Effects of oral contraceptives on body fluid regulation

To test the hypothesis that estrogen reduces the operating point for osmore gulation of arginine vasopressin (AVP), thirst, and body water balance, we studied nine women (25 +/- 1 yr) during 150 min of dehydrating exercise fol lowed by 180 min of ad libitum rehydration. Subjects were tested six differ ent times, during the early follicular (twice) and midluteal (twice) menstr ual phases and after 4 wk of combined [estradiol-norethindrone (progestin), OC E + P] and 4 wk of norethindrone (progestin only, OC P) oral contracept ive administration, in a randomized crossover design. Basal plasma osmolali ty (P-osm) was lower in the luteal phase (281 +/- 1 mosmol/kgH(2)O, combine d means, P < 0.05), OC E + P (281 +/- 1 mosmol/kgH(2)O, P < 0.05), and OC P (282 +/- 1 mosmol/kgH(2)O, P < 0.05) than in the follicular phase (286 +/- 1 mosmol/kgH(2)O, combined means). High plasma estradiol concentration low ered the P-osm threshold for AVP release during the luteal phase and during OC E + P [x-intercepts, 282 +/- 2, 278 +/- 2, 276 +/- 2, and 280 +/- 2 mos mol/kgH(2)O, for follicular, luteal (combined means), OC E + P, and OC P, r espectively; P < 0.05, luteal phase and OC E + P vs. follicular phase] duri ng exercise dehydration, and 17 beta-estradiol administration lowered the P -osm threshold for thirst stimulation [x-intercepts, 280 +/- 2, 279 +/- 2, 276 +/- 2, and 280 +/- 2 mosmol/kgH(2)O for follicular, luteal, OC E + P, a nd OC P, respectively; P < 0.05, OC E + P vs. follicular phase], without af fecting body fluid balance. When plasma 17 beta-estradiol concentration was high, P-osm was low throughout rest, exercise, and rehydration, but plasma arginine vasopressin concentration, thirst, and body fluid retention were unchanged, indicating a lowering of the osmotic operating point for body fl uid regulation.