The pituitary hormone vasopressin (AVP) is a cyclic nonapeptide that acts by binding to a family of vasopressin receptors that includes the V1a, V1b, and V2 receptors. V1a receptors have been shown to be expressed in vascular smooth muscle, hepatocytes, various blood cells, brain, and retina. These receptors mediate cell proliferation, platelet aggregation, coagulation factor release and glycogenolysis.