HOCl causes airway substance P hyperresponsiveness and neutral endopeptidase hypoactivity

We investigated whether exposure of guinea pig tracheal tissue to hypochlorous acid (HOCl) or hydrogen peroxide (H2O2) by perfusion through the airway lumen affected the responsiveness of airway muscle to ACh, KCl, or substance P in the presence or absence of 1 microM phosphoramidon, an inhibitor of neutral endopeptidase (NEP). Pairs of tracheal segments were immersed in a Krebs solution (pH 7.40 at 37 degrees C) and connected to perfusion circuits so that the lumen of one segment of each pair could be perfused with Krebs solution while the other was perfused for the same time (10 min) with either 0.1 microM HOCl or 10 mM H2O2. Segments after perfusion were cut into rings of similar size and placed in muscle chambers so that airway muscle force generation in vitro could be measured on stimulation by cumulative agonist doses. In addition, cell homogenates were made from other, similarly perfused tracheal segments to assess NEP activity using reverse-phase, high-pressure liquid chromatography (HPLC). We found that smooth muscle of mucosa-intact guinea pig airways perfused with HOCl, but not H2O2, was hyperresponsive to substance P but not to ACh or KCl. HOCl-perfused rings were not different from Krebs solution-exposed rings pretreated with phosphoramidon. There was no increase in substance P responsiveness of HOCl-exposed airways in which the mucosa had been removed before testing in vitro. The substance P hyperresponsiveness of HOCl-exposed, mucosa-intact airways was associated with decreased NEP activity.(ABSTRACT TRUNCATED AT 250 WORDS)