Effect of Heat Acclimation on Lactate Excretion Rate and Sodium Ion Reabsorption Rate during Exercise

Abstract

Sweat production requires two Na+/K+-ATPase energy dependent processes: (i) isoosmotic precursor sweat formation in the secretory coil, and (ii) sodium ion reabsorption along the distal sweat duct. Sweat lactate production has been hypothesized to reflect glycolytic ATP-resynthesis for both processes. Research suggests that heat acclimation does not alter sweat lactate concentration ([Lactate]). This is hard to rationalize considering heat acclimation is reported to increase sweat production and sodium ion reabsorption, which are the energy dependent processes. PURPOSE: (1) To determine the relationships during exercise between sweat rate (SR) and sodium ion reabsorption rate (NaRR), and exercise intensity and lactate excretion rate (LER), and (2) to determine whether heat acclimation affects these relationships. METHODS: Six subjects completed an eight-day heat acclimation protocol comprised of 90-minutes treadmill walking in an environmental chamber set at 39°C and 40% relative humidity. Subjects performed three, 30-minute exercise bouts at 30, 40, and 50% of their previously determined VO2max. Sweat rate was measured volumetrically during each bout on the flexor surface of the proximal half of the forearm. [Lactate] was measured and multiplied by forearm SR to calculate LER. Variables of interest were analyzed using non-parametric Friedman tests. RESULTS: LER, and ratios of LER to NaRR, and [Lactate] to SR were not significantly different between the 30, 40, and 50% workloads or between pre and post-heat acclimation based on 95% confidence intervals. Additionally, these relationships were not significantly different between the three workloads on the first or last day of heat acclimation except for LER to NaRR (p<0.05), which increased from 0.05 ± 0.02 to 0.08 ± 0.02 from 30 to 50% of VO2max. CONCLUSION: These data suggest that sweat gland ATPase activity does not increase as a function of exercise intensity, as demonstrated by LER and NaRR remaining relatively constant. Heat acclimation does not appear to affect these relationships.