Submitted on December 16, 2007
Accepted on April 23, 2008
Endogenous and Exogenous female sex hormones and renal electrolyte handling: effects of an acute sodium load on plasma volume at rest
Stacy T Sims1*, Nancy J Rehrer2, Melanie L Bell3, and James D Cotter4
1 Stanford Prevention Research Center, Stanford University, Stanford, California, United States
2 School of Physical Education, University of Otago, Dunedin, Otago, New Zealand; Human Nutrition, University of Otago, Dunedin, Otago, New Zealand
3 Preventive and Social Medicine, University of Otago, Dunedin, Otago, New Zealand
4 School of Physical Education, University of Otago, Dunedin, Otago, New Zealand
* To whom correspondence should be addressed. E-mail: stsims{at}stanford.edu.
This study was conducted to investigate effects of an acute sodium load on resting plasma volume and renal mechanisms across the menstrual cycle of endurance-trained women with natural or oral contraceptive pill (OCP) controlled cycles. Twelve women were assigned to one of two groups according to their usage status: 1) oral contraceptive pill (OCP, n = 6, 29 y (SD 6), 59.4 kg (SD 3.2)), or 2) natural cycle (NAT, n = 6, 24 y (5), 61.3 kg (3.6)). The sodium load was administered as a concentrated sodium chloride/citrate beverage (164 mmol Na+·L-1, 253 mOsm·kg H2O-1, 10 ml·kg-1 BM) during the last high hormone week of the OCP cycle (OCPhigh) or late-luteal phase of the natural cycle (NAThigh) and during the low hormone sugar pill week of OCP (OCPlow) or early follicular phase of the natural cycle (NATlow). The beverage (~628 ml) was ingested in 7 portions across 60 min. Over the next 4 h plasma volume (PV) expanded more in the low-hormone phase for both groups: (time-averaged change) OCPlow 
6.1 (SD 1.1) and NATlow and 5.4% (1.2) vs. OCPhigh 3.9 (0.9) and NAThigh 3.5% (0.8), P = 0.02. The AVP increased less in the low hormone phase (1.63 (0.2) and 1.30 pg·ml-1 (0.2) vs. 1.82 (0.3) and 1.57 pg·ml-1 (0.5), P = 0.0001), as did plasma aldosterone concentration (~64% lower, P=0.0001). Thus, PV increased more and renal hormone sensitivity was decreased in the low-hormone menstrual phase following sodium/fluid ingestion, irrespective of OCP usage.
