC activator thymeleatoxin for 60 min. After hypoxia, with or without PKC inhibition, or PKC activation alone, pulmonary artery rings were subjected to mRNA analysis for TNF-alpha and IL-1beta via RT-PCR. Our results showed that, in isolated pulmonary arteries, hypoxia caused a biphasic contraction and increased expression of TNF-alpha and IL-1beta mRNA. Both effects were inhibited by PKC inhibition. PKC activation resulted in pulmonary artery contraction and increased the pulmonary artery expression of TNF-alpha and IL-1beta mRNA. These findings suggest that hypoxia induces the expression of inflammatory cytokines and causes vasoconstriction via a PKC-dependent mechanism. We conclude that PKC may have a central role in modulating hypoxic pulmonary vasoconstriction, and further elucidation of its involvement may lead to therapeutic application.

Hypoxia and acidosis impair cGMP synthesis in microvascular coronary endothelial cells

To characterize the effects of ischemia on cGMP synthesis in microvascular endothelium, cultured endothelial cells from adult rat hearts were exposed to hypoxia or normoxia at pH 6.4 or 7.4. Cellular cGMP and soluble (sGC) and membrane guanylyl cyclase (mGC) activities were measured after stimulation of sGC (S-nitroso-N-acetyl-penicillamine) or mGC (urodilatin) or after no stimulation. Cell death (lactate dehydrogenase release) was negligible in all experiments. Hypoxia at pH 6.4 induced a rapid approximately 90% decrease in cellular cGMP after sGC and mGC stimulation. This effect was reproduced by acidosis. Hypoxia at pH 7.4 elicited a less pronounced (approximately 50%) and slower reduction in cGMP synthesis. Reoxygenation after 2 h of hypoxia at either pH 6.4 or 7.4 normalized the response to mGC stimulation but further deteriorated the sGC response; normalization of pH rapidly reversed the effects of acidosis. At pH 7.4, the response to GC stimulation correlated well with cellular ATP. We conclude that simulated ischemia severely depresses cGMP synthesis in microvascular coronary endothelial cells through ATP depletion and acidosis without intrinsic protein alteration.

Chronic hypoxia attenuates cGMP-dependent pulmonary vasodilation

Chronic hypoxia (CH) augments endothelium-derived nitric oxide (NO)-dependent pulmonary vasodilation; however, responses to exogenous NO are reduced following CH in female rats. We hypothesized that CH-induced attenuation of NO-dependent pulmonary vasodilation is mediated by downregulation of vascular smooth muscle (VSM) soluble guanylyl cyclase (sGC) expression and/or activity, increased cGMP degradation by phosphodiesterase type 5 (PDE5), or decreased VSM sensitivity to cGMP. Experiments demonstrated attenuated vasodilatory responsiveness to the NO donors S-nitroso-N-acetylpenicillamine and spermine NONOate and to arterial boluses of dissolved NO solutions in isolated, saline-perfused lungs from CH vs. normoxic female rats. In additional experiments, the sGC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, blocked vasodilation to NO donors in lungs from each group. However, CH was not associated with decreased pulmonary sGC expression or activity as assessed by Western blotting and cGMP radioimmunoassay, respectively. Consistent with our hypothesis, the selective PDE5 inhibitors dipyridamole and T-1032 augmented NO-dependent reactivity in lungs from CH rats, while having little effect in lungs from normoxic rats. However, the attenuated vasodilatory response to NO in CH lungs persisted after PDE5 inhibition. Furthermore, CH similarly inhibited vasodilatory responses to 8-bromoguanosine 3'5'-cyclic monophosphate. We conclude that attenuated NO-dependent pulmonary vasodilation after CH is not likely mediated by decreased sGC expression, but rather by increased cGMP degradation by PDE5 and decreased pulmonary VSM reactivity to cGMP.

Nitric oxide production in the hypoxic lung

Nitric oxide (NO) is a potent vasodilator and inhibitor of vascular remodeling. Reduced NO production has been implicated in the pathophysiology of pulmonary hypertension, with endothelial NO synthase (NOS) knockout mice showing an increased risk for pulmonary hypertension. Because molecular oxygen (O2) is an essential substrate for NO synthesis by the NOSs and biochemical studies using purified NOS isoforms have estimated the Michaelis-Menten constant values for O2 to be in the physiological range, it has been suggested that O2 substrate limitation may limit NO production in various pathophysiological conditions including hypoxia. This review summarizes numerous studies of the effects of acute and chronic hypoxia on NO production in the lungs of humans and animals as well as in cultured vascular cells. In addition, the effects of hypoxia on NOS expression and posttranslational regulation of NOS activity by other proteins are also discussed. Most studies found that hypoxia limits NO synthesis even when NOS expression is increased.

Role of calpain in hypoxic inhibition of nitric oxide synthase activity in pulmonary endothelial cells

Pulmonary artery endothelial cells (PAEC) were exposed to normoxia or hypoxia (0% O(2)-95% N(2)-5% CO(2)) in the presence and absence of calpain inhibitor I or calpeptin, after which endothelial nitric oxide synthase (eNOS) activity and protein content were assayed. Exposure to hypoxia decreased eNOS activity but not eNOS protein content. Both calpain inhibitor I and calpeptin prevented the hypoxic decrease of eNOS activity. Incubation of calpain with total membrane preparations of PAEC caused dose-dependent decreases in eNOS activity independent of changes in eNOS protein content. Exposure of PAEC to hypoxia also caused time-dependent decreases of heat shock protein 90 (HSP90) that were prevented by calpain inhibitor I and calpeptin. Moreover, the HSP90 content in anti-eNOS antibody-induced immunoprecipitates from hypoxic PAEC lysates was reduced, and repletion of HSP90 reversed the decrease of eNOS activity in these immunoprecipitates. Incubation of PAEC with a specific inhibitor of HSP90 (geldanamycin) mimicked the hypoxic decrease of eNOS activity. These results indicate that the hypoxia-induced reduction in eNOS activity in PAEC is due to a decrease in HSP90 caused by calpain activation.

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.

Hypoxic pulmonary blood flow redistribution and arterial oxygenation in endotoxin-challenged NOS2-deficient mice

Sepsis and endotoxemia impair hypoxic pulmonary vasoconstriction (HPV), thereby reducing arterial oxygenation and enhancing hypoxemia. Endotoxin induces nitric oxide (NO) production by NO synthase 2 (NOS2). To assess the role of NO and NOS2 in the impairment of HPV during endotoxemia, we measured in vivo the distribution of total pulmonary blood flow (QPA) between the right (QRPA) and left (QLPA) pulmonary arteries before and after left mainstem bronchus occlusion (LMBO) in mice with and without a congenital deficiency of NOS2. LMBO reduced QLPA/QPA equally in saline-treated wild-type and NOS2-deficient mice. However, prior challenge with Escherichia coli endotoxin markedly impaired the ability of LMBO to reduce QLPA/QPA in wild-type, but not in NOS2-deficient, mice. After endotoxin challenge and LMBO, systemic oxygenation was impaired to a greater extent in wild-type than in NOS2-deficient mice. When administered shortly after endotoxin treatment, the selective NOS2 inhibitor L-NIL preserved HPV in wild-type mice. High concentrations of inhaled NO attenuated HPV in NOS2-deficient mice challenged with endotoxin. These findings demonstrate that increased pulmonary NO levels (produced by NOS2 or inhaled at high levels from exogenous sources) are necessary during the septic process to impair HPV, ventilation/perfusion matching and arterial oxygenation in a murine sepsis model.

Hypoxia inhibits gene expression of voltage-gated K+ channel alpha subunits in pulmonary artery smooth muscle cells

Activity of voltage-gated K+ channels (KV) in pulmonary arterial smooth muscle cells (PASMC) is pivotal in controlling membrane potential, cytoplasmic free Ca2+ concentration ([Ca2+]cyt, and pulmonary vasomotor tone. Acute hypoxia selectively inhibits KV channels, depolarizes PASMC, raises [Ca2+]cyt, and causes pulmonary vasoconstriction and vascular remodeling. Prolonged hypoxia (24-60 h) decreased significantly the mRNA levels of KV channel alpha subunits, KV1.2 and KV1.5. Consistently, the protein levels of KV1.2 and KV1.5 were also decreased significantly by hypoxia (48-72 h). Nevertheless, hypoxia affected negligibly the mRNA levels of KV channel beta subunits (KVbeta1, KVbeta2, and KVbeta3). The native K+ channels are composed of pore-forming alpha and auxiliary beta subunits. Assembly of KV beta subunits with alpha subunits confers rapid inactivation on the slowly or non-inactivating delayed rectifier KV channels. KV beta subunits also function as an open-channel blocker of KV channels. Thus, the diminished transcription and expression of KV alpha subunits may reduce the number of KV channels and decrease KC currents. Unchanged transcription of KV beta subunits may increase the fraction of the KV channel alpha subunits that are associated with beta subunits and further reduce the total KV currents. These data demonstrate a novel mechanism by which chronic hypoxia may cause pulmonary vasoconstriction and hypertension.

Involvement of endothelin-1 in hypoxic pulmonary vasoconstriction in the lamb

1. Using isolated pulmonary resistance vessels from mature fetal lamb and chronically instrumented lambs (8-17 days old), we have examined whether hypoxic pulmonary vasoconstriction is sustained by activation of a constrictor mechanism or suppression of a dilator mechanism. 2. Hypoxia contracted both arteries and veins in vitro, and the contraction was greater with the former. After removing the endothelium, arteries responded faster to hypoxia, but the magnitude of the response remained unchanged. 3. Hypoxic arteries, unlike normally oxygenated arteries, did not contract with either indomethacin (2.8 microM) or N omega-nitro-L-arginine methyl ester (L-NAME, 100 microM). The same vessels relaxed with sodium nitroprusside (SNP, 0.001-10 microM) but not with bradykinin (0.1-100 nM). 4. Endothelin-1 (ET-1, 0.01-10 nM) contracted isolated arteries and veins under normoxic and hypoxic conditions. In both vessels, the contraction was fast in onset and subsidence, and was inhibited by the ETA receptor antagonist BQ123 (1 microM). The ET-1 precursor, big ET-1 (100 nM), also contracted arteries and veins, but compared with ET-1 its action was slower in development. Big ET-1 contraction, unlike ET-1 contraction, was curtailed by the inhibitor of the ET-1-converting enzyme, phosphoramidon (50 microM). 5. ET-1 (0.1-10 nM) had no effect on isolated arteries precontracted with a thromboxane A2 (TXA2) analogue (ONO-11113) and treated with BQ123 (10 microM). Under the same conditions, ET-1 relaxed the veins. Accordingly, in the absence of BQ123 treatment, the selective ETB receptor agonist IRL-1620 (0.1-100 nM) relaxed the contracted veins but not the arteries. 6. BQ123 (10 microM) inhibited the constriction of isolated arteries and veins to hypoxia. Likewise, in the conscious lamb a bolus of BQ123 (0.4 mg kg-1, injected into the pulmonary artery) curtailed the rise in pulmonary vascular resistance (Rpa) brought about by alveolar hypoxia without changing significantly systemic vascular resistance (Rao). Under normoxia, Rpa was insignificantly affected by BQ123. 7. The results indicate that pulmonary resistance arteries are more susceptible to hypoxia than the veins, and that hypoxic vasoconstriction does not require an intact endothelium to occur. Hypoxic tone is ascribed primarily to intramural generation of ET-1, while removal of the tonic action of a relaxant may only have an accessory role in the response.

Effects of hypoxia, mechanical and chemical endothelium denudation on guinea-pig isolated pulmonary arteries

1. The isolated unstimulated main trunk, extralobar and intralobar branches of the pulmonary artery of the guinea-pig developed well-sustained contractions upon exposure to hypoxia (95% N2-5% CO2 gas mixture; PO2 11-15 mm Hg). The contractions were readily reversible by reoxygenation (95% O2-5% CO2). 2. Mechanical removal of the endothelium did not significantly affect the magnitude of the hypoxia-induced contractions in rings obtained from the main trunk of the pulmonary artery but reduced those of rings obtained from the proximal and distal extralobar branches. 3. Mechanical removal of the endothelium also did not affect the magnitude of contractions induced by BaCl2 in the main but significantly reduced contractions induced by the same agent in the proximal and distal extralobar branches of the pulmonary artery, suggesting that the reduction of hypoxia-induced contractions in the endothelium-denuded rings is due to impairment of vascular reactivity. 4. Pretreatment with L-N-nitro arginine, an inhibitor of the synthesis of the endothelium-derived relaxing factor, did not significantly affect the hypoxia-induced contractions but increased the magnitude of BaCl2-induced contractions in the main and the extralobar branches. 5. These observations demonstrate that isolated pulmonary artery rings of the guinea-pig develop slow contractions in response to hypoxia without prior contraction with an agonist, and that the endothelium plays little role in the hypoxia-induced contractions of guinea-pig isolated large pulmonary arteries. 6. Furthermore, these observations suggest that the effect of mechanical endothelium denudation or pharmacological manipulation, such as EDRF inhibition, on vascular reactivity should be considered when the effect of hypoxia is studied in isolated pulmonary arteries.

Endothelin-1 does not mediate hypoxic vasoconstriction in canine isolated blood vessels: effect of BQ-123

1. The role of endothelin-1 in mediating the phenomenon of hypoxic vasoconstriction was examined in canine, isolated pulmonary, circumflex coronary and femoral arterial rings. 2. In tissues with an intact endothelium, the exogenous application of endothelin-1 (0.1-300 nM) caused concentration-dependent increases in canine, isolated pulmonary artery tone. Endothelin-3 (1-300 nM) was approximately 30 fold less potent than endothelin-1 as a vasoconstrictor in this tissue. In contrast, the selective ETB-receptor agonist, sarafotoxin S6c (0.01-1 microM), failed to elicit vasoconstriction in this tissue. Thus, endothelin isopeptide-induced vasoconstriction of the canine isolated pulmonary artery is mediated exclusively by the ETA-receptor subtype. 3. The concentration-dependent increases in isometric tension induced by endothelin-1 (0.1-300 nM) were antagonized by the ETA-selective antagonist, BQ-123 (10 microM); this concentration of antagonist caused a shift to the right in the concentration-response curve for endothelin-1 of approximately two orders of magnitude. This concentration of BQ-123 did not unmask any ETB-receptor-mediated vasoconstriction since sarafotoxin S6c (0.01-1 microM) still failed to elicit contraction in the presence of this concentration of BQ-123. 4. The hypoxia-induced vasoconstriction of canine, isolated pulmonary, circumflex coronary and femoral arterial rings was unaffected by pretreatment with the endothelin receptor antagonist, BQ-123 (10 microM), a concentration shown previously to antagonize the contractile actions of exogenously applied endothelin-1 in the isolated pulmonary artery. 5. These results are the first to provide direct evidence showing that the endothelium-dependent vasoconstriction observed during acute periods of hypoxia in vitro is not mediated by an endothelin-related isopeptide.

Endogenous endothelium-derived relaxing factor opposes hypoxic pulmonary vasoconstriction and supports blood flow to hypoxic alveoli in anesthetized rabbits

Agents that inhibit nitric oxide synthesis augment hypoxic pulmonary vasoconstriction. In an animal model of unilateral alveolar hypoxia, we investigated the hypothesis that endogenous endothelium-derived relaxing factor/nitric oxide opposes hypoxic pulmonary vasoconstriction and supports blood flow to hypoxic alveoli, resulting in a reduction in arterial oxygen tension (PO2). In pentobarbital-anesthetized rabbits, unilateral alveolar hypoxia was produced by ventilation of one lung with 100% oxygen and the other with 100% nitrogen (O2/N2). NG-Nitro-L-arginine methyl ester (0.03 followed by 1.0 mg/kg i.v.) resulted in dose-dependent decreases in the percent of pulmonary blood flow to the N2-ventilated lung and increases in arterial PO2. L-Arginine (1 mg.kg-1.min-1 i.v.) prevented the NG-nitro-L-arginine methyl ester-induced redistribution of blood flow away from hypoxic alveoli and improvement in arterial PO2. Indomethacin (5 mg/kg i.v.) administered during O2/N2 ventilation resulted in a reduction in the percentage of total blood flow to the hypoxic lung and an increase in arterial PO2. However, NG-nitro-L-arginine methyl ester administered in the presence of indomethacin caused additional diversion of blood flow away from the hypoxic lung. The magnitude of the changes suggests that the endothelium-derived relaxing factor/nitric oxide system has the capacity to make a greater contribution than products of cyclooxygenase-mediated arachidonic acid metabolism in supporting blood flow to hypoxic alveoli in the rabbit.

Evidence that epithelium-dependent relaxation of vascular smooth muscle detected by co-axial bioassays is not attributable to hypoxia

1. The present study was undertaken to examine further the contribution of hypoxia to airway epithelium-dependent relaxation of rat aorta in the co-axial bioassay. 2. Endothelium-denuded rat aorta contracted with phenylephrine (0.05 microM) relaxed in a time-dependent manner (t1/2 = 8.3 +/- 0.4 min, n = 38) when the bathing solution was bubbled with 95% N2 and 5% CO2. In co-axial bioassays, the t1/2 for histamine (100 microM; guinea-pig trachea)- and methacholine (100 microM; rabbit bronchus)- induced relaxation was 1.9 +/- 0.2 min (n = 14) and 1.2 +/- 0.1 min (n = 26), respectively. 3. Hypoxia-induced relaxation was not associated with a rise in intracellular guanosine 3':5'-cyclic monophosphate (cyclic GMP). This contrasts with previous findings of an elevation in cyclic GMP associated with epithelium-dependent relaxation of rat aorta in co-axial bioassays. 4. Hypoxia-induced vascular relaxation was antagonized by the ATP-sensitive K+ channel blocker, glibenclamide (100 microM). In contrast, glibenclamide (100 microM) failed to inhibit histamine (100 microM; guinea-pig trachea)- and methacholine (0.1-100 microM; rabbit bronchus)-induced release of epithelium-derived inhibitory factor (EpDIF), in co-axial bioassays. Glibenclamide (100 microM) antagonized BRL 38227 (lemakalin), but not isoprenaline-induced relaxation of phenylephrine-contracted rat aorta. 5. These data strongly suggest that the airway epithelium-dependent relaxant responses observed in co-axial bioassays cannot be attributed to hypoxia.

Endothelial modulation and changes in endothelin pressor activity during hypoxia in the rat isolated perfused superior mesenteric arterial bed

1. The isolated superior mesenteric arterial bed of the rat, perfused with Krebs-Henseleit solution containing 10 microM indomethacin, was used to study the effects of reducing dissolved O2 tension on the pressor responses to endothelin-1, endothelin-3 and sarafotoxin S6b. The modulation of these responses by the endothelium was investigated by removing the intima with the detergent CHAPS and, for endothelin-1, by inhibiting nitric oxide production with N omega-nitro-L-arginine methyl ester (L-NAME). Comparison was made with the effects of lowering O2 tension on the pressor responses to noradrenaline and 5-hydroxytryptamine. 2. Lowering the perfusate O2 tension from 551 +/- 2 mmHg to 14.0 +/- 0.5 mmHg did not change the ED50 for endothelin-1 but its maximal responses (Rmax) were increased by 2.1 and 2.7 fold, respectively, in the presence and absence of endothelium. The Rmax values for endothelin-3 were also greater in hypoxia either in the presence (by 2.3 fold) or absence of the endothelium (by 1.6 times) but those for sarafotoxin S6b were only enhanced significantly by hypoxia in the absence of the intima. hypoxia reduced the potencies of endothelin-3 and sarafotoxin S6b whether or not endothelium was present. 3. Endothelial destruction, whether in hypoxic or oxygenated conditions, increased the Rmax values for endothelin-1 and endothelin-3; at both O2 tensions those for endothelin-3 increased more than those for endothelin-1. The ED50 for endothelin-1 was unchanged by destroying the endothelium but endothelin-3 was less potent in the absence of an endothelium than in its presence. Removal of the endothelium did not change the R.ax for sarafotoxin S6b but increased its potency in both hypoxic and oxygenated tissues. 4. In hypoxia, and in the presence of both the endothelium and 100 microM L-NAME, the Rmax for endothelin-1 was 1.6 times greater than that in hypoxia in the absence of L-NAME. Co-infusion of 100 microM L-arginine, but not of 100 mircoM D-arginine, with 100 microM L-NAME reversed this effect. The presence of L-NAME decreased the potency of endothelin-1. 5. Destroying the endothelium did not affect the Rmax for noradrenaline in either oxygenated conditions or hypoxia. Changing 02 tension when the endothelium was intact had no effect on the Rmax but it was 11% greater in oxygenated, than in hypoxic, endothelium denuded preparations. Endothelial destruction decreased the potency of noradrenaline in hypoxia but increased it in oxygenated tissues. In hypoxia, L-NAME had no effect on the ED50 relative to control preparations with endothelium but the Rmax was 30% greater. 6. 5-Hydroxytryptamine gave very small pressor responses in the presence of endothelium in both oxygenated and hypoxic tissues but the Rmax was 1.7 times greater in hypoxia. L-NAME increased the R,,x by 9.8 times in oxygenated preparations and 6.3 fold in hypoxia. The ED5o values were the same in all conditions. 7. It is concluded that, although hypoxia generally increased the R.. for the endothelin/sarafotoxin peptides, the changes could not be explained by a simple increase in receptor number since hypoxia decreased the potency of endothelin-3 and sarafotoxin S6b. Thus alterations in receptor binding or activation properties, or both, also occurred. The changes associated with hypoxia were not common to all vasoconstrictor agonists since, in the absence of endothelial function, hypoxia did not affect the Rmax values for either noradrenaline or 5-hydroxytryptamine. Also, the pressor responses to the peptides and both the amines can be modulated by the endothelium in hypoxia as well as in oxygenated conditions.