Synthesis and some pharmacological properties of [4-threonine,7-glycine]oxytocin, [1-(L-2-hydroxy-3-mercaptopropanoic acid),4-threonine,7-glycine]oxytocin (hydroxy[thr4, gly7]oxytocin), and [7-glycine]oxytocin, peptides with high oxytocic-antidiuretic selectivity