Heme oxygenase and nitric oxide synthase in human middle ear epithelium indicates local carbon monoxide and nitric oxide production

The gas mixture of the middle ear differs from that of the atmosphere, a fact that has been attributed to gas exchange across the middle ear mucosa. Several diseases of the middle ear seem to be related to impaired ventilation together with conjunctional changes in pressure and gas composition. Carbon monoxide (CO) and nitric oxide (NO) have recently been shown to be endogenously produced in the human lung as well as in the nasal airways. The production of CO and NO is enzymatically regulated by heme oxygenase (HO) and NO synthase (NOS), respectively. These enzymes display isoforms that are both constitutively expressed [HO-2, endothelial NOS (eNOS), neuronal NOS (nNOS)] and inducible [HO-1, inducible NOS (iNOS)] following different types of stimulation. The present study was designed to investigate the presence of HO-1, HO-2 and eNOS in the middle ear epithelium, using immunocytochemistry. Specimens from human middle ear mucosa obtained at autopsy and during surgery revealed HO-1-, HO-2- and eNOS-like immunoreactivity, indicating the possibility of local CO and NO production in the middle ear. If this assumption is true, it may affect our understanding of middle ear physiology and give new insights into the mechanisms behind middle ear pathology.