Thermoregulatory responses to exercise in relation to the phase of the menstrual cycle were studied in ten women taking oral contraceptives (P) and in ten women not taking oral contraceptives (NP). Each subject was tested for maximal aerobic capacity (VO2max) and for 50% VO2max exercise in the follicular (F) and luteal (L) phases of the menstrual cycle. Since the oral contraceptives would have prevented ovulation a quasi-follicular phase (q-F) and a quasi-luteal phase (q-L) of the menstrual cycle were assumed for P subjects. Exercise was performed on a cycle ergometer at an ambient temperature of 24-degrees-C and relative air humidity of 50%. Rectal (T(re)), mean skin (T(sk)BAR), mean body (T(b)BAR) temperatures and heart rate (f(c)) were measured. Sweat rate was estimated by the continuous measurement of relative humidity of air in a ventilated capsule placed on the chest, converted to absolute pressure (PH2O(chest)). Gain for sweating was calculated as a ratio of increase in PH2O(chest) to the appropriate increase in T(re) for the whole period of sweating (G) and for unsteady-state (G(u)) separately. The VO2max did not differ either between the groups of subjects or between the phases of the menstrual cycle. In P, rectal temperature threshold for sweating (T(re,td)) was 37.85-degrees-C in q-L and 37.60-degrees-C in q-F (P<0.01) and corresponded to a significant difference from T(re) at rest. The T(re), T(sk)BAR, T(b)BAR and f(c) increased similarly during exercise in q-F and q-L. No menstrual phase-related differences were observed either in the dynamics of sweating or in G. In NP, T(re,td) was shorter in L than in F (37.70 vs 37.47-degrees-C, P<0.02) with a significantly greater value from T(re), at rest. The dynamics and G for sweating were also greater in L than in F. The G(u) was 36.8 versus 16.6 kPa.degrees-C-1 (P<0.01) while G was 6.4 versus 3.8 kPa.degrees-C-1 (P<0.05), respectively. The T(re), T(sk)BAR, T(b)BAR and f(c) increased significantly more in phase F than in phase L. It was concluded that in these women performing moderate exercise, there was a greater temperature threshold and larger gains for sweating in phase L than in phase F. Intake of oral contraceptives reduced the differences in the gains for sweating making the thermoregulatory responses to exercise more uniform.
