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

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.

Exercise training and vascular heterogeneity in db/db mice: evidence for regional- and duration-dependent effects

Exercise training (ET) has several health benefits; however, our understanding of regional adaptations to ET is limited. We examined the functional and molecular adaptations to short- and long-term ET in elastic and muscular conduit arteries of db/db mice in relation to changes in cardiovascular risk factors. Diabetic mice and their controls were exercised at moderate intensity for 4 or 8 weeks. The vasodilatory and contractile responses of thoracic aortae and femoral arteries isolated from the same animals were examined. Blood and aortic samples were used to measure hyperglycemia, oxidative stress, inflammation, dyslipidemia, protein expression of SOD isoforms, COX, eNOS, and Akt. Short-term ET improved nitric oxide (NO) mediated vasorelaxation in the aortae and femoral arteries of db/db mice in parallel with increased SOD2 and SOD3 expression, reduced oxidative stress and triglycerides, and independent of weight loss, glycemia, or inflammation. Long-term ET reduced body weight in parallel with reduced systemic inflammation and improved insulin sensitivity along with increased SOD1, Akt, and eNOS expression and improved NO vasorelaxation. Exercise did not restore NOS- and COX-independent vasodilatation in femoral arteries, nor did it mitigate the hypercontractility in the aortae of db/db mice; rather ET transiently increased contractility in association with upregulated COX-2. Long-term ET differentially affected the aortae and femoral arteries contractile responses. ET improved NO-mediated vasodilation in both arteries likely due to collective systemic effects. ET did not mitigate all diabetes-induced vasculopathies. Optimization of the ET regimen can help develop comprehensive management of type 2 diabetes.

Transforming growth factor-β downregulates sGC subunit expression in pulmonary artery smooth muscle cells via MEK and ERK signaling

TGFβ activation during newborn lung injury decreases the expression of pulmonary artery smooth muscle cell (PASMC)-soluble guanylate cyclase (sGC), a critical mediator of nitric oxide signaling. Using a rat PASMC line (CS54 cells), we determined how TGFβ downregulates sGC expression. We found that TGFβ decreases sGC expression through stimulating its type I receptor; TGFβ type I receptor (TGFβR1) inhibitors prevented TGFβ-1-mediated decrease in sGCα1 subunit mRNA levels in the cells. However, TGFβR1-Smad mechanisms do not regulate sGC; effective knockdown of Smad2 and Smad3 expression and function did not protect sGCα1 mRNA levels during TGFβ-1 exposure. A targeted small-molecule kinase inhibitor screen suggested that MEK signaling regulates sGC expression in TGFβ-stimulated PASMC. TGFβ activates PASMC MEK/ERK signaling; CS54 cell treatment with TGFβ-1 increased MEK and ERK phosphorylation in a biphasic, time- and dose-dependent manner. Moreover, MEK/ERK activity appears to be required for TGFβ-mediated sGC expression inhibition in PASMC; MEK and ERK inhibitors protected sGCα1 mRNA expression in TGFβ-1-treated CS54 cells. Nuclear ERK activity is sufficient for sGC regulation; heterologous expression of a nucleus-retained, constitutively active ERK2-MEK1 fusion protein decreased CS54 cell sGCα1 mRNA levels. The in vivo relevance of this TGFβ-MEK/ERK-sGC downregulation pathway is suggested by the detection of ERK activation and sGCα1 protein expression downregulation in TGFβ-associated mouse pup hyperoxic lung injury, and the determination that ERK decreases sGCα1 protein expression in TGFβ-1-treated primary PASMC obtained from mouse pups. These studies identify MEK/ERK signaling as an important pathway by which TGFβ regulates sGC expression in PASMC.

Antenatal BAY 41-2272 reduces pulmonary hypertension in the rabbit model of congenital diaphragmatic hernia

Infants with congenital diaphragmatic hernia (CDH) fail to adapt at birth because of persistent pulmonary hypertension (PH), a condition characterized by excessive muscularization and abnormal vasoreactivity of pulmonary vessels. Activation of soluble guanylate cyclase by BAY 41-2272 prevents pulmonary vascular remodeling in neonatal rats with hypoxia-induced PH. By analogy, we hypothesized that prenatal administration of BAY 41-2272 would improve features of PH in the rabbit CDH model. Rabbit fetuses with surgically induced CDH at day 23 of gestation were randomized at day 28 for an intratracheal injection of BAY 41-2272 or vehicle. After term delivery (day 31), lung mechanics, right ventricular pressure, and serum NH2-terminal-pro-brain natriuretic peptide (NT-proBNP) levels were measured. After euthanasia, lungs were processed for biological or histological analyses. Compared with untouched fetuses, the surgical creation of CDH reduced the lung-to-body weight ratio, increased mean terminal bronchial density, and impaired lung mechanics. Typical characteristics of PH were found in the hypoplastic lungs, including increased right ventricular pressure, higher serum NT-proBNP levels, thickened adventitial and medial layers of pulmonary arteries, reduced capillary density, and lower levels of endothelial nitric oxide synthase. A single antenatal instillation of BAY 41-2272 reduced mean right ventricular pressure and medial thickness of small resistive arteries in CDH fetuses. Capillary density, endothelial cell proliferation, and transcripts of endothelial nitric oxide synthase increased, whereas airway morphometry, lung growth, and mechanics remained unchanged. These results suggest that pharmacological activation of soluble guanylate cyclase may provide a new approach to the prenatal treatment of PH associated with CDH.

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.

Regulation of the Pulmonary Circulation in the Fetus and Newborn

During the development of the pulmonary vasculature in the fetus, many structural and functional changes occur to prepare the lung for the transition to air breathing. The development of the pulmonary circulation is genetically controlled by an array of mitogenic factors in a temporo-spatial order. With advancing gestation, pulmonary vessels acquire increased vasoreactivity. The fetal pulmonary vasculature is exposed to a low oxygen tension environment that promotes high intrinsic myogenic tone and high vasocontractility. At birth, a dramatic reduction in pulmonary arterial pressure and resistance occurs with an increase in oxygen tension and blood flow. The striking hemodynamic differences in the pulmonary circulation of the fetus and newborn are regulated by various factors and vasoactive agents. Among them, nitric oxide, endothelin-1, and prostaglandin I2 are mainly derived from endothelial cells and exert their effects via cGMP, cAMP, and Rho kinase signaling pathways. Alterations in these signaling pathways may lead to vascular remodeling, high vasocontractility, and persistent pulmonary hypertension of the newborn.

Nitric Oxide in the Pulmonary Vasculature

Homeostasis in the pulmonary vasculature is maintained by the actions of vasoactive compounds, including nitric oxide (NO). NO is critical for normal development of the pulmonary vasculature and continues to mediate normal vasoregulation in adulthood. Loss of NO bioavailability is one component of the endothelial dysfunction and vascular pathology found in pulmonary hypertension (PH). A broad research effort continues to expand our understanding of the control of NO production and NO signaling and has generated novel theories on the importance of pulmonary NO production in the control of the systemic vasculature. This understanding has led to exciting developments in our ability to treat PH, including inhaled NO and phosphodiesterase inhibitors, and to several promising directions for future therapies using nitric oxide-donor compounds, stimulators of soluble guanylate cyclase, progenitor cells expressing NO synthase (NOS), and NOS gene manipulation.

An in situ evidence for autocrine function of NO in the vasculature

The concept of endothelium derived relaxing factor (EDRF) implies that nitric oxide (NO) generated by NO synthase in the endothelium diffuses to the underlying vascular smooth muscle cells (VSMC) modulating thereby vascular tone. VSMC were regarded as passive recipients of NO from endothelial cells. However, this paradigm of a paracrine function of NO became currently subject to considerable debate. To address this issue, we examined the localization of enzymes engaged in l-arginine-NO-cGMP signaling in the rat blood vessels. Employing multiple immunocytochemical labeling complemented with signal amplification, electron microscopy, Western blotting, and RT-PCR, we found that NO synthase was differentially expressed in blood vessels depending on the blood vessel type. Moreover, the expression pattern of NO synthase in VSMC showed striking parallels with arginase and soluble guanylyl cyclase. Our findings challenge the commonly accepted view that the expression of NO synthase is restricted to vascular endothelial cells and lends further support to an alternative mechanism, by which constitutive local NOS expression in VSMC may modulate vascular functions in an endothelium-independent manner. Moreover, the co-expression of enzymes engaged in l-arginine-NO-cGMP signaling (NO synthase, arginase, and soluble guanylyl cyclase) in VSMC is indicative of an autocrine fashion of NO signaling in the vasculature in addition to the paracrine role of NO generated in the endothelium.

Hypoxia Decreases Expression of Soluble Guanylate Cyclase in Cultured Rat Pulmonary Artery Smooth Muscle Cells

Nitric oxide (NO) has an important role in modulating the pulmonary vascular tone. NO acts, in part, by stimulating soluble guanylate cyclase (sGC) to synthesize the intracellular second messenger cyclic GMP. In vascular smooth muscle cells, sGC is a heterodimer composed of alpha1 and beta1 subunits. The objective of this study was to test whether oxygen concentration regulates sGC expression in cultured rat pulmonary artery smooth muscle cells (rPaSMC). rPaSMC were exposed to 0, 3, and 20% oxygen for 1-48 h, and sGC subunit mRNA levels were measured. Compared with rPaSMC exposed to 20% oxygen, sGC alpha1 and beta1 subunit mRNA levels were markedly decreased in rPaSMC exposed to 0% and 3% oxygen. The decrease in sGC subunit mRNA levels in hypoxic rPaSMC was detected as early as 6 h of exposure. Compared with rPaSMC exposed to 20% oxygen, exposure of rPaSMC to 3% oxygen progressively decreased sGC subunit protein levels at 24 and 48 h. There was also a 30% and 50% decrease in sGC enzyme activity in cells exposed to hypoxia for 24 and 48 h (P < 0.05 and P < 0.001, respectively, as compared with cells maintained in normoxia). These results demonstrate that hypoxia decreases sGC expression in cultured pulmonary artery smooth muscle cells and suggest that, in hypoxic vascular smooth muscle, decreased cyclic GMP synthesis may limit the vasodilator response to NO.

Pulmonary vascular dysfunction in preterm lambs with chronic lung disease

Chronic lung injury from prolonged mechanical ventilation after premature birth inhibits the normal postnatal decrease in pulmonary vascular resistance (PVR) and leads to structural abnormalities of the lung circulation in newborn sheep. Compared with normal lambs born at term, chronically ventilated preterm lambs have increased pulmonary arterial smooth muscle and elastin, fewer lung microvessels, and reduced abundance of endothelial nitric oxide synthase. These abnormalities may contribute to impaired respiratory gas exchange that often exists in infants with chronic lung disease (CLD). Nitric oxide inhalation (iNO) reduces PVR in human infants and lambs with persistent pulmonary hypertension. We wondered whether iNO might have a similar effect in lambs with CLD. We therefore studied the effect of iNO on PVR in lambs that were delivered prematurely at approximately 125 days of gestation (term = 147 days) and mechanically ventilated for 3 wk. All of the lambs had chronically implanted catheters for measurement of pulmonary vascular pressures and blood flow. During week 2 of mechanical ventilation, iNO at 15 parts/million for 1 h decreased PVR by approximately 20% in 12 lambs with evolving CLD. When the same study was repeated in eight lambs at the end of week 3, iNO had no significant effect on PVR. To see whether this loss of iNO effect on PVR might reflect dysfunction of lung vascular smooth muscle, we infused 8-bromo-guanosine 3',5'-cyclic monophosphate (cGMP; 150 microg. kg(-1). min(-1) iv) for 15-30 min in four of these lambs at the end of week 3. PVR consistently decreased by 30-35%. Lung immunohistochemistry and immunoblot analysis of excised pulmonary arteries from lambs with CLD, compared with control term lambs, showed decreased soluble guanylate cyclase (sGC). These results suggest that loss of pulmonary vascular responsiveness to iNO in preterm lambs with CLD results from impaired signaling, possibly related to deficient or defective activation of sGC, the intermediary enzyme through which iNO induces increased vascular smooth muscle cell cGMP and resultant vasodilation.

Paracrine role of soluble guanylate cyclase and type III nitric oxide synthase in ovine fetal pulmonary circulation: a double labeling immunohistochemical study

Endothelial nitric oxide synthase (eNOS) or NOS-III in the endothelium catalyzes production of nitric oxide (NO). Nitric oxide diffuses freely into vascular smooth muscle, where it activates soluble guanylate cyclase (sGC) to produce guanosine 3',5'-cyclic monophosphate (cGMP) and causes vasorelaxation. The NO/cGMP pathway is an important signaling pathway in the control of perinatal pulmonary circulation. An exact colocalization of NOS-III in the pulmonary endothelium and sGC in the vascular smooth muscle was demonstrated using a double immunolabeling technique. The sGC immunoreactivity was higher in resistant pulmonary vessels and veins than in conduit arteries, whereas NOS-III immunoreactivity was higher in conduit arteries than in veins. These results demonstrated anatomically in situ a paracrine role of NOS-III and sGC in the regulation of fetal pulmonary circulation and suggested a heterogeneous distribution of NOS-III and sGC within fetal ovine pulmonary vasculature. Our results provided an anatomic basis that supported previous functional studies on perinatal control of pulmonary circulation.

Type I nitric oxide synthase is decreased in the fetal pulmonary circulation of hypertensive lambs

The nitric oxide (NO)/guanosine 3',5'-cyclic monophosphate (cGMP) pathway plays an essential role in mediating pulmonary vasodilatation during transition of the pulmonary circulation at birth. We used immunoblot analysis (Western) and semiquantitative immunohistochemistry to study the presence, distribution, and relative amounts of type I nitric oxide synthase (NOS-I). Immunoblots were performed on normal fetal sheep lungs, whereas immunohistochemistry for NOS-I was compared between lungs from normal fetal lambs vs. fetal lambs with persistent pulmonary hypertension of the newborn (PPHN) induced by ligation of the ductus arteriosus. Western blot analysis using a polyclonal antibody detected NOS-I protein in homogenates of normal fetal sheep lungs. Abundant NOS-I immunoreactivity was observed exclusively in the precapillary resistance vessels, i.e., terminal bronchiole-associated arteries (TA) and respiratory bronchiole-associated arteries (RA) in normal fetal lung. In marked contrast, immunoreactivity for NOS-I was significantly reduced in the TA and RA of hypertensive lungs. We conclude that there is a heterogeneous distribution of NOS-I in the normal fetal sheep lung, but that NOS-I staining is significantly reduced in lambs with PPHN.

Homogeneous segmental profile of carbon monoxide-mediated pulmonary vasodilation in rats

Carbon monoxide (CO) has been proposed to attenuate the vasoconstrictor response to local hypoxia that contributes to pulmonary hypertension. However, the segmental response to CO, as well as its mechanism of action in the pulmonary circulation, has not been fully defined. To investigate the hemodynamic response to exogenous CO, lungs from male Sprague-Dawley rats were perfused with physiological saline solution. Measurements were made of pulmonary arterial, venous, and capillary pressures. Lungs were constricted with the thromboxane mimetic U-46619. To examine the vasodilatory response to CO, 500 microl of CO-equilibrated physiological saline solution or vehicle were injected into the arterial line. Additionally, CO and vehicle responses were examined in the presence of the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM) or the larger conductance calcium-activated K(+) (BK(Ca)) channel blockers tetraethylammonium chloride (10 mM) and iberiotoxin (100 nM). CO administration decreased vascular resistance to a similar degree in both vascular segments. This vasodilatory response was completely abolished in lungs pretreated with ODQ. Furthermore, CO administration increased whole lung cGMP content, which was prevented by ODQ. Neither tetraethylammonium chloride nor iberiotoxin affected the CO response. We conclude that exogenous CO administration causes vasodilation in the pulmonary vasculature via a soluble guanylyl cyclase-dependent mechanism that does not likely involve activation of K(Ca) channels.

Pulmonary arterial dilation by inhaled NO: arterial diameter, NO concentration relationship

The objective of this study was to determine the nitric oxide (NO) concentration and vessel diameter dependence of the pulmonary arterial dilation induced by inhaled NO. Isolated dog lung lobes were situated between a microfocal X-ray source and X-ray detector and perfused with either blood or plasma. Boluses of radiopaque contrast medium were injected into the lobar artery under control conditions, when the pulmonary arteries were constricted by infusion of serotonin and when the serotonin infusion was accompanied by inhalation of from 30 to 960 parts/million NO. Arterial diameter measurements were obtained from X-ray images of vessels having control diameters in the 300- to 3,400-microm range. Serotonin constricted the vessels throughout the size range studied, with an average decrease in diameter of approximately 20%. The fractional reversal of the serotonin-induced constriction by inhaled NO was directly proportional to inhaled NO concentration, inversely proportional to vessel size, and greater with plasma than with blood perfusion in vessels as large as 3 mm in diameter. The latter indicates that intravascular hemoglobin affected the bronchoalveolar-to-arterial luminal NO concentration gradient in fairly large pulmonary arteries. The data provide information regarding pulmonary arterial smooth muscle accessibility to intrapulmonary gas that should be useful as part of the database for modeling the communication between intrapulmonary gas and pulmonary arterial smooth muscle cells in future studies.

Development of fetal vascular responses to endothelin-1 and acetylcholine in the sheep

Responses to K(+), endothelin-1 (ET-1), and acetylcholine (ACh) of isolated adrenal, femoral, middle cerebral, and renal arteries from fetal [110--145 days gestational age (dGA, term approximately 148 dGA)] and 0- to 24-h newborn (NB) lambs were evaluated using the technique of wire myography. Responses at distinct developmental ages for each vascular bed were compared. In all arteries sensitivity to K(+)-induced vasoconstriction was similar at all fetal age points examined. In contrast, sensitivity to ET-1 increased with increasing fetal age in arteries from all vascular beds. The magnitude of the maximal vasoconstriction was positively correlated with GA for K(+) in adrenal, femoral, and cerebral arteries and for ET-1 in femoral, cerebral, and renal arteries. Cerebral arteries showed a greater sensitivity when compared with the other systemic arteries to K(+) and ET-1 at all fetal ages and to K(+) in NB. ACh evoked relaxatory responses in fetal and NB femoral and adrenal arteries. However, renal arteries relaxed comparatively less in response to ACh, and no vasodilation was noted in middle cerebral arteries at any age points examined. For femoral arteries ACh-induced vasorelaxation decreased with increasing GA but was restored in arteries from NB lambs. In summary, the responsiveness of isolated resistance arteries varies with developmental age in the fetal and perinatal sheep and these effects are both agonist and vascular bed specific. The augmented sensitivity in response to ET-1 of middle cerebral compared with other systemic arteries may reflect the importance of cerebral blood flow control during this critical developmental period.

Pulmonary hypertension alters soluble guanylate cyclase activity and expression in pulmonary arteries isolated from fetal lambs

The nitric oxide (NO)-guanosine 3',5'-cyclic monophosphate (cGMP) signaling pathway plays an important role in the pulmonary vascular transition at birth. We studied pulmonary arteries and veins isolated from normal late-gestation fetal lambs and from fetal lambs with persistent pulmonary hypertension (PPHN) following prenatal ligation of the ductus arteriosus. We additionally used double immunolabeling and immunoblot analysis to determine relative vascular contents of endothelial nitric oxide synthase (NOS-III) and soluble guanylate cyclase (sGC). Cyclic GMP content and sGC activity were significantly lower in arteries from hypertensive lambs than controls. A rank order for contents of both soluble guanylate cyclase and NOS-III was observed by both immunolabeling and immunoblotting: Control vein = Hypertensive vein > Control artery > Hypertensive artery. Our data demonstrate that the relative expression of sGC correlates well with the relative expression of NOS-III, and indicate the potential importance of soluble guanylate cyclase in the regulation of the perinatal pulmonary circulation. These data may help us understand vascular mechanisms producing PPHN, as well as patterns of response to exogenous NO.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Form and function of fetal and neonatal pulmonary arterial bifurcations

Bifurcation is a basic form of vascular connection. It is composed of a parent vessel of diameter d(0), and two daughter vessels, d(1) and d(2), where d(0) > d(1) >/= d(2). Optimal values for the bifurcation area ratio, beta = (d(1)(2) + d(2)(2))/d(0)(2), and the junction exponent, x, in d(0)(x) = d(1)(x) + d(2)(x), are postulated to be universal in nature. However, we have hypothesized that the perinatal pulmonary arterial circulation is an exception. Arterial diameters were measured in pulmonary vascular casts of a fetal lamb (140 days gestation/145 days term) and a neonatal lamb (1 day old). The values for beta and x were evaluated in 10,970 fetal and 846 neonatal bifurcations sampled from the proximal and intermediate arterial regions. Mean values and confidence intervals (CI) for the fetus were beta = 0.890 (0.886-0.895 CI) and x = 1.75 (1.74-1.76 CI); and for the newborn were beta = 0.913 (0.90-0.93 CI) and x = 1. 79 (1.75-1.82 CI). These values are significantly different from Murray's law (beta > 1, x = 3) or the West-Brown-Enquist law (beta = 1, x = 2). Therefore, perinatal pulmonary bifurcation design appears to be distinctive and exceptional. The decreasing cross-sectional area with branching leads to the hemodynamic consequence of shear stress amplification. This structural organization may be important for facilitating vascular development at low flow rates; however, it may be the origin of unstable reactivity if elevated blood flow and pressure occurs.

Oxygen-induced vasodilation in pulmonary arterioles from fetal rats

BACKGROUND

Oxygen is a potent stimulus for pulmonary vasodilation in the perinatal period. Little information is available regarding mediators of oxygen-induced pulmonary vasodilation in fetal rats. We have investigated the effect of blocking several proposed mediators of oxygen-induced vasodilation on the responses of isolated, third-generation pulmonary arterioles from term fetal rats to an increase in oxygen tension.

MATERIALS AND METHODS

Third-generation pulmonary arterioles were isolated from fetal rats at term. Arterioles were preconstricted by suffusion with "hypoxic" (pO(2) 25-40 mm Hg) solution containing 40 mM KCl. The vasodilation induced by suffusion with "normoxic" (pO(2) 90-150 mm Hg) 40 mM KCl solution was recorded for control pulmonary arterioles and for arterioles pretreated with inhibitors of nitric oxide synthase and cyclooxygenase, and blockers of bradykinin receptors and purinergic receptors. Responses to oxygen suffusion were also recorded for pulmonary arterioles denuded of endothelium.

RESULTS

Control arterioles dilated 113 +/- 28% of the potassium-induced preconstriction after 60 min of normoxic suffusion. Pretreatment with indomethacin completely blocked oxygen-induced vasodilation. Inhibitors of nitric oxide synthase, blockers of bradykinin and purinergic receptors, and removal of the endothelium did not significantly change normoxic vasodilation.

CONCLUSIONS

Our results are most consistent with a vasodilator product of cyclooxygenase metabolism as a primary stimulus for oxygen-induced vasodilation in isolated, third-generation pulmonary arterioles from fetal rats.

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.

Soluble guanylate cyclase gene expression and localization in rat lung after exposure to hypoxia

The nitric oxide (NO)-cGMP signal transduction pathway plays an important role in the regulation of pulmonary vascular tone and resistance in pulmonary hypertension. A number of studies have demonstrated that endothelial (e) and inducible nitric oxide synthases (NOS) are upregulated in hypoxia-exposed rat lung. These changes in NOS expression have been found to correlate with the process of pulmonary vascular remodeling in hypoxia-induced pulmonary hypertension, and remodeling is increased in the absence of eNOS. In this study, we examined the expression and localization of soluble guanylate cyclase (sGC), the primary receptor for NO, in hypoxia- and normoxia-treated rat lungs. Male Sprague-Dawley rats were exposed to hypoxia (10% O(2), normobaric) or normoxia for 1, 3, 5, and 21 days. The lungs were used for Western analysis of sGC protein, sGC enzyme activity, immunohistochemistry using antiserum against sGC alpha(1)- and beta(1)-subunits, and nonradioactive in situ hybridization (NRISH) using a digoxigenin-labeled sGC alpha(1)-subunit cRNA probe. Western blot analysis revealed a more than twofold increase of sGC protein alpha(1)-subunit in rat lungs exposed to 3, 5, and 21 days of hypoxia, correlating well with sGC enzyme activity. Immunohistochemistry and NRISH demonstrated increased expression of sGC in the smooth muscle cells of the pulmonary arteries and arterioles in the hypoxic rat lungs when compared with normoxic controls. Based on our results, the upregulation of sGC may play an important role in the regulation of smooth muscle tone and pressure in the pulmonary circulation during chronic hypoxia.

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.