Heterogeneity of endothelium-dependent regulation in ophthalmic and ciliary arteries

Endothelial cells modulate vascular tone by releasing the vasodilator nitric oxide (NO) or the vasoconstrictor endothelin-1. From one vascular bed to another and between vessels of different diameter, heterogeneities of endothelium-dependent regulatory mechanisms exist. Hence, the current study compared the effects of NO and endothelin-1 in the porcine ophthalmic artery and one of its branches, the ciliary artery.

Porcine eyes were obtained at the slaughterhouse. The ophthalmic and ciliary arteries were dissected free under a microscope and suspended in myograph systems (95% O2 and 5% CO2, 37 degrees C) for isometric tension recording.

In both vessels, bradykinin stimulated the release of NO, but the sensitivity to bradykinin increased with decreasing vascular diameter. By contrast, the basal release of NO became less efficient in inhibiting contractions to serotonin and endothelin-1 in ciliary versus ophthalmic artery. Endothelin-1 induced potent contractions that were more pronounced in ciliary than in ophthalmic artery. Serotonin-induced contractions also were more efficient in ciliary artery but less than to those to endothelin-1. Contractions to serotonin were inhibited in both blood vessels by the 5-HT2 serotonergic antagonist ketanserin.

Thus endothelium-derived vasoactive substances are potent regulators of porcine extraocular ophthalmic circulation. Their effects increase with decreasing vascular diameter, suggesting an important role of NO and endothelin-1 in the regulation of ophthalmic circulation. A dysfunction of these regulatory mechanisms could have implications about the pathogenesis of ophthalmic complications seen in diabetes, hypertension, and in certain forms of glaucoma associated with ocular vasospasms.