Structural and functional alterations in the rat lung following whole thoracic irradiation with moderate doses: injury and recovery

Abstract

Purpose: To characterize structural and functional injuries following a single dose of whole-thorax irradiation that might be survivable after a nuclear attack/accident.

Methods: Rats were exposed to 5 or 10 Gy of X-rays to the whole thorax with other organs shielded. Non-invasive measurements of breathing rate and arterial oxygen saturation, and invasive evaluations of bronchoalveolar lavage fluid, (for total protein, Clara cell secretory protein), vascular reactivity and histology were conducted for at least 6 time points up to 52 weeks after irradiation.

Results: Irradiation with 10 Gy resulted in increased breathing rate, a reduction in oxygen saturation, an increase in bronchoalveolar lavage fluid protein and attenuation of vascular reactivity between 4-12 weeks after irradiation. These changes were not observed with the lower dose of 5 Gy. Histological examination revealed perivascular edema at 4-8 weeks after exposure to both doses, and mild fibrosis beyond 20 weeks after 10 Gy.

Conclusions: Single-dose exposure of rat thorax to 10 but not 5 Gy X-irradiation resulted in a decrease in oxygen uptake and vasoreactivity and an increase in respiratory rate, which paralleled early pulmonary vascular pathology. Vascular edema resolved and was replaced by mild fibrosis beyond 20 weeks after exposure, while lung function recovered.

Figure 1

Hematoxylin-eosin-stained lung sections from a minimum of 7 rats were examined at each time point. Representative images for the major findings are shown as follows: a) control (0 Gy) at 3 days after irradiation; b) 0 Gy at 52 wks after irradiation; c) 5 Gy at 8 wks after irradiation, vessel wall hyalinization marked with arrow; d) 5 Gy at 20 wks after irradiation, chronic vessel changes marked with arrow; e) 10 Gy at 4 wks after irradiation, perivascular edema marked with arrow; f) 10 Gy at 52 wks after irradiation, mononuclear cells marked with arrow.

Figure 2

The breathing rate of rats was measured non-invasively using an airtight plethysmograph. Breaths per min following 5 Gy (panel a) and 10 Gy (panel b) were compared to controls. Values are mean breaths per min ± sem; n=4–6 (5 Gy), n=6–15 (10 Gy). No increase in breathing rate was observed following exposure to 5 Gy up to 8 wks. A significant elevation (p≤0.05 vs Ctrl) in breathing rate was noted at 7, 11 and 12 wks following irradiation with 10 Gy.

Figure 3

Oxygen saturation was measured non-invasively using a veterinary pulse oximeter 10 min after exposure to a hypoxic gas mixture of 12% oxygen in nitrogen. Rats were compared to controls at each time point. Data is shown for exposure to 10 Gy. The difference of mean oxygen saturation values between the age-matched controls and irradiated rats is presented for 10 Gy only. The mean ± sem values recorded were: wk1–10 Gy: 83.0 ±1.6, 0 Gy: 84.2 ± 0.7; wk2–10 Gy: 82.8 ± 1.0, 0 Gy: 84.0 ± 1.0; wk5–10 Gy: 80.3 ± 1.3, 0 Gy: 83.7 ± 1.4; wk8–10 Gy: 81.3 ± 1.5, 0 Gy: 85.0 ± 1.8; wk11–10 Gy: 79.5 ± 1.2, 0 Gy: 85.3 ± 1.5; wk13–10 Gy: 80.8 ± 0.9, 0 Gy: 86.1 ± 1.6; wk20–10 Gy: 85.7 ± 2.4, 0 Gy: 83.3 ± 1.2; wk52–10 Gy: 85.2 ± 1.4, 0 Gy: 86.3 ± 1.3; n=6–15. A significant reduction (p≤0.05 vs Ctrl) in oxygen saturation was observed in rats exposed to a dose of 10 Gy at 11 and 13 wks after irradiation.

Figure 4

Total protein levels in the BALF of rats exposed to a dose of 5 Gy (panel a) and 10 Gy (panel b) compared to controls. Each data point represents mean ± sem; n=3–8. Following exposure to 5 Gy, no significant increase in total proteins was observed up to 8 wks after irradiation. Total Proteins were significantly elevated (p≤0.05 vs Ctrl) at 4 wks and at 8 wks after irradiation in rats exposed to a dose of 10 Gy compared to un-irradiated control rats.

Figure 4

Total protein levels in the BALF of rats exposed to a dose of 5 Gy (panel a) and 10 Gy (panel b) compared to controls. Each data point represents mean ± sem; n=3–8. Following exposure to 5 Gy, no significant increase in total proteins was observed up to 8 wks after irradiation. Total Proteins were significantly elevated (p≤0.05 vs Ctrl) at 4 wks and at 8 wks after irradiation in rats exposed to a dose of 10 Gy compared to un-irradiated control rats.

Figure 5

Representative graphs showing vascular reactivity to Ang II in pulmonary artery rings with a dose of 5 Gy at different Ang II concentrations (panel a) and time points (panel b). Each data point represents mean ± sem and is expressed as % of baseline contraction; n=32 rings from 8 rats (panel a), n=20–34 rings from 5–9 rats (panel b). No significant differences were observed at 8 wks after irradiation (panel a). The response of pulmonary artery rings to 10⁻⁷ M Ang II at the different times after irradiation found no significant differences between the two groups (panel b).

Figure 5

Representative graphs showing vascular reactivity to Ang II in pulmonary artery rings with a dose of 5 Gy at different Ang II concentrations (panel a) and time points (panel b). Each data point represents mean ± sem and is expressed as % of baseline contraction; n=32 rings from 8 rats (panel a), n=20–34 rings from 5–9 rats (panel b). No significant differences were observed at 8 wks after irradiation (panel a). The response of pulmonary artery rings to 10⁻⁷ M Ang II at the different times after irradiation found no significant differences between the two groups (panel b).

Figure 6

Representative graphs showing vascular reactivity to Ang II in pulmonary artery rings with a dose of 10 Gy at different Ang II concentrations (panel a) and time points (panel b). Each data point represents mean ± sem and is expressed as % of baseline contraction; n=27 rings from 7 rats (panel a), n=16–40 rings from 4–11 rats (panel b). There was a fall in reactivity in irradiated vessels at 8 wks after irradiation (panel a). The response of the rings to 10⁻⁷ M Ang II at different time points following exposure showed significant attenuation to vasoreactivity at 4, 8 and 52 wks after irradiation (* p<0.01 vs Ctrl, panel b).

Figure 6

Representative graphs showing vascular reactivity to Ang II in pulmonary artery rings with a dose of 10 Gy at different Ang II concentrations (panel a) and time points (panel b). Each data point represents mean ± sem and is expressed as % of baseline contraction; n=27 rings from 7 rats (panel a), n=16–40 rings from 4–11 rats (panel b). There was a fall in reactivity in irradiated vessels at 8 wks after irradiation (panel a). The response of the rings to 10⁻⁷ M Ang II at different time points following exposure showed significant attenuation to vasoreactivity at 4, 8 and 52 wks after irradiation (* p<0.01 vs Ctrl, panel b).

Figure 7

The effect of KCl on rat PA rings at different doses and time points. The blank and black columns represent 5 Gy and 10 Gy respectively. No significant differences were observed between 5 Gy and control rings (striped columns). Significant attenuation to KCl was noted at 8 wks and 52 wks following exposure to 10 Gy compared to controls (grey-striped columns). Each data is mean ± sem and expressed as % of baseline contraction (* p<0.05 vs Ctrl); n=16–40 rings from 4–11 rats.