Surface-active substances have been previously shown to stimulate mucociliary transport in in vitro
and in vivo
systems. In the frog palate model, mucociliary transport of airway sputum was significantly increased when the surface of the palate was sprayed with physiologic surfactant compared with a saline spray (17
). Intrabronchial administration of Curosurf (a porcine lung isolate containing alveolar phospholipids and apoproteins including SP-B and SP-C but not SP-A) in healthy anesthetized dogs increased mucociliary transport by ~ 5-fold compared with saline administration (15
). Consistent with these findings, we observed in the present study that two different surface-active substances, Tween80 and calfactant, induced substantial increases in mucociliary transport in our in vitro
model of the fluid-depleted porcine trachea. Although we observed that isotonic and hypertonic saline were capable of restoring mucociliary transport in some tissues, it seemed that application of surface-active substances provided additional benefit. Because Tween80 was at least the equal of calfactant at restoring mucociliary transport, the beneficial effects could not have been due to the presence of surfactant-associated proteins, which were present only in calfactant.
In our model of the fluid-depleted pig trachea, mucociliary transport is impaired by depletion of the periciliary “sol” fluid layer, which normally surrounds the cilia, providing a low-viscosity medium through which the cilia can freely move (10
). The depletion of this layer is caused by the pharmacologic inhibition of Cl−
secretion, which blocks secretion of liquid by the submucosal glands (11
) and perhaps by the surface epithelium. When liquid secretion is inhibited, the periciliary fluid is likely to be quickly absorbed by ongoing Na+
-dependent absorptive processes. Because a similar impairment of anion and fluid secretion is likely to occur in CF, we speculate that this preparation serves as a useful model of this pathologic consequence of CF disease. Transmission electron micrographs of the airway surface of tissues treated with these inhibitors show that a thin layer of dense mucus compresses the cilia between the mucus layer and the apices of the epithelial cells (10
) resembling that seen in scanning electron micrographs of CF airways (18
We reasoned that simple addition of physiologic salt solution to the airway mucosa might replenish this periciliary liquid and thus restore mucociliary transport. Our results indicate that restoration of mucociliary transport occurs in some tissues after such treatment. We found that the addition of hypertonic saline solution to the lumen also proved to be effective in some tissues. Hypertonic saline treatment of CF sputum in vitro
has been shown to reduce its viscosity and elasticity and increase its transportability when placed on bovine tracheas (14
). Inhaled aerosols of hypertonic saline have also been shown to increase mucociliary clearance from the lungs of patients with CF (19
). It was not apparent in our study, however, that hypertonic saline provided additional benefit to instillation of isotonic physiologic salt solution.
It is unclear how surfactants aid in the restoration of mucociliary transport in the present study. In our model, the mucus gel layer seems to become adherent to the surface epithelial cells (10
). Breaking these mucus plaques free of the epithelial surface may be the critical step in the recovery of mucociliary transport activity. The adhesivity property of mucus, which is assessed by measuring the contact angle of a mucus droplet on a surface, has been shown to be increased in CF mucus (20
). Surfactant application reduces the adhesivity of CF mucus (21
); thus, these surface-active substances may play the important role of reducing mucus adhesion to the epithelial surface and may allow greater access of free liquid to the periciliary space. Surfactants are also capable of altering the viscoelastic properties of mucus in ways that favor stimulation of mucociliary transport. Martin and coworkers (22
) showed that sodium deoxycholate, sodium taurodeoxycholate, sodium glycocholate, and lysophosphatidylcholine decreased the viscosity and elasticity of bronchial mucus and suggested that this effect was due to structural breakdown of the mucins. However, De Sanctis and associates (15
) demonstrated that in vivo
administration of Curosurf to normal dog airways substantially increased mucociliary transport without significantly altering the viscoelastic properties of airway mucus. Surfactants may aid in solubilizing lipids or reducing the hydrophobicity of nonpolar domains in airway mucus. This may be of particular benefit to patients with CF because the airway mucus in CF has a higher lipid content than normal mucus (23
), accounting for nearly 40% of its dry weight (24
). This higher lipid content of CF mucus seems to be related to degree of infection and is sufficient to significantly increase the viscosity of this material (25
). An additional benefit is that surfactants have been shown to have a stimulatory effect on ciliary beat frequency (15
), an effect that could partially account for the stimulation of mucociliary transport with these substances. We cannot completely discount the possibility that the observed stimulation of mucociliary transport by the instillates was a toxic response. We believe that this is unlikely with KRB, which is a buffered physiologic salt solution, or calfactant, which is an isolate of bovine lung surfactant. Hypertonic saline could induce osmotic cell shrinkage of the mucosal epithelium, but this effect should be transient and well tolerated. Of the four instillate solutions, Tween80 is the most likely to have toxicity issues because it is a nonionic detergent. However, hypertonic saline and Tween80 induced nearly identical responses to KRB and calfactant, respectively; these treatments have minimal expectations for toxicity.
Severe impairment of mucociliary transport is considered by many to be a hallmark of CF lung disease, yet literature reports of clearance rates in patients with CF are widely variable, with some studies showing similar and even increased clearance relative to normal subjects (27
). Differences in experimental design and methodology, such as patient breathing patterns, particle deposition patterns, tracers, cough, posture, time-frame for clearance, and whole lung versus regional clearance, contribute to this variability. A recent retrospective study of patients with CF whose airway function ranged from normal to severe obstruction carefully controlled for many of these variables and demonstrated that a consistent reduction in clearance of ~ 50% was evident in CF whole, central, and intermediate lung regions (28
). Clearance in this study seemed to be independent of disease severity (28
). Similar magnitude reductions of mucociliary transport are seen in CF nasal passages (29
), which are less likely to be influenced by obstructive disease and asymmetric tracer distribution than the intrapulmonary airways. These studies suggest that a defect in mucociliary clearance exists in CF airways even though baseline levels of transport seem to be higher (4.5–5.5 mm/min for the nose) than we report in our volume-depleted trachea model (0.3–0.5 mm/min). Thus, our model of mucociliary stasis may be more severe than the defect in CF airways in vivo
. This could be explained if (1
) the anion secretion inhibitors used in the present study block significant quantities of CFTR-independent anion and liquid secretion or (2
) the inflammatory milieu of the CF airway induces passive fluid leakage into the airway lumen sufficient to support a minimal level of mucociliary transport. More study is needed to resolve this issue.
In the present study, we observed that the high rates of mucociliary transport achieved with surface-active substances were short lived. Within the 90-min period of exposure, mucociliary transport rates in most tissues returned to the low rates of the initial control periods. We expect that this occurred because the high rates of stimulated transport rapidly swept the replenished airway liquid to the cranial end of the trachea. Because the secretion inhibitors were present in the bath, the tracheas quickly returned to their initial state of periciliary fluid depletion. This observation raises an important point—The beneficial effects of surfactants on restoring mucociliary transport in a disease such as CF depends upon delivery of liquid volume to the airway surface. This would not be a problem with a short-term interventional procedure, such as bronchoalveolar lavage, where large volumes of liquid are instilled into the lung. However, accomplishing this task in ambulatory patients would be much more difficult. Because the capability of CF airways to secrete liquid by normal physiologic means is likely to be pathologically limited, the problem remains as to how quantities of liquid adequate to support mucociliary transport can be safely delivered to airway surfaces over long periods of time. A successful solution to this problem will hopefully arise from numerous ongoing studies investigating the use of isosmotic and hyperosmotic aerosols, inhibitors of Na+ and liquid absorption, and stimulators of CFTR-independent pathways for airway anion and liquid secretion.