Evidence for a tight blood‐labyrinth barrier

The entry of a small tracer ion (FW 135.6) from blood into perilymph and cerebrospinal fluid (CSF) has been monitored using ion‐selective electrodes. The tracer, trimethylphenylammonium (TMPA) can be detected in extremely low concentrations (< 1 μm) by electrodes, allowing entry to be accurately quantified. Electrodes were sealed into the cochlea to ensure that artifactual fluid leakages did not distort the results. A constant level of TMPA in plasma (approximately 0.5 mM) was maintained by continuous venous infusion of tracer. TMPA entry into CSF was extremely slow, reaching only 13% of the plasma level within 90 min (n = 3). Tracer entered perilymph even more slowly, reaching 6.5% (n = 5) and 3.7% (n = 5) of the plasma level in 90 min for ST and SV, respectively. The slow entry of such a small ion into perilymph demonstrates the existence of a tight barrier between blood and perilymph, and eliminates the possibility that perilymph is generated by mechanisms such as ultrafiltration. In two animals where bleeding was noted at the electrode insertion site, a much more rapid rate of tracer entry was recorded, probably as a result of plasma leakage into perilymph. Treatment with epinephrine (giving a brief blood pressure increase) also opens the barrier, allowing rapid tracer entry.