We previously showed that when pulmonary capillaries in anesthetized rabbits are exposed to a transmural pressure (Ptm) of approximately 40 mmHg, stress failure of the walls occurs with disruption of the capillary endothelium, alveolar epithelium, or sometimes all layers. The present study was designed to test whether stress failure occurred more frequently at high than at low lung volumes for the same Ptm. Lungs of anesthetized rabbits were inflated to a transpulmonary pressure of 20 cmH2O, perfused with autologous blood at 32.5 or 2.5 cmH2O Ptm, and fixed by intravascular perfusion. Samples were examined by both transmission and scanning electron microscopy. The results were compared with those of a previous study in which the lung was inflated to a transpulmonary pressure of 5 cmH2O. There was a large increase in the frequency of stress failure of the capillary walls at the higher lung volume. For example, at 32.5 cmH2O Ptm, the number of endothelial breaks per millimeter cell lining was 7.1 +/- 2.2 at the high lung volume compared with 0.7 +/- 0.4 at the low lung volume. The corresponding values for epithelium were 8.5 +/- 1.6 and 0.9 +/- 0.6. Both differences were significant (P less than 0.05). At 52.5 cmH2O Ptm, the results for endothelium were 20.7 +/- 7.6 (high volume) and 7.1 +/- 2.1 (low volume), and the corresponding results for epithelium were 32.8 +/- 11.9 and 11.4 +/- 3.7. At 32.5 cmH2O Ptm, the thickness of the blood-gas barrier was greater at the higher lung volume, consistent with the development of more interstitial edema. Ballooning of the epithelium caused by accumulation of edema fluid between the epithelial cell and its basement membrane was seen at 32.5 and 52.5 cmH2O Ptm. At high lung volume, the breaks tended to be narrower and fewer were oriented perpendicular to the axis of the pulmonary capillaries than at low lung volumes. Transmission and scanning electron microscopy measurements agreed well. Our findings provide a physiological mechanism for other studies showing increased capillary permeability at high states of lung inflation.

Three independent assay methods were used to investigate the activities of antimicrobial peptides (human and rabbit defensins and protegrin from porcine leukocytes) against Mycobacterium tuberculosis in vitro. M. tuberculosis H37Ra was cultured in the presence of human neutrophil peptide 1, synthetic rabbit neutrophil peptide 1, or porcine protegrin 1 at 37؇C for 6 to 48 h, and antimycobacterial activity was measured by CFU assay. These peptides at a concentration of 50 g/ml showed significant antibacterial effects on M. tuberculosis after 24 and 48 h of incubation (85.9 to 97.5% at 24 h and 91.6 to 99.4% at 48 h). A radiometric method and a radial diffusion assay confirmed these observations. Antibacterial activity against M. tuberculosis was independent of calcium (1.0 mM) or magnesium (1.0 mM) and not inhibited by sodium chloride (100 mM). The optimal pH for antibacterial activity against M. tuberculosis was greater than 4.0. Three clinical isolates of M. tuberculosis were also studied, and these peptides showed 86.3 to 99.0% reduction in CFU of these organisms. Morphological studies using scanning electron microscopy showed that defensins caused lesions on the surface of H37Ra. These observations suggest that antimicrobial peptides such as defensins and protegrins may represent an important component of the host defense mechanism against M. tuberculosis and offer a potential new approach to therapy. Mycobacterium tuberculosis is still a leading cause of worldwide disease morbidity and is responsible for more deaths each year than any other single pathogen (32). In addition, the AIDS epidemic has exacerbated the problem. Interest in tuberculosis has been rekindled by the recent resurgence of cases both in the United States and worldwide. The increasing number of multidrug-resistant M. tuberculosis isolates that can be exceedingly difficult and expensive to treat is of particular concern (17). The combination of the increased frequency of infection due to M. tuberculosis and the increase in multidrugresistant M. tuberculosis isolates in patients with AIDS has raised a great deal of concern across the country. Defensins are endogenous antimicrobial peptides (AMPs) that contain 29 to 35 amino acid residues (27). These peptides were first recognized in rabbit and guinea pig neutrophils and in rabbit alveolar macrophages as ''lysosomal cationic proteins'' with antimicrobial properties (42, 43). Much earlier, calf thymus peptide (7) and lysozyme (34) were shown to inhibit the growth of pathogenic mycobacteria. More than 15 mammalian defensins derived from five species have been purified and sequenced, and human neutrophils were found to contain four defensin peptides (15). Defensins have been shown to possess antifungal (10), antibacterial (13, 16, 37), and antiviral (4) activities in vitro. We have previously demonstrated that the defensins human neutrophil peptide 1 (HNP-1), HNP-2, and HNP-3 have activity against Mycobacterium avium-Mycobacterium intracellulare (33). More recently, we have synthesized rabbit neutr...

Tissue response to these corneal inserts includes keratocyte activation, intracellular lipid accumulation, and new collagen formation. Our histologic findings suggest that these deposit formations are not lipofuscin accumulations.