Late asthmatic response causes peripheral airway hyperresponsiveness in dogs treated with metopirone

To determine if late asthmatic response (LAR) is associated with hyperrespon-sivcness of airway smooth muscle itself, we performed antigen challenge in dogs treated with Metopirone. We studied the contractile response to acetylcholine (ACh) in isolated bronchial and bronchiolar segments 8 h after either saline inhalation (the control group) or antigen challenge in dogs demonstrating immediate asthmatic response (IAR) alone and in dogs demonstrating both IAR and LAR. Airway responses to Ascaris suum antigen were assessed by changes in respiratory resistance measured with the forced oscillation technique at 3 Hz. Concentration-response curves of bronchial preparations to ACh did not differ significantly among three groups consisting of the control, IAR and LAR. However, the contractile response of bronchiolar preparations to ACh was significantly greater in the LAR group when compared to the control and IAR groups at the concentrations of ACh ranging from 10⁻⁶ to 3 × 10⁻⁴M (p <0.01). SQ 29548, a receptor antagonist of thromboxane A₂ and prostaglandin D₂ (PGD₂), inhibited LAR-induced hyperresponsiveness to ACh in a concentration-dependent fashion. The bronchiolar preparations obtained from dogs showing LAR contained a significantly higher amount of PGD₂ than those obtained from dogs showing IAR alone (p <0.01, n = 6). These results suggest that LAR is associated with hyperresponsiveness of peripheral airway smooth muscle to ACh, and this augmented response to ACh mediates via PGD₂ released during LAR.