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The John B. Pierce Laboratory and Departments of Epidemiology and Public Health, Yale University School of Medicine, and Women and Infants Hospital, Brown University School of Medicine, New Haven, Connecticut 06519
To test
the hypothesis that progesterone, independent of estrogen, decreases
the plasma osmotic threshold for arginine vasopressin (AVP) release and
thirst onset, we compared AVP and thirst responses to hypertonic saline
infusion (HSI) during administration of oral contraceptives (OCs)
containing progesterone (OCP) with responses to infusion of OCs
containing progesterone and estrogen (OCEP). Eight women (29 ± 2 yr) were infused with 3% NaCl (120 min, 0.1 ml · kg body
wt
1 · min1) and consumed fluid (90 min, 15 ml/kg body wt) in the early follicular and midluteal phases of
a 28-day menstrual cycle and also after 4 wk of OCP and after 4 wk of
OCEP in a randomized crossover design. Baseline plasma osmolality
(Posm) was lower in the luteal phase (280 ± 1 mosmol/kgH2O) and during OCEP (283 ± 1 mosmol/kgH2O) than in the follicular phase (286 ± 1 mosmol/kgH2O, P < 0.05) but was unaffected
by OCP (284 ± 1 mosmol/kgH2O). Posm
remained lower in the follicular phase than in the luteal phase and
with OCEP throughout the first 50 min of HSI. The mean abscissal plasma AVP concentration-Posm intercept was unaffected by OCP
(267 ± 1 mosmol/kgH2O) but was greater in the
follicular phase (273 ± 2 mosmol/kgH2O) than in the
luteal phase (266 ± 4 mosmol/kgH2O) and with OCEP
(268 ± 2 mosmol/kgH2O, P < 0.05).
There were no differences in osmotic thresholds for thirst onset across
experimental days. Despite the lower osmotic threshold for AVP release
during the luteal phase and with OCEP, fluid balance, renal free water clearance, and Na+ regulation during HSI were unaffected by
menstrual phase or OC treatment, indicating a lower osmotic operating
point for body water balance. OCP did not affect osmotic AVP
regulation, suggesting that progesterone does not affect osmotic fluid
regulation through a mechanism independent of estrogen.
body fluid regulation; estrogen; arginine vasopressin; osmolality; thirst; oral contraceptives
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