Background: Magnetic resonance imaging (MRI) is a powerful technique for visualizing tissues with a high water content. In this study we used MRI to evaluate the effect of antigen and mediators of allergic responses on the nasopharyngeal airway of guinea pigs.
Methods and results: Longitudinal relaxation time-weighted transverse images of the nasopharyngeal airway revealed a clearly defined airway lumen and mucosa. Topical administration of ovalbumin (0.00006% to 0.06%) to the nasopharyngeal airway of sensitized guinea pigs caused a concentration-dependent reduction (34% +/- 1%, maximum) in airway luminal volume and a corresponding increase (28% +/- 3%, maximum) in the volume of the airway mucosa. These effects were duplicated by histamine (10(-5) to 10(-3) mol/L), but not by methacholine (10(-3) mol/L). The antigen-induced changes in airway and mucosal volumes were dose-dependently inhibited by the H1-antagonist loratadine (0.3 to 3 mg/kg, administered orally). On the other hand, topical administration of a decongestant drug, oxymetazoline (250 micrograms per guinea pig), had no effect on the response to antigen.
Conclusions: These results indicate that MRI is a useful technique to measure allergic responses in the airways and identify that histamine is an important mediator of the obstruction that occurs in the nasopharyngeal airway of guinea pigs after antigen challenge.