The level of spontaneous myogenic tone in isolated human posterior ciliary arteries decreases with age.

The spontaneous myogenic activity of rings of human posterior ciliary arteries was studied in vitro. All arteries from six patients had an intrinsic myogenic tone, which was dependent on extracellular calcium and inversely correlated with age. The myogenic tone was stable without rhythmic variations, except in one artery. The level of spontaneous myogenic tone was higher in vessels where the endothelium was removed than in normal arteries. Acetylcholine induced a complete and concentration dependent relaxation of endothelium-intact but not endothelium-denuded arteries. Indomethacin induced a small concentration dependent contraction of the arteries. Methylene blue did also contract the arteries concentration dependently. The experiments indicate that the myogenic tone of human posterior ciliary arteries is regulated by prostaglandin formation (prostacyclin?) as well as release of endothelium-derived relaxing factor in the arteries. The results indicate that the blood flow regulatory capacity of these arteries may be endangered by aging and endothelial damage, which could increase the probability of developing, e.g. ischemic damage of the optic nerve head.