THE ORIGINS AND MECHANISM OF GLUTAMATE RELEASE IN COCHLEA

我们应用微透析技术研究了外淋巴液中的谷氨酸。用酶联荧光法结合微透析法连续测定谷氨酸浓度。(1)观察了外淋巴液中谷氨酸含量变化的时程，发现卡那霉素和乙丙氨酸对内耳毛细胞有损伤作用。豚鼠皮下注射卡那霉素负荷量800 mg/kg，3小时后静脉注射40 mg/kg乙氰酸，在注射乙氰酸约2小时后明显释放谷氨酸。目前的发现为谷氨酸在氨基糖苷类药物所致耳毒性中的加重作用提供了进一步的证据。(2)上述模型中观察到的谷氨酸释放在无细胞外钙离子的情况下被完全抑制。这一观察结果表明，细胞外钙在药物耳毒性中起重要作用。(3)研究正常豚鼠和耳聋豚鼠在心跳骤停时外淋巴液中谷氨酸的变化规律。在正常动物中，在心脏骤停后10分钟内观察到轻微的谷氨酸外流。在没有细胞外钙离子的情况下，这一作用被完全抑制。显著的谷氨酸外流仅发生在心脏骤停后10分钟，且不依赖于钙。在失聪动物中，在心脏骤停后1小时内没有谷氨酸释放。目前的研究结果表明，在心脏骤停过程中，释放的谷氨酸大多来自非囊泡来源，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.