Abstract
Reactive airway disease is mediated by smooth muscle contraction initiated through several agonist-dependent pathways. Activation of type 1 N-methyl-D-aspartate receptors (NMDA-R1s) by plasminogen activators (PAs) has been linked to control of vascular tone, but their effect on airway smooth muscle contractility has not previously been studied to our knowledge. We observed that NMDA-R1s are expressed by human airway smooth muscle cells and constitutively inhibit the contraction of isolated rat tracheal rings in response to acetylcholine (Ach). Both tissue-type PA (tPA) and urokinase-type PA (uPA) bind to NMDA-R1 and reverse this effect, thereby enhancing Ach-induced tracheal contractility. Tracheal contractility initiated by Ach is reduced in rings isolated from tPA(-/-) and uPA(-/-) mice compared with their wild-type counterparts. The procontractile effect of uPA or tPA was mimicked and augmented by the nitric oxide synthase inhibitor, l-NAME. uPA and tPA further enhanced the contractility of rings denuded of epithelium, an effect that was inhibited by the NMDA-R antagonist, MK-801. Binding of PAs to NMDA-R1 and the subsequent activation of the receptor were inhibited by PA inhibitor type 1, by a PA inhibitor type 1-derived hexapeptide that recognizes the tPA and uPA docking domains, as well as by specific mutations within the docking site of tPA. These studies identify involvement of PAs and NMDA-R1 in airway contractility, and define new loci that could lead to the development of novel interventions for reactive airway disease.