Identification and characterization of middle ear vascular leakage sites in experimental otitis media

The site of leakage in middle ear vessels was determined and characterized in experimental otitis media in rats. Middle ear effusion was induced by intravenous administration or local application in the tympanic bulla of substance P (SP), acetylcholine, and histamine. In another experiment, a 14 degrees C airstream was blown into the external auditory canal. Colloidal carbon was used to trace leakage sites at the light and electron microscopic levels. All mediators tested and the 14 degrees C airstream resulted in an increased number of leaking vessels in the pars flaccida and the middle ear mucosa. The leakage sites were restricted to capillaries and postcapillary venules. Increased numbers of degranulated pars flaccida mast cells were observed for SP only. Interendothelial gaps formed in leakage vessels after administration of mediators and stimulation of the external auditory canal with a 14 degrees C airstream. Also, cytoplasmic vesicle-like structures within the endothelial cells increased in number following SP and histamine treatment, suggesting that an increased permeability in experimental otitis media does not occur exclusively through interendothelial gaps.