The effects of exercise and passive heating on the sweat glands ion reabsorption rates

The sweat glands maximum ion reabsorption rates were investigated (n = 12, 21.7 ± 3.0 years, 59.4 ± 9.8 kg, 166.9 ± 10.4 cm and 47.1 ± 7.5 mL/kg/min) during two separate endogenous protocols; cycling at 30% (LEX) and 60% VO2max(MEX) and one exogenous trial; passive heating (PH) (43°C water lower leg immersion) in 27°C, 50%RH. Oesophageal temperature (Tes), skin temperature (Tsk), and forearm, chest and lower back sweat rate (SR) and galvanic skin conductance (GSC) were measured. Salivary aldosterone was measured pre-and postheating (n = 3). Using the ∆SR threshold for an increasing ∆GSC to identify maximum sweat ion reabsorption rate revealed higher reabsorption rates during MEX compared to PH (mean of all regions: 0.63 ± 0.28 vs. 0.44 ± 0.3 mg/cm2/min, P < 0.05). It was not possible to identify the ion reabsorption rate during LEX for some participants. Tesand mean Tskwere different between conditions but mean body temperature (Tb) and local Tsk(forearm, chest and back) were similar (P > 0.05). Aldosterone increased more during MEX (72.8 ± 36.6 pg/mL) compared to PH (39.2 ± 17.5 pg/mL) and LEX (1.8 ± 9.7 pg/mL). The back had a higher threshold than the forearm (P < 0.05) but it was similar to the chest (P > 0.05) (mean of all conditions; 0.64 ± 0.33, 0.42 ± 0.25, 0.54 ± 0.3 mg/cm2/min, respectively). Although the differences between conditions may be influenced by thermal or nonthermal mechanism, our results indicate a possibility that the sweat glands maximum ion reabsorption rates may be different between exercise and passive heating without mediating skin regional differences.