cell biochemistry and function Cell Biochem Funct 2006; 24: 471–473. Published online 15 November 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1027/cbf.1299 SHORT COMMUNICATION Nitric oxide synthase activity and nitric oxide level in erythrocytes of guinea pigs with experimental otitis media with effusion Adnan Yilmaz 1 *, Celil Uslu 2 and Mehmet Akyuz 3 1 Nenehatun Obstetric and Gynecology Hospital, Erzurum, Turkey 2 Department of Otorhinolaryngology-Head and Neck Surgery, Haydarpasa Numune Hospital, Istanbul, Turkey 3 Department of Biochemistry, Atatu ¨rk University, Medical School, Erzurum, Turkey Free radicals have been implicated in the pathogenesis of an increasing number of disease and inflammatory states. They may cause cell and tissue damage by chemical modification of proteins, carbohydrates, nucleotides and lipids. Under phy- siological conditions free radicals are parts of normal regulatory circuits and are neutralized by antioxidants. Infections are one cause of increased free radicals production. The aim of our study was to assess whether increased oxidative stress is reflected by erythrocyte nitric oxide synthase activity and nitric oxide levels in guinea pigs with experimental otitis media with effusion (n ¼ 6) and in a control group (n ¼ 6). Erythrocyte nitric oxide synthase activity and nitric oxide levels were measured in both groups. The nitric oxide synthase activity and nitric oxide level in the experimental otitis media with effusion were significantly higher than those of the control group. There was a significant positive correlation between the nitric oxide synthase activity and nitric oxide in the experimental otitis media with effusion group. Thus, increased nitric oxide levels may play an important role in cell and tissue damage due to experimental otitis media with effusion. Copyright # 2005 John Wiley & Sons, Ltd. key words — nitric oxide synthase; nitric oxide; free radicals; xanthine oxidase; otitis media with effusion; oxidative stress INTRODUCTION Otitis media with effusion (OME) is a very common disease, especially in childhood and infancy and is char- acterized by non-purulent fluid in the middle ear and fluctuating conductive hearing loss. OME is an inflam- matory response of the middle ear caused by multiple factors such as viral or bacterial infection, eustachian tube dysfunction or allergy. Inflammatory mediators seem to play a major role in the pathogenesis of OME, but the pathogenesis is still not fully understood. 1 In response to inflammatory stimuli released during OME, the middle ear mucosa undergoes intense cell proliferation and differentiation, the middle ear is rapidly populated by leukocytes, and effusion appears in the middle ear cavity. These responses are mediated by the action of a number of bioactive molecules that interact with the cells of the middle ear mucosa and its vasculature. 2 Some studies have documented the roles of several cytokines, inflammatory mediator and growth factors in contributing to inflammatory res- ponses in the middle ear. 2–4 The interaction of cytokines and growth factors with their receptors has multiple intra- and extra-cellular effects. Among these can be the production and release of the free radical nitric oxide (NO  ), which is produced by nitric oxide synthase. 2 Recently, attention has focused on the importance of NO  in the respiratory system, and in particular the importance of epithelial-derived NO  in the regulation Received 30 May 2005 Revised 16 August 2005 Copyright # 2005 John Wiley & Sons, Ltd. Accepted 1 September 2005 *Correspondence to: Dr. A. Yilmaz, Nenehatun Obstetric and Gynecology Hospital, C ¸at yolu, Erzurum, Turkey. Tel: þ90 442 2333562. Fax: þ90 442 3172294. E-mail: yilmazadnan69@hotmail.com