The expansion of plasma volume (PV) is thought to be the most critical event in the heat acclimation process. The principal mechanism underlying this adaptive response is the translocation of proteins (albumin and globulin) from the lymph to the vascular space, thus providing an osmotic base for the retention of water. A critical stimulus for this translocation of proteins is exercise induced hypohydration whereby viscous lymph is produced resulting in a reduction in lymphatic flow that increases interstitial oncotic pressure, facilitating a net outward filtration. PV may increase ~20% with repeated exercise-heat exposure, however little is known about the rate of decay in PV following an acclimation programme. We investigated PV changes, together with thermoregulatory and cardiorespiratory adaptations induced by a humid-heat acclimation programme and subsequently measured PV changes with the withdrawal of the humid-heat stimulus. Methods Five endurance trained cyclists (age 29±3yrs; VO2max 58±11ml.kg-1.min-1) performed 1h of cycling at 70%VO2max for 6 consecutive days within a Climate Chamber at 300C and 65%rh. PV measures (CO re-breathing, Hct, Hb) were made pre, and 1 day post acclimation and then at 10 and ~25 days post acclimation. Cardiac output (CO2 re-breathing), heart rate, respiratory gas exchange, sweat loss, together with skin and rectal temperatures were monitored throughout the 1h exercise during acclimation and ~25 days post acclimation. Results PV determined by the CO re-breathing method increased from 4306±307ml to 4707±335ml (9.3±1.4%) after acclimation and declined to 4567±330ml at 10 days post (5.9±1.2% above pre-acclimation value) and 4346±384ml at ~25 days post acclimation (0.6±0.3% below the pre-acclimation value).The changes in PV were associated with changes in heart rate at 60 minutes of exercise. Heart rate was 171±4bpm at the end of the initial 60 minute bout of exercise, 160±8bpm immediately post acclimation and 166±10bpm following ~25 days de-acclimation. Rectal temperature at the conclusion of the initial 60 minutes of exercise was 39.3±0.20C, and reduced to 38.7±0.20C following acclimation with little further change at ~25 days post acclimation (38.8±0.30C). Discussion The expansion in PV induced by humid-heat acclimation was no longer evident ~25 days post acclimation. This contraction in PV was reflected in an upward drift in heart rate which was similar to the pre-acclimation exercise heart rate response. However the decrease in exercise rectal temperature immediately following acclimation remained evident ~25 days post acclimation. These observations suggest that an expanded PV augments cardiac filling pressure but has little impact on body heat storage during exercise.
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