Ferret tracheal mucus rheology, clearability and volume following administration of substance P or methacholine.

We studied the effects of substances P administration on tracheal mucus viscoelasticity, water content, clearability and secretion rate. Six neutered adult male ferrets (weight 1.1-1.5 kg) were studied, on four occasions each. They were anaesthetized with ketamine and xylazine, and intubated shallowly. Control mucus (pre- and post-Ringer instillation) was compared with the mucus obtained following instillation of 200 microliters of 10(-6), 10(-5), and 10(-4) M substance P (SP), and 10(-4) M methacholine chloride as a reference. Tracheal mucus was collected by inserting a soft-bristled cytology brush to the level of the carina, and leaving it in contact with the mucosa for 30 min. After withdrawing the brush, the adherent mucus was quickly scraped off and layered with paraffin oil to prevent evaporation. The mucus was analysed for viscoelasticity by magnetic rheometry and solids content by evaporation to dryness. Mucus transportability was assessed by comparison with model gels, and also by means of the frog palate assay, which indicates how well mucus is cleared by normal ciliary action. There was a dose-related increase in mucus volume, and a dose-related decrease in mucus viscosity and elasticity, with substance P administration. Mucus transportability increased with both substance P and methacholine. The effects of 10(-4) SP and methacholine were comparable in terms of viscoelasticity and volume; with both methacholine and SP, there appeared to be an additive effect. The mucus solids content did not correlate with the viscoelastic changes for SP administration, whereas it did for methacholine. These observations suggest that the hypersecretion induced by these two agents might involve different pathways.