Renal effects of recombinant prolactin in anaesthetized rats

Objective: To re-examine the controversial possibility that prolactin exert s renal effects, using recombinant mouse prolactin (rmP), in the presence a nd absence of circulating vasopressin. Design: In experiment 1, the renal effects of rmP were examined in anaesthe tized Brattleboro rats with hereditary hypothalamic diabetes insipidus (BDI ) lacking circulating vasopressin and normal animals of the parent Long Eva ns (LE) strain. In experiment 2, salt and water excretion were studied in f luid-loaded normal Sprague-Dawley (SD) rats, some of which received rmP. Methods: In experiment 1, BDI and LE rats maintained in fluid balance were infused i.v. with each of three concentrations of rmP (10, 20 and 40 mug/ml per h) or maintained on 150 mmol/l NaCl vehicle (controls), In experiment 2, the SD rats were infused with 75 mmol/l NaCl in order to induce a state of diuresis comparable to that of BDI rats, some of them then receiving the rmP i,v. Results: A profound rmP-induced dose-dependent decrease in urine excretion (P < 0.005) and a lesser decrease in sodium excretion in the BDI rats was i n marked contrast with the small but significant increase in urine excretio n in the LE rats compared with controls (P < 0.025). The rmP-infused fluid- loaded SD rats also demonstrated a significant (P < 0.05) dose-related anti diuresis compared with the control animals, in addition to a decrease in so dium excretion. Conclusions: These results show that prolactin has a profound antidiuretic effect in the absence of circulating vasopressin. In contrast, when vasopre ssin is present in the circulation rmP has a small. but opposite, diuretic effect. Thus the use of a recombinant prolactin has provided evidence for r enal effects of this hormone which are modified in the presence of the circ ulating neurohypophysial hormone vasopressin.