THE ORIGINS AND MECHANISM OF GLUTAMATE RELEASE IN COCHLEA

应用微透析技术研究外淋巴谷氨酸的变化。采用酶联荧光法结合微透析技术连续测定谷氨酸浓度。(1)在应用卡那霉素和依他尼酸期间观察外淋巴谷氨酸变化的时间过程，已知这两种物质会损伤内耳毛细胞。在豚鼠皮下接受800 mg/kg负荷剂量的卡那霉素，3小时后静脉注射40 mg/kg依他尼酸，在注射依他尼酸后约2小时清楚地发现显著的谷氨酸释放。目前的研究结果提供了额外的证据表明，谷氨酸作为一个加重因素，氨基糖苷类诱导的耳毒性。(2)在上述相同模型中观察到的谷氨酸释放在细胞外Ca^<2+>缺失时被完全抑制。这一观察结果表明细胞外Ca^2+在药物诱导的耳毒性中起重要作用。(3)本文观察了正常和耳聋豚鼠耳蜗在心脏骤停时外淋巴谷氨酸的变化。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排。在细胞外Ca^<2+>缺乏的情况下，这一作用被完全抑制。仅在心脏停搏后10分钟出现显著的谷氨酸外排，且不依赖于Ca^<2+>。耳聋动物心脏骤停后1小时内无谷氨酸释放。目前的研究结果表明，在心脏骤停，大多数释放的谷氨酸是非囊泡的起源，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.