The functional resemblance between kidney proximal tubular and inner ear epithelial cells which has often been pointed out in the literature led us to hypothesize that nephrotoxic agents that cause renal tubular injury might also impair the function of inner ear cells. As one of the most toxic environmental nephrotoxic agents is cadmium, we aimed to study its effects on hearing experimentally in rats. In this study, increased blood and renal cortical cadmium levels were associated with high cadmium accumulation in ear ossicles and labyrinth in rats exposed to cadmium. The changes in auditory brainstem response (ABR) and otoacoustic emission in 2-month-old male rats exposed to drinking water containing 5 and 15 ppm CdCl2 for 30 days showed that cadmium-induced nephrotoxicity was associated with signs of defective hearing at a concentration of 15 ppm CdCl2 but that 5 ppm CdCl2 caused hearing loss without affecting kidney function. The mean latency of ABR wave 1, which indicates the function of the cochlea, was 1.335 +/- 0.31 ms in the control group and 1.641 +/- 0.052 and 1.74 +/- 0.88 ms in the rats subjected to 5 and 15 ppm CdCl2, respectively (p < 0.001). In the cadmium-treated groups short interpeak wave I-III latencies (p < 0.01) indicated cochlear dysfunction and this was also supported by the distortion product otoacoustic emission results (p < 0.001). Non-significant changes in wave III and V latencies were accepted as evidence of unaltered function of the other parts of the auditory system. These results suggest that hair cells are more sensitive to cadmium than kidney tubule cells and that the cochlear component of hearing is more vulnerable to cadmium toxicity than other parts of the auditory system.
Cadmium, which has adverse effects on many physiological systems, is an important environmental pollutant. Our previous experimental study showed that cadmium also has a dose-dependent deleterious effect on the auditory system in rats. Because zinc reverses cadmium cytotoxicity in many systems, we investigated the possible preventive effect of a zinc-enriched diet given isochronally on cadmium-induced hearing loss in rats. Fifty-four male rats were divided into three equal groups. Control rats were fed normal rat food and tap water, whereas the cadmium group was subjected to 15 ppm cadmium-containing water as CdCl2. The third group received 15 ppm CdCl2 and food enriched with 200 ppm zinc as ZnSO4 for 30 d. On d 30, eight animals from each group were used for the measurement of kidney functions. In the remaining animals, hearing functions were measured by auditory brainstem response and distortion product otoacoustic emission. Blood cadmium increased from 1.87+/-1.69 to 6.08+/-2.62 microg/dL and elevated cadmium contents of ear ossicles and kidney cortex were associated with a decreased glomerular filtration rate in rats subjected to high cadmium. A zinc-enriched diet obviously reduced cadmium accumulation in the kidney and prevented the nephrotoxicity. Our data indicated that cadmium-induced ototoxicity seems to be partially zinc preventable and zinc addition to diet without altering cadmium content in ear ossicles may help to prevent cadmium-induced hearing loss.
Reactive oxygen metabolites are products of oxidative metabolism that are continuously generated in vivo, and are known to produce serious cellular, tissue and genomic damage. l-carnitine is an endogenous amine that has been shown to have an effect on the synthesis of reactive oxygen metabolites. Twenty Wistar rats, 24 months of age, were randomly assigned to two groups as control and l-carnitine treatment groups. One millilitre of distilled water was administered to control rats and 50 mg/kg l-carnitine to rats of l-carnitine treatment groups by intragastric gavage once a day for 30 days. At the end of 30 days, all groups underwent auditory brainstem response testing after administration of intraperitoneal urethane anaesthesia. l-carnitine treatment reduced III, V latencies and I-III, III-V and I-V interpeak latencies (IPL) significantly compared with the control group. l-carnitine treatment improved age-related deterioration in auditory pathways and hence may be a new alternative for the treatment of presbyacusis.
The efficiency of steroid treatment in patients with severe hearing loss was low. It was statistically ascertained that adding hyperbaric oxygen or ozone therapy to the treatment contributed significantly to treatment success.
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