Airway Smooth Muscle Contraction
Hyperresponsiveness to mast cell-derived mediators (eg, histamine) is a distinctive feature of asthma, but its etiology is unknown. We have studied the effects of mast cell enzymes on airway smooth muscle responsiveness. The original studies are described elsewhere. Addition of mastocytoma supernatant to a muscle bath containing dog bronchial smooth muscle greatly increased the contractile response of the smooth muscle to histamine, an effect that was decreased by an inhibitor of mast cell tryptase, apro-tinin, and was diminished by nedocromil sodium. Contractile effects of acetylcholine were unaffected, suggesting that mast cell proteases act on airway smooth muscle membranes to modify membrane potential-dependent contractile mechanisms. Incubation of the smooth muscle with purified tryptase alone (0.09 μg/ml) did not affect resting tension, but tryptase potentiated markedly the contractile effect of histamine. The significance of these observations is that mast cell tryptase, which is released with histamine from the mast cell secretory granules during degranulation, causes bronchial hyperresponsiveness. Bronchial hyperresponsiveness is a hallmark of asthma, and thus, tryptase may play an important role in the pathogenesis of this disease.
Cleavage of Substance P and Vasoactive
Intestinal Peptide by Tryptase and Chymase
Substance P (SP) is released from sensory neurons in the airways and is implicated as a mediator involved in neurogenic inflammation. Vasoactive intestinal peptide, a neuropeptide that relaxes bronchial smooth muscle, has been localized to efferent autonomic neurons. Studies implicate VIP as a mediator of nonadrenergic smooth muscle relaxation in the airways. Furthermore, there is a close association of some mast cells and peptidergic nerve terminals. Therefore, Caughey and his associates examined the kinetics and sites of cleavage of SP and VIP by both tryptase and chymase: tryptase cleaved VIP rapidly at two sites with k^/K™ of 2.2 X 105 s_1 M, but it had no effect on SP Chymase cleaved both SP and VIP at primarily a single site with k^/K^ of 3.9 x 104 and 5.4 X 104 s~ M ~l, respectively.These differences between tryptase and chymase suggest that the location and phenotype of mast cells are important determinants of their peptidase activity. For example, 90 percent of human lung mast cells are reported to contain tryptase alone. Thus, tryptase, by cleaving the bronchodilator VIP but not the bronchoconstrictor SP, might promote bronchospasm in asthma.
Reversal of Vasoactive Intestinal Peptide
Induced Airway Smooth Muscle Relaxation
Because mast cell proteases cleave the VIP molecule, we studied their effect on VIP-induced relaxation of ferret airway smooth muscle. Mast cell supernatant containing tryptase and chymase abolished VIP-induced relaxation, and inhibitors of tryptase and chymase (aprotinin and soybean trypsin inhibitor) eliminated this effect of the supernatant. These studies confirm the finding that mast cell proteases cleave VIP and suggest that they may contribute to bronchospasm in diseases such as asthma.