INHALED TRYPTASE CAUSES BRONCHOCONSTRICTION IN SHEEP VIA HISTAMINE-RELEASE

Allergen-induced bronchoconstriction involves mast cell activation. Tr yptase is a mast cell serine protease that is released during this pro cess, but little is known about the action of tryptase in the airway. The purpose of this study was to determine: (1) if aerosolized tryptas e causes bronchoconstriction, and (2) the mechanism by which this occu rs. We measured mean pulmonary flow resistance (RL) in five allergic s heep before and after consecutive inhalations of 100 and 500 ng trypta se (in 2 mi total volume). Inhaled tryptase at 100 and 500 ng increase d RL (mean +/- SE) by 33 +/- 12 and 122 +/- 8% (p < 0.05) over baselin e. The response was reproducible upon repeat challenges. These studies were repeated in the same animals after pretreatment with aerosolized APC 366 (9 mg/3 mi), a specific tryptase inhibitor. In APC-366-treate d sheep, tryptase increased RL by 10 +/- 3 and 6 +/- 2% (p < 0.05 vers us control values) at 100 and 500 ng, respectively. The response to tr yptase was also blocked by pretreating the sheep intravenously with th e histamine H-1-antagonist chlorpheniramine (2 mg/kg), in which RL inc reased only 5 +/- 4 and 7 +/- 6% after 700 and 500 ng tryptase. APC 36 6, however, did not block histamine-induced bronchoconstriction. Consi stent with these findings was the observation that segmental bronchial challenge with tryptase (1 mu g) resulted in a significant increase i n histamine levels in bronchoalveolar lavage. Inhaled tryptase (500 mu g) also caused airway hyperresponsiveness to aerosolized carbachol 2 h after tryptase challenge. This tryptase-induced airway hyperresponsi veness could be blocked either by pretreating the sheep with APC 366 ( 30 min before challenge) or by treating the sheep 30 min after challen ge. These results indicate that inhaled tryptase causes bronchoconstri ction and airway hyperresponsiveness in allergic sheep by an event tha t may involve mast cell activation.