RHEOLOGICAL ASPECTS OF MUCOCILIARY FLOW

Bronchial mucus is a solution of complex polyelectrolytes, principally glycoproteins, which possesses markedly viscoelastic flow properties, i.e., these solutions are gels and exhibit a flow behavior that has both solid-like or elastic components and flow-like or viscous components. ' The storage shear moduli curves of such material, which characterize the elastic component, exhibit a marked plateau region which indicates a high degree of entanglement between the molecules, thought principally to be due to hydrogen bonding. Secretions from subjects with bronchial disease also show these properties with much more pronounced elastic behavior. ' At the beat frequency of normal cilia, the elastic component of mucus predominates, indicating that the coupling between cilia and mucus is primarily of an elastic nature. :' Mucociliary clearance rates will therefore be a complex function of the viscoelastic properties of the mucus coupled to the cilia as delineated by the Lucas-Douglas model of ciliary transport. In this model, the mucus is assumed to be in a sol layer which has little, if any, elastic or gel character, with only the tips of the cilia, if at all, touching the mucus. A recent model by Ross has shown that adequate prediction of mucus transport rates can be obtained even if the cilia do not touch the bottom of the mucus blanket. The work of the Rehovoth group,' however, has shown that, using a frog palate model, mucus transport occurs only in the presence of mucus. Materials with gel-like properties similar to mucus were not adequate to provide mucus transport, indicating a fine relationship between the rheological properties of mucus itself and the ciliary mechanism. In a more recent report from this group,'; they have shown that adequate mucociliary transport would still be obtained if the mucus was treated with sialidase, ie. , the sialic acid residues were not essential to the adequate transport of mucus. More recent work in our laboratory has shown that it is the glycoprotein fraction of the mucus which is responsible for the rheological behavior of the entire solution, confirming the observations of the Rehovoth group. " However, many questions still remain unresolved-particularly, the origin and properties of the hypothetical sol layer. No one has yet directly measured the properties of this material, nor shown adequately where it originates. Another unresolved question is that of the relation between mucociliary and alveolar clearance. It has been assumed that, somehow, materials are transported to the beginning of the mucociliary escalator, but no adequate mechanism for this has been postulated or demonstrated. It is also not clear whether, in obstructive pulmonary disease, the observed changes in rheological properties of mucus are a primary contributor to the obstruction or occur secondarily as a result of inflammation and infection. It has also been suggested :$ that increased elasticity in obstructive disease may cause mucostasis, primarily by preventing clearance of elastic materials from the mouths of the bronchial glands. Of particular interest to this conference would be alterations in mucus