Factors explaining individual variations in whole-body sweat sodium ion concentration ([Na+]) during exercise are not fully understood. Galvanic skin conductance (GSC) reflects the electrical properties of the skin influenced by sweat rate (SR) and the presence of ions. Initiation of increases in this response to elevating sweating may reflect exceeding the maximal capacity of sweat ion reabsorption in sweat glands. We investigated whether the SR threshold for increasing GSC, an indirect measure of maximum ion reabsorption rates of sweat glands, explains the variations in whole-body sweat [Na+]. Thirty young healthy males cycled for 90 min at incremental exercise intensities of 30, 45, and 60% peak oxygen uptake (30 min each) in the heat (32°C, 50%RH). Whole-body sweat [Na+] was measured using a whole-body washdown technique. The SR threshold for increasing GSC was determined from the relationship between the local SR (ventilated capsule) and GSC on the forearm and chest. The average whole-body sweat [Na+] was 42.8 ± 18.9 (range, 14.4-81.0) mmol L-1, and the SR threshold for increasing GSC was 0.29 ± 0.20 (range, 0.02-0.62) and 0.35 ± 0.30 (range, 0.01-1.40) mg cm-2 min-1 for the forearm and chest, respectively. Whole-body sweat [Na+] was not correlated with the SR threshold for increasing GSC in the forearm or chest (r2 {less than or equal to} 0.001, P {greater than or equal to} 0.921). We conclude that the SR threshold for increasing GSC at the forearm and chest does not explain the individual variation in whole-body sweat [Na+] during exercise in the heat.
Keywords: Eccrine sweat glands; dehydration; rehydration; sodium loss; sweating; thermoregulation.