Effects of intravenous hypertonic urea on distorion product otoacoustic emissions in cats

The present study was conducted to measure the effects of IV hypertonic urea on cochlear function in seven normal cats as measured using distortion product otoacoustic emissions (DPOAEs). DPOAE measurements were made at 1/12 octave intervals from 1000 - 8000 Hz on cats sedated with ketamine and xylazine. Three measurement trials (8 minutes per trial) were made before the infusion of hypertonic urea and four measurements trials were made while urea was being infused at a rate of 2-7 mg/kg. The infusion of urea resulted in a mean reduction of 45% in intraocular pressure (lOP), which remained constant over the four post-infusion periods. The mean results indicated that hypertonic urea produced small but significant (3.1 dB) reductions on DPOAE levels in the 1-2 kHz octave- frequency band. In the 2-4 kHz and 4-8 kHz octave-frequency band, the mean DPOAE levels were 4.6 and 1.7 dB, respectively; but these mean differences were not significant. However, for one subject DPOAE levels decreased very substantially at all frequencies. For another subject, DPOAE levels changed in two well-defined regions; they increased in one region and decreased in the other. Urea did not appear to produce DPOAE changes that could be characterized as shifts in place of stimulation or in frequency selectivity.