Hyperoxic conditions inhibit airway smooth muscle myosin phosphatase in rat pups

Rho-kinase inhibitors protect against neonatal hyperoxia-induced airway hyperreactivity in a rat pup model: Role of prostaglandin F2α

BACKGROUND

Oxygen therapy in preterm neonates is associated with airway hyperreactivity. The role of Rho/Rho-kinase smooth muscle signaling in hyperoxia-induced airway hyperreactivity remains understudied. We hypothesized that inhibition of Rho-kinase will attenuate airway hyperreactivity induced by neonatal hyperoxia.

METHODS

Newborn rats were raised in hyperoxia (>95% O₂ ) or ambient air (AA) for 7 days. Subgroups were injected with a Rho-kinase inhibitor: Y-27632 (10 mg·kg^-1 ·day^-1 ) or fasudil (10 mg·kg^-1 ·day^-1 ), or a FP receptor antagonist - AS604872 (30 mg·kg^-1 ·day^-1 ). After exposures, tracheal cylinders were prepared for in vitro wire myography. Contraction to methacholine or PGF_2α was measured in the presence or absence of tissue-bath Y-27632, fasudil, or AS604872. Lung PGF_2α levels, Rho-kinase protein level and Rho-kinase 1 activity were measured by ELISA.

RESULTS

Tracheal smooth muscle contraction was significantly greater in hyperoxic compared to AA groups. Both, Y-27632 and fasudil significantly decreased contractility to MCh or PGF_2α in hyperoxic groups versus hyperoxic controls (p < 0.001), but did not alter AA group responses. Inhibition of FP receptors attenuated responses to PGF_2α . Hyperoxia significantly increased lung PGF_2α compared to AA (p < 0.01), but Rho-kinase inhibition did not influence PGF_2α level. Rho-kinase protein level (p < 0.001) and activity (p < 0.01), were increased by hyperoxia, but blockade of FP receptor reduced the Rho-kinase 1 activity (p < 0.05) under hyperoxic condition.

CONCLUSIONS

This study demonstrates an active role of Rho/Rho-kinase signaling on hyperoxia-induced airway hyperreactivity. These findings suggest that Rho-kinase inhibitors might serve as an effective therapy for hyperoxia-induced airway hyperreactivity.

Vulnerability of the developing airway

Longer term respiratory morbidity is a frequent concern for former preterm infants. Increased airway reactivity and wheezing disorders are extremely common in this population, both in infants who meet diagnostic criteria for bronchopulmonary dysplasia [BPD], and in the absence of this diagnosis. It is, therefore, imperative to gain a better understanding of normal and abnormal postnatal development of the immature airway. Airway hyperreactivity may be secondary to abnormal bronchoalveolar attachments in the face of parenchymal lung injury, or secondary to an imbalance between constrictor and dilator neural pathways. Finally, the airway itself may undergo functional and/or structural changes, including increased airway smooth muscle mass, and changes in airway extracellular matrix which may, in turn, modulate downstream signaling pathways to hyperoxia or pressure exposed vulnerable airways.

Effects of Hyperoxia on the Developing Airway and Pulmonary Vasculature

Although it is necessary and part of standard practice, supplemental oxygen (40-90% O₂) or hyperoxia is a significant contributing factor to development of bronchopulmonary dysplasia, persistent pulmonary hypertension, recurrent wheezing, and asthma in preterm infants. This chapter discusses hyperoxia and the role of redox signaling in the context of neonatal lung growth and disease. Here, we discuss how hyperoxia promotes dysfunction in the airway and the known redox-mediated mechanisms that are important for postnatal vascular and alveolar development. Whether in the airway or alveoli, redox pathways are important and greatly influence the neonatal lung.

S-Nitrosoglutathione Attenuates Airway Hyperresponsiveness in Murine Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD) is characterized by lifelong obstructive lung disease and profound, refractory bronchospasm. It is observed among survivors of premature birth who have been treated with prolonged supplemental oxygen. Therapeutic options are limited. Using a neonatal mouse model of BPD, we show that hyperoxia increases activity and expression of a mediator of endogenous bronchoconstriction, S-nitrosoglutathione (GSNO) reductase. MicroRNA-342-3p, predicted in silico and shown in this study in vitro to suppress expression of GSNO reductase, was decreased in hyperoxia-exposed pups. Both pretreatment with aerosolized GSNO and inhibition of GSNO reductase attenuated airway hyperresponsiveness in vivo among juvenile and adult mice exposed to neonatal hyperoxia. Our data suggest that neonatal hyperoxia exposure causes detrimental effects on airway hyperreactivity through microRNA-342-3p–mediated upregulation of GSNO reductase expression. Furthermore, our data demonstrate that this adverse effect can be overcome by supplementing its substrate, GSNO, or by inhibiting the enzyme itself. Rates of BPD have not improved over the past two decades; nor have new therapies been developed. GSNO-based therapies are a novel treatment of the respiratory problems that patients with BPD experience.

Early exposure to hyperoxia or hypoxia adversely impacts cardiopulmonary development

Preterm infants are at high risk for long-term abnormalities in cardiopulmonary function. Our objectives were to determine the long-term effects of hypoxia or hyperoxia on cardiopulmonary development and function in an immature animal model. Newborn C57BL/6 mice were exposed to air, hypoxia (12% oxygen), or hyperoxia (85% oxygen) from Postnatal Day 2-14, and then returned to air for 10 weeks (n = 2 litters per condition; > 10/group). Echocardiography, blood pressure, lung function, and lung development were evaluated at 12-14 weeks of age. Lungs from hyperoxia- or hypoxia-exposed mice were larger and more compliant (compliance: air, 0.034 ± 0.001 ml/cm H2O; hypoxia, 0.049 ± 0.002 ml/cm H2O; hyperoxia, 0.053 ± 0.002 ml/cm H2O; P < 0.001 air versus others). Increased airway reactivity, reduced bronchial M2 receptor staining, and increased bronchial α-smooth muscle actin content were noted in hyperoxia-exposed mice (maximal total lung resistance with methacholine: air, 1.89 ± 0.17 cm H2O ⋅ s/ml; hypoxia, 1.52 ± 0.34 cm H2O ⋅ s/ml; hyperoxia, 4.19 ± 0.77 cm H2O ⋅ s/ml; P < 0.004 air versus hyperoxia). Hyperoxia- or hypoxia-exposed mice had larger and fewer alveoli (mean linear intercept: air, 40.2 ± 0. 0.8 μm; hypoxia, 76.4 ± 2.4 μm; hyperoxia, 95.6 ± 4.6 μm; P < 0.001 air versus others; radial alveolar count [n]: air, 11.1 ± 0.4; hypoxia, 5.7 ± 0.3; hyperoxia, 5.6 ± 0.3; P < 0.001 air versus others). Hyperoxia-exposed adult mice had left ventricular dysfunction without systemic hypertension. In conclusion, exposure of newborn mice to hyperoxia or hypoxia leads to cardiopulmonary abnormalities in adult life, similar to that described in ex-preterm infants. This animal model may help to identify underlying mechanisms and to develop therapeutic strategies for pulmonary morbidity in former preterm infants.

The importance of complete tissue homogenization for accurate stoichiometric measurement of myosin light chain phosphorylation in airway smooth muscle

The standard method for measuring the phosphorylation of the regulatory myosin light chain (MLC20) in smooth muscle is extraction of the light chain using a urea extraction buffer, urea-glycerol gel electrophoresis of the soluble portion of the extract (supernatant) and Western blot analysis. The undissolved portion of the tissue during extraction (the pellet) is usually discarded. Because the pellet contains a finite amount of MLC20, omission of the pellet could result in inaccurate measurement of MLC20 phosphorylation. In this study we compared the level of tracheal smooth muscle MLC20 phosphorylation in the supernatant alone, with that in the complete tissue homogenate (supernatant and pellet) using the standard method. The supernatant fraction showed the well-known double bands representing phosphorylated and un-phosphorylated MLC20. The dissolved pellet fraction showed varying amounts of un-phosphorylated and phosphorylated MLC20. There was a small but statistically significant overestimation of the percent MLC20 phosphorylation if the pellet was not taken into consideration. The overestimation was 7% ± 2% (mean ± SEM) (p < 0.05) in unstimulated muscle and 2% ± 1% (p < 0.05) in acetylcholine (10(-6) mol/L) stimulated muscle. This finding suggests that for accurate estimation of the stoichiometry of MLC20 phosphorylation it is necessary to consider the contribution from the pellet portion of the muscle tissue homogenate.

Intra-amniotic LPS amplifies hyperoxia-induced airway hyperreactivity in neonatal rats

BACKGROUND

We previously showed that intra-amniotic lipopolysaccharide (LPS) amplifies alveolar hypoplasia induced by postnatal hyperoxia. We determined whether the priming effect of intra-amniotic LPS amplifies hyperoxia-induced airway hyperreactivity (AHR).

METHODS

LPS or normal saline was injected into the amniotic cavities of pregnant rats at the 20th day of gestation. After birth, rat pups were exposed to 60% O₂ or air for 14 d. On postnatal day 14, rat pups underwent forced oscillometry, which included a challenge with nebulized methacholine, and the lungs were harvested for morphological studies.

RESULTS

Hyperoxia significantly increased airway reactivity and decreased compliance. Intra-amniotic LPS further increased hyperoxia-induced AHR but did not further impair respiratory system compliance. Hyperoxia-induced changes in lung parenchymal and small airway morphology were not further altered by intra-amniotic LPS. However, combined exposure to intra-amniotic LPS and hyperoxia increased the proportion of degranulating mast cells in the hilar airways.

CONCLUSION

Intra-amniotic LPS amplified postnatal hyperoxia-induced AHR. This was associated with increased airway mast cell degranulation, which has previously been linked with hyperoxia-induced AHR. There were no morphologic changes of parenchyma or airways that would account for the LPS augmentation of hyperoxia-induced AHR.