Endothelin evokes efflux of glutamate in cultures of rat astrocytes.

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 mM KCI 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 nM ET-1 in a concentration-dependent manner, causing a maximal inhibition of 60% at 1 microM. However, the ET(A)-R antagonist BQ-123 did not cause significant inhibition even at 10 microM. 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.