Mucociliary clearance in the rat lung and the effect of age

Mucociliary clearance rate was measured in groups of rats (n = 25) at 2, 5, 8 and 12 months of age. Animals inhaled an aerosol of indium-111 labeled iron oxide particles (median dia. 1.95 u) for a period of 5 min. and thoracic activity was monitored externally during the next 48 hours. Clearance curves, generated as loss of initial activity versus time, were found to be biphasic and represented the sum of 2 exponential curves with separate rate constants (slopes). The first phase lasted 6 +/- 1.5 hours whereas the second was much slower and continued into the second day. Comparison of individual values obtained for the first phase at 2 months showed considerable variation among animals ranging +/- 20% from the mean. These variations were not due to experimental error or sex and persisted in each animal with aging. Absolute clearance rates, however, progressively decreased with age. This phenomenon was at least in part the result of a greater penetration of aerosol within the airways as the animal ages. Percent of initial radioactivity remaining within the lungs 48 hours following aerosol inhalation was 15.1 +/- 0.82% at 2 months and progressively increased to 28.4 + 0.60% at 12 months. The authors conclude that in rats mucociliary clearance is in part genetically determined and regional deposition patterns of particulates within the airways varies with age