Hyperoxia-induced lung structure-function relation, vessel rarefaction, and cardiac hypertrophy in an infant rat model

Abstract

Background: Hyperoxia-induced bronchopulmonary dysplasia (BPD) models are essential for better understanding and impacting on long-term pulmonary, cardiovascular, and neurological sequelae of this chronic disease. Only few experimental studies have systematically compared structural alterations with lung function measurements.

Methods: In three separate and consecutive series, Sprague-Dawley infant rats were exposed from day of life (DOL) 1 to 19 to either room air (0.21; controls) or to fractions of inspired oxygen (FiO₂) of 0.6, 0.8, and 1.0. Our primary outcome parameters were histopathologic analyses of heart, lungs, and respiratory system mechanics, assessed via image analysis tools and the forced oscillation technique, respectively.

Results: Exposure to FiO₂ of 0.8 and 1.0 resulted in significantly lower body weights and elevated coefficients of lung tissue damping (G) and elastance (H) when compared with controls. Hysteresivity (η) was lower due to a more pronounced increase of H when compared with G. A positive structure-function relation was demonstrated between H and the lung parenchymal content of α-smooth muscle actin (α-SMA) under hyperoxic conditions. Moreover, histology and morphometric analyses revealed alveolar simplification, fewer pulmonary arterioles, increased α-SMA content in pulmonary vessels, and right heart hypertrophy following hyperoxia. Also, in comparison to controls, hyperoxia resulted in significantly lower plasma levels of vascular endothelial growth factor (VEGF). Lastly, rats in hyperoxia showed hyperactive and a more explorative behaviour.

Conclusions: Our in vivo infant rat model mimics clinical key features of BPD. To the best of our knowledge, this is the first BPD rat model demonstrating an association between lung structure and function. Moreover, we provide additional evidence that infant rats subjected to hyperoxia develop rarefaction of pulmonary vessels, augmented vascular α-SMA, and adaptive cardiac hypertrophy. Thus, our model provides a clinically relevant tool to further investigate diseases related to O₂ toxicity and to evaluate novel pharmacological treatment strategies.

Keywords: Animal model; Bronchopulmonary dysplasia; Digital pathology; Forced oscillation technique; Hyperoxia; Hysteresivity eta (η); Respiratory system mechanics; Vascular endothelial growth factor (VEGF); α-Smooth muscle actin (α-SMA).

Fig. 1

Body weight gain from DOL 5 to 19. Curves with white and grey circles indicate the normoxic and FiO₂ 0.6 (a), FiO₂ 0.8 (b), and FiO₂ 1.0 (c) groups, respectively. Asterisk displays a significant difference on DOL 19 between study groups, p < 0.05

Fig. 2

Respiratory system mechanics on DOL 19. Normoxic and hyperoxic study groups are illustrated with white and grey box plots, respectively. a–c, g R_aw, G, H, and η in normoxia (n = 6) and hyperoxia (FiO₂ 0.8) (n = 6). d–f, h R_aw, G, H, and η in normoxia (n = 7) and hyperoxia (FiO₂ 1.0) (n = 6). R_aw: airway resistance; G: coefficient of tissue damping; H: coefficient of tissue elastance; η: lung tissue hysteresivity. Data are expressed as vertical box plots with median, 10th, 25th, 75th, and 90th percentiles. Asterisk displays a significant difference between study groups, p < 0.05

Fig. 3

Lung histology and morphometric analysis. Normoxic and hyperoxic study groups are illustrated with white and grey box plots, respectively. a, b Representative microscopic photographs of lung sections in normoxia (a) and hyperoxia (b). Magnification ×10. c–e Alveolar count, mean linear intercept, and ratio of α-SMA-positive lung tissue area/total parenchymal tissue area in normoxia (n = 8) and hyperoxia (n = 8). Data are expressed as vertical box plots with median, 10th, 25th, 75th, and 90th percentiles. Asterisk displays a significant change between study groups, p < 0.05

Fig. 4

Histomorphometry of pulmonary vessels and heart. Normoxic and hyperoxic study groups are illustrated with white and grey box plots, respectively. a–d Count of pulmonary vessels (n = 8 + 8), α-SMA content in the medial wall of the pulmonary arterioles (n = 7 + 8), right-to-left ventricle wall thickness ratio (n = 3 + 6), cardiomyocyte cross-sectional area (n = 7 + 8) in normoxia and hyperoxia, respectively. Data are expressed as vertical box plots with median, 10th, 25th, 75th, and 90th percentiles. Asterisk displays a significant difference between study groups, p < 0.05

Fig. 5

Scatter plot of α-SMA to lung tissue ratio (a) and mean linear intercept (b) against lung tissue elastance (H). White and grey circles indicate the normoxic (n = 7) and FiO₂ 1.0 (n = 6) groups, respectively; r² = coefficient of determination

Fig. 6

VEGF and ET-1 concentrations in plasma. Normoxic and hyperoxic study groups are illustrated with white and grey box plots, respectively. a VEGF concentration in normoxia (n = 8) and hyperoxia (FiO₂ 1.0) (n = 8). b ET-1 concentration in normoxia (n = 8) and hyperoxia (n = 7). Data are expressed as vertical box plots with median, 10th, 25th, 75th, and 90th percentiles. Asterisk displays a significant change between study groups, p < 0.05