Alteration of oxidative enzymes (LDH and MDH) in perilymph after noise exposure.

Chinchillas were exposed to noise (123 dB, 1/2 h, 700--2800 Hz). The activity of lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) was studied immediately after exposure or 1, 7, 14, 21, or 28 days later. LDH activity in perilymph doubled immediately after exposure, gradually decreasing to control values in about a month. The activity of MDH not only doubled immediately after the exposure but continued to rise for at least another 24 h, reaching a peak value of 3.5 times normal at 24 h. The activity was still 1.6 times normal even after a month of recovery. The enzyme activity of serum and CSF remained within the control range throughout the experimental period except for an initial increase of serum enzyme activity due to anesthetic. The results demonstrate that the morphological changes induced by a noise exposure that is known to produce permanent threshold shift (PTS) are reflected in a significant elevation of oxidative enzymes in perilymph. Possible mechanisms of the elevation of enzymes in perilymph are discussed.