ESTROGEN INFLUENCES OSMOTIC SECRETION OF AVP AND BODY-WATER BALANCE IN POSTMENOPAUSAL WOMEN

To determine if estrogen upregulates osmotic secretion of arginine vas opressin (AVP) and alters body water balance, we infused hypertonic (3 % NaCl) saline in 6 women (68 +/- 3 yr) after 14 days of 17 beta-estra diol (transdermal patch, similar to 0.1 mg/day, E-2) and placebo (cont rol) administration. Hypertonic saline was infused at 0.1 ml.kg(-1).mi n(-1) for 120 min, and after a 30-min equilibration period, the subjec ts drank water ad libitum for 180 min. E-2 increased basal plasma estr adiol concentration from less than or equal to 12 to 80 +/- 12 pg/ml a nd plasma AVP concentration (P-[AVP]) from 2.1 +/- 0.7 to 3.1 +/- 0.8 pg/ml (P < 0.05), but not plasma osmolality (P-osm, 288 +/- 1 and 287 +/- 1, for control and E-2, respectively). Hypertonic saline infusion increased P-osm by 18 +/- 1 and 17 +/- 1 mosmol/kgH(2)O and P-[AVP] by 5.2 +/- 0.5 and 4.9 +/- 0.4 pg/ml for control and E-2 treatments, res pectively. The P-[AVP]-P-osm relationship shifted upward after E-2, wi th no change in sensitivity (slope, 0.36 +/- 0.02 and 0.33 +/- 0.03 pg .ml(-1).mosmol(-1) for control and E-2, respectively). Water intake wa s similar between control and E-2 (24 vs. 22 ml/kg), but by 180 min of drinking, urine output and free water clearance (C-H2O) were reduced by 5.6 +/- 2.3 ml/kg and 2.6 +/- 2.0 ml/min, respectively (P < 0.05) a fter E-2. Plasma aldosterone concentration was unaffected by E-2, but fractional sodium excretion was reduced from 2.7 +/- 0.5 to 1.7 +/- 0. 4% (P < 0.05) at 180 min of drinking. Our data suggest that E-2 augmen ts osmotic AVP secretion, thereby implicating elevated AVP as a contri butor to water retention in high E-2 states; however, an increase in r enal sodium reabsorption was a major component of the enhanced fluid r etention.