Mast cell tryptase causes homologous desensitization of β-adrenoceptors by Ca2+ sensitization in tracheal smooth muscle

Background Recent studies have revealed that in asthma, mast cells infiltrate to the smooth muscle layer and release tryptase, an enzymatic activator of protease-activated receptor 2 (PAR2). This phenomenon, mast cell myositis, is proposed as a new feature of asthma. However, little is known about the involvement of mast cell myositis in the pathophysiology of asthma. Objective This study was designed to determine whether mast cell degranulation has any functional impact on beta-adrenoceptors via PAR2 in airway smooth muscle. Moreover, we focused on Ca2+ signalling as a mechanism underlying alteration of smooth muscle tone and responsiveness. Methods Isometric tension and F-340/F-380, an indicator of the concentration of intracellular Ca2+ ([Ca2+](i)), were simultaneously measured using fura-2-loaded tissues isolated from guinea-pig tracheal smooth muscle. Results Tryptase (1-100nm) caused tension with elevated F-340/F-380, and after exposure to tryptase for 15 min the inhibitory effect of isoprenaline (ISO) against methacholine was attenuated without elevating F-340/F-380 in a concentration-dependent manner. Tryptase (<1 nM) had a modest effect on tension, but prolonged treatment (<= 120 min) with 0.1 nm tryptase also reduced the effects of ISO in a time-dependent manner. When tissues were incubated with tryptase in the presence of Y-27632, a Rho-kinase inhibitor, reduced responsiveness to ISO by tryptase was reversed without affecting F-340/F-380. In contrast, pre-treatment with SKF96365, a non-selective inhibitor of Ca2+ channels, did not antagonize the effect of tryptase. Moreover, pre-treatment with SLIGKV-NH2, a non-enzymatic activator of PAR2, resulted in a loss of P-adrenergic efficacy, similar to tryptase. The effect of cAMP-related agents bypassing beta-adrenoceptors was not attenuated after exposure to tryptase. Conclusion In mast cell myositis, tryptase released from mast cells acts on air-way smooth muscle, leading to homologous P-adrenergic desensitization mediated by [Ca2+](i)-independent mechanisms via PAR2 activation.