Effect of prolactin on diluting and concentrating ability in the rat

The effect of prolactin on free water clearance (C(H2O)) and reabsorption (T(cH2O)) was assessed in hereditary hypothalamic diabetes insipidus (HHDI) and hydropenic Sprague-Dawley rats. Infusion of ovine prolactin (200 nmol/kg per h) ablated C(H2O) in HHDI rats but had no effect on T(cH2O) in hydropenic rats. Additional experiments in HHDI rats employing submaximal infusions of both ovine and rat prolactin indicated that with increasing infusion rates of these hormones, urine osmolality progressively increased with a maximum effect being reached at 30-40 nmol/kg per h with ovine and 15-20 nmol/kg per h with rat prolactin. In similar experiments, using synthetic lysine-arginine vasopressin, maximum effect on urine osmolality was achieved with an infusion rate of 25-75 pmol/kg per h. During infusion of all three hormones, urinary cyclic AMP excretion increased significantly in a similar fashion. These data suggested that prolactin had a direct effect on collecting tubule permeability and acted through stimulation of cyclic AMP production.