Adult male Sprague-Dawley rats were irradiated to the right hemithorax with a range of total doses delivered in 10 equal daily fractions of 4 MeV X rays. Half of each dose group consumed control feed, and half consumed feed containing the
angiotensin-converting enzyme inhibitor captopril (50 mg/kg/day) continuously after the last irradiation. High-resolution computed tomography (CT) of the entire thorax was performed at 4 and 8 weeks after the last irradiation, and the findings with CT were correlated with hemodynamic data, heart weight, and pulmonary histopathology. Rats exposed to 20 or 40 Gy in 10 fractions exhibited no acute changes in right lung density. After 60 Gy in 10 fractions, however, right lung density in rats on the control diet increased significantly at 4 weeks, and then returned to normal at 8 weeks.
Captopril-treated rats exposed to 60 Gy/10 fractions did not exhibit this transient increase in right lung density. After 80 Gy/10 fractions, right lung density increased to 0.60-0.65 g/cm3 at 4 weeks regardless of diet. At 8 weeks after 80 Gy/10 fractions, right lung density increased further in rats given the control diet, but decreased to near normal levels in
captopril-treated animals. The density of the shielded left lung based on the CT was independent of both contralateral radiation dose and diet. Histological examination of the irradiated lungs indicated that these acute changes detected by CT were associated with the exudative and edematous phases of
radiation pneumonitis, and that
captopril reduced the severity of these changes. Irradiated (40-80 Gy/10 fractions) animals fed the control diet exhibited a significant increase in central venous and pulmonary artery pressure, and cardiac
right ventricular hypertrophy.
Captopril prevented or attenuated these hypertensive reactions. These data demonstrate that high-resolution CT can detect radiation reactions in rat lung within 4 weeks after 60 Gy/10 fractions, and that
captopril spares these acute changes detected by CT. The mechanism of
captopril action is not clear, but may be due in part to a reduction in pulmonary arterial pressure, resulting in less severe
edema in the irradiated lung.