Changes in plasma arginine vasopressin concentration during lactation in rats

Background and Purpose: The objectives of the study reported here were to determine whether a change in the plasma arginine vasopressin (AVP) concentration occurred in lactating, compared with non-lactating rats and to examine the involvement of suckling with plasma AVP concentration. Methods: Experiments were performed on 86 female Wister Imamichi rats, 12 weeks old at parturition, with fast lactation. On day 13 of lactation, AVP concentration and plasma osmotic pressure were measured in lactating and non-lactating rats. Results: Plasma AVP concentration was always higher in rats of the lactating groups than in non-lactating controls (1.06 +/- 9.28 pg/ml), and a conspicuous increase in ANP concentration was seen during the postsuckling period (1.70 +/- 0.61 pg/ml before vs. 2.56 +/- 1.31 pg/ml after suckling, P < 0.05). Plasma osmotic pressure in lactating rats with 12 pups (296.6 +/- 5.2 mOsmol/kg . H2O) was lower than that in rats of the removed control groups (306.7 +/- 5.7 mOsmol/kg . H2O). Conclusion: On the basis of these results, it appears that "low plasma osmotic pressure-high AVP status" develops in the lactating period, similar to pregnancy, through resetting of the regulatory mechanism of the AVP system. It was concluded that suckling stimulation could release AVP,which could dilute the blood with water resulting in the increase in circulating blood volume.