Acute lung injury secondary to hydrochloric acid instillation induces small airway hyperresponsiveness

Abstract

Background: Acute respiratory distress syndrome (ARDS) is a severe form of respiratory failure characterized by altered lung mechanics and poor oxygenation. Bronchial hyperresponsiveness has been reported in ARDS survivors and animal models of acute lung injury. Whether this hyperreactivity occurs at the small airways or not is unknown.

Objective: To determine ex-vivo small airway reactivity in a rat model of acute lung injury (ALI) by hydrochloric acid (HCl) instillation.

Methods: Twelve anesthetized rats were connected to mechanical ventilation for 4-hour, and randomly allocated to either ALI group (HCl intratracheal instillation; n=6) or Sham (intratracheal instillation of 0.9% NaCl; n=6). Oxygenation was assessed by arterial blood gases. After euthanasia, tissue samples from the right lung were harvested for histologic analysis and wet-dry weight ratio assessment. Precision cut lung slice technique (100-200 μm diameter) was applied in the left lung to evaluate ex vivo small airway constriction in response to histamine and carbachol stimulation, using phase-contrast video microscopy.

Results: Rats from the ALI group exhibited hypoxemia, worse histologic lung injury, and increased lung wet-dry weight ratio as compared with the sham group. The bronchoconstrictor responsiveness was significantly higher in the ALI group, both for carbachol (maximal contraction of 84.5±2.5% versus 61.4±4.2% in the Sham group, P<0.05), and for histamine (maximal contraction of 78.6±5.3% versus 49.6±5.3% in the Sham group, P<0.05).

Conclusion: In an animal model of acute lung injury secondary to HCL instillation, small airway hyperresponsiveness to carbachol and histamine is present. These results may provide further insight into the pathophysiology of ARDS.

Keywords: ALI (acute lung injury); ARDS (acute respiratory distress syndrome); acid aspiration; small airway hyperresponsiveness.

Figure 1

Diagram showing animal groups, intervention tissue sampling, and analysis.

Figure 2

HCl instillation induces impaired gas exchange and pulmonary mechanics. (A) Partial pressure of O₂ (PaO₂), (B) Partial pressure of CO₂ (PaCO₂) and (C) Peak airway pressure (Paw_peak) in 4 hours of experimental period. #P<0.05, compared to baseline of their group; *P<0.05, ALI compared to Sham. ALI: Acute Lung Injury.

Figure 3

HCl instillation induces severe acute lung injury. A. Pulmonary edema represented by lung wet/dry weight ratio. B. Representative hematoxylin and eosin stain from 3 rats, left Sham and right ALI samples (scale 200 µm). C. Semiquantitative score for lung injury by a semiquantitative score based on severity (0= not present, 4= severe) in 5 regions of right lower lobe. Values are represented as median and interquartile ranges. #P<0.05, compared to baseline of their group; *P<0.05, ALI compared to Sham. ALI: Acute Lung Injury.

Figure 4

Acute lung injury induces small airway hyperresponsiveness to carbachol in precision cut lung slices (PCLS). A. Representative microscopic images of a lung slice showing contraction of a single airway treated with increasing concentrations of carbachol (from 0.001 to 100 micromolar) and graphical representation of the airway area (percentage relative to initial area). B. Concentration-response curve calculated by percentage of airway contraction with respect to the baseline. 37 lung slices from sham group and 36 from ALI group were used. Data are represented in mean ± SE. *P<0.05. Black line: Sham group, red line: ALI group. ALI: Acute Lung Injury; HBSS: Hanks’ Balanced Salt Solution.

Figure 5

Acute lung injury induces small airway hyperresponsiveness to histamine in precision cut lung slices (PCLS). A. Representative microscopic images of a lung slice showing contraction of a single airway treated with increasing concentrations of histamine and graphical representation of the airway area (percentage relative to initial area). B. Concentration-response curve calculated by percentages of airway contraction with respect to the baseline. 30 lung slices from sham group and 29 from ALI group were used. Data are represented in mean ± SE. *P<0.05. Black line: Sham group, red line: ALI group. ALI: Acute Lung Injury; HBSS: Hanks’ Balanced Salt Solution.