Endogenous and exogenous female sex hormones and renal electrolyte handling: Effects of an acute sodium load on plasma volume at rest

This study was conducted to investigate effects of an acute sodium load on resting plasma volume (PV) and renal mechanisms across the menstrual cycle of endurance-trained women with natural (NAT) or oral contraceptive pill (OCP) controlled cycles. Twelve women were assigned to one of two groups, according to their usage status: 1) OCP [n = 6, 29 yr (SD 6), 59.4 kg (SD 3.2)], or 2) NAT [n = 6, 24 yr (SD 5), 61.3 kg (SD 3.6)]. The sodium load was administered as a concentrated sodium chloride/citrate beverage (164 mmol Na⁺/l, 253 mosmol/kgH₂O, 10 ml/kg body mass) during the last high-hormone week of the OCP cycle (OCP_high) or late luteal phase of the NAT cycle (NAT_high) and during the low-hormone sugar pill week of OCP (OCP _low) or early follicular phase of the NAT cycle (NAT_low). The beverage (∼628 ml) was ingested in seven portions across 60 min. Over the next 4 h, PV expanded more in the low-hormone phase for both groups (time-averaged change): OCP_low 6.1% (SD 1.1) and NAT_low 5.4% (SD 1.2) vs. OCP_high 3.9% (SD 0.9) and NAT_high 3.5% (SD 0.8) (P = 0.02). The arginine vasopressin increased less in the low-hormone phase [1.63 (SD 0.2) and 1.30 pg/ml (SD 0.2) vs. 1.82 (SD 0.3) and 1.57 pg/ml (SD 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.