Longitudinal endolymph flow associated with acute volume increase in the guinea pig cochlea

Endolymph volume disturbances were induced by microinjections of artificial endolymph into the second turn of the guinea pig cochlea at rates less than 60 nl/min. Induced longitudinal movements and area changes of endolymph were quantified in the basal turn using an ionic flow marker technique. Tetramethylammonium (TMA) was used as a flow marker by iontophoresing it into endolymph in micromolar amounts. TMA movements in the apical and basal directions were monitored by ion-selective electrodes. Changes in endolymph flow and cross-sectional area of scala media were derived using a mathematical model to interpret the recorded tracer time courses. The model was validated by performing comparable volume injections and flow measurements in fine-diameter plastic tubes. The rate of flow of endolymph measured prior to injection was close to zero, in agreement with prior studies. Based on the injection of different volumes into endolymph over a 15 min period, we found that injection of up to 80 nl of artificial endolymph into the second turn would not induce flow in the basal turn. However, above this amount, flow towards the base increased at a rate which correlated with the injected volume, with endolymph moving basally by a distance of 0.0067 mm/nl of artificial endolymph injected. Flow rates measured in the third turn, on the apical side of the injection were far lower and showed characteristics consistent with there being no outlet at the apex. These results suggest that small volume disturbances are corrected locally in the cochlea, but larger disturbances produce a longitudinal flow of endolymph out of the cochlea which represents a significant mechanism contributing to homeostasis. It can be concluded that structures outside the cochlea, such as the endolymphatic sac, do play a role in the correction of endolymph volume disturbances. Although the maintenance of endolymph composition is dominated by local ion transport mechanisms, the capacity of these local mechanisms to maintain normal endolymph volume appears to be limited.