Regression of hypoxic hypertension-induced changes in the elastic laminae of rat pulmonary arteries

The elastic laminae of the pulmonary arteries (PAs) undergo a progressive structural change in hypoxic hypertension. This study focused on the reversibility of altered PA elastic laminae of the rat due to hypoxic hypertension. The structure and cross-sectional area of the PA roedial elastic laminae were examined by using electron-microscopic and image- analytic approaches during recovery from 12 h and 10 days of hypoxic hypertension. At 12 h of hypoxic hypertension, the elastic laminae, which appeared homogeneous in normal control animals, were reorganized into structures composed of randomly oriented filaments, with an increase in the cross-sectional area of 70%. At 10 days of hypoxic hypertension, the elastic laminae appeared homogeneous in structure and normal in cross-sectional area despite continuous exposure to hypoxia. During recovery from 12 h of hypoxic hypertension, the medial elastic laminae regained their homogeneous structure and normal cross-sectional area after day 2. During recovery from 10 days of hypoxic hypertension, the medial elastic laminae changed from homogeneous to filamentous structures, with a progressively altered cross-sectional area that increased by 89% from recovery day 0 to day 10 and returned to the normal level on day 30. These changes were associated with alterations in the PA wall tensile stress. These results indicated that structural changes in the PA elastic laminae were reversible and that the regression process depended on the duration of exposure to hypoxia, the state of the elastic laminae, and possibly the tensile stress level in the PA wall.