Pituitary Oxytocin and Vasopressin Content of Pregnant Rats Before, During, and After Parturition

Oxytocin and vasopressin content of the neurohypophysis was determined by means of bioassays in pregnant rats before, during, and after spontaneous as well as oxytocin- induced parturition. Before parturition the levels of oxytocin and vasopressin in the neural lobe were equal on Days 21 and 22 of gestation. Immediately after spontaneous parturition the content of both hormones was about 60% of the prelabor values. At the delivery of the first fetus the oxytocin level had decreased by only about 10%, which is not a significant difference, while the vasopressin content of the posterior lobe was almost as low as at the end of parturition. When parturition was induced by oxytocin or combined oxytocin-arginine-vasopressin infusions, the pituitary oxytocin level was unchanged but the level of vasopressin was as low as after spontaneous delivery. Infusions of exogenous hormone to pregnant rats before term did not alter the endogenous hormone levels, indicating that no neurophypophysial accumulation of oxytocin occurs in spite of high circulating blood levels. It is concluded that significant amounts of both oxytocin and vasopressin are released during spontaneous parturition but that these hormones are secreted independently. The release of oxytocin is not a consequence of labor since it did not occur in induced deliveries, whereas vasopressin release is associated with spontaneous as well as induced delivery, probably in response to the pain and stress of labor. No negative feedback effect occurred with respect to vasopressin. (Endocrinology88: 574, 1971)