[1-(.beta.-mercapto-.beta.,.beta.-cyclopentamethylenepropionic acid),2-(O-methyl)tyrosine]arginine-vasopressin and [1-(.beta.-mercapto-.beta.,.beta.-cyclopentamethylenepropionic acid)]arginine-vasopressin, two highly potent antagonists of the vasopressor response to arginine-vasopressin