Cyclic variation of oxytocin in the blood of pituitary portal vessels of rats.

The release of oxytocin (OT) into pituitary portal blood during the estrous cycle was studied in female rats under Althesin anesthesia. On each afternoon of the 4-day estrous cycle, animals were anesthetized with Althesin, and pituitary portal blood samples were collected. The concentration of OT in portal plasma collected between 14.00 and 16.00 h of proestrus (1,079 +/- 185 pg/ml) was significantly higher than that at any other time studied. There were no appreciable differences between the mean OT concentration on the days of estrus (198 +/- 29 pg/ml), metestrus (283 +/- 55 pg/ml), and diestrus (267 +/- 49 pg/ml). The concentration of OT in pituitary portal plasma was also greater than that in blood withdrawn from the external jugular vein (less than 41-67 pg/ml). Comparison of the portal plasma OT with peripheral plasma prolactin in Althesin-anesthetized rats indicated an association between the release of these hormones during the estrous cycle.