Sodium ion concentration vs. sweat rate relationship in humans

Abstract

The purpose of this study was to determine the effect of active heat acclimation on the sweat osmolality and sweat sodium ion concentration vs. sweat rate relationship in humans. Eight healthy male volunteers completed 10 days of exercise in the heat. The mean exercising heart rate and core temperature were significantly decreased (P < 0.05) by 18 beats/min and 0.6 degrees C, respectively, following heat acclimation. Furthermore, sweat osmolality and the sweat sodium ion concentration vs. sweat rate relationships were shifted to the right. Specifically, the slopes of the relationships were not affected by heat acclimation. Rather, heat acclimation significantly reduced the y-intercepts of the sweat osmolality and sweat sodium relationships with sweat rate by 28 mosmol/kgH(2)O and 15 mmol/l, respectively. Thus there was a significantly lower sweat sodium ion concentration for a given sweat rate following heat acclimation. These results suggest that heat acclimation increases the sodium ion reabsorption capacity of the human eccrine sweat gland.

Determination of the maximum rate of eccrine sweat glands’ ion reabsorption using the galvanic skin conductance to local sweat rate relationship

“…Furthermore, the ΔSR threshold was significantly shifted to the right following heat acclimation, but this did not affect the slope of the GSC-SR relationship above the threshold. This is in agreement with previous studies that reported the effect of exercise heat acclimation on sweated sodium ion versus SR (Allan and Wilson 1971;Buono et al 2007). Based on these findings, we believe that the ΔSR threshold for increasing ΔGSC as evident in the GSC-SR relationship can serve as a new index of the maximum rate of sweated ion reabsorption of eccrine sweat glands.…”

“…These results suggest that the GSC-SR relationship is valid for assessing a threshold in SR reflecting the maximum rate of sweat ion reabsorption of the eccrine sweat glands, irrespective of differences between sweat and skin conductance. Buono et al (2007) measured sweat sodium concentrations and SR relationships during exercise, as well as pre-and post-exercise heat acclimation. A macroduct collector was used to collect sweat for 30 min during three exercise bouts, data from which were then plotted to evaluate the influence of exercise heat acclimation on sweat sodium concentration.…”

“…Furthermore, it has been reported that exercise heat acclimation improves the ability of sweat glands to reabsorb sweat sodium, as evidenced by a rightward shift of the regression line obtained when the sweated sodium concentration -SR relationship is graphed, with or without changes in the slope (Buono et al 2007;Allan and Wilson 1971). In light of the aforementioned observations, this study was designed to investigate whether the GSC-SR relationship is able to reproduce the characteristics of sweated ion reabsorption by human eccrine sweat glands of non-glabrous skin during thermal stimulation.…”

“…When the sodium secretion rate exceeds that of reabsorption, the sweated sodium concentration increases linearly with sweating rate (SR) (Shamsuddin et al 2005b;Buono et al 2008). Therefore, the sodium concentration in sweat should always be evaluated with respect to the actual SR (Sato et al 1989;Allan and Wilson 1971;Buono et al 2007;Buono et al 2008;Shamsuddin et al 2005b;Kuno 1956). Furthermore, the SR at which the sweated ion (e.g., sodium) concentration begins to rapidly increase is believed to reflect the point where the maximum rate of sweated ion reabsorption in sweat gland (SR threshold) is reached (Shamsuddin et al 2005a;Shamsuddin et al 2005b).…”

“…Conventionally, filter paper, gauze pads, or commercial sweat collectors have been used to collect sweat for measurement of electrolyte concentrations (Buono et al 2007;Inoue et al 1998;Allan and Wilson 1971;Verde et al 1982;Araki et al 1981;Hamouti et al 2011). …”