Glutamate, of the endogenous primary alpha-amino acids, is specifically released from hair cells by elevated extracellular potassium.

A hair cell (octavolateralis mechanoreceptor cell) sheet preparation from the trout saccular macula was superfused with bicarbonate-based physiological saline. Among the primary amine-containing compounds resolved by cation-exchange HPLC, glutamate alone was released in a statistically significant manner with elevation of extracellular [K+] from 3.5 to 14 mM in the presence of 1.8 mM calcium. Release of glutamate averaged 10.9 +/- 2.5 pmol (mean +/- SEM) over a 10-min period for a hair cell sheet preparation representing 20 micrograms of cell protein. No potassium-evoked release of glutamate was observed in 0 mM calcium/10 mM magnesium saline, suggesting calcium dependency. Because the sheet preparation, by the method of its isolation, contained only the hair cell as the intact cell type, release of glutamate, induced by relatively small increases in extracellular potassium, can be attributed directly to the receptor cell. The specific release of glutamate and its block by magnesium are consistent with the hypothesis that glutamate is one neurotransmitter/neuromodulator mediating receptoneural transmission in the octavolateralis periphery.