Endothelium-derived nitric oxide plays a larger role in pulmonary veins than in arteries of newborn lambs

Nitric oxide regulation of fetal and newborn lung development and function

In the developing lung, nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) signaling are essential in regulating lung formation and vascular tone. Animal studies have linked many anatomical and pathophysiological features of newborn lung disease to abnormalities in the NO/cGMP signaling system. They have demonstrated that driving this system with agonists and antagonists alleviates many of them. This research has spurred the rapid clinical development, testing, and application of several NO/cGMP-targeting therapies with the hope of treating and potentially preventing significant pediatric lung diseases. However, there are instances when the therapeutic effectiveness of these agents is limited. Studies indicate that injury-induced disruption of several critical components within the signaling system may hinder the promise of some of these therapies. Recent research has identified basic mechanisms that suppress NO/cGMP signaling in the injured newborn lung. They have also pinpointed biomarkers that offer insight into the activation of these pathogenic mechanisms and their influence on the NO/cGMP signaling system's integrity in vivo. Together, these will guide the development of new therapies to protect NO/cGMP signaling and safeguard newborn lung development and function. This review summarizes the important role of the NO/cGMP signaling system in regulating pulmonary development and function and our evolving understanding of how it is disrupted by newborn lung injury.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.

Venous Vasomotion

Veins exhibit spontaneous contractile activity, a phenomenon generally termed vasomotion. This is mediated by spontaneous rhythmical contractions of mural cells (i.e. smooth muscle cells (SMCs) or pericytes) in the wall of the vessel. Vasomotion occurs through interconnected oscillators within and between mural cells, entraining their cycles. Pharmacological studies indicate that a key oscillator underlying vasomotion is the rhythmical calcium ion (Ca²⁺) release-refill cycle of Ca²⁺ stores. This occurs through opening of inositol 1,4,5-trisphosphate receptor (IP₃R)- and/or ryanodine receptor (RyR)-operated Ca²⁺ release channels in the sarcoplasmic/endoplasmic (SR/ER) reticulum and refilling by the SR/ER reticulum Ca²⁺ATPase (SERCA). Released Ca²⁺ from stores near the plasma membrane diffuse through the cytosol to open Ca²⁺-activated chloride (Cl^-) channels, this generating inward current through an efflux of Cl^-. The resultant depolarisation leads to the opening of voltage-dependent Ca²⁺ channels and possibly increased production of IP₃, which through Ca²⁺-induced Ca²⁺ release (CICR) of IP₃Rs and/or RyRs and IP₃R-mediated Ca²⁺ release provide a means by which store oscillators entrain their activity. Intercellular entrainment normally involves current flow through gap junctions that interconnect mural cells and in many cases this is aided by additional connectivity through the endothelium. Once entrainment has occurred the substantial Ca²⁺ entry that results from the near-synchronous depolarisations leads to rhythmical contractions of the mural cells, this often leading to vessel constriction. The basis for venous/venular vasomotion has yet to be fully delineated but could improve both venous drainage and capillary/venular absorption of blood plasma-associated fluids.

Intrapulmonary arteriovenous anastomoses. Physiological, pathophysiological, or both?

Large-diameter, intrapulmonary arteriovenous anastomoses exist in human lungs. In developing fetuses, blood flows physiologically through pulmonary arteriovenous channels that appear to regress during lung maturation. Blood flow through intrapulmonary arteriovenous anastomoses is a normal occurrence during exercise or inhalation of reduced oxygen gas mixtures in most healthy humans. However, the importance of blood flow through these anastomoses to the efficiency of pulmonary gas exchange in normal and pathological states remains controversial. Newly reported three-dimensional dissections of human lung samples provide direct anatomic evidence of intrapulmonary arteriovenous anastomoses in the lungs of prematurely born infants, and suggest that these vessels contribute consequentially to the severe arterial hypoxemia experienced by infants who die of bronchopulmonary dysplasia. Surgical construction of a cavopulmonary anastomosis can also induce pathological arteriovenous shunting suggestive of a regression to the fetal state, possibly implicating an enigmatic hepatic factor in arteriovenous shunt regulation. These two observations support an important contribution of blood flow through intrapulmonary arteriovenous anastomoses to arterial hypoxemia under at least some pathological conditions. The degree to which these vessels contribute to arterial hypoxemia in other disease states where intrapulmonary shunting is present, such as hepatopulmonary syndrome, remains unknown.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Hypoxic pulmonary hypertension of the newborn

Hypoxic pulmonary hypertension of the newborn is characterized by elevated pulmonary vascular resistance and pressure due to vascular remodeling and increased vessel tension secondary to chronic hypoxia during the fetal and newborn period. In comparison to the adult, the pulmonary vasculature of the fetus and the newborn undergoes tremendous developmental changes that increase susceptibility to a hypoxic insult. Substantial evidence indicates that chronic hypoxia alters the production and responsiveness of various vasoactive agents such as endothelium-derived nitric oxide, endothelin-1, prostanoids, platelet-activating factor, and reactive oxygen species, resulting in sustained vasoconstriction and vascular remodeling. These changes occur in most cell types within the vascular wall, particularly endothelial and smooth muscle cells. At the cellular level, suppressed nitric oxide-cGMP signaling and augmented RhoA-Rho kinase signaling appear to be critical to the development of hypoxic pulmonary hypertension of the newborn.

Hypoxic regulation of pulmonary vascular smooth muscle cyclic guanosine monophosphate-dependent kinase by the ubiquitin conjugating system

We previously reported that hypoxia attenuates nitric oxide-cyclic guanosine monophosphate (NO-cGMP)-mediated fetal pulmonary vessel relaxation by inhibiting cGMP-dependent protein kinase 1 (PKG1) activity, but not all the mechanisms by which acute hypoxia inhibits PKG1 activity have been delineated. Here we demonstrate for the first time, to the best of our knowledge, that acute hypoxia induces an accumulation of ubiquitinated PKG1 in ovine fetal and newborn pulmonary artery smooth muscle cells. Such a modification was not evident in ovine fetal systemic (cerebral) artery smooth muscle cells. The accumulation of polyubiquitinated PKG1 observed after 4 hours of hypoxia was affected neither by the activation of PKG1 kinase activity with the cell-permeable cGMP analogue 8-bromo-cGMP, nor by its inhibition with DT-3 in fetal pulmonary artery smooth muscle cells. Ubiquitinated PKG1α was unable to bind the cGMP analogue 8-(2-aminoethyl)thioguanosine-3',5' (AET)-cGMP, a ligand for the unmodified protein. Inhibition of the proteasomal complex with MG132 led to the accumulation of polyubiquitinated PKG1 in normoxia, indicating the involvement of the ubiquitin-26S proteasomal system in degradation and clearance of this protein under normoxic conditions. The ubiquitinated PKG1 under hypoxic conditions, however, was not predominantly targeted for proteasomal degradation. Importantly, reoxygenation reversed the acute hypoxia-induced accumulation of ubiquitinated PKG1. Our results suggest that the PKG1 ubiquitination induced by acute hypoxia plays a unique role in the regulation of the pulmonary vascular smooth muscle cell vasoreactivity and relaxation mediated by the NO-cGMP-PKG1 pathway.

Increased degradation of MYPT1 contributes to the development of tolerance to nitric oxide in porcine pulmonary artery

Myosin phosphatase target subunit 1 (MYPT1) is the regulatory subunit of myosin light chain phosphatase (MLCP). It plays a critical role in vasodilatation induced by cGMP-elevating agents such as nitric oxide (NO). The present study was performed to determine the role of MYPT1 in the development of tolerance of the pulmonary artery to NO. Incubation of isolated porcine pulmonary arteries for 24 or 48 h with DETA NONOate (DETA NO) significantly reduced protein levels of MYPT1 and the leucine zipper-positive (LZ+) isoform of MYPT1 but not that of PP1cdelta. The extent of reduction in total MYPT1 protein level was comparable to that of MYPT1 (LZ+). The decrease in MYPT1 protein caused by 48-h DETA NO incubation was prevented by ODQ, an inhibitor of guanylyl cyclase, and by inhibitors of proteasomes (MG-132 and lactacystin) but was not affected by the inhibitor of protein synthesis, cycloheximide. A reduction in MYPT1 protein was also obtained with 8-bromo-cGMP, but this was prevented by Rp-8-bromo-PET-cGMP [inhibitor of cGMP-dependent protein kinase (PKG)]. Incubation for 48 h with DETA NO also reduced dephosphorylation of myosin light chain and relaxation of the artery in response to DETA NO, which was prevented by MG-132. These results suggest that the reduction in MYPT1 protein contributes to the development of tolerance of pulmonary arteries to NO. This may result from increased degradation of MYPT1 after prolonged PKG activation.

Protein kinase G regulates the basal tension and plays a major role in nitrovasodilator-induced relaxation of porcine coronary veins

BACKGROUND AND PURPOSE

Coronary venous activity is modulated by endogenous and exogenous nitrovasodilators. The present study was to determine the role of protein kinase G (PKG) in the regulation of the basal tension and nitrovasodilator-induced relaxation of coronary veins.

EXPERIMENTAL APPROACH

Effects of a PKG inhibitor on the basal tension and responses induced by nitroglycerin, DETA NONOate, and 8-Br-cGMP in isolated porcine coronary veins were determined. Cyclic cGMP was measured with radioimmunoassay. PKG activity was determined by measuring the incorporation of 32P from gamma-32P-ATP into the specific substrate BPDEtide.

KEY RESULTS

Rp-8-Br-PET-cGMPS, a specific PKG inhibitor, increased the basal tension of porcine coronary veins and decreased PKG activity. The increase in tension was 38% of that caused by nitro-L-arginine. Relaxation of the veins induced by nitroglycerin and DETA NONOate was accompanied with increases in cGMP content and PKG activity. These effects were largely eliminated by inhibiting soluble guanylyl cyclase with ODQ. The increase in PKG activity induced by the nitrovasodilators was abolished by Rp-8-Br-PET-cGMPS. The relaxation caused by these dilators and by 8-Br-cGMP at their EC50 was attenuated by the PKG inhibitor by 51-66%.

CONCLUSIONS AND IMPLICATIONS

These results suggest that PKG is critically involved in nitric oxide-mediated regulation of the basal tension in porcine coronary veins and that it plays a primary role in relaxation induced by nitrovasodilators. Since nitric oxide plays a key role in modulating coronary venous activity, augmentation of PKG may be a therapeutic target for improving coronary blood flow.

Regulation of cGMP-dependent protein kinase-mediated vasodilation by hypoxia-induced reactive species in ovine fetal pulmonary veins

We previously reported that hypoxia attenuates cGMP-dependent protein kinase (PKG)-mediated relaxation in pulmonary vessels (Am J Physiol Lung Cell Mol Physiol 279: L611-L618, 2003). To determine whether hypoxia-induced reactive oxygen and nitrogen species (ROS and RNS, respectively) may be involved in the downregulation of PKG-mediated relaxation, ovine fetal intrapulmonary veins were exposed to 4 h of normoxia or hypoxia, with or without scavengers of ROS [N-acetylcysteine (NAC)] or peroxynitrite (quercetin and Trolox) and preconstricted with endothelin-1. Hypoxia decreased the relaxation response to 8-bromo-cGMP, PKG protein expression, and kinase activity and increased tyrosine nitration in PKG. However, ROS and RNS scavengers prevented these changes. To determine whether increased PKG nitration diminishes PKG activity, pulmonary vein smooth muscle cells (PVSMC) were exposed to shorter-term (30 min) hypoxia, which increased PKG nitration and decreased PKG activity but did not alter PKG protein expression. Increased dihydro-2,7-dichlorofluorescein diacetate (DCFH(2)-DA) fluorescence in PVSMC after 4 h or 30 min of hypoxia was not observed in the presence of NAC, quercetin, or Trolox, suggesting increased ROS and RNS production. Increased PKG nitration and the associated decrease in PKG activity in PVSMC after 30 min of hypoxia were also reversed on reoxygenation. The consequences of PKG nitration were assessed by exposure of purified PKG-Ialpha to peroxynitrite, which caused increased 3-nitrotyrosine immunoreactivity and inhibition of kinase activity. Our data suggest that, after 30 min of hypoxia, reversible covalent modification of PKG by hypoxia-induced reactive species may be an important mechanism by which the relaxation response to cGMP is regulated. However, after 4 h of hypoxia, PKG nitration and decreased PKG expression are involved.

Pulmonary endothelium dependent vasodilation emerges after birth in mice

At birth, with the first breath, pulmonary vessels undergo profound adaptive processes. A failure in the ability of pulmonary vessels to adapt at birth results in persistent pulmonary hypertension of the new born. The mechanisms regulating pulmonary adaptation at birth are still unclear. Progress in this area is slow, not least because genetically modified mice have not yet been used to address questions in this area of research, principally because pulmonary vessels in new born mice are very small making the study of dilator response in vitro difficult. In the current study we have used precision cut lung slices to characterise the functional vasomotor changes in lung vessels of new born mice (1-4 days old), 8-15 day old mice or adult mice. The internal luminal area of pulmonary artery and airways was measured digitally. Vasoconstriction and vasodilatation were expressed as the percentage change in internal luminal area compared with the control area. The thromboxane A(2) mimetic U-46619 (3x10(-7) M) caused a significant vasoconstriction in vessels of all groups. However, acetylcholine (3x10(-5) M) induced arterial dilation only in the 8-15 days, and adult groups. By contrast, sodium nitroprusside, which acts independently of the endothelium, was an effective vasodilator in lung vessels from neonates. These data are the first to characterise the development of endothelium dependent vasodilatation in lung after birth in mice. This approach can be used with genetically modified mice, which is important to further our understanding of the physiology in this area.

Propofol and thiopental attenuate adenosine triphosphate-sensitive potassium channel relaxation in pulmonary veins

Pulmonary veins (PV) make a significant contribution to total pulmonary vascular resistance. We investigated the cellular mechanisms by which the intravenous anesthetics propofol and thiopental alter adenosine triphosphate-sensitive potassium (KATP+) channel relaxation in canine PV. The effects of KATP+ channel inhibition (glybenclamide), cyclooxygenase inhibition (indomethacin), nitric oxide synthase inhibition (L-NAME), and L-type voltage-gated Ca2+ channel inhibition (nifedipine) on vasorelaxation responses to levcromakalim (KATP+ channel activator) alone and in combination with the anesthetics were assessed. The maximal relaxation response to levcromakalim was attenuated by removing the endothelium and by L-NAME, but not by indomethacin. Propofol (10(-5), 3x10(-5), and 10(-4) M) and thiopental (10(-4) and 3x10(-4) M) each attenuated levcromakalim relaxation in endothelium-intact (E+) rings, whereas propofol (3x10(-5) and 10(-4) M) and thiopental (3x10(-4) M) attenuated levcromakalim relaxation in endothelium-denuded (E-) rings. In E+ rings, the anesthesia-induced attenuation of levcromakalim relaxation was decreased after pretreatment with L-NAME but not with indomethacin. In E-strips, propofol (10(-4) M) and thiopental (3x10(-4) M) inhibited decreases in tension and intracellular Ca2+ concentration ([Ca2+]i) in response to levcromakalim, and these changes were abolished by nifedipine. These findings indicate that propofol and thiopental attenuate the endothelium-dependent component of KATP+ channel-induced PV vasorelaxation via an inhibitory effect on the nitric oxide pathway. Both anesthetics also attenuate the PV smooth muscle component of KATP+ channel-induced relaxation by reducing the levcromakalim-induced decrease in [Ca2+]i via an inhibitory effect on L-type voltage-gated Ca2+ channels.

Role of NO and EDHF-mediated endothelial function in the porcine pulmonary circulation: Comparison between pulmonary artery and vein

OBJECTIVE

To compare electrophysiological measurement of nitric oxide (NO) release and endothelium-derived hyperpolarizing factor (EDHF)-mediated endothelial function in porcine pulmonary arteries and veins.

METHODS

Isolated pulmonary interlobular arteries (PA) and veins (PV) were obtained from a local slaughterhouse. By using a NO-specific electrode and a conventional intracellular microelectrode, the amount of NO released from endothelial cells and hyperpolarization of smooth muscle cells were investigated. The bradykinin (BK)-induced relaxation in the precontraction by U(46619) was examined in the absence or presence of N(G)-nitro-l-arginine (l-NNA), indomethacin (INDO) plus oxyhemoglobin (HbO).

RESULTS

The basal release of NO was 7.0+/-1.2 nmol/L in PA (n=8) and 5.5+/-1.6 nmol/L in PV (n=8, p<0.01). BK-induced release of NO was 160.4+/-10.3 nmol/L in PA (n=8) and 103.0+/-14.7 nmol/L in PV (n=8, p<0.001) with longer releasing duration in PA than in PV (14.3+/-1.3 vs. 12.1+/-0.8 min, p<0.01). BK evoked an endothelium-dependent hyperpolarization and relaxation that were reduced by l-NNA, INDO, and HbO (hyperpolarization: 12.8+/-1.3 vs. 8.0+/-1.4 mV in PA, n=6, p<0.001 and 8.3+/-1.4 vs. 3.0+/-0.8 mV in PV, n=6, p<0.001; relaxation: 92.8+/-3.1% vs. 19.6+/-11.1% in PA n=8, p<0.001 and 70.3+/-7.9% vs. 6.0+/-6.8% in PV, n=8, p<0.001). Both hyperpolarization (8.0+/-1.4 vs. 3.0+/-0.8 mV, p<0.001) and relaxation (19.6+/-11.1% vs. 6.0+/-6.8%, p<0.01) were greater in PA than in PV.

CONCLUSIONS

Both NO and EDHF play an important role in regulation of porcine pulmonary arterial and venous tones. The more significant role of NO and EDHF is revealed in pulmonary arteries than in veins.

Pulmonary arterial contractility in neonatal lambs increases with 100% oxygen resuscitation

The optimal Fi(O2) during neonatal resuscitation is a subject of controversy. The effect of exposure to high levels of inspired oxygen on pulmonary arterial (PA) contractility is not known. We studied differences in PA vasoreactivity in term lambs initially ventilated with 21% or 100% oxygen, followed by continued ventilation using oxygen as needed for 24 h, or ventilated with 100% oxygen for 24 h and room air breathing 1-d-old lambs. Term lambs were delivered by cesarean section, intubated, and ventilated with 21% (21%Res) or 100% oxygen (100%Res) for the first 30 min of life. Subsequently, the ventilator Fi(O2) was adjusted to maintain a Pa(O2) between 45 and 65 mm Hg for 24 h. Five lambs were ventilated continuously with 100% oxygen (100%24h). Six spontaneously breathing newborn lambs (RA Spont) were studied for comparison. Lambs were killed at 24 h of life and PA rings were isolated and contracted with norepinephrine (NE) and KCl and some were relaxed with A23187 and SNAP in tissue baths. NE and KCl induced contractions were highest in PA isolated from 100%24h lambs, and were significantly higher in 100%Res lambs than PA from 21%Res lambs. Contraction responses in PA from RA Spont lambs were similar to 21%Res lambs. Relaxations to A23187 and SNAP were similar among all ventilated groups. PA contractility to NE and KCl is increased following both brief (30 min) and prolonged (24 h) exposure to 100% oxygen during mechanical ventilation. In contrast, normoxic resuscitation and ventilation do not increase PA contractility.

Hypoxia-induced reactive oxygen species downregulate ETB receptor-mediated contraction of rat pulmonary arteries

Production of reactive oxygen species (ROS) may be increased during hypoxia in pulmonary arteries. In this study, the role of ROS in the effect of hypoxia on endothelin (ET) type B (ETB) receptor-mediated vasocontraction in lungs was determined. In rat intrapulmonary (approximately 0.63 mm ID) arteries, contraction induced by IRL-1620 (a selective ETB receptor agonist) was significantly attenuated after 4 h of hypoxia (30 mmHg Po2) compared with normoxic control (140 mmHg Po2). The effect was abolished by tiron, a scavenger of superoxide anions, but not by polyethylene glycol (PEG)-conjugated catalase, which scavenges H2O2. The hypoxic effect on ETB receptor-mediated vasoconstriction was also abolished by endothelium denudation but not by nitro-L-arginine and indomethacin. Exposure for 4 h to exogenous superoxide anions, but not H2O2, attenuated the vasoconstriction induced by IRL-1620. Confocal study showed that hypoxia increased ROS production in pulmonary arteries that were scavenged by PEG-conjugated SOD. In endothelium-intact pulmonary arteries, the ETB receptor protein was reduced after 4 h of exposure to hypoxia, exogenous superoxide anions, or ET-1. BQ-788, a selective ETB receptor antagonist, prevented these effects. ET-1 production was stimulated in endothelium-intact arteries after 4 h of exposure to hypoxia or exogenous superoxide anions. This effect was blunted by PEG-conjugated SOD. These results demonstrate that exposure to hypoxia attenuates ETB receptor-mediated contraction of rat pulmonary arteries. A hypoxia-induced production of superoxide anions may increase ET-1 release from the endothelium and result in downregulation of ETB receptors on smooth muscle.

A pilot study of nitric oxide blood levels in patients with chronic orofacial pain

BACKGROUND

Control of pain is the major goal in the management of chronic orofacial pain (COP) patients. The pathogenesis of COP is currently not well understood. Consequently, the treatment of COP may be suboptimal or even harmful. Based on independent observations, we propose that local elevated levels of nitric oxide (NO) may have a central role in the pathogenesis of COP.

HYPOTHESIS

NO level in the orofacial region of COP patients is elevated. A regional increased level of NO causes excessive vasodilatation. This hyperperfusion is manifested by hyperthermia of the overlying skin, while NO enhances nociception, aggravating orofacial pain. An alternative mechanism involving NO as a neurotransmitter at the CNS level may contribute to orofacial pain, but seems not to account for all the known clinical observations.

METHODS

Two groups of subjects were studied: 5 patients with COP and 59 control subjects. For each subject we collected blood samples for analysis of nitrite\nitrate (or NOx).

RESULTS

(1) NOx blood levels for 5 patients diagnosed with COP was 65.9 microM (SD of 10.4) verses 42.7 microM (SD of 24.2) for 59 control subjects, the difference being statistically significant, t-statistic = -2.12 (P > .05). (2) No statistical difference was found for NOx blood levels for 59 control subjects divided by gender (male vs female), with 23 female controls having NOx blood levels of 42.6 microM (SD of 25.2) and male controls having NOx blood levels of 42.8 microM (SD of 24.0), t-statistic = -0.03, P = .98.

CONCLUSION

This pilot study suggests that NO blood levels may have an association with COP. A better understanding of the mechanism of chronic orofacial pain is expected to lead to more precise diagnostic staging and management of this disorder.

Parathyroid hormone-related protein-mediated responses in pulmonary arteries and veins of newborn lambs

PTHrP has important roles in lung development and function. Here we determined the vasomotor responses of isolated pulmonary arteries and veins of newborn and adult sheep to PTHrP. In vessels constricted with endothelin-1, PTHrP (PTHrP 1-34) caused greater relaxation of veins than of arteries. In both vessel types, relaxation to the peptide was less in adult than in newborn vessels. In newborn lambs, PTHrP-induced relaxation was not affected by endothelium removal, inhibition of eNOS, or inhibition of adenylyl cyclases by SQ-22536. However, relaxation was attenuated by 4-aminopyridine, inhibitor of voltage-dependent potassium channels, in both arteries and veins, and by charybdotoxin, inhibitor of calcium-activated potassium channels, in veins. When vessels were saturated with 8-BrcAMP (3 x 10(-4) M), to eliminate relaxation mediated by endogenous cAMP, PTHrP-induced relaxation was partially attenuated. In vessels treated with 8-BrcAMP (3 x 10(-4) M), 4-aminopyridine but not charybdotoxin inhibited relaxation induced by PTHrP 1-34 in both arteries and veins. Radioimmunoassay showed that, in the presence of a general phosphodiesterase inhibitor, PTHrP caused a concentration-dependent increase in intracellular cAMP content in arteries and veins, which was largely abolished by SQ-22536. Our results demonstrate that PTHrP is a potent vasodilator of pulmonary vessels, with a greater effect in veins than in arteries. Relaxation induced by the peptide contains both cAMP-dependent and -independent components. In both arteries and veins, voltage-dependent potassium channels mediate the response to PTHrP, at least in part, in a cAMP-independent fashion; and in veins, calcium-activated potassium channels may be stimulated by elevated cAMP levels.

Role of veins in regulation of pulmonary circulation

Pulmonary veins have been seen primarily as conduit vessels; however, over the past two decades, a large amount of evidence has accumulated to indicate that pulmonary veins can exhibit substantial vasoactivity. In this review, the role of veins in regulation of the pulmonary circulation, particularly during the perinatal period and under certain pathophysiological conditions, is discussed. In the fetus, pulmonary veins contribute a significant fraction to total pulmonary vascular resistance. At birth, the veins as well as the arteries relax in response to endothelium-derived nitric oxide and dilator prostaglandins, thereby assisting in the fall in pulmonary vascular resistance. These effects are oxygen dependent and modulated by cGMP-dependent protein kinase. Under chronic hypoxic conditions, pulmonary veins undergo remodeling and demonstrate substantial constriction and hypertrophy. In a number of species, including the human, pulmonary veins are also the primary sites of action of certain vasoconstrictors such as endothelin and thromboxane. In various pathological conditions, there is an increased synthesis of these vasoactive agents that may lead to pulmonary venous constriction, increased microvascular pressures for fluid filtration, and formation of pulmonary edema. In conclusion, the significant role of veins in regulation of the pulmonary circulation needs to be appreciated to better prevent, diagnose, and treat lung disease.

Regulation of pulmonary venous tone in response to muscarinic receptor activation

We investigated cellular mechanisms that mediate or modulate the vascular response to muscarinic receptor activation (ACh) in pulmonary veins (PV). Isometric tension was measured in isolated canine PV rings with endothelium (E+) and without endothelium (E−). Tension and intracellular Ca2+concentration ([Ca2+]i) were measured simultaneously in fura-2-loaded E− PV strips. In the absence of preconstriction, ACh (0.01–10 μM) caused dose-dependent contraction in E+ and E− rings. ACh contraction was potentiated by removing the endothelium or by nitric oxide (NO) synthase inhibition ( N-nitro-l-arginine methyl ester, P = 0.001). Cyclooxygenase inhibition (indomethacin) reduced ACh contraction in both E+ and E− PV rings ( P = 0.013 and P = 0.037, respectively). ACh contraction was attenuated by inhibitors of voltage-operated Ca2+channels (nifedipine, P < 0.001), inositol-1,4,5-trisphosphate (IP3)-mediated Ca2+release (2-aminoethoxydiphenyl borate, P = 0.001), PKC (bisindolylmaleimide I, P = 0.001), Rho-kinase (Y-27632, P = 0.002), and tyrosine kinase (TK; tyrphostin 47, P = 0.015) in E− PV rings. ACh (1 μM) caused a leftward shift in the [Ca2+]i-tension relationship ( P = 0.015), i.e., ACh increased myofilament Ca2+sensitivity. Inhibition of PKC, Rho-kinase, and TK attenuated the ACh-induced increase in myofilament Ca2+sensitivity ( P < 0.001, P < 0.001, and P = 0.024, respectively). These findings indicate that in canine PV, ACh contraction is modulated by NO and partially mediated by metabolites of the cyclooxygenase pathway and involves Ca2+influx through voltage-operated Ca2+channels and IP3-mediated Ca2+release. In addition, ACh induces increased myofilament Ca2+sensitivity, which requires the PKC, Rho-kinase, and TK pathways.

Role of cGMP-dependent protein kinase in development of tolerance to nitric oxide in pulmonary veins of newborn lambs

Continuous exposure to nitrovasodilators and nitric oxide induces tolerance to their vasodilator effects in vascular smooth muscle. This study was done to determine the role of cGMP-dependent protein kinase (PKG) in the development of tolerance to nitric oxide. Isolated fourth-generation pulmonary veins of newborn lambs were studied. Incubation of veins for 20 h with DETA NONOate (DETA NO; a stable nitric oxide donor) significantly reduced their relaxation response to the nitric oxide donor and to beta-phenyl-1,N2-etheno-8-bromo-cGMP (8-Br-PET-cGMP, a cell-permeable cGMP analog). Incubation with DETA NO significantly reduced PKG activity and protein and mRNA levels in the vessels. These effects were prevented by 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (an inhibitor of soluble guanylyl cyclase) and Rp-8-Br-PET-cGMPS (an inhibitor of PKG). A decrease in PKG protein and mRNA levels was also observed after continuous exposure to cGMP analogs. The PKG inhibitor abrogated these effects. The decrease in cGMP-mediated relaxation and in PKG activity caused by continuous exposure to DETA NO was not affected by KT-5720, an inhibitor of cAMP-dependent protein kinase. Prolonged exposure to 8-Br-cAMP (a cell-permeable cAMP analog) did not affect PKG protein level in the veins. These results suggest that continuous exposure to nitric oxide or cGMP downregulates PKG by a PKG-dependent mechanism. Such a negative feedback mechanism may contribute to the development of tolerance to nitric oxide in pulmonary veins of newborn lambs.

Effect of oxygen on cyclic GMP-dependent protein kinase-mediated relaxation in ovine fetal pulmonary arteries and veins

Cyclic GMP-dependent protein kinase (PKG) plays an important role in regulating pulmonary vasomotor tone in the perinatal period. In this study, we tested the hypothesis that a change in oxygen tension affects PKG-mediated pulmonary vasodilation. Isolated intrapulmonary arteries and veins of near-term fetal lambs were first incubated for 4 h under hypoxic and normoxic conditions (Po2 of 30 and 140 mmHg, respectively) and then contracted with endothelin-1. 8-Bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP), a cell membrane-permeable analog of cGMP, induced a greater relaxation in vessels incubated in normoxia than in hypoxia. beta-Phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate, Rp isomer (Rp-8-Br-PET-cGMPS), a selective inhibitor of PKG, attenuated relaxation induced by 8-BrcGMP (10-4 and 3 x 10-4 M). In the presence of Rp-8-Br-PET-cGMPS, the differential responses to 8-BrcGMP between hypoxia and normoxia treatment were abolished in veins but not in arteries. cGMP-stimulated PKG activity was present in arteries but not in veins after 4 h of hypoxia. Both vessel types showed significant increase in cGMP-stimulated PKG activity after 4 h of normoxia. PKG protein (Western blot analysis) and PKG mRNA levels (quantitative RT-PCR) were greater in veins but not in arteries after 4-h exposure to normoxia vs. hypoxia. These results demonstrate that oxygen augments cGMP-mediated vasodilation of fetal pulmonary arteries and veins. Furthermore, the effect of oxygen on response of the veins to cGMP is due to an increase in the activity, protein level, and mRNA of PKG.

Oxygen-dependent signaling in pulmonary vascular smooth muscle

The pulmonary circulation constricts in response to acute hypoxia, which is reversible on reexposure to oxygen. On exposure to chronic hypoxia, in addition to vasoconstriction, the pulmonary vasculature undergoes remodeling, resulting in a sustained increase in pulmonary vascular resistance that is not immediately reversible. Hypoxic pulmonary vasoconstriction is physiological in the fetus, and there are many mechanisms by which the pulmonary vasculature relaxes at birth, principal among which is the acute increase in oxygen. Oxygen-induced signaling mechanisms, which result in pulmonary vascular relaxation at birth, and the mechanisms by which chronic hypoxia results in pulmonary vascular remodeling in the fetus and adult, are being investigated. Here, the roles of cGMP-dependent protein kinase in oxygen-mediated signaling in fetal pulmonary vascular smooth muscle and the effects of chronic hypoxia on ion channel activity and smooth muscle function such as contraction, growth, and gene expression were discussed.

Mechanisms of bradykinin-mediated dilation in newborn piglet pulmonary conducting and resistance vessels

Bradykinin (BK) is a potent dilator of the perinatal pulmonary circulation. We investigated segmental differences in BK-induced dilation in newborn pig large conducting pulmonary artery and vein rings and in pressurized pulmonary resistance arteries (PRA). In conducting pulmonary arteries and veins, BK-induced relaxation is abolished by endothelial disruption and by inhibition of nitric oxide (NO) synthase with nitro-L-arginine (L-NA). In PRA, two-thirds of the dilation response is L-NA insensitive. Charybdotoxin plus apamin and depolarization with KCl abolish the L-NA-insensitive dilations, findings that implicate the release of endothelium-derived hyperpolarizing factor (EDHF). However, endothelium-disrupted PRA retain the ability to dilate to BK but not to ACh or A-23187. In endothelium-disrupted PRA, dilation was inhibited by charybdotoxin. Thus in PRA, BK elicits dilation by multiple and duplicative signaling pathways. Release of NO and EDHF contributes to the response in endothelium-intact PRA; in endothelium-disrupted PRA, dilation occurs by direct activation of vascular smooth muscle calcium-dependent potassium channels. Redundant signaling pathways mediating pulmonary dilation to BK may be required to assure a smooth transition to extrauterine life.

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.

Effects of SQ 22536, an adenylyl cyclase inhibitor, on isoproterenol-induced cyclic AMP elevation and relaxation in newborn ovine pulmonary veins

The effects of inhibition of adenylyl cyclase on isoproterenol-induced relaxation were determined in isolated pulmonary veins of newborn lambs (7-12 days old). In veins constricted with endothelin-1, isoproterenol at concentrations < or = 3 x 10(-9) M had no effect on the cyclic AMP (cAMP) content but caused up to 56% relaxation. At higher concentrations (> or = 10(-8) M), isoproterenol elevated cAMP content and caused further relaxation. In veins constricted with endothelin-1 or U46619 (9,11-dideoxy-11, 9-epoxymethanoprostaglandin prostaglandin F2alpha), the cAMP elevation but not relaxation caused by isoproterenol was abolished by SQ 22536 [9-(tetrahydro-2-furanyl)-9H-purin-6-amine; an adenylyl cyclase inhibitor]. The effects of isoproterenol on vessel tension and cAMP content were inhibited by propranolol. Rp-8-CPT-cAMPS [8-(4-Chlorophenylthio)-adenosine-3',5'-cyclic monophosphorothioate, Rp-isomer] and Rp-8-Br-PET-cGMPS [beta-phenyl-1, N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate, Rp-isomer], inhibitors of cAMP- and guanosine-3',5'-cyclic monophosphate (cGMP)-dependent protein kinases, respectively, attenuated relaxation caused by a cAMP analog but not that by isoproterenol. In the crude membrane preparations of pulmonary veins, an increase in the activity of adenylyl cyclase caused by isoproterenol was abolished by propranolol and SQ 22536. These results suggest that cAMP may not play a critical role in isoproterenol-induced relaxation of pulmonary veins of newborn lambs.

Differential responses of newborn pulmonary arteries and veins to atrial and C-type natriuretic peptides

Atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are important dilators of the pulmonary circulation during the perinatal period. We compared the responses of pulmonary arteries (PA) and veins (PV) of newborn lambs to these peptides. ANP caused a greater relaxation of PA than of PV, and CNP caused a greater relaxation of PV than of PA. RIA showed that ANP induced a greater increase in cGMP content of PA than CNP. In PV, ANP and CNP caused a similar moderate increase in cGMP content. Receptor binding study showed more specific binding sites for ANP than for CNP in PA and more for CNP than for ANP in PV. Relative quantitative RT-PCR for natriuretic peptide receptor A (NPR-A) and B (NPR-B) mRNAs show that, in PA, NPR-A mRNA is more prevalent than NPR-B mRNA, whereas, in PV, NPR-B mRNA is more prevalent than NPR-A mRNA. In conclusion, in the pulmonary circulation, arteries are the major site of action for ANP, and veins are the major site for CNP. Furthermore, the differences in receptor abundance and the involvement of a cGMP-independent mechanism may contribute to the heterogeneous effects of the natriuretic peptides in PA and PV of newborn lambs.

5-Hydroxytryptamine-induced microvascular pressure transients in lungs of anaesthetized rabbits

We determined lung microvascular pressure transients induced by 5-hydroxytryptamine (5HT), by the micropuncture technique. We mechanically ventilated anaesthetized (halothane 0.8%), open-chested rabbits, in which we recorded pulmonary artery (PA), left atrial (LA) and carotid artery pressures and cardiac output. For 4-min periods of stopped ventilation, we constantly inflated the lung with airway pressure of 7 cmH2O, then micropunctured the lung to determine pressures in arterioles and venules of 20-25 microm diameter. An intravenous bolus infusion of 5HT (100 microg), increased total pulmonary vascular resistance by 59%. Prior to 5HT infusion, the arterial, microvascular and venous segments comprised 30, 50 and 19% of the total pulmonary vascular pressure drop, respectively. However 14 s after 5HT infusion, the PA-arteriole pressure difference (arterial pressure drop) increased 46%, while the venule-LA pressure difference (venous pressure drop) increased >100%. The arteriole-venule pressure difference (microvascular pressure drop) was abolished. The increase in the arterial pressure drop was maintained for 4.8 min, whereas the increased venous pressure drop reverted to baseline in <1 min. We conclude that in the rabbit lung in situ, a 5HT bolus causes sustained arterial constriction and a strong but transient venous constriction.

SQ22536 and W-7 inhibit forskolin-induced cAMP elevation but not relaxation in newborn ovine pulmonary veins

The role of cAMP in forskolin-induced relaxation was studied in isolated pulmonary veins of newborn lambs (7-12 days). In vessels preconstricted with endothelin-1, forskolin at concentrations < or =10(-7) M had no effect on cAMP content and adenylyl cyclase activity but caused up to 50% relaxation. At higher concentrations, forskolin markedly elevated cAMP content and adenylyl cyclase activity and caused a further relaxation. SQ22536 [9-(tetrahydro-2-furanyl)-9H-purin-6-amine; an adenylyl cyclase inhibitor] and W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalensulfonamide; a calmodulin-dependent adenylyl cyclase inhibitor] had no significant effect on forskolin-induced relaxation but markedly inhibited the elevation of cAMP content and adenylyl cyclase activity caused by forskolin. Rp-8-CPT-cAMPS [8-(4-chlorophenylthio)-adenosine-3',5'-cyclic monophosphorothioate; an inhibitor of cAMP-dependent protein kinases] and Rp-8-Br-PET-cGMPS (beta-phenyl-1, N(2)-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate; an inhibitor of cGMP-dependent protein kinases) attenuated the relaxation caused by a cAMP analog but not that caused by forskolin. These results suggest that cAMP may not play a major role in forskolin-induced relaxation of pulmonary veins of newborn lambs.

Relaxant effects of carbon monoxide compared with nitric oxide in pulmonary and systemic vessels of newborn piglets

Nitric oxide (NO) has been implicated in a number of diverse physiologic processes, including regulation of vascular tone. Carbon monoxide (CO) is another endogenously generated diatomic gas that may play an important physiologic role in vascular smooth muscle homeostasis. The purpose of this study was to compare the responses to exogenous NO and CO in isolated vessels (pulmonary arteries, pulmonary veins, and mesenteric arteries) from 12- to 24-h-old and 2-wk-old piglets. Vessels precontracted with the thromboxane A(2) mimetic U46619 (10(-7) M) relaxed in response to CO (2 x 10(-6) to 2 x 10(-4) M) and NO (2 x 10(-9) to 2 x 10(-7) M); these effects were not affected by endothelium removal but were completely abolished by the soluble guanylate cyclase inhibitor ODQ (10(-5) M). In pulmonary arteries, the maximal relaxation to NO increased with postnatal age from 33 +/- 4% of the precontraction value to 56 +/- 5%, in 12- to 24-h-old and 2-week-old piglets, respectively (p < 0.01), but the response to CO decreased from 25 +/- 3% to 12 +/- 1%, respectively (p < 0.01). The maximal response to CO was greater in pulmonary veins than in pulmonary or mesenteric arteries for both age groups (p < 0.01). Vasorelaxation induced by endogenous NO (stimulated by acetylcholine) was also greater in pulmonary veins when compared with pulmonary arteries and increased with postnatal age in both vessels. In contrast, no age-related differences were observed in the vasorelaxation induced by the cGMP analog 8-bromo cGMP in pulmonary arteries. When the response to NO was analyzed under three different extracellular O(2) concentrations (PO(2) 4.51 +/- 0.03, 19. 32 +/- 0.17, and 86 +/- 0.62, kPa), no significant differences were found. However, in the presence of superoxide dismutase (100 U/mL). the response to CO remained unchanged, and the response to NO improved in pulmonary arteries from 2-week-old but not from newborn piglets. In conclusion, both NO and CO relaxed neonatal vessels through soluble guanylate cyclase activation. However, when compared with NO, CO exhibited a poor vasorelaxant activity. Pulmonary vasorelaxation induced by NO increased with postnatal age, whereas that induced by CO decreased. Changes in extracellular oxygen concentration did not alter the pulmonary vascular response to NO. However, the presence of superoxide dismutase improved the response to NO, indicating that oxidant activity limits the vasorelaxant response to NO but not to CO.

Involvement of cGMP-dependent protein kinase in the relaxation of ovine pulmonary arteries to cGMP and cAMP

Agonist-induced smooth muscle relaxation occurs following an increase in intracellular concentrations of cGMP or cAMP. However, the role of protein kinase G (PKG) and/or protein kinase A (PKA) in cGMP- or cAMP-mediated pulmonary vasodilation is not clearly elucidated. In this study, we examined the relaxation responses of isolated pulmonary arteries of lambs (age = 10 +/- 1 days), preconstricted with endothelin-1, to increasing concentrations of 8-bromo-cGMP (8-BrcGMP) or 8-BrcAMP (cell-permeable analogs), in the presence or absence of Rp-8-beta-phenyl-1,N(2)-etheno-bromoguanosine cyclic monosphordthioate (Rp-8-PET-BrcGMPS) or KT-5720, selective inhibitors of PKG and PKA, respectively. When examined for specificity, Rp-8-Br-PET-cGMPS abolished PKG, but not PKA, activity in pulmonary arterial extracts, whereas KT-5720 inhibited PKA activity only. 8-BrcGMP-induced relaxation was inhibited by the PKG inhibitor only, whereas 8-BrcAMP-induced relaxation was inhibited by both inhibitors. A nearly fourfold higher concentration of cAMP than cGMP was required to relax arteries by 50% and to activate PKG by 50%. Our results demonstrate that relaxation of pulmonary arteries is more sensitive to cGMP than cAMP and that PKG plays an important role in both cGMP- and cAMP-mediated relaxation.

Physiologic significance of nitric oxide and endothelin-1 in circulatory adaptation

BACKGROUND

The purpose of the present paper was to evaluate the physiologic significance of nitric oxide (NO) and endothelin (ET)-1 in circulatory adaptation in the neonate.

METHODS

The serum levels of NO metabolites (NOx; the sum of nitrites and nitrates) and the plasma level of ET-1 were determined in 14 healthy full-term infants at 0-6 h, 24 h and 5 days after birth. We measured the heart rate, the mean systemic blood pressure and the mean pulmonary arterial pressure, estimated by pulsed Doppler echocardiography, at each time point.

RESULTS

The serum concentration of NOx was lowest at birth and increased with age. The plasma concentration of ET-1 was highest at birth and decreased with age. The ratio of NOx to ET-1 was inversely related to the estimated mean pulmonary arterial pressure in the early neonatal period. The ratio of NOx to ET-1 was not correlated with the systemic blood pressure.

CONCLUSION

Increased NO synthesis and decreased production of ET-1 during the early neonatal period may contribute to the decrease in pulmonary arterial pressure.

Functional studies of leukotriene receptors in vascular tissues

The paradoxical effects of cysteinyl-leukotrienes, namely contraction and relaxation, are now well documented in a number of vascular preparations from various species. The vascular smooth muscle contractions are associated with activation of a single receptor subtype and in some vascular smooth muscles with activation of two receptor subtypes. However, the receptors implicated in the contraction of vessels such as pig pulmonary arteries and veins, dog inferior vena cava, and dog splenic and mesenteric veins remain to be established. There are sufficient data concerning some vascular tissues to suggest that relaxations induced by cysteinyl-leukotrienes are via the stimulation of specific receptors present on the endothelium. The endothelium in human pulmonary arteries has one receptor (CysLT2) and activation induced the release of NO. However, in isolated human pulmonary veins two receptors are present, CysLT1 and CysLT2 (Figure 1). Activation of the former induced the release of a contractile factor whereas activation of the CysLT2 receptor released NO. In guinea pig pulmonary artery and guinea pig thoracic aorta, one receptor has been demonstrated since the relaxations are blocked by ICI-198615. These data suggest the presence of a CysLT1 receptor. Activation of this receptor leads to the release of a relaxant factor, namely, nitric oxide. In contrast, in human pulmonary arteries and veins activation of a receptor that is resistant to ICI-198615 is associated with NO release. These results suggest that there may be species differences even when analogous vascular preparations are examined. While the cysteinyl-leukotrienes are known to relax vascular smooth muscle in a variety of preparations from different species, there are presently two pathways known to be involved in this response. One involves the metabolites of arachidonic acid via the cyclooxygenase enzymatic pathway and the other implicates products of the L-arginine enzymatic pathway. Although both pathways may be present and active in the endothelium of the vascular preparations only one of these enzymatic pathways may be dominant and responsible for the relaxations observed. Ortiz and coworkers have demonstrated that in pulmonary veins the dominant pathway for cysteinyl-leukotriene relaxations is the NO pathway. There are some reports from animal studies that support a dominant role for NO in pulmonary veins. In contrast, Allen and co-workers demonstrated that the LTC4-induced relaxations in isolated human saphenous veins were not modified by treatment of tissues with an NO inhibitor but were significantly enhanced after treatment with indomethacin. These authors suggested that a contracting factor derived from the arachidonic acid pathway was released in preparations challenged with LTC4. In addition, these investigators demonstrated that the NO inhibitor had no effect on the LTC4 relaxations. Together, these results suggest that cysteinyl-leukotriene effects in human pulmonary veins are dominated by the NO pathway whereas in human systemic veins these mediator effects are modified by metabolites of the cyclooxygenase pathway. Unfortunately, most studies involving the actions of cysteinyl-leukotrienes on vessels have been performed in the presence of indomethacin, making interpretation of the relative contribution of the cyclooxygenase and NO pathways difficult. In any event, the cysteinyl-leukotrienes may have a prominent role in the activation of these pathways and the receptors involved have not been clearly established.

Newborn intrapulmonary veins are more reactive than arteries in normal and hypertensive piglets

The reactivity of pulmonary veins during adaptation from pre- to postnatal life is not well characterized. With an in vitro organ bath technique, the responses to the contractile and relaxant agonists U-46619 (10(-10) to 3 x 10(-6) M) and acetylcholine (10(-9) to 10(-4) M) were compared in adjacent conduit pulmonary vein and artery rings from 66 piglets aged 1 wk preterm to 14 days of postnatal life and from adult tissue. Five additional piglets were made hypertensive by exposure to chronic hypoxia for 3 days after birth. Both arteries and veins showed smaller contractile and relaxant responses before birth than after. By 5 min after birth, the contraction by arteries and relaxation by veins had increased (P < 0.05). By 3 days of age, arterial relaxation increased, but in all animals, venous relaxation exceeded that in arteries (P < 0.05). Veins contracted more than arteries in animals aged 3-14 days. Neonatal hypoxia diminished the responses to both agonists in the veins (P < 0.05), whereas the response in the arteries remained similar to that in the normal newborn. We speculate that veins may be more important in postnatal adaptation than previously suggested.

Developmental change in magnesium sulfate-induced relaxation of rabbit pulmonary arteries

Magnesium causes a variety of vascular smooth muscle to relax. The present study was designed to determine whether there is a developmental change in the magnesium-induced response of pulmonary vasculature. Isolated pulmonary arteries (PA) of newborn (1- to 3-day-old) and juvenile (4- to 6-wk-old) rabbits were suspended in organ chambers filled with modified Krebs-Ringer bicarbonate solution (95% O(2)-5% CO(2), 37.0 degrees C), and their isometric tension was recorded. In arteries preconstricted with endothelin-1 to a similar tension level, MgSO(4) caused greater relaxation of juvenile rabbit PA than that of the newborn rabbit PA. Verapamil, a voltage-dependent Ca(2+) channel blocker, attenuated magnesium-induced relaxation in juvenile rabbit PA but not in newborn PA. The uptake of Ca(2+) of juvenile rabbit PA was inhibited by MgSO(4), and the inhibition was attenuated by verapamil. The uptake of Ca(2+) of newborn rabbit PA was smaller than that of the juvenile PA and was not significantly affected by MgSO(4) and verapamil. These results demonstrate that there is a developmental increase in the dilator effect of MgSO(4) in rabbit PA. In newborn rabbit PA, an incomplete maturation of the voltage-dependent Ca(2+) channels may contribute to the smaller vasodilation induced by MgSO(4).

Effect of homocysteine on the production of nitric oxide in endothelial cells

1. Homocysteine decreased the release of nitric oxide in cultured human umbilical vein endothelial cells. 2. Homocysteine did not affect constitutive and inducible nitric oxide synthase activity.

Role of protein kinase G in nitric oxide- and cGMP-induced relaxation of newborn ovine pulmonary veins

In a variety of systemic blood vessels, protein kinase G (PKG) plays a critical role in mediating relaxation induced by agents that elevate cGMP, such as nitric oxide. The role of PKG in nitric oxide- and cGMP-induced relaxation is less certain in the pulmonary circulation. In the present study, we examined the effects of inhibitors of PKG on the responses of isolated fourth-generation pulmonary veins of newborn lambs (10 +/- 1 days of age) to nitric oxide and cGMP. In vessels preconstricted with endothelin-1, nitric oxide and 8-bromo-cGMP (a cell-membrane-permeable cGMP analog) induced concentration-dependent relaxation. The relaxation was significantly attenuated by beta-phenyl-1, N(2)-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothionate (Rp-8-Br-PET-cGMPS; a PKG inhibitor) and N-[2-(methylamino)ethyl]5-isoquinolinesulfonamide [H-8; an inhibitor of PKG and protein kinase A (PKA)] but was not affected by KT-5720 (a PKA inhibitor). Biochemical study showed that PKG activity in newborn ovine pulmonary veins was inhibited by 8-Br-PET-cGMPS and H-8 but not by KT-5720. PKA activity was not affected by 8-Br-PET-cGMPS but was inhibited by H-8 and KT-5720. These results suggest that PKG is involved in relaxation of pulmonary veins of newborn lambs induced by nitric oxide and cGMP.

Activation of soluble guanylate cyclase and potassium channels contribute to relaxations to nitric oxide in smooth muscle derived from canine femoral veins

Experiments were designed to examine mechanisms of relaxations to nitric oxide (NO) in venous smooth muscle. Rings of canine femoral veins without endothelium were suspended for measurement of isometric force in organ chambers. Concentration-response curves to NO and 8-Br-cyclic guanosine monophosphate (cGMP) were obtained in veins contracted with KCl (60 mM) or prostaglandin F2alpha (PGF2alpha; 2x10(-6) M) in the absence and presence of inhibitors of soluble or particulate guanylate cyclase or K+ channel antagonists. In rings contracted with PGF2alpha, relaxations to NO were reduced significantly by inhibition of soluble but not particulate guanylate cyclase. Relaxations to NO were reduced in rings contracted with KCI. Tetraethylammonium (10(-2) M) and glibenclamide (10(-7) M) + charybdotoxin (10(-7) M) significantly reduced relaxations to NO in rings contracted with PGF2alpha. Relaxations to 8-Br-cGMP were decreased significantly only by charybdotoxin. These results suggest that relaxations to NO in canine femoral veins involve at least two intracellular processes: activation of soluble guanylate cyclase and activation of adenosine triphosphate (ATP)-sensitive and large-conductance, Ca+2-activated K+ channels. The large-conductance, Ca+2-activated K+ channels seem to be activated by cGMP-dependent mechanisms. Therefore relaxations to NO in venous smooth muscle involve intracellular processes similar to those in arterial smooth muscle.

Phosphodiesterase activity in intrapulmonary arteries and veins of perinatal lambs

The transition from fetal to newborn life is marked by a reduction in pulmonary vascular tone mediated by the intracellular second messengers, cGMP and cAMP. We have compared the rates of phosphodiesterase (PDE)-catalyzed hydrolysis of cGMP and cAMP in intrapulmonary vessels of fetal (146 +/- 2 days gestation) and newborn (3-7-day-old) lambs, each n = 6. Lung vessels of second to sixth generations were dissected and cytosol was prepared by differential centrifugation. PDE activity in cytosol was determined by radiometric assay of the hydrolysis of exogenous nucleotides at 30 degrees C for 10 min. Rates of hydrolysis (pmol/min/mg protein) of cGMP were 225 +/- 38 in fetal arteries and different from 151 +/- 7 in veins. In newborn vessels, the rates were 155 +/- 49 and 63 +/- 13 in arteries and veins, respectively. Rates of cAMP hydrolysis by the fetus were 80 +/- 11 in arteries and 45 +/- 16 veins. In newborn lambs the rates were 69 +/- 10 in arteries and different from 18 +/- 4 in veins. Inhibition of PDE activity by zaprinast, a cGMP-specific PDE inhibitor, and rolipram, a cAMP-specific PDE inhibitor, was more in veins of fetal and newborn lambs. Our data show that rates of hydrolysis of the cyclic nucleotides were faster in fetal vessels than in the newborn. We speculate that this would result in a greater accumulation of the cyclic nucleotides in newborn vessels, particularly the veins, and therefore endow the veins with less vascular tone.

Heterogeneity in endothelium-derived nitric oxide-mediated relaxation of different sized pulmonary arteries of newborn lambs

Endothelium-derived nitric oxide (EDNO) plays a pivotal role in regulating pulmonary circulation. To determine whether there is a heterogeneity in EDNO-mediated responses of different sized pulmonary vessels, we studied small and large isolated pulmonary arteries of newborn lambs (diameter, 0.4-0.7 and 1.5-2.5 mm, respectively). The isometric tension of vessel rings were recorded while suspended in organ chambers filled with modified Krebs-Ringer bicarbonate solution (95% O2-5% CO2, 37 degrees C). In vessels preconstricted with norepinephrine, acetylcholine and bradykinin induced a greater relaxation of small pulmonary arteries than of large pulmonary arteries. Acetylcholine, bradykinin, and nitric oxide also induced a greater increase in cGMP content in small arteries than in large ones. The responses to acetylcholine and bradykinin were endothelium-dependent and inhibited by nitro-L-arginine, an inhibitor of nitric oxide synthase. In vessels without endothelium, the response to nitric oxide was inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylate cyclase. The activity of soluble guanylyl cyclase of small arteries was greater than that of large arteries under basal conditions and after stimulation with S-nitroso-N-acetylpenicillamine, a nitric oxide donor. These results demonstrate that heterogeneity exists in EDNO-mediated relaxation of small and large pulmonary arteries in newborn lambs. A difference in the soluble guanylate cyclase activity of vascular smooth muscle may have contributed to this phenomenon.

5,6-Epoxyeicosatrienoic acid reduces increases in pulmonary vascular resistance in the dog

We recently reported that canine pulmonary microsomes metabolize arachidonic acid to all four regioisomeric epoxyeicosatrienoic acids (EET). 5,6-EET dilates blood vessels in several nonpulmonary vascular beds, often in a cyclooxygenase-dependent manner. The present study was designed to determine whether 5,6-EET can decrease pulmonary vascular resistance (PVR) in the intact pulmonary circulation. In isolated canine lungs perfused with physiological salt solution, a constant infusion of U-46619 (3.28 +/- 0.99 nmol/min) increased PVR 62.1 +/- 4.5%. Administration of 5,6-EET (10(-5) M) into the perfusate reduced the U-46619-mediated increase in PVR by 23.6 +/- 6.1%. These effects of U-46619 and 5,6-EET were limited to changes in resistance solely in the pulmonary venous segment. In contrast, venous as well as arterial segmental resistances were increased in 5-hydroxytryptamine (5-HT)-treated lungs. However, in the latter instance, 5,6-EET reduced arterial but not venous segmental resistance. 5,6-EET increased pulmonary PGI2 synthesis from 70.5 +/- 18.4 to 675.9 +/- 125.4 ng/min. In the presence of indomethacin (10(-4) M), 5,6-EET did not increase PGI2 synthesis nor did it decrease U-46619- or 5-HT-mediated increases in PVR. In canine intrapulmonary vessels, 5,6-EET decreased active tension in veins contracted with U-46619. 5,6-EET decreased active tension in arteries but not veins contracted with 5-HT, consistent with results in the perfused lungs. These results demonstrate that 5, 6-EET is a vasodilator in the intact pulmonary circulation. Its dilator activity depends on the constrictor agent present, the segmental resistance, and cyclooxygenase activity.

The possible role of nitric oxide in the physiopathology of pain associated with temporomandibular joint disorders

Temporomandibular joint disorders (TMD) pose a significant challenge to the practice of oral and maxillofacial surgery. When painful, TMD are generally associated with hyperthermia of the overlying skin. It is hypothesized that this skin hyperthermia, caused by regional vasodilation, is induced by nitric oxide (NO) produced in the extravascular space of the joint. Extravascular NO can be produced by osteoblasts, chondrocytes and macrophages, or by stimulated neurons. It is suggested that this kind of pain is associated with NO-enhanced sensitivity of the peripheral nociceptors. Verification and clinical implications of the proposed mechanism are discussed.

Contractile properties of intralobar pulmonary arteries and veins in the fetal lamb model of congenital diaphragmatic hernia

BACKGROUND/PURPOSE

Pulmonary hypertension plays a significant role in the pathophysiology of congenital diaphragmatic hernia (CDH). Although there has been an intensive research effort directed at mediators that may cause pulmonary vasoconstriction, no single agent has been identified. The authors hypothesize that there may be an alteration in the cGMP-nitric oxide (NO) pathway of vasodilatation contributing to the pulmonary hypertension observed in CDH. The purpose of these studies is to begin to elucidate vasoactive properties of pulmonary vessels with particular attention to the cGMP-NO pathway of vasodilatation in fetal lambs with CDH.

METHODS

Fourth-generation pulmonary arteries and pulmonary veins were dissected from both right and left lungs of eight, 139-day gestational fetuses with surgically created CDH. Vessels were studied with standard isolated tissue bath techniques. Experiments examined basal release of NO in endothelium-intact PVs and PAs of both right and left lungs by measuring the contractile force of vessels constricted with norepinephrine (NE) in the presence and absence of the nitric oxide synthase (NOS) inhibitor N(omega)-nitro-L-arginine (L-NA). Concentration-response curves to the vasodilating agents zaprinast and A23187 were also obtained in vessels contracted by NE.

RESULTS

Left and right pulmonary artery responses to NE are enhanced over those of historic controls. Pretreatment of left pulmonary arteries with L-NA enhances the vasoconstrictor response to NE, whereas right PAs show no increased response. Relaxation responses to A23187 and zaprinast, in both left and right pulmonary arteries were not different from control lambs. Relaxation responses of both left and right pulmonary veins to A23187 and zaprinast are blunted compared with controls. This blunting is significantly more in left pulmonary veins than right. Further, right but not left pulmonary veins display enhanced vasoconstrictive response to NE after L-NA pretreatment.

CONCLUSIONS

The NO-cGMP pathway of vasodilatation is abnormal in the near term, fetal lamb with CDH. These abnormalities were most apparent in pulmonary veins and may reflect abnormal NOS activity or content between left and right lungs of the fetal lamb with CDH. Pulmonary arteries from CDH lambs have basal and stimulated NO release equal to that of historic controls but appear to be hypersensitive to exogenous vasoconstrictors.

Developmental change in isoproterenol-mediated relaxation of pulmonary veins of fetal and newborn lambs

beta-Adrenergic agonists are important regulators of perinatal pulmonary circulation. They cause vasodilation primarily via the adenyl cyclase-adenosine 3',5'-cyclic monophosphate (cAMP) pathway. We examined the responses of isolated fourth-generation pulmonary veins of term fetal (145 +/- 2 days gestation) and newborn (10 +/- 1 days) lambs to isoproterenol, a beta-adrenergic agonist. In vessels preconstricted with U-46619 (a thromboxane A2 analog), isoproterenol induced greater relaxation in pulmonary veins of newborn lambs than in those of fetal lambs. The relaxation was eliminated by propranolol, a beta-adrenergic antagonist. Forskolin, an activator of adenyl cyclase, also caused greater relaxation of veins of newborn than those of fetal lambs. 8-Bromoadenosine 3',5'-cyclic monophosphate, a cell membrane-permeable analog of cAMP, induced a similar relaxation of all vessels. Biochemical studies show that isoproterenol and forskolin induced a greater increase in cAMP content and in adenyl cyclase activity of pulmonary veins in the newborn than in the fetal lamb. These results demonstrate that beta-adrenergic-agonist-mediated relaxation of pulmonary veins increases with maturation. An increase in the activity of adenyl cyclase may contribute to the change.

Differential responses of pulmonary arteries and veins to histamine and 5-HT in lung explants of guinea-pigs

1. The mechanisms by which histamine and 5-HT differentially contract pulmonary arteries and veins are unclear. In lung explants from 26 guinea-pigs, we compared responses of pulmonary arteries and vein to histamine, 5-HT and KCI, and examined potential determinants for the differential responses. Lungs were filled with agarose, sectioned into approximately 1 mm thick slices, and vascular luminal areas measured by image analysis. 2. Histamine and 5-HT produced a concentration-dependent constriction in arteries and veins, greater in the latter. KCl constricted arteries and veins equally. 3. The histamine H1 antagonist chlorpheniramine (10(-4) M) abolished contractions to histamine; the H2 antagonist cimetidine enhanced maximal responses and sensitivity of arteries and veins to histamine, and diminished the differences between their maximal responses; the NO synthase inhibitor Nomega-nitro-L-arginine (L-NOARG) increased the maximal responses of arteries and veins, and the differences between their responses; indomethacin had no effect. 4. Contractions to 5-HT were abolished in arteries and markedly reduced in veins by the 5-HT2 antagonist ketanserin (10(-4) M); L-NOARG potentiated the maximal responses of arteries but not of veins; indomethacin increased the maximal responses of arteries but reduced them in veins. 5. By morphometry, arteries had a greater medial thickness and luminal diameter than veins. 6. The data suggest that in guinea-pigs, H2 receptors are responsible for the differential contractile responses of pulmonary arteries and veins to histamine, whereas endothelium-derived vasoactive substances are responsible for their differential contractile responses to 5-HT.

Effect of selective phosphodiesterase inhibitors on response of ovine pulmonary arteries to prostaglandin E2

Several adenosine 3',5'-cyclic monophosphate (cAMP)-hydrolyzing phosphodiesterase isozymes are present in the pulmonary vasculature. The present study was designed to determine the effect of selective inhibitors of phosphodiesterase subtypes on prostaglandin E2 (PGE2)-induced relaxation of isolated fourth-generation pulmonary arteries of newborn lambs. PGE2 and forskolin caused pulmonary arteries to relax and induced an increase in the intracellular cAMP content in the vessels. The relaxation and change in cAMP content were augmented by milrinone and rolipram, inhibitors of phosphodiesterase type 3 (PDE3) and type 4 (PDE4), respectively. The augmentation in relaxation and the increase in cAMP content caused by milrinone plus rolipram was greater than the sum of the responses caused by either of the inhibitors alone. 8-Methoxymethyl-1-methyl-3-(2-methylpropyl)xanthine, an inhibitor of phosphodiesterase type 1, had no effect on relaxation and change in cAMP induced by PGE2 and forskolin. Acetylcholine alone had no effect on cAMP content in the vessels but augmented the relaxation and the increase in cAMP induced by PGE2 and forskolin in arteries with endothelium. This effect was not observed in arteries without endothelium or in arteries with endothelium treated with NG-nitro-L-arginine. These results suggest that PDE3 and PDE4 are the primary enzymes hydrolyzing cAMP of pulmonary arteries of newborn lambs and that an inhibition of both PDE3 and PDE4 would result in a greater effect than that caused by inhibition of either one of the subtype isozymes alone. Furthermore, endothelium-derived nitric oxide may enhance cAMP-mediated relaxation by inhibition of PDE3.

Heterogeneous distribution of soluble guanylate cyclase in the pulmonary vasculature of the fetal lamb

BACKGROUND

Vascular segments in the fetal lung differ anatomically and functionally from one another. At birth, the nitric oxide (NO) pathway plays an integral role in reducing pulmonary vascular resistance through a marked vasodilation. However, the contributions of each vascular segment to this dilation are unclear. We sought to determine the distribution of soluble guanylate cyclase (sGC), the enzyme NO activates to induce vasodilation across the pulmonary vasculature.

METHODS

Pulmonary airspaces were expanded with freezing compound and the pulmonary arterial tree was infused with barium sulfate gelatin. Soluble guanylate cyclase was localized by immunohistochemistry across the pulmonary vasculature of four near-term fetal lambs and its immunoreaction product was assessed by a semiquantitative method. The physiologic response of fourth- and fifth-generation arteries and veins isolated from age-matched lambs to NO was measured using standard tissue bath techniques.

RESULTS

Clear differences in sGC immunostaining were present throughout the pulmonary vasculature: very weak to absent in large arteries accompanying bronchi, but intensely positive for veins. This pronounced staining for sGC in preacinar veins correlated with a 100-fold greater sensitivity to NO in veins compared to arteries of the same generation. The percentage of arteries staining positively approached 100% at the level of respiratory bronchioles and alveoli.

CONCLUSIONS

These findings suggest that the increased response to NO in preacinar veins compared to that of arteries is in part due to increased sGC within venous vascular smooth muscle. Furthermore, intense staining within distal arteries implies a greater role for NO-mediated vasodilation within these segments.