Endothelin-1 enhances glutamate-induced retinal cell death, possibly through ETA receptors

To determine the modification of the glutamate-induced death of retinal neurons by endothelin (ET)-1.

Cultured retinal neurons from fetal rats were exposed to glutamate (1.0 mM) alone or glutamate with ET-1 (10(-10)-10(-7)M) for 10 minutes. Neuronal death was assessed by the trypan blue exclusion or TUNEL assays at 2, 6, and 24 hours after the exposure. The effects of adding BQ-123 or BQ-788, ET(A), and ET(B) receptor antagonists, respectively, in combination with ET-1 was also assessed.

Immunohistochemical analyses showed that the ETs as well as ET(A) and ET(B) receptors were expressed on cultured retinal neurons consisting mainly of amacrine cells. A brief exposure of the cultured retinal neurons to glutamate alone significantly increased the number of dead cells, and the addition of ET-1 with glutamate caused a further significant increase in retinal neuronal death compared with the cells exposed to glutamate alone. A significant increase in neuronal death was detected at doses of 10 nM of ET-1 and higher after a 24-hour exposure (P < 0.05, Dunnett), whereas brief exposure of neurons to up to 1 microM ET-1 alone did not cause delayed cell death of neurons. BQ-123 (10 nM) suppressed the enhancement of retinal toxicity caused by ET-1 (10 nM), whereas BQ-788 had no significant effect.

These results indicate that ET-1 enhances glutamate-induced retinal cell death, possibly through ET(A) receptors. ET-1 may act synergistically with glutamate to damage retinal neurons under hypoxic conditions.