Mucus Hypersecretion and Viscoelasticity Changes in Cigarette-Smoking Dogs

Abstract

Chronic cigarette smoke exposure is well known to cause mucus hypersecretion in experimental animals, but the alterations in mucus rheology have not been described. We studied mucus hypersecretion and viscoelasticity changes in nine tracheostomized beagle dogs exposed to cigarette smoke. The dogs were trained to stand quietly in a harness, and smoke was delivered via a cuffed tracheostomy tube. A 35-cm bolus was introduced to the inspiratory line each 20 s, using unfiltered 70-mm cigarettes (20 mg tar, 1.2 mg nicotine). Each dog smoked 10 cigarettes per day over 2.5 h, 5 days per week. Two dogs were exposed for 6 months; 7 dogs were exposed for 10 months. Five dogs served as sham-smoking controls. Mucus was collected twice weekly without drugs by resting a cytology brush on the lower trachea for 2-5 min. The rheological properties of the mucus samples were determined by magnetic rheometry, which yields elasticity and viscosity as a function of frequency. The mucus was also weighed, and the galactose content was determined by phenolsulfuric acid assay. The mucus collection rate served as an index of tracheal mucus flux, and the galactose assay as a marker of mucous glycoprotein content. The tracheal mucus linear velocity (TMV) was determined periodically under xylazine analgesia by observing charcoal particle transport bronchoscopically. Eight of 9 smoking dogs developed mucus hypersecretion (flux > 2 × control) versus 1 of 5 controls (p < .01). TMV did not change significantly in 10 months of exposure. In the first 2-4 months of smoking, the elasticity and viscosity of the mucus both decreased (mean at 4 months = 42% control, p < .001), as did the galactose content (mean at 4 months = 48% control, p < .01). At this stage, according to model studies, the mucus should have been more easily clearable by ciliary action. After 6 months, the viscoelasticity returned toward the initial control level, while the galactose content remained low, suggesting an alteration in the nature of the mucous glycoprotein.