Effect of adrenergic agents and their mode of action on mucociliary clearance in man

The aerosolized adrenergic (ADR) agent, isoproterenol (ISO), was found markedly to accelerate mucus clearance within the human tracheobronchial tree. Mucus transport was measured by external gamma counting of aerosolized Fe2O3 particles deposited on the mucous membrane during inhalation. Aerosolized epinephrine (EPI), despite its alpha-ADR bronchial vasoconstrictor activity, increased mucus clearance to the same degree as did aerosolized ISO, with its beta-ADR bronchial vasodilator activity. The vehicle used for delivery of the ADR agents, i.e., a H2O aerosol by intermittent positive-pressure breathing (IPPB), itself increased mucus clearance slightly, but did not elicit the enormous increases produced by the ADR agents. Parenteral ISO caused increases in clearances similar to aerosolized ISO and EPI (consistent with different tissue drug levels achieved). Oral atropine delayed clearance, but atropine prior to an ADR aerosol did not alter the mucus transport effect of the ADR agent. ISO and atropine, despite opposite effects on mucus clearance, caused equal bronchodilation. Thus, increased clearance following aerosolized ISO was not dependent on bronchial vasodilation, aqueous aerosol droplets, reflex parasympathetic activation, or bronchodilation, and seems best attributable to increased ciliary beat rate.