Density and ultrastructure of mast cells in lung vessels of aging rats exposed to and recovering from chronic hypoxia.

A decrease in pulmonary vascular responsiveness in aging animals during exposure to chronic hypoxia has been previously reported; however, morphological documentation is lacking. Lungs from young (3-5 months) and aging (12-14 months) Sprague-Dawley rats, exposed to and recovering from chronic hypoxia, were morphometrically analyzed at the light-microscopic level for changes in perivascular mast cells, and at the electron-microscopic level for cellular alterations. While young rat lungs showed proliferation of mast cells around elastic and muscular pulmonary arteries and arterioles, perivascular mast cell density in lungs of aging rats was significantly greater than in young rat lungs. At the ultrastructural level, perivascular mast cells in aging hypoxic rats showed numerous profiles of cellular extensions that contained remnants of discharged secretory vesicles. The results suggest that increased proliferation of perivascular mast cells as well as increased secretory activity of vasoactive substances in aging animals might represent a humoral determinant of the hyporesponsiveness of pulmonary vessels that occurs with increasing age during chronic hypoxia.