Na⁺ secretion rate increases proportionally more than the Na⁺ reabsorption rate with increases in sweat rate

Abstract

The purpose of this study was to measure the in vivo Na(+) secretion and Na(+) reabsorption rates of the human eccrine sweat gland with increases in sweat rate. Such data should help to elucidate the physiological mechanism responsible for the previously reported linear relationship between increases in sweat rate and Na(+) concentration in sweat. On 5 days, each subject (n = 10) completed a 30-min exercise bout in an environmental chamber set at 35 degrees C and 40% relative humidity. The intensity for the five exercise bouts in the heat was set to approximate 50, 60, 70, 80, and 90% of age-predicted maximum heart rate. Forearm sweat samples and capillary blood samples were collected during each of the five 30-min exercise bouts. The sweat and blood samples were analyzed for Na(+) concentration in sweat and serum, which were used to calculate the rate of Na(+) secretion and Na(+) reabsorption. The mean correlation between sweat rate and Na(+) concentration in sweat was found to be r = 0.73. Within the sweat rate range of the present study, both Na(+) secretion rate and Na(+) reabsorption rate increased linearly; however, the Na(+) secretion rate increased almost twice as fast (slope = 141 vs. 80). Thus the rate at which Na(+) escaped reabsorption increased with increases in sweat rate and was significantly (P < 0.05) correlated to the Na(+) concentration in sweat (mean r = 0.90). Such results strongly suggest that the physiological mechanism responsible for the previously reported linear increase in Na(+) concentration in sweat seen with increases in sweat rate is that the Na(+) secretion rate increases proportionally more than the Na(+) reabsorption rate.

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

“…Two processes are relevant to the amount and concentration of various ions (Na + , Cl -) secretion of sweat from the sweat gland. Firstly, isosmotic precursor sweat is produced in the proximal secretory coil (Buono et al 2008;Shamsuddin et al 2005b;Sato et al 1989). Secondly, sodium and chloride ions are reabsorbed from precursor sweat in the distal duct, rendering sweat appearing on the skin to be hypo-osmotic (Buono et al 2008;Shamsuddin et al 2005b;Sato et al 1989).…”

“…Firstly, isosmotic precursor sweat is produced in the proximal secretory coil (Buono et al 2008;Shamsuddin et al 2005b;Sato et al 1989). Secondly, sodium and chloride ions are reabsorbed from precursor sweat in the distal duct, rendering sweat appearing on the skin to be hypo-osmotic (Buono et al 2008;Shamsuddin et al 2005b;Sato et al 1989). 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).…”

“…Secondly, sodium and chloride ions are reabsorbed from precursor sweat in the distal duct, rendering sweat appearing on the skin to be hypo-osmotic (Buono et al 2008;Shamsuddin et al 2005b;Sato et al 1989). 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).…”

“…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).…”

“…We previously reported that the maximum SR at which the eccrine sweat gland can completely reabsorb sweated ion ranges from 0.2 to 0.5 mg/cm 2 /min during passive heating and cycling (Shamsuddin et al 2005a;Shamsuddin et al 2005b). In addition, the sodium in sweat is likely reabsorbed to the extent of approximately 90% by the duct when the SR attains 0.2 mg/cm 2 /min (Buono et al 2008). …”