Long-term retention of inhaled fused aluminosilicate particles in the trachea and first bifurcation of the rat

Abstract

Rats inhaled by nose-only exposure an aerosol of fused aluminosilicate particles, 3.95 μm activity median aerodynamic diameter, labelled with 57Co. The amount remaining on the airway surface at 7, 28 and 112 days after inhalation was determined at sacrifice by measuring the 57CO content in successive washes of trachea and of the region of the first bifurcation. Similar low levels were found in both cases. From 57Co excretion measurements, the rate at which particles were being cleared through the trachea was calculated, and hence the amount on the tracheal surface that was expected from rapid mucociliary clearance, T f. As expected, T f decreased markedly during the study period. However, at all times the amount of 57Co washed from the tracheal surface was greater than T f. The difference was a measure of the amount remaining on the surface long-term, T s. At 7 and 28 days after inhalation, T s was approx. 4 x 10 −4 of the thoracic content at 1 day, decreasing to 7 × 10 −5 after 112 days. The fraction sequestered in the tracheal wall, T w, remained approximately constant at around 1 × 10 −4 At 7 days, T s and T w were 1.81 and 0.45%, respectively, of the estimated amount deposited on the trachea. Much of the 57Co activity in the tracheal washes was associated with macrophages, the proportion increasing with time.