Search

Autologous serum with or without platelet-activating factor (PAF) was instilled into one lung lobe of an anesthetized cat, and changes in the regional lung water content were monitored for 4 h with proton nuclear magnetic resonance (NMR) images and relaxation time measurements. With serum as an instillate, water was cleared with a half time of approximately 670 min; after 4 h, 86 +/- 6% of that instilled remained. With PAF added to the instillate, clearance was biphasic with an initial clearance half time of approximately 30 min followed by clearance similar to that observed after serum instillation; after 4 h, 35 +/- 4% of that instilled remained. In contrast, 4 h after instillation of serum or serum plus PAF, 91 +/- 3% and 82 +/- 5%, respectively, of the instilled 125I-labeled albumin remained in the lung (P = 0.06). From transverse magnetization relaxation curves we were able to resolve two relaxation components, which we have attributed to the instilled fluid in the air spaces (relaxation time = 177 +/- 7 ms) and the tissue-bound fluid (relaxation time = 25 +/- 1 ms).

Platelet-activating factor (PAF) has been implicated as a mediator of airway hyperresponsiveness. PAF, infused intra-arterially into the canine cervical trachea, causes adherence of neutrophils to vascular endothelium, increases vascular permeability, and increases the responsiveness of tracheal muscle to parasympathetic stimulation. We hypothesized that the increase in airway responsiveness induced by PAF in this model depends on the presence of neutrophils. To test this hypothesis, we perfused a cervical tracheal segment with autologous blood depleted of leukocytes or with similar leukocyte-depleted blood that had been repleted with its neutrophils. Fifteen minutes after the onset of perfusion with either leukocyte-depleted or neutrophil-repleted blood, PAF infusion was begun into the tracheal arterial vasculature. The contractile response of the tracheal muscle to parasympathetic stimulation was measured before and 15 and 30 min after the onset of PAF infusion. PAF did not significantly change the response of tracheal muscle during perfusion with neutrophil-depleted blood but increased the response of tracheal muscle during perfusion with neutrophil-repleted blood. We conclude that the increase in canine tracheal muscle response induced by intra-arterial PAF depends on neutrophils.

Platelet-activating factor (PAF) increases the bronchoconstrictor response of mammalian airways to cholinergic agonists and is thus implicated as a potential mediator of airway hyperreactivity. This study further defines the nature of the increase in airway responsiveness induced by PAF. We employed an in situ canine tracheal preparation, which allows differentiation of the effects PAF has on airway smooth muscle contraction from confounding effects it has on inducing airway edema and secretions. We found that PAF, infused regionally into tracheal arteries, increases the responsiveness of the trachealis muscle to parasympathetic stimuli in a dose-dependent manner. This effect occurred within 15 min. To determine whether the increase in trachealis muscle responsiveness resulted from effects localized to the trachea, we compared the effect of PAF on the tracheal segment with effects of the lower airways of the lung. Delivered to the arteries perfusing the tracheal segment, PAF did not increase lung resistance during vagus nerve stimulation. These data indicate that airway hyperresponsiveness elicited by PAF results from regional stimulation and/or release of mediators that augment tracheal contractility and that this effect is distinct from systemic effects elicited by PAF.

Endotoxin causes pulmonary vascular neutrophil sequestration and injures the lung. Whether this is primarily due to a direct effect of endotoxin on the endothelium or is mediated by an action on the neutrophil is unclear. Canine neutrophils, isolated on plasma-Percoll gradients in vitro, were incubated with Salmonella enteriditis endotoxin, washed, and injected via wedged pulmonary arterial catheters into discrete lung zones of anesthetized dogs, whereas untreated neutrophils were administered into contralateral control lung zones. 113mIn-transferrin was administered intravenously 2 h before the animals were killed. Morphometry and extravascular protein accumulation were assessed at 4 h. Endotoxin treatment of neutrophils ex vivo induced a two- to three-fold increase in neutrophils in these lung zones (0.096 +/- 0.012 vs. 0.05 +/- 0.002 neutrophils/alveolar septal intercept, P less than 0.05). Extravascular-to-intravascular protein ratios in zones receiving endotoxin-treated neutrophils were significantly increased compared with control zones (0.146 +/- 0.02 vs. 0.079 +/- 0.02, P less than 0.05). Because complement fragments increase injury to endothelium in vitro, exogenous C5 fragments were administered to other dogs before administration of neutrophils but failed to significantly increase the extravascular protein signal (0.154 +/- 0.03 vs. 0.124 +/- 0.04). In summary, endotoxin treatment of neutrophils leads to neutrophil sequestration and increased pulmonary extravascular protein accumulation. C5 fragments failed to further enhance the protein accumulation. These data are consistent with an effect of endotoxin on the neutrophil to initiate neutrophil-endothelial interaction and subsequent lung injury.

Although the lung is known to be a major site of neutrophil margination, the anatomic location of these sequestered cells within the lung is controversial. To determine the site of margination and the kinetics of neutrophil transit through the pulmonary microvasculature, we infused fluorescein isothiocyanate-labeled canine neutrophils into the pulmonary arteries of 10 anesthetized normal dogs and made fluorescence videomicroscopic observations of the subpleural pulmonary microcirculation through a window inserted into the chest wall. The site of fluorescent neutrophil sequestration was exclusively in the pulmonary capillaries with a total of 951 labeled cells impeded in the capillary bed for a minimum of 2 s. No cells were delayed in the arterioles or venules. Transit times of individual neutrophils varied over a wide range from less than 2 s to greater than 20 min with an exponential distribution skewed toward rapid transit times. These observations indicate that neutrophil margination occurs in the pulmonary capillaries with neutrophils impeded for variable periods of time on each pass through the lung. The resulting wide distribution of transit times may determine the dynamic equilibrium between circulating and marginated neutrophils.