The round window membrane is regarded to be the main route for passage of potentially ototoxic substances from the middle ear cavity into the inner ear. This is of clinical importance in acute otitis media and chronic otitis media, where sensorineural hearing impairments sometimes develop. The accuracy and reliability of an in vitro round window membrane permeability model was studied. The round window membrane of the mongolian gerbil was resected, together with its bone niche. The preparation was mounted between two glass chambers, representing the middle ear cavity and the perilymphatic space respectively. Passage through the round window membrane did not occur within 3 h for low-density lipoprotein with a molecular weight of 2,300 kD and high-density lipoprotein with a molecular weight of 115-350 kD. Horseradish peroxidase, with a molecular weight of 40 kD, passed the round window membrane at a mean rate of 6 micrograms/h when the concentration of the protein was 10 g/l in the middle ear chamber. The present in vitro model is considered to be free of leakage artifacts between the chambers. Passage rates for different substances through the round window membrane can be calculated under controlled conditions using this type of in vitro model.
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