Contamination of prolactin preparations by antidiuretic hormone and oxytocin

Since impurities consisting of neurohypophysical hormones in prolactin powder may be responsible for the vascular and renal effects attributed to prolactin, rat (NIH-RP-1), ovine (NIH-P-S-10, S-12), and bovine (NIH-P-B4) prolactin preparations were examined for their content of ADH and oxytocin by rat antidiuresis, milk-ejection, and blood pressure assays. Activities were identified as due to ADH or oxytocin by incubation of prolactin solutions with antisera against ADH, oxytocin, and prolactin, or with pregnancy plasma. The ADH content of rat, ovine (P-S-10, P-S-12) and bovine prolactin was found to be 104.5 +/- 7.1 (means +/- SE), 2.5 +/- 0.2, 1.6 +/- 0.1, and 1.6 +/- 0.5 mU/mg powder, respectively; the corresponding values for oxytocin content were 155.3 +/- 3.5, 1.2 +/- 0.1, 0.5 +/- 0.1, and 1.2 +/- 0.01 mU/mg powder, respectively. Because antidiuretic, milk-ejection, and blood pressure activities of the various prolactins were eliminated after incubation with antisera against ADH and oxytocin, or with pregnancy plasma, but not with prolactin antisera, it is concluded that the reported vascular and renal prolactin effects are attributable to ADH contamination of the prolactin preparation rather than to the prolactin molecule itself. These findings have implications for renal and vascular prolactin research.