NOS2 and airway hyperresponsiveness: In the early phase of the asthmatic response, NO plays an important role in reducing airway hyperresponsiveness (AHR), and the hyperreactive response is related to NO deficiency, either due to the greater activity of the enzyme arginase, which prevents the conversion to NO and L-citrulline by NOS, or due to the lower activity of NOS constitutive isoforms.