EFFECTS OF HIGH-INTENSITY IMPULSE NOISE ON IONIC CONCENTRATIONS IN COCHLEAR ENDOLYMPH OF THE GUINEA-PIG

Objective To investigate the effect of acoustic overstimulation on con centrations of cations in cochlear endolymph and analyze the relations hip between the ionic changes in endolymph and the hearing loss. Metho ds The endocochlear potentials (EP), K+, Na+ and Ca2+ concentration in cochlear endolymph were examined in vivo for normal and 167 +/- 2 dB SPL impulse noise exposed groups of guinea pig bored on time course by means of double-barreled ion-selective microelectrodes. Brain stem au ditory evoked potential (BAEP) was used to evaluate the auditory funct ion. Ca2+-ATPase activity was demonstrated cytochemically in the later al cochlear wall as induced by Ando et al with slight modification. Re sults The K+ and Ca2+ concentration exhibited significant changes in 8 -hour groups (P<0. 05 for K+ and P<0. 01 for Ca2+). Then the K+ concen tration was eventually resumed to the initial levels in accordance wit h the EP recovery in 7 days, while during the same period Ca2+ concent ration was always significantly higher than that in control group (P<0 . 01). The BAEP threshold shifts were correlated well with changes in ionic concentrations, especially Ca2+ (P<0. 001), in the endolymph ind uced by acoustic trauma. Although the normal positive EP was observed 7 days after noise exposure, the function of the vascular stria was no t completely restored as revealed by the fact that the Ca2+-ATPase was diffused to the apical membrane surface. Conclusions Endolymph compar tment intrinsic mechanism for maintaining ionic composition is serious ly deteriorated after high impulse noise stimulation. The changes of t he unique environment of endolymph may play an important role in the m echanism of sensorineural hearing loss induced by acoustic trauma.