J Appl Physiol 105: 1044 –1048, 2008.
First published July 24, 2008; doi:10.1152/japplphysiol.90503.2008.
Na⫹ secretion rate increases proportionally more than the Na⫹ reabsorption
rate with increases in sweat rate
Michael J. Buono,1,2 Ryan Claros,2 Teshina DeBoer,2 and Janine Wong2
1
Department of Biology and 2School of Exercise and Nutritional Sciences, San Diego State University, San Diego, California
Submitted 7 April 2008; accepted in final form 18 July 2008
sodium reabsorption; sodium secretion; eccrine sweat gland; sweat
sodium concentration
with increases in sweat rate. For example, Inoue et al. (11)
reported significant positive correlations (r ⫽ 0.67– 0.98) between local sweat rate and [Na⫹]sweat measured on the chest,
back, forearm, and thigh. Over the years, three different physiological mechanisms have been proposed in the literature to
explain this relationship. Taylor (30) hypothesized that increases in sweat rate will increase [Na⫹]sweat because of a
reduction in ductal Na⫹ reabsorption. Next, several investigators (2, 4, 22, 27) suggested that Na⫹ reabsorption has a
maximum limit that becomes saturated with increases in sweat
rate, thus causing an increase in [Na⫹]sweat. Finally, it has been
proposed that with increases in sweat rate the sweat Na⫹
reabsorption rate increases proportionally less than the Na⫹
secretion rate, thus resulting in an elevated [Na⫹]sweat (11, 16).
Therefore, depending on the reference chosen, it could be
argued that increases in sweat rate will cause the Na⫹ reabsorption rate to either 1) decrease, 2) become saturated and
thus demonstrate a plateau effect, or 3) increase. However,
such data have not been previously measured in the human
eccrine sweat gland. Thus the purpose of this study was to
measure the in vivo sweat Na⫹ secretion and reabsorption rates
with increasing sweat rates. Such data should help to elucidate
the physiological mechanism responsible for the previously
reported linear relationship between increases in sweat rate
and [Na⫹]sweat.
METHODS
THE FORMATION OF SWEAT IN the human eccrine gland is the result
of a two-step process. The first step involves the secretion of an
isosmotic precursor sweat by the proximal secretory coil. Next
sodium (Na⫹) and chloride ions are reabsorbed from the
precursor sweat during its passage along the distal duct segment of the gland (21, 23, 28). Thus, in healthy humans,
because ductal water reabsorption is small (13) the final composition of sweat that appears at the skin surface is always
hyposmotic (12, 17, 29) and has recently been reported by
Patterson et al. (19), using the whole body wash-down technique, to have a mean Na⫹ concentration of 24 mmol/l.
Conceptually, the final composition of sweat therefore depends
largely on the relationship between the amount of Na⫹ secreted
during precursor sweat formation minus the rate of ductal Na⫹
reabsorption (2, 23).
Most previous studies (1, 2, 4, 11, 15, 32) have shown that
the Na⫹ concentration in sweat ([Na⫹]sweat) increases linearly
The subjects for the study were 10 healthy volunteers (4 men and
6 women). They had a mean ⫾ SE age, height, weight, and body
surface area of 25 ⫾ 1 yr, 168.7 ⫾ 2.4 cm, 66.5 ⫾ 3.6 kg, and 1.76 ⫾
0.06 m2, respectively. The subjects were recruited from the San Diego
area using posted flyers and via word of mouth. They were all
recreationally active; however, none was formally heat acclimated.
They were instructed to refrain from initiating new exercise programs
during the duration of the study. No restrictions were placed on their
diet; however, the subjects were asked to report to the laboratory well
hydrated and to not have exercised in the preceding 12 h. The study
was approved by the San Diego State University Institutional Review
Board, and written informed consent was obtained from each subject
before participation in the study.
On 5 days, during a 2-wk period of time, each subject completed a
30-min exercise bout in an environmental chamber set at 35°C and
40% relative humidity. Before each exercise bout the subject’s urinary
specific gravity was measured and had to be ⬍1.028 to ensure that
they were not dehydrated before the start of each trial. This was done
because dehydration is known to affect [Na⫹]sweat (15). In addition,
water was allowed ad libitum during each of the five exercise trials.
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
Address for reprint requests and other correspondence: M. J. Buono, San
Diego State Univ., Mail Code 7251, San Diego, CA 92182-7251 (e-mail:
mbuono@mail.sdsu.edu).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1044
8750-7587/08 $8.00 Copyright © 2008 the American Physiological Society
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Buono MJ, Claros R, DeBoer T, Wong J. Na⫹ secretion rate
increases proportionally more than the Na⫹ reabsorption rate with
increases in sweat rate. J Appl Physiol 105: 1044 –1048, 2008. First
published July 24, 2008; doi:10.1152/japplphysiol.90503.2008.—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°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.
SODIUM ION SECRETION VS. REABSORPTION RATES
Na ⫹ reabsorption rate ⫽ 关共sweat rate 䡠 关Na⫹兴serum 兲
⫺ 共sweat rate 䡠 关Na⫹兴sweat 兲兴
Last, the rate at which Na⫹ escaped reabsorption was calculated as
the difference between the Na⫹ secretion rate minus the Na⫹ reabsorption rate.
For the previously mentioned dependent variables, Pearson
product-moment correlations were calculated for each subject and the
mean r was the average of the 10 values. A two ⫻ five repeatedmeasures ANOVA was used to compare mean Na⫹ secretion vs.
reabsorption data, while a one-way repeated-measures ANOVA was
used to analyze the mean relative Na⫹ reabsorption rate data. Post hoc
analyses were conducted using dependent t-tests and were adjusted
using the Bonferonni correction. Overall significance was set at the
P ⬍ 0.05 level.
RESULTS
In agreement with numerous past studies (1, 2, 4, 11, 14, 27,
32) the results showed that within the range studied there was
a significant linear relationship between sweat rate and
[Na⫹]sweat. There were no differences in responses between
men and women; thus all data were grouped together. As can
be seen in Fig. 1 the mean r was 0.73. At the lowest sweat rate,
the mean ⫾ SE [Na⫹]sweat was 19 ⫾ 5 mmol/l, whereas at the
highest sweat rate it increased to 59 ⫾ 10 mmol/l. These values
are similar in magnitude to other studies that sampled sweat
locally (1, 2, 4). Figure 2 shows that the rate at which Na⫹ was
secreted into the coil and the Na⫹ reabsorption rate during
J Appl Physiol • VOL
Fig. 1. Sweat sodium ion concentration vs. sweat rate relationship. Values are
means ⫾ SE for 10 subjects. The mean r for the group was 0.73 (P ⬍ 0.05).
y ⫽ 59.7(x) ⫹ 6.7. Sweat rate in mg 䡠 cm⫺2 䡠 min⫺1 can be converted to
g 䡠 m⫺2 䡠 min⫺1 by multiplying by 10.
passage of sweat down the distal duct of the eccrine gland both
increased linearly with increases in sweat rate. However, the
Na⫹ secretion rate increased significantly faster, as evidenced
by a significant interaction (P ⬍ 0.05) and by having a slope
that was almost twice as great (141 vs. 80) as the Na⫹
reabsorption rate. Conversely, as shown in Fig. 3, the mean
relative Na⫹ reabsorption rate, expressed as a percentage of
Na⫹ secreted during precursor sweat formation, decreased
significantly with increases in sweat rate. Specifically, 86 ⫾
3% of the secreted Na⫹ was reabsorbed at the lowest sweat
rate, and this decreased to 65 ⫾ 6% at the highest sweat rate.
Conceptually, the difference between the two lines presented
in Fig. 2 is the rate at which Na⫹ escaped reabsorption during
passage along the distal duct. In other words, it represents the
amount of Na⫹ not reabsorbed that ultimately would appear as
Na⫹ in the sweat. Figure 4 illustrates this relationship and
shows that the rate at which Na⫹ escaped reabsorption was
highly correlated (mean r ⫽ 0.90) with [Na⫹]sweat.
DISCUSSION
Numerous studies (1, 2, 4, 9, 11, 14, 32) have previously
reported that as sweat rate increases so does the [Na⫹]sweat.
However, the physiological mechanism responsible for this
phenomenon is currently unknown. Conceptually, [Na⫹]sweat
should be the difference between the amount of Na⫹ secreted
during precursor sweat formation in the proximal coil minus
the rate of ductal Na⫹ reabsorption (2, 24, 26). The data from
the present study are believed to be the first to measure these
two parameters in vivo over a wide range of different sweat
rates and are presented in Fig. 2. As can be seen, within the
range of sweat rates reported in the current study the rate of
both Na⫹ secretion and Na⫹ reabsorption increased in a linear
fashion. Such findings support the work of others (3, 5, 25)
who have indirectly determined ductal Na⫹ reabsorption by
calculating the maximum free water clearance, which is proportional to maximum water-free Na⫹ reabsorption. These
studies reported that there was a significant linear relationship
between maximum free water clearance and maximum sweat
rate (3, 5, 25). However, in the current study, the rate of
increase in Na⫹ secretion was proportionally greater, as evi-
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rate (220 minus age). The absolute workloads were not critical and
were simply designed to produce various sweat rates, ranging from
very low to very high, from each of the subjects. Heart rate was
continuously measured during exercise using a Polar heart rate monitor. The order of the five exercise bouts was randomly assigned for all
subjects.
Forearm sweat samples were collected for 20 min (from minute 10
to minute 30) during each of the five 30-min exercise bouts using
macroduct sweat collectors (Wescor, Logan, UT). This protocol was
designed to allow the subjects to start sweating before the initiation of
sweat collection and has been successfully used in the past (25). The
collectors were placed on the flexor surface of the proximal forearm
and were held in place by a Velcro strap that prevented leakage and
sample contamination. The collection site was identified so that the
macroduct could be placed at the same location for all trials. The skin
was cleaned with deionized water and dried immediately before
securing each collector. Forearm sweat rate, for the approximate
5-cm2 area under the macroduct sweat collector, was calculated by the
change in volume within the sample collection tubing and expressed
in milligrams per square centimeter per minute. Following collection
into the sample tubing, sweat was expressed into a test tube using a
positive-pressure air bellows and stored frozen for later analysis. The
sweat samples were analyzed, in duplicate, for [Na⫹]sweat using a
calibrated flame photometer (Cole-Palmer, Chicago, IL).
Blood samples were collected from free-flowing fingertip punctures at minute 20 of each exercise bout. They were allowed to clot
and centrifuged for 10 min to obtain samples that were analyzed for
Na⫹ concentration in serum ([Na⫹]serum) using flame photometry.
Because it has been previously shown (5, 24, 25) that precursor
sweat is isosmotic to serum and that Na⫹ concentration of precursor
sweat is independent of sweat rate (24), the Na⫹ secretion rate,
expressed in nanomoles per square centimeter per minute, was calculated as the product of sweat rate and [Na⫹]serum. Ductal Na⫹
reabsorption rate, expressed in nanomoles per square centimeter per
minute, was calculated according to the following formula developed
by Sato (23):
1045
1046
SODIUM ION SECRETION VS. REABSORPTION RATES
denced by having a slope that was almost double that of Na⫹
reabsorption (141 vs. 80). Such findings do not support Taylor’s (30) contention that “Elevated flow will increase sodium
and chloride sweat content, possibly because of reduced ductal
reabsorption at higher flow rates.” Clearly the absolute rate of
Na⫹ reabsorption was not reduced with increases in sweat rate
in the present study. Furthermore, several previous investigators (2, 4, 27) have hypothesized that Na⫹ reabsorption would
become saturated at high sweat rates, thus causing the observed
increase in [Na⫹]sweat. Again, the data presented in Fig. 2 do
not support such a hypothesis as Na⫹ reabsorption did not
demonstrate a plateau effect with increases in sweat rate up to
0.82 mg䡠cm⫺2 䡠min⫺1 or ⬃1 l/h.
It could be argued that possibly the sweat rates attained with
the present study were not large enough and thus failed to elicit
a plateau effect in Na⫹ reabsoption. However, the mean fore-
Fig. 3. Mean ⫾ SE sodium ion reabsorption expressed as a percentage of Na⫹
secretion for the 10 subjects. *Value was significantly (P ⬍ 0.05) different
than value obtained at the lowest sweat rate.
J Appl Physiol • VOL
Fig. 4. Rate of Na⫹ escaping reabsorption vs. the sweat sodium ion concentration relationship. Values are means ⫾ SE for 10 subjects. The mean r for the
group was 0.90 (P ⬍ 0.05). y 䡠 16(x) ⫹ 16.
arm sweat rate produced by the most intense workload (i.e.,
90% of maximal heart rate) in the present study (0.82
mg䡠cm⫺2 䡠min⫺1) was similar in magnitude to previous results
that were produced by exercise in the heat or maximal pharmacological stimulation (5, 8, 10, 18, 19). For example,
Gonzalez et al. (10) reported a mean forearm sweat rate of 0.75
mg䡠cm⫺2 䡠min⫺1 in six healthy men during exercise in a hot
(40°C) humid (90% relative humidity) environment that produced a mean esophageal temperature of 38.9°C. In addition,
Crandall et al. (8) reported that pharmacological-induced maximal forearm sweat rate was 0.63 mg䡠cm⫺2 䡠min⫺1. Thus
lower than expected forearm sweat rates do not seem to be the
likely cause for not observing a plateau in the Na⫹ reabsorption vs. sweat rate relationship data presented in Fig. 2.
Rather the present data suggest that within the range of
sweat rates studied, the absolute rate of Na⫹ reabsorption from
the distal duct increases continuously with increases in sweat
rate. However, when expressed as a relative percentage of Na⫹
secretion, it decreases with increases in sweat rate. Specifically, Fig. 2 shows that the absolute rate of Na⫹ reabsorption
at the highest sweat rate was almost three times that seen
during the lowest sweat rate (77 vs. 29 nmol䡠cm⫺2 䡠min⫺1).
However, as can be seen in Fig. 3, at the lowest sweat rate
⬃86 ⫾ 3% of the Na⫹ secreted during precursor sweat formation were able to be reabsorbed during passage along the
distal duct of the eccrine gland. This value was significantly
reduced to 65 ⫾ 6% at the highest sweat rate. Such data clearly
show that the faster the precursor sweat travels along the distal
duct (i.e., the greater the sweat rate) the smaller the percentage
of total Na⫹ secreted that can be reabsorbed. This decrease in
relative Na⫹ reabsorption with increases in sweat rate could be
the result of a number of factors, including decreased contact
time of the fluid column with the luminal membrane of the
distal duct, a decreased lumen-intracellular diffusion gradient,
and/or saturation of various transport proteins. An alternate
mechanism may involve the fact that decreases in the cytoplasmic pH have been shown (6) to reduce the activity of the
amiloride-sensitive epithelium Na⫹ channel (ENaC), which is
the primary luminal path used for ductal Na⫹ reabsorption.
Because glycolysis appears to be the predominate way that
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Fig. 2. Rates of sodium ion secretion during precursor sweat formation and
sodium ion reabsorption from the distal duct vs. sweat rate relationships.
Values are means ⫾ SE for 10 subjects. Na⫹ secretion ⫽ 140.9(x) ⫺ 1.1. Na⫹
reabsorption ⫽ 79.9(x) ⫹ 8.4. There was a significant (P ⬍ 0.05) interaction
between the 2 relationships. *Significant (P ⬍ 0.05) difference between the
rate of Na⫹ secretion and reabsorption at that sweat rate.
SODIUM ION SECRETION VS. REABSORPTION RATES
J Appl Physiol • VOL
concentration in sweat vs. serum as an indicator of potential
leaching. In the present study, 18 randomly selected paired
sweat and serum samples were analyzed for K⫹ concentration
using flame photometry. The mean K⫹ concentrations in sweat
and. serum were both 5.1 mmol/l. These data strongly suggest
that leaching of Na⫹ was negligible in the current study.
In conclusion, the results of the present study provide insight
into the physiological mechanism responsible of the well
documented increase in [Na⫹]sweat with increasing sweat rates.
The results show that within the sweat rate range of the current
study the absolute rates of both Na⫹ secretion and Na⫹
reabsorption increase linearly with increases in sweat rate.
However, Na⫹ secretion increases proportionally faster, thus,
the rate at which Na⫹ escape reabsorption during passage
along the distal duct of the sweat gland increases at higher
sweat rates. Lastly, the rate of Na⫹ escaping reabsorption was
linearly related to the [Na⫹]sweat. Such data strongly support
that with increasing sweat rates, the rate of Na⫹ secretion
during precursor sweat formation increases faster than Na⫹
reabsorption in the distal duct. This difference allows for a
greater percentage of the secreted Na⫹ to escape reabsorption,
thus causing [Na⫹]sweat to increase.
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escaped reabsorption, should be highly related to the
[Na⫹]sweat. As can be seen in Fig. 4, these two variables were
linearly related and had a mean r of 0.90. Thus the more Na⫹
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examining the x- and y-axis, the rate of Na⫹ escaping reabsorption increased about sixfold, whereas the [Na⫹]sweat only
increased about twofold. At first glance this seems illogical;
however, it must be remembered that during the same time the
sweat rate also increased threefold. Thus a sixfold increase in
Na⫹ escaping reabsorption should only increase the [Na⫹]sweat
about twofold, if the sweat rate simultaneously increased
threefold.
There are three assumptions that were made in the present
study that need to be further addressed. First, it was assumed
that the measured [Na⫹]serum provides a valid measure of the
precursor [Na⫹]sweat. The support for this assumption is based
on the fact that numerous previous studies, using three different techniques, have all shown that precursor sweat is isosmotic to serum. Specifically, Sato and Dobson (25) using the
backward extrapolation technique reported that the mean ⫾ SE
precursor [Na⫹]sweat in the forearm of 14 healthy volunteers
was 140 ⫾ 1.8 mmol/l. Additionally, precursor sweat samples
collected directly from the secretory coil were shown to be
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142 mmol/l. Finally, sweat samples collected following the
application of amiloride, which selectively inhibits Na⫹ reabsorption from the distal duct, were isosmotic to serum (20).
Taken together these three studies clearly support the validity
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1047
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