COORDINATED CLEARANCE OF PERICILIARY LIQUID AND MUCUS FROM AIRWAY SURFACES

Airway surface liquid is comprised of mucus and an underlying, watery periciliary liquid (PCL). In contrast to the well-described axial tran sport of mucus along airway surfaces via ciliary action, theoretical a nalyses predict that the PCL is nearly stationary. Conventional and co nfocal microscopy of fluorescent microspheres and photoactivated fluor escent dyes were used with well-differentiated human tracheobronchial epithelial cell cultures exhibiting spontaneous, radial mucociliary tr ansport to study the movements of mucus and PCL, These studies showed that the entire PCL is transported at approximately the same rate as m ucus, 39.2+/-4.7 and 39.8+/-4.2 mu m/sec, respectively. Removing the m ucus layer reduced PCL transport by > 80%, to 4.8+/-0.6 mu m/sec, a va lue close to that predicted from theoretical analyses of the ciliary b eat cycle. Hence, the rapid movement of PCL is dependent upon the tran sport of mucus. Mucus-dependent PCL transport was spatially uniform an d exceeded the rate expected for pure frictional coupling with the ove rlying mucus layer; hence, ciliary mixing most likely accelerates the diffusion of momentum from mucus into the PCL. The cephalad movement o f PCL along airway epithelial surfaces makes this mucus-driven transpo rt an important component of salt and water physiology in the lung in health and disease.