Figure 2: The dual effects of nitric oxide in asthma pathology. In asthma, nitric oxide can have both beneficial and adverse effects. The production of NO by constitutive isoforms can relax the smooth muscle of airways and vessels via cyclic GMP regulation inducing bronchodilation and vasodilation. However, by acting in the postcapillary venule, it can induce plasma extravasation. Nitric oxide can also regulate the mucosal glands, increasing the mucus secretion. High amounts of nitric oxide produced by iNOS in pathological situations induce the inflammatory cell chemotaxis, particularly recruiting eosinophils and T-lymphocytes to the lung. The reaction of nitric oxide with anion superoxide increases the oxidative stress pathway and can induce cellular injury by protein dysfunction or DNA injury and airway hyperresponsiveness. By substrate competition, nitric oxide can control the arginase pathway and induces airway remodeling, smooth muscle contraction and mucus production.