SEX HORMONES LOWER THE OPERATING POINT FOR BODY FLUID TONICITY.:

1311 To test the hypothesis that sex hormones reduce the operating point for osmoregulation of arginine vasopressin (AVP) and body water balance, we studied these variables in 8 women (25±1 yr) during 150 min of exercise at 36 °C, 30 min of equilibration and 180 min of ad libitum rehydration during the follicular (F) and luteal (L) menstrual phases. Plasma estrogen and progesterone concentrations were 26.5±5.9 and 1.2±0.5 pg/ml in F, and 106.3±22.7 and 10.3±2.4 pg/ml in L. Plasma osmolality (Osmp) was higher in F than L (286±1 vs. 281±1 mOsm/kg H2O, P<0.05), but menstrual phase did not affect plasma AVP concentration ([AVP]p, 1.2 ± 0.4 and 1.3±0.1 pg/ml) or renal concentrating capacity (plasma/urine osmolality, 1.6±0.3 and 1.3±0.3). Exercise caused similar changes in body water (-24±2 and -25±2 ml/kg), plasma volume (-9.5±1.1 and -8.2± 1.8%), Osmp (7±1 and 6±1 mOsm/kg H2O) and [AVP]p (2.0 ±0.6 and 3.1±1.2 pg/ml) in F and L, respectively. Post exercise fluid intake (18±2 ml/kg and 17±3 ml/kg) and restoration (12±2 and 15±3 ml/kg) were similar between the menstrual phases. During 180 min of drinking, Osmp remained greater in F vs L (area under the curve, 17.3·104 vs. 17.1·104 mOsm/kg H2O, P<0.05), but area under the curve for [AVP]p and urine concentrating capacity were unaffected by menstrual phase. During L, Osmp was lower throughout rest, exercise and rehydration, but [AVP]p and fluid balance were unchanged, suggesting that osmoregulation of AVP and body water balance are reset at a lower body tonicity when endogenous sex hormones are elevated.