THE ORIGINS AND MECHANISM OF GLUTAMATE RELEASE IN COCHLEA

我们应用微透析技术研究淋巴管周围谷氨酸。采用酶联荧光法联合微透析连续分析谷氨酸浓度。(1)观察了卡那霉素和乙酸对内耳毛细胞的损伤作用过程中淋巴周围谷氨酸盐变化的时间过程。在豚鼠皮下注射800 mg/kg卡那霉素负荷剂量，3小时后静脉注射40 mg/kg乙酸，在注射乙酸约2小时后明显发现谷氨酸释放明显。目前的研究结果提供了额外的证据，表明谷氨酸在氨基糖苷诱导的耳毒性中起加重作用。(2)在上述模型中观察到的谷氨酸释放在没有细胞外Ca^<2+>的情况下完全被抑制。这表明细胞外Ca^<2+>在药物性耳毒性中起重要作用。(3)研究了正常或耳聋豚鼠心脏骤停时耳蜗淋巴周围谷氨酸变化的时间过程。正常动物在心脏骤停后10分钟内可观察到谷氨酸轻微外排。在没有细胞外Ca^<2+>的情况下，这种作用完全被抑制。谷氨酸外排仅在心脏骤停后10 min出现，且与Ca^<2+>无关。耳聋动物在心脏骤停后1小时内无谷氨酸释放。本研究结果提示，在心脏骤停过程中，大部分释放的谷氨酸是非水疱性的，可能来自内毛细胞和外毛细胞。

We applied microdialysis technique to investigate the perilymphatic glutamate. The glutamate concentration was analyzed continuously by enzyme-linked fluorometric assay combined with microdialysis. (1) The time course of changes in perilymphatic glutamate were observed during the application of kanamycin and ethacrynic acid, which are known to damage the hair cells in the inner ear. In guinea pigs receiving a loading dose of 800 mg/kg of kanamycin subcutaneously, followed three hours later by an intravenous injection of 40 mg/kg of ethacrynic acid, a marked glutamate release was clearly found about 2 hours after the injection of ethacrynic acid. The present findings provide additional evidence that glutamate acts as an aggravating factor in aminoglycoside-induced ototoxicity. (2) A glutamate release observed in the same model shown above was completely inhibited in the absence of extracellular Ca^<2+>. This observation shows extracellular Ca^<2+> play an important role in drug-induced ototoxicity. (3) The time course of changes in perilymphatic glutamate were studied in the normal or deaf guinea pig cochlea during cardiac arrest. In normal animal slightly efflux of glutamate was observed within 10 min after cardiac arrest. This was completely inhibited in the absence of extracellular Ca^<2+>. Marked glutamate efflux only occurred 10 min after cardiac arrest in Ca^<2+>-independent manner. In deaf animals there was no glutamate release in 1 hr after cardiac arrest. The present findings suggest that during the cardiac arrest, most of the released glutamate is of non-vesicular origin, probably from both inner and outer hair cells.