EDN1 and diabetes mellitus: Endothelial cells also regulate vasodilation and contraction by secreting and synthesizing various vasoactive substances, including endothelial-derived relaxing factor, prostaglandin I2 (PGI2), endothelin-1, and a series of prostaglandin metabolites such as prostaglandin F2a.[10,11] The synthesis of PGI2 is decreased and that of endothelin-1 (ET-1) is increased in diabetic patients, which is more significantly in DR.[12,13]