Failure of oxytocin and lysine-vasopressin to stimulate prolactin release in humans.

In order to test a possible stimulatory effect of oxytocin and lysine-vasopressin on prolactin (PRL) release six female volunteers received either saline or 1 IU oxytocin (Syntocinon) i.v.. Blood sampling was performed at intervals over 90 minutes. Nine puerperae were treated with oral oxytocin (Sandopart) 50 IU t.i.d. for seven days. Nine other untreated postpartum women served as controls. Blood was collected before delivery and on subsequent postpartum days, and the milk volumes produced were estimated by increases in the babies' weight. In addition, seven volunteers received 5 or IU lysine-vasopressin (Vasopressin) i.m. Blood was withdrawn at regular intervals for 180 minutes, the urine volumes excreted in 30-minute intervals were measured and the clearance of free water calculated. Oxytocin failed to modify basal plasma PRL in the acute trial and in the post-partum women, and milk amounts produced were not influenced by treatment. Vasopressin did not change the PRL plasma profile despite a significant (p less than 0.05) reduction in the excretion of free water. Results indicate that neuropeptides are not implicated in the control of PRL release.