A NOVEL MECHANISM FOR VASOCONSTRICTOR ACTION OF 8-ISOPROSTAGLANDIN F2-ALPHA ON RETINAL-VESSELS

Using a videoimaging technique, we characterized the effects of 8-isop rostaglandin F-2 alpha (8-iso-PGF(2 alpha)) on retinal vasculature fro m piglets. 8-Iso-PGF(2 alpha) potently contracted (EC50 = 5.9 +/- 0.5 nM) retinal vessels. These effects were completely antagonized by the cyclooxygenase inhibitor indomethacin, the thromboxane synthase blocke r CGS-12970, the thromboxane receptor antagonist L-670596, and the put ative inhibitor of the non-voltage-dependent receptor-operated Ca2+ pa thway SKF-96365; constrictor effects of 8-iso-PGF(2 alpha) were also p artly attenuated by the ETA-receptor blocker BQ-123 and an inhibitor o f endothelin-converting enzyme, phosphoramidon, but was negligibly aff ected by the L-type voltage-gated Ca2+ channel blocker nifedipine. Cor respondingly, 8-iso-PGF(2 alpha) elicited endothelin release from reti nal preparations, which was markedly reduced by SKF-96365. 8-Iso-PGF(2 alpha) also increased thromboxane production in the retina and cultur ed endothelial cells, but not on retinovascular smooth muscle cells; t hese effects of 8-iso-PGF(2 alpha) were blocked by indomethacin, CGS-1 2970, SKF-96365, and EGTA, but not by nifedipine. 8-Iso-PGF(2 alpha) a lso increased Ca2+ transients in retinal endothelial cells, which were inhibited by SKF-96365 and EGTA, but not by nifedipine, whereas in sm ooth muscle cells U-46619, but not 8-iso-PGF(2 alpha), stimulated a ri se in Ca2+ transients. Finally, H2O2 + FeCl2 (in vitro) and anoxia fol lowed by reoxygenation (in vivo) stimulated formation of 8-iso-PGF(2 a lpha), in the retina. In conclusion, 8-iso-PGF(2 alpha)-induced retina l vasoconstriction is mediated by cyclooxygenase-generated formation o f thromboxane and, to a lesser extent, by endothelin after Ca2+ entry into cells, possibly through receptor-operated channels. Retinal vasoc onstriction to 8-isoprostanes might play a role in the genesis of isch emic retinopathies.