Ionic and potential changes of the endolymphatic sac induced by endolymph volume changes.

The endolymphatic sac (ES) is believed to be the locus for endolymph volume regulation in the inner ear. It has recently been shown that induced endolymph volume changes in the cochlea result in anatomical changes in the ES, suggesting that function of the sac varies according to endolymph volume status. In the present study we have recorded luminal concentrations of K(+) and Na(+) from the ES and the endolymphatic sac potential (ESP) during cochlear endolymph volume changes. ES recordings were made by an extradural approach, thereby preserving normal cerebrospinal fluid resting pressure. Cochlear endolymph volume changes were generated by performing injections or withdrawals through a pipette inserted into endolymph by a round window approach. The pre-treatment concentrations of K(+) and Na(+) in the ES were found to be 8.4 mM (S.D. 3.3, n=8) and 128. 6 mM (S.D. 18.4, n=10) respectively, and the mean ESP was 14.4 mV (S. D. 5.2, n=18). Endolymphatic injections were found to produce a sustained increase in the K(+) content of the ES by an average of 19. 9 mM and to decrease Na(+) by 30.7 mM measured 50 min after the start of injection. The time for K(+) increase to occur was found to correlate with the injected volume, with larger injected volumes producing a more rapid increase. Endolymphatic withdrawals were found to induce a slow decline in endolymphatic K(+) by an average of 3.4 mM measured at 50 min after withdrawal, although no significant change of Na(+) was detected. Volume-induced ESP changes were highly variable. Injections produced a small increase in the mean ESP and withdrawals produced a small decrease but neither change was statistically significant and some animals showed potential changes in the opposite direction. These data show that a change in cochlear endolymph volume status results in a physiologic response of the ES which is sustained for a considerable period. If the ES plays a part in the restoration of normal endolymph volume, this process appears to proceed slowly, based on the prolonged time courses of ionic changes observed.