Mechanisms Underlying Airway Hypersensitivity Induced by Hydrogen Sulfide

Hydrogen sulfide (H2S) has long been considered as an exogenous irritant gas but now is regarded as an endogenous pro-inflammatory mediator in various pulmonary pathophysiological conditions. However, the role of H2S in “inflammatory reaction” remains controversial, with anti-inflammatory effects having been documented in several pulmonary inflammatory diseases. Therefore, the exogenous H2S supplement is now strongly suggested to offer therapeutic potential. Regardless of its controversial role in inflammatory reaction, we recently reported that H2S exerts a distinct sensitizing effect on pulmonary C fibers. Pulmonary C fibers are nociceptive-like sensory nerve endings innervating all levels of the respiratory tract and play a vital role in detecting chemical stimuli to lungs. Stimulation of the afferents triggers various respiratory reflexes such as cough, bronchoconstriction, and mucus secretion. Sensitization of these afferents is known to lead to airway hypersensitivity characterized by enhanced airway reflex responses such as sustained cough and severe bronchospasm. However, whether H2S induces airway hypersensitivity and its underlying mechanisms are still largely unknown. The proposed study will take 3 years 1) to determine whether lung exposure to H2S induces airway hypersensitivity using in vivo model; and if so, 2) to investigate the role of pulmonary C fibers in this H2S-induced airway hypersensitivity; 3) to investigate Ca2+-permeable ion channels and Ca2+-sensitive signaling pathways involved in the H2S-induced sensitization of pulmonary C neurons using in vitro model; and 4) whether airway hypersensitive effects can also be induced by “endogenous release” of H2S using in vivo model. The possible Ca2+-permeable ion channels include transient receptor potential ankyrin 1 channels (TRPA1 channels) and T-type Ca2+ channels. The possible Ca2+-sensitive signaling pathways will include extracellular signal-regulated kinase (ERK), protein kinase C (PKC), Ca2+-activated Cl- channels. Exogenous H2S is givens by sodium hydrosulfide (a donor of H2S) delivered by aerosol inhalation in vivo and by bath application in vitro, whereas endogenous H2S is applied by its endogenous donor, lipopolysaccharide. In the in vivo study of 1st- and 3rd-year, airway hypersensitivity will be determined by elevation of cough and apnea reflexes measured in conscious guinea pigs and spontaneously breathing anesthetized rats, respectively. The afferent activity of the pulmonary C fibers will be measured using “single-fiber” recording technique in artificially ventilated anesthetized rats. In the in vitro study of 2nd-year, neuronal responses will be determined using Ca2+ imaging and patch-clamp techniques in rat cultured primary pulmonary sensory neurons. Our preliminary results reveal that 1) inhalation of H2S enhances cough and apnea reflexes triggered by activation of pulmonary C fibers; 2) H2S sensitizes pulmonary C-fiber afferents; 3) H2S enhances Ca2+ transients and inward currents triggered by chemical stimulant in pulmonary C neurons; 4) H2S-induced elevations of the C-fiber excitability and Ca2+ transients mediated through the TRPA1 channels; 5) lipopolysaccharide sensitizes the pulmonary C fibers through a H2S-dependent manner; and 6) lipopolysaccharide sensitizes the apnea reflex via TRPA1 channels. The results thus suggest the feasibility of this proposed study. The outcomes of this proposed study should provide a better understanding of the clinically significant physiology of H2S and its effects on the pulmonary C-fiber sensitization. It may potentially lead to novel targets for improving the airway hypersensitivity during lung inflammation.