Localization of Nitric Oxide Synthase Isoforms (I, II and III) in the Endolymphatic Sac of the Guinea Pig

The localization of nitric oxide (NO) synthase (NOS) isoforms was examined in the endolymphatic sac (ES) of the pigmented guinea pig by indirect immunohistochemistry. The cytoplasm of the epithelial cells in the ES showed both NOS-I and -III immunoreactivity, whereas their nuclei appeared negative. NOS-III staining was also observed in the endothelial lining of the blood vessels. These findings support the hypothesis that NO in the epithelial cells may play an important role for the active intracellular transport of the endolymph and ion. NO may also be crucially involved in the regulation of ES blood flow. Immunostaining for NOS II revealed no reactivity in general, while in lipopolysaccharide-inoculated animals, intense reactivity was found in the cytoplasm of the ES epithelial cells as well as macrophages in the lumen. Thus it has been indicated that NO also may play an important role in the immunodefensive mechanisms in the ES.