Background: Inhalation of hypertonic saline and mannitol improve mucociliary clearance in patients with bronchiectasis, but little is known about how the relative osmotic strengths of these compounds affect ciliary beat frequency (CBF) of ciliated human bronchial epithelial cells (HBEC). Our aim was to compare in vitro the direct effects of osmotically equivalent solutions on CBF of HBEC.
Methods: HBEC were acutely (10, 30 min) exposed to comparable osmolar solutions of saline (0.03-0.48%), mannitol (0.19-3%) and dextran (10%-39.39%). Effects of higher % solutions, reversibility of responses, and prolonged treatments (15-20 h) were also compared. CBF was measured using digital videomicroscopy at baseline and at all time points.
Results: CBF of HBEC increased significantly after acute exposure to mannitol (0.19%, 0.38%), decreased with dextran and remained unchanged with saline. Prolonged exposure to mannitol at high (3%, 6%) osmolar concentrations reversibly suppressed CBF. In comparison, acute and prolonged treatment with 39.39% dextran (equivalent to 3% mannitol) reduced CBF irreversibly. Furthermore, acute and prolonged treatment with 1% saline (equivalent to 6% mannitol) suppressed CBF with only the acute effect being reversible.
Conclusions: Mannitol had a direct osmolarity-independent cilio-stimulatory effect at lower % solutions and a reversible cilio-inhibitory effect at higher % solutions, and prolonged exposure to mannitol inhibited CBF reversibly. Both the acute and prolonged effects of mannitol compared to dextran and saline on CBF of HBEC in vitro, imply a unique mechanism of action for mannitol on ciliary beating and might contribute to the improved clearance observed in mannitol-treated patients.