Relationships between plasma volume, fluid regulatory hormones, and maximal oxygen uptake following fixed-workload heat acclimation in men and women

Plasma volume (PV) expansion may support various adaptations associated with heat acclimation (HA), including a potential increase in temperate maximal oxygen uptake (VO _2max ). The PV response to HA is highly variable, and mechanisms that contribute to this variability are poorly understood, including the potential influence of sex. 26 volunteers (11M;15F) underwent a 10-d HA program that consisted of 120 min of fixed-workload treadmill exercise at 40 °C and 40 %RH. Temperate (~20 °C) VO _2max and PV (carbon monoxide rebreathing) were measured before and after HA. Serum aldosterone, estradiol, and progesterone, as well as plasma copeptin, were measured on the first and last day of HA. HA was successfully induced, as evidenced by reductions in body core temperature, heart rate, and sweating rate during exercise in the heat. PV (Pre: 3280 ± 552 mL; Post: 3380 ± 685 mL, P = 0.096) and VO _2max (3.06 ± 0.82 L/min; 3.09 ± 0.77 L/min, P = 0.453) were unchanged with HA at the cohort level, and sex did not modify responses to HA (Interaction effects: P > 0.05). ∆Aldosterone was correlated with ∆PV (P = 0.047, r = 0.393, r ² = 0.155,), and ∆progesterone with ∆PV in women (P = 0.039, r = 0.537, r ² = 0.289,). In our young, healthy cohort of men and women, PV and VO _2max were unchanged following fixed-workload HA, and the overall adaptive response did not differ based on sex. The PV response was highly variable across individuals such that increases in PV was not observed in most individuals. Inter-individual variability in circulating progesterone may contribute to some of the response in women.