Noteworthy is that the high Ap4A concentration (about 300 nM) found in the aqueous humor of glaucoma patients is unable to reduce IOP, probably because this concentration is enough to stimulate the hypertensive P2Y2 receptor, but insufficient to activate the hypotensive P2Y6, which requires micromolar concentrations (Guzman-Aranguez et al., 2007). This evidence concerns the gene P2RY6 and glaucoma.