Ermert, L., H.-R. Duncker, H. Brückner, F. Grimminger, T. Hansen, R. Rössig, K. Aktories, and W. Seeger. Ultrastructural changes of lung capillary endothelium in response to botulinum C₂ toxin. J. Appl. Physiol. 82(2): 382-388, 1997.The role of the endothelial cytoskeleton for the structural integrity of the pulmonary gas exchange area was probed with the use of Clostridium botulinum C₂ toxin. This agent causes selective loss of nonmuscle F-actin. In buffer-perfused rabbit lungs, vascular pressures were kept within physiological ranges. In different groups, low-dose [0.3 (C_2,I)/0.6 (C_2,II) ng/ml] and high-dose [10 (C_2,I)/20 (C_2,II) ng/ml] toxin were applicated into the buffer fluid; experiments were terminated after a total weight gain of either 1 or 7.5 g. Electron microscopy revealed extensive attenuations, undulations, and protrusions of the endothelial layer, suggestive of "remodeling" and "flowing" of the cell membrane in low C₂ toxin-treated lungs accompanied by few disruptions of the endothelial layer and edema formation. In addition, endothelial cells displayed vesiculation and bleb formation. Lungs that were exposed to high-toxin doses displayed marked attenuations of the endothelial layer in addition to large endothelial cell disruptions, which did not include interendothelial junctions. Interestingly, type II epithelial cells displayed fusion of lamellar bodies. Collectively, these data suggest that the actin microfilament system is instrumental in supporting endothelial cell membrane configuration and integrity and maintains the intimal barrier function of the lung microvasculature.