Effect of prolactin on diluting and concentrating ability in the rat

The effect of prolactin on free water clearance (C(H₂O)) and reabsorption (Tc(H₂O)) 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 Tc(H₂O) 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.