Suppression of AVP release by drinking despite hypertonicity during and after gestation

Water drinking suppresses arginine vasopressin (AVP) release before changes in tonicity or volume. To further characterize this oropharyngeal neuroendocrine reflex, we took advantage of ongoing studies of osmoregulation in pregnancy as follows: five women studied serially in early (8 +/- 1 wk, mean +/- SD) and late (32 +/- 3 wk) pregnancy and 8-10 wk postpartum were slowly infused with hypertonic saline for 150 min while drinking water to satiety (drinking started at 120 min). Plasma osmolality (Posmol) at 120 min was increased (277 +/- 3 to 291 +/- 2, 280 +/- 6 to 295 +/- 5, and 287 +/- 1 to 307 +/- 1 mosmol/kg in 1st and 3rd trimesters and postpartum; P less than 0.01), accompanied by increments in plasma AVP (PAVP) up to 7 +/- 5, 5 +/- 3, and 8 +/- 3 pg/ml, respectively (P less than 0.01). PAVP declined as soon as drinking began (P less than 0.01 at 5 min), despite maintenance of peak osmolality for another 30 min. Similar events were recorded in three males. Calculation of normal disappearance rates (t1/2) demonstrated that AVP levels decreased more rapidly (P less than 0.01) in the 3rd trimester compared with early pregnancy and postpartum. Two other subjects became nauseated during their 1st trimester and postpartum tests which permitted comparison of nausea and drinking which evoke central reflexes with opposing action on AVP release. PAVP, which had increased markedly (range 17-42 pg/ml; P less than 0.01), was not abolished by drinking. These data demonstrate the potency of the drinking reflex on osmoregulation in pregnant and nonpregnant subjects.(ABSTRACT TRUNCATED AT 250 WORDS)