Progesterone does not alter osmotic regulation of AVP

To test the hypothesis that progesterone, independent of estrogen, decrease s the plasma osmotic threshold for arginine vasopressin (AVP) release and t hirst onset, we compared AVP and thirst responses to hypertonic saline infu sion (HSI) during administration of oral contraceptives (OCs) containing pr ogesterone (OCP) with responses to infusion of OCs containing progesterone and estrogen (OCEP). Eight women (29 +/- 2 yr) were infused with 3% NaCl (1 20 min, 0.1 ml.kg body wt(-1).min(-1)) and consumed fluid (90 min, 15 ml/kg body wt) in the early follicular and midluteal phases of a 28-day menstrua l cycle and also after 4 wk of OCP and after 4 wk of OCEP in a randomized c rossover design. Baseline plasma osmolality (P-osm) was lower in the luteal phase (280 +/- 1 mosmol/kgH(2)O) and during OCEP (283 +/- 1 mosmol/kgH(2)O ) than in the follicular phase (286 +/- 1 mosmol/kgH(2)O, P < 0.05) but was unaffected by OCP (284 +/- 1 mosmol/kgH(2)O). P-osm remained lower in the follicular phase than in the luteal phase and with OCEP throughout the firs t 50 min of HSI. The mean abscissal plasma AVP concentration-P-osm intercep t was unaffected by OCP (267 +/- 1 mosmol/kgH(2)O) but was greater in the f ollicular phase (273 +/- 2 mosmol/kgH(2)O) than in the luteal phase (266 +/ - 4 mosmol/kgH(2)O) and with OCEP (268 +/- 2 mosmol/kgH(2)O, P < 0.05). The re were no differences in osmotic thresholds for thirst onset across experi mental 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 treatme nt, indicating a lower osmotic operating point for body water balance. OCP did not affect osmotic AVP regulation, suggesting that progesterone does no t affect osmotic fluid regulation through a mechanism independent of estrog en.