Demonstration that rat prolactin has no intrinsic antidiuretic activity in the rat

The proposition that rat prolactin has an intrinsic antidiuretic activity was examined using a conscious rat model (Brattleboro homozygotes, DI rats) which does not appear to produce arginine vasopressin (AVP). A preparation of rat prolactin obtained from NIAMDD, rat prolactin B1, produced a prompt antidiuresis when administered intravenously to these animals. Prolactin B1 was found to contain 150 ng of AVP/mg protein by RIA. All of the measurable AVP was removed from prolactin B1 by polyacrylamide gel electrophoresis and the electrophoresed prolactin was completely devoid of any antidiuretic activity. Addition of synthetic AVP to electrophoresed prolactin in an amount equivalent to the AVP contamination of prolactin B1 completely restored the originally observed antidiuretic activity. Synthetic AVP given alone produced a similar antidiuresis, but in the presence of electrophoresed prolactin these effects were produced by much smaller amounts of AVP. We believe that our data demonstrate that rat prolactin has no intrinsic antidiuretic activity, and that the antidiuretic activity associated with different prolactin preparations could be entirely due to their contamination with AVP. The data also show that the presence of prolactin enhanced by about five times the antidiuretic activity of AVP.