Suppressive action of prolactin on renal response to volume expansion

The effects of prolactin on rat renal sodium and water handling during volume expansion were studied using clearance techniques. Both control and experimental adult male Wistar rats were prehydrated with an oral water load of volume equal to 2.5% body weight (BW). At least 3 h later, a continuous intravenous infusion of ovine prolactin (NIH-P-S8), 7.1 mug/h per 100 g, was started in the experimental group. After a 1-h steady-state period, the rats were given an intravenous expansion infusion of either hypotonic saline (2.5% BW), isotonic saline (2.5% and 7.5% BW), or blood (2.5% BW). In all control hypotonic and isotonic saline-expanded animals, within 1 h the rats excreted a volume of urine equal to over 50% of the volume of saline infused. The diuretic and natriuretic responses to saline expansion of prolactin-treated rats were significantly smaller than controls. In contrast to the effects of prolactin on the renal response to saline infusions, it did not alter the natriuretic or diuretic response to blood infusion. Prolactin may be counteracting the effects of physical factors on the regulation of sodium reabsorption in the proximal tubule.