Endothelin Evokes Efflux of Glutamate in Cultures of Rat Astrocytes

Abstract: Excessive release of glutamate, from glial cells as well as neurons, is thought to be a major cause of neuronal death in ischemia. To investigate glutamate release from glial cells, we measured glutamate efflux from cultures of rat astrocytes preloaded with l -³H]-glutamate. Glutamate efflux was induced by either 60 m M KCl or Na⁺-free medium, suggesting that the efflux is due to the reversed operation of a Na⁺- and K⁺-coupled glutamate uptake machinery. While investigating various neuropeptides and neurotransmitters, we found that endothelin (ET) specifically induced efflux of glutamate. Northern blot analysis and binding study showed that the ET type B receptor (ET_B-R) subtype was expressed two to three times more densely than the ET type A receptor (ET_A-R) in astrocytes. The ET_B-R antagonist IRL 2500 partially inhibited efflux of glutamate induced by 1 n M ET-1 in a concentration-dependent manner, causing a maximal inhibition of 60% at 1 µ M . However, the ET_A-R antagonist BQ-123 did not cause significant inhibition even at 10 µ M . Combination of both antagonists completely inhibited the ET-1-induced efflux. These results indicate that both receptor subtypes are involved in efflux of glutamate with a major contribution from the ET_B-R. Our findings suggest that ET, which is known to be released in ischemia, may exacerbate neurodegeneration by stimulating efflux of glutamate.