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Volume flow rate of perilymph in the guinea-pig cochlea
K Ohyama1, A N Salt, R Thalmann
1Department of Otolaryngology, Washington University School of Medicine, St. Louis, MO 63110.
Abstract:
The rate of longitudinal flow of perilymph has been measured using an ionic tracer technique. Spread of the tracer trimethylphenylammonium (TMPA) along the perilymphatic scalae was monitored with ion-selective microelectrodes following injection of a minute bolus (approximately 50 nl) of 150 mM TMPAC1 one turn away. This amount of TMPA had virtually no toxic effect on cochlear function. The spread of tracer by longitudinal volume flow and passive diffusion were separated by comparing tracer movements in both apical and basal directions along the scalae in two groups of animals. Experimental findings were compared with a mathematical model which combined diffusion and volume flow. Our results demonstrated that when electrodes were completely sealed into the cochlea, the rate of longitudinal volume flow in scala tympani was extremely slow, approximately 1.6 nl/min in the apical direction. Longitudinal flow was not detectable in scala vestibuli. When the otic capsule was perforated, flow rates of over 1 microliter/min were recorded in scala tympani, probably as a result of cerebrospinal fluid entry through the cochlear aqueduct. When the cochlea was sealed (with recording electrodes in place) and cerebrospinal fluid pressure was released, there was no significant basally-directed flow of perilymph in scala tympani. These findings support the concept that perilymph composition is maintained by local, cochlear mechanisms which do not involve longitudinal volume flow. They provide strong evidence that perilymph is not secreted in one region and resorbed at a spatially distant site.
Insights
Perilymph longitudinal flow is extremely slow in a sealed cochlea, suggesting local mechanisms maintain its composition. This study measured flow using an ionic tracer, revealing minimal movement without cochlear perforation.
Area of Science:
- Otolaryngology
- Neuroscience
- Physiology
Background:
- Perilymphatic fluid dynamics are crucial for cochlear function and homeostasis.
- Understanding perilymph flow is key to elucidating mechanisms of hearing and inner ear disorders.
Purpose of the Study:
- To quantify the rate of longitudinal perilymph flow in the cochlea.
- To differentiate between volume flow and diffusion in perilymph transport.
- To investigate the influence of cochlear sealing and cerebrospinal fluid pressure on perilymph flow.
Main Methods:
- An ionic tracer technique using trimethylphenylammonium (TMPA) was employed.
- Ion-selective microelectrodes monitored tracer spread along perilymphatic scalae.
- Tracer movement was analyzed in apical and basal directions, with and without cochlear perforation, and compared to a mathematical model.
Main Results:
- Longitudinal volume flow in scala tympani was extremely slow (approx. 1.6 nl/min apically) when the cochlea was sealed.
- No significant longitudinal flow was detected in scala vestibuli or scala tympani when cerebrospinal fluid pressure was released.
- Perforation of the otic capsule led to high flow rates (>1 µl/min) in scala tympani, likely due to cerebrospinal fluid entry.
Conclusions:
- Perilymph composition is maintained by local cochlear mechanisms, not significant longitudinal volume flow.
- Evidence suggests perilymph is not secreted and resorbed at spatially distant sites.
- Cochlear sealing is critical for accurate measurement of intrinsic perilymph flow.