Enhanced airway smooth muscle (ASM) contractility contributes to increased resistance to airflow in diseases such as bronchitis and asthma that occur in passive smokers exposed to secondhand smoke. Little information exists on the cellular mechanisms underlying such airway hyperreactivity. Sputum samples of patients with chronic sinusitis, bronchitis, and asthma show increased concentrations of growth factors called neurotrophins, including brain-derived growth factor (BDNF), but their physiological significance remains unknown. In human ASM, we tested the hypothesis that BDNF contributes to increased contractility with cigarette smoke exposure. The exposure of ASM to 1% or 2% cigarette smoke extract (CSE) for 24 hours increased intracellular calcium ([Ca 21 ] i ) responses to histamine, and further potentiated the enhancing effects of a range of BDNF concentrations on such histamine responses. CSE exposure increased the expression of the both high-affinity and lowaffinity neurotrophin receptors tropomyosin-related kinase (Trk)-B and p75 pan-neurotrophin receptor, respectively. Quantitative ELISA showed that CSE increased BDNF secretion by human ASM cells. BDNF small interfering (si)RNA and/or the chelation of extracellular BDNF, using TrkB-fragment crystallizable, blunted the effects of CSE on [Ca 21 ] i responses as well as the CSE enhancement of cell proliferation, whereas TrkB siRNA blunted the effects of CSE on ASM contractility. These data suggest that cigarette smoke is a potent inducer of BDNF and TrkB expression and signaling in ASM, which then contribute to cigarette smoke-induced airway hyperresponsiveness.Keywords: neurotrophin; asthma; TrkB; environmental tobacco exposure; secondhand smoke Airway diseases such as asthma and chronic obstructive pulmonary disease (COPD) can be triggered or exacerbated by cigarette smoke and secondhand smoke exposure (1-4). Furthermore, in people with preexisting airway diseases such as asthma, acute exposure to cigarette smoke can produce a bronchospastic response. Considerable evidence already exists that airway inflammation occurs with cigarette smoke exposure, and contributes to airway disease even in passive smokers. Regardless of the underlying causes for airway inflammation (of which there are many), increased airway smooth muscle (ASM) contractility is certainly a key factor contributing to the pathological airway narrowing and increased airflow resistance observed in asthma and bronchitis. Although smoke exposure is recognized to increase airway contractility, the mechanisms by which cigarette smoke enhances ASM contractility remain under investigation.In the nervous system, growth factors called neurotrophins (NTs) are well recognized for their role in the generation, growth, and maintenance of different neuronal populations (5-9). Both short-term (seconds or minutes) and long-term (hours or days) effects of NTs on nuclear and cytosolic signals have been recognized as favoring increased synaptic transmission and plasticity, cell proliferation, and surviva...