|
|
||||||||
Department of Animal Physiology, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden
| |
ABSTRACT |
|---|
|
|
|---|
The effect
of feeding frequency and voluntary sodium intake (VSI) on fluid shifts
and plasma aldosterone concentration (PAC) were studied at rest and
after exercise in six athletic horses. The horses were fed twice a day
(2TD) and six times a day (6TD) for 25 days for each protocol,
according to a changeover design. VSI was measured by
weighing each horse's salt block daily. Feeding 2TD or 6TD caused no
major alterations in fluid shifts, but in the 2TD treatment there was a
postprandial increase in plasma protein concentration and osmolality
that lasted <1 h. PAC and VSI were not affected by feeding frequency.
VSI ranged from 0 to 62 mg · kg body
weight
1 · day
1
and caused significant alterations in PAC. At VSI <26
mg · kg body
weight
1 · day
1,
a diurnal rhythm for PAC was noted. Water intake, fecal concentrations of sodium and potassium, and packed cell volume during exercise were
influenced by VSI. The response to exercise did not differ between treatments. In conclusion, VSI, but not feeding frequency, has
significant effects on fluid and electrolyte regulation in athletic horses.
aldosterone; exercise; feces; fluid balance; sodium deficiency
| |
INTRODUCTION |
|---|
|
|
|---|
WILD HERBIVORES spend most of their time grazing or ruminating. This behavior ensures a regular flow of digesta and causes small, momentary fluxes of digestive fluids across the alimentary tract. In cattle, sheep, and ponies on a hay diet, the daily parotid secretion is [in liters/100 kg body weight (body wt)] ~9-17 (15), 6-21 (15), and 7-8 (2), respectively. This salivary secretion corresponds to ~1-4 times the plasma volume, and if this fluid shift were to take place within a short period, it would certainly have the potential to affect fluid-balance regulation. Accordingly, a large single meal fed to sheep (5) and ponies (6) has been shown to cause a significant postprandial hypovolemia that did not occur in animals fed more frequently. Single meals can also increase the plasma renin level in sheep (5) and levels of both renin and aldosterone in goats (8). It has been concluded that, in nonexercising horses (7) and ponies (6), feeding is a major stimulus for the renin-angiotensin-aldosterone system (RAAS).
Athletic horses undergo regular exercise training and competition, and the associated sweating results in substantial losses of sodium, which cannot be compensated for by a grass and grain diet. However, herbivores are known to have a well-developed appetite for sodium, and one of the most common ways of supplying sodium to them is by offering a salt block. The mechanisms regulating sodium appetite are not fully understood, but, in some species, aldosterone and angiotensin II may play a role. If feeding frequency affects the RAAS, then sodium appetite may also be influenced by feeding.
Previous studies of the effect of feeding frequency have been made in animals fed concentrated feed at a maintenance level. These studies have also been made under laboratory conditions, with animals kept in metabolism cages or subjected to some other type of restraint. In addition, few studies have been made on the voluntary sodium intake of animals subjected to physiological sodium losses, such as sweating. In the present study, the effects of feeding frequency on fluid shifts, plasma aldosterone concentration, and voluntary sodium intake were investigated in athletic horses without having to encroach on their daily routine to any great extent.
| |
METHODS |
|---|
|
|
|---|
Animals. Six Standardbred geldings (CD, CW, LS, MK, SB, WS) were used (body wt, 407-522 kg; age, 5-10 yr). The horses belonged to the Swedish Academy of Trotting and Thoroughbred Racing in Örebro, Sweden. They were used in the education of apprentices and professional race trainers and performed a simulated 2,140-m trotting race or interval training once a week. On days when no intensive exercise was performed, the horses spent the morning hours in paddocks or had lighter exercise. The study took place between April and June in 1995. During this period, the average ambient temperature per 24 h increased from 10 to 16°C. The study was approved by the Uppsala Local Ethics Committee.
Diets and feeding frequencies. Two feeding frequencies were studied during two 25-day-periods according to a change-over design. The horses were fed either twice a day (2TD; at 1730 and 0530) or six times per day (6TD; at 1730, 2130, 0130, 0530, 0930, and 1330). The horses had been fed the experimental diet for at least 4 wk before the study began. The diet consisted of 7.4 ± 0.3 kg grass hay and 4.1 ± 0.2 kg concentrates per day, corresponding to the Swedish recommendations (15a) for energy and protein supply for hard-working horses (21.6 MJ/100 kg body wt, 124 g digestible protein/100 kg body wt). The daily amounts of hay and concentrates were evenly distributed among the meals. The stable was divided into two compartments, and horses subjected to the same treatment were housed together to avoid psychological disturbances. The horses were housed in individual boxes (10 m2) and had had free access to a 2-kg salt block (99% NaCl) for several months before the study. The salt block was weighed every afternoon throughout the study. Total sodium intake was calculated as the sum of the daily weight loss of the salt block and the amounts of sodium in the hay, concentrates, and mineral supplement. Concentrations of sodium and potassium in the drinking water were negligible. Water was offered ad libitum from graduated buckets, and water intake was measured every fourth hour throughout the study.
Blood sampling and fecal collection.
On day 17 in both treatments, blood
samples were taken every hour for 24 h to determine a resting profile
for each animal. Additional blood samples were taken every 15 min for 1 h after the 1730 and 0530 meals were provided. The blood samples were taken via a catheter introduced into one of the jugular veins (Intranule, 2.0 × 105 mm, SweVet-Piab, Sjöbo, Sweden). An
extension tube was attached to the catheter to facilitate blood
sampling. The horses could move freely in their boxes during the
experiment, and, to avoid the animals' discomfort, no samples were
taken when the horses were lying down. At 0330, almost all
horses were lying down; therefore, this sample was omitted. All feces
produced during 3 days (days
19-21) were
collected in both feeding-frequency treatments. Each horse was fitted
with a collecting device, consisting of a bag hanging under the tail
just below the anus and attached to a girth and a brest. The bag was
emptied manually at least every 90 min, and the feces were frozen at
20°C. After the feces from each horse and treatment were
mixed, one sample was taken. These samples were dried (65°C, 24 h),
milled (1 mm), ashed (600°C, 12 h), and dissolved in 1 M
HNO3 (1 h). After neutralization
of the samples (1 M NH3), the
sodium and potassium concentrations were measured by using an
ion-selective electrode. Evaluation of this method has shown that the
results are similar to those obtained with fresh samples boiled in
HNO3 and perchloric acid. Occasionally, the collecting device was displaced in horse
CW; therefore, fecal data were
excluded from the feeding-frequency analysis. The horses were hand
walked or lunged for 15 min during the collection days.
Exercise test. In the morning (0800-1200) of day 25 in both experimental periods, an exercise test (ET) was performed on a field track. In the ET, the horses trotted five 500-m intervals at a speed of 11.1 ± 0.2 m/s (heart rate ~205 beats/min; heart-rate recorder Optipuls, Borlänge, Sweden). After each interval, the horses returned to the start in a slow trot. This return trip took 2-4 min, depending on whether or not blood samples were taken (see below). All ETs were preceded by a warm-up in a slow trot (14 min). The horses were returned to the stable in slow trot (13 min) immediately after the last interval. In the evening before the ET days, a catheter was inserted into a jugular vein and flushed with heparinized saline (0.2% heparin) overnight and with isotonic saline between samplings. Blood samples were taken at rest, after the two last intervals (within 60 s), and 30 min after the last interval. The ETs were performed at the same time of day for each horse on both occasions. The horses were exercised in fixed pairs and driven by the same person on both occasions. The weather and track conditions were almost identical on the two ET days (11-17°C, 62-69% relative humidity, and sunshine). Four of the six horses (CD, CW, MK, SB) completed both ETs, and only these horses are included in the statistical analysis of exercise data for the two feeding frequencies.
Each horse was weighed before and after a simulated 2,140-m race when being fed 2TD as well as 6TD (weight indicator U-137, UNI Systems and Vågspecialisten, Skara, Sweden).Analysis.
The blood samples were collected in lithium- heparinized
tubes kept on ice until centrifugation and thereafter
frozen at
20°C. The packed cell volume (PCV) was determined
in duplicate by centrifugation of blood in capillary tubes (12,000 rpm;
ALC microhematocrit centrifuge, Milan, Italy). Total
plasma protein concentration (TPP) was measured with a refractometer
(Cambridge Instruments, Buffalo, NY). Plasma osmolality was analyzed by
freezing-point depression (3-W Wide-Range Osmometer, Advanced
Osmometer, Roebling, Berlin, Germany). Analyses of fecal and plasma
sodium and potassium concentrations were made by using an ion-selective
electrode method (system E2A electrolyte analyzer; Beckman Instruments,
Brea, CA). Plasma aldosterone concentration (PAC) was determined after
extraction of fat and proteins (acetone and petroleum ether extraction)
by using a commercially available radioimmunoassay kit for aldosterone
(Coat-a-Count; Diagnostic Products, Los Angeles, CA). The samples for
the standard curve were extracted in the same way. The quality control
was run by using MultiCalc software version 2.0 (Wallac, Turku,
Finland). The minimum detectable concentration was 27.4 pmol/l, the
within-assay variation was 4.1%, and the between-assays variation was
7.5%.
Statistical analysis. Values are presented as means ± SE. All data were subjected to analysis of variance (GLM procedure in the Statistical Analysis Systems package, SAS Institute, Cary, NC) by using the model
|
i is the effect of animal,
j is the effect of treatment,
k is the effect of sample,
(
)j k is the effect of interaction between treatment and sample,
ei j k
is the residuals (independent with mean = 0 and variance =
2e). The
P value for significance within and
between treatments was <0.05. The individual diurnal data for PAC and
plasma sodium concentrations were pooled into three groups of sodium
intake, regardless of feeding frequency: low sodium intake (12-16
mg · kg body
wt
1 · day
1),
medium sodium intake (17-26 mg · kg body
wt
1 · day
1),
and high sodium intake (37-70 mg · kg body
wt
1 · day
1).
Intake-level groups were the same during the two periods for each of
the horses except for horse CD, which had a high intake on 2TD
and a medium intake on 6TD. Individual data were subjected to linear
(linear least-squares-regression analysis) and nonlinear regression
analysis (Slide Write Plus version 3.0, Advanced Graphics Software,
Carlsbad, CA). The P value for
significance was <0.05.
| |
RESULTS |
|---|
|
|
|---|
Short-term effects of feeding 2TD and 6TD.
All horses consumed the concentrate first, immediately after feeding in
both the 2TD and 6TD treatments. The hay was consumed next, but some of
it was sometimes left uneaten for hours in 2TD. TPP and plasma
osmolality increased after feeding in 2TD (Table 1). The PCV, PAC, and plasma sodium
concentration were not affected by feeding 2TD or 6TD, but some changes
were seen in plasma potassium concentration (Table 1).
|
Diurnal effects of feeding 2TD and 6TD.
The TPP and plasma osmolality were lower in 2TD than in 6TD on several
occasions in the late night and morning (Fig.
1). The PCV was not affected by feeding
frequency (Fig. 1). Individual variation in TPP and PCV were greater in
2TD than in 6TD (Fig. 1).
|
|
|
Individual sodium intake and its effects.
The daily individual sodium and water intakes are shown in Table
3. Voluntary sodium intake from the salt
block was in the range of 0-62 mg · kg body
wt
1 · day
1,
but, on an individual basis, the daily variation was small. The mean
voluntary sodium intake was not affected by feeding frequency. Individual water intake and individual sodium intake showed a positive
linear correlation (y = 36.5 + 0.22x;
R2 = 0.54, P < 0.01).
|
1 · day
1)
PAC increased markedly between 2130 and 0430. A medium intake (17-26 mg · kg body
wt
1 · day
1)
also led to an increased PAC at night, whereas PAC only increased occasionally on the high sodium intake (37-70
mg · kg body
wt
1 · day
1).
Plasma sodium concentrations were not affected by sodium intake.
|
|
Effects on exercise.
All horses completed the ETs without signs of fatigue. There were no
differences between 2TD and 6TD in TPP, PCV, osmolality, plasma sodium,
potassium, or aldosterone concentration before, during or after the ET
(Table 4). The simulated races caused a
similar weight loss of 11 ± 3 and 13 ± 4 kg for 2TD and 6TD feeding, respectively (NS).
|
x/8.89);
R2 = 0.65, P < 0.05], with the highest
PACs in horses with a low sodium intake. The individual mean PCV after
the two last intervals was inversely correlated to daily sodium intake
(Fig. 5). The individual difference between
rest and the mean for the two last intervals in TPP and plasma
osmolality, respectively, showed no correlation to sodium intake.
|
| |
DISCUSSION |
|---|
|
|
|---|
Contrary to earlier reports (6, 7), fluid shifts and PAC showed no
major alterations in relation to the number of times per day that the
horses were fed (2TD vs. 6TD). Although voluntary sodium intake was not
affected by feeding frequencies, it did induce significant alterations
in the PAC, PCV, water intake, and fecal excretion of sodium and
potassium. There was great individual variation in sodium intake, and
four of the horses consumed sodium in amounts less than or equal to the
suggested maintenance requirement of 20 mg · kg body
wt
1 · day
1
(16). Considering that these were athletic horses, the
intakes were surprisingly low.
As expected, the highest PACs were found in the horses with the lowest
sodium intake. The capacity to minimize urinary sodium losses is good
in horses, both at rest (21) and after exercise (13). However, in
herbivores, the sodium balance is not necessarily under primary control
of the kidneys, because, under some conditions, it may be affected
mainly by fecal excretion (18). In horses (1, 21) and in sheep (18),
the sodium excretion in feces may well exceed that in the urine.
Aldosterone has been shown to stimulate sodium absorption in the
intestinal tract of horses (4), and this mechanism is dependent on the
exchange of either potassium or hydrogen ions. In this study, the
sodium and potassium contents of the feces were reciprocally related.
The most dramatic interactions occurred at sodium intakes <26 mg
Na · kg body
wt
1 · day
1
when PAC was also elevated. At this level, potassium excretion was
about six times higher than sodium excretion. In addition, at sodium
intakes <26 mg · kg body
wt
1 · day
1,
aldosterone was released according to a diurnal rhythm, with elevated
plasma levels occurring between 2130 and 0430. To our knowledge, this
is the first observation of a diurnal rhythm in PAC in the horse,
although a tendency for PAC to be higher in the early morning has been
reported earlier (7). It is widely accepted that there is a circadian
rhythm in PAC in humans and that body posture affects the RAAS. In
humans, it has also been shown that the circadian rhythm is amplified
by a restricted sodium intake (14), but the origin of the rhythm seems
to be complex. The present study shows that changes in body posture
will not have any sustainable effect on PAC in horses, because this
level did not change between 0230 and 0430, a period during which
almost all horses had been lying down.
Increases in both plasma renin and PAC in response to feeding in horses were shown by Clarke et al. (7); therefore, meal intake was affirmed as a physiological event that causes significant diurnal variation in the RAAS. Increased postprandial renin secretion has also been reported in ruminants (5, 8). In all of these reports, it is suggested that the activation of RAAS is due to the secretion of digestive fluids and the resulting hypovolemia. Therefore, the difference in postprandial PAC between earlier investigations and the present study might be related to the magnitude of the postprandial decrease in plasma volume. In the present study, TPP increased by 3-6% in animals receiving 2TD feeding. In earlier studies on horses (7) and ponies (12) in which the feeding interval was similar to that in our study (12 h) or longer (24 h), TPP increased by as much as 11-12% after feeding. There can be several explanations for the comparatively low increase in TPP during feeding in the present study. First, our horses were athletic and were offered almost twice as much feed per kilogram body weight as the animals in the studies mentioned above. As a result, our horses might have been less excited during feeding and might have fed less voraciously. Blair-West and Brook (5) noticed that renin was secreted in sheep that ate rapidly but not in slow feeders. Nevertheless, in the present study, TPP increased by 10% in the most voracious individual, without any increase in PAC. Second, the ponies and horses in the earlier studies were offered only pelleted feed, which has been shown to be consumed more rapidly than long hay (17). If feed is consumed at a slower rate, the digestive fluids will be reabsorbed, and an equilibrium between fluid secretion and reabsorption will be reached.
There were no differences in daily water intake between 2TD and 6TD protocols. This is in accordance with a study on ponies fed once per day or 6TD (12). However, the amounts of water were more evenly distributed throughout the day when the horses were fed 6TD. This pattern of water intake may have supported the less variable maintenance of the fluid homeostasis, illustrated by the small individual variation in TPP and PCV in 6TD. There was a positive correlation between water intake and sodium intake. Body water content or plasma volume was not measured in the present study, but the increased PCV during exercise may indicate that a reduction of the plasma volume and/or cellular hypervolemia had occurred in horses with a low sodium intake. An increase in PCV, while on low-sodium diets, has been shown in resting rats (10) and humans (22) as well as after exercise in rats (11). That the increase in PCV in the present study was due to a decreased plasma volume and/or to cellular hypervolemia rather than to differences in fluid shifts during exercise is supported by the finding that the individual changes in TPP and osmolality that occurred when animals shifted from rest to exercise were similar, irrespective of the sodium intake. It has previously been reported that a comparatively high red cell volume during exercise could be related to a decrease in the work capacity in Swedish Standardbred horses (20).
In the present study, we did not observe any alterations in the response to exercise when feeding frequency was changed. The horses were not exercised within 2.5 h after feeding, and, according to the results from resting conditions, this should have been long enough to prevent prior feed intake from having any effect on fluid and electrolyte regulation. The exercise-induced increase in TPP probably reflects a decrease in plasma volume due to increased hydrostatic pressure and a true fluid loss due to sweating. The increase in plasma osmolality was mainly due to the accumulation of lactate (data not shown).
The variation in individual voluntary sodium intake is intriguing. Why did so many of the horses regulate their sodium losses through internal strategies instead of increasing their sodium intake? The daily voluntary sodium intake was almost identical during the two periods in four of six horses, suggesting that these horses regulated their sodium intake by some physiological signal. It has been shown in sheep (9) and rats (19) that sodium-deficient animals will not replete their sodium reserves when offered concentrated sodium solutions. These authors suggest that consuming concentrated salt solutions causes taste aversion and/or a temporary inhibition of the sodium appetite. A salt block might, therefore, not be an optimal source of sodium for athletic horses.
The present study has shown that feeding athletic horses large meals had a short-term effect on their hemodynamics but did not influence their diurnal PAC or voluntary sodium intake. PAC, on the other hand, was significantly influenced by voluntary sodium intake.
| |
ACKNOWLEDGEMENTS |
|---|
We thank Åsa Rytthammar, Ulf Nilsson, and Ulrika Hultberg at the Swedish Academy of Trotting and Thoroughbred Racing in Örebro, Sweden, for providing horses and assistance during the experiments. We also thank Gunilla Drugge-Boholm and Veronica Hjertqvist for technical assistance.
| |
FOOTNOTES |
|---|
This project was supported by grants from Aktiebolaget Trav och Galopp (Swedish Racing Board).
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. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: A. Jansson, Dept. of Animal Physiology, Box 7045, S-750 07 Uppsala, Sweden (E-mail: Anna.Jansson{at}djfys.slu.se).
Received 13 April 1998; accepted in final form 22 January 1999.
| |
REFERENCES |
|---|
|
|
|---|
1.
Alexander, F.
Diuretics and faecal electrolytes in horses.
Br. J. Pharmacol.
60:
589-593,
1977[Medline].
2.
Alexander, F.,
and
J. C. D. Hickson.
The salivary and pancreatic secretions of the horse.
In: Physiology of Digestion and Metabolism in the Ruminant, edited by A. T. Phillipson. New Castle, IN: Oriel, 1970, p. 375-389.
4.
Argenzio, R. A.,
and
L. L. Clarke.
Electrolyte and water absorption in the hind gut of herbivores.
Acta Vet. Scand. Suppl.
86:
159-167,
1989[Medline].
5.
Blair-West, J. R.,
and
A. H. Brook.
Circulatory changes and renin secretion in sheep in response to feeding.
J. Physiol. (Lond.)
204:
15-30,
1969.
6.
Clarke, L. L.,
R. A. Argenzio,
and
M. C. Roberts.
Effect of meal feeding on plasma volume and urinary electrolyte clearance in ponies.
Am. J. Vet. Res.
51:
571-576,
1990[Medline].
7.
Clarke, L. L.,
B. Ganjam,
B. Fichtenbaum,
D. Hatfield,
and
H. E. Garner.
Effect of feeding on renin-angiotensin-aldosterone system of the horse.
Am. J. Physiol.
254 (Regulatory Integrative Comp. Physiol. 23):
R524-R530,
1988
8.
Dahlborn, K.
Diurnal changes with different feeding intervals in the goat.
In: Proc. 5th Federation of Eur. Laboratory Animal Assoc. Symposium: Welfare and Science, edited by J. Bunyan., 1995, p. 457-459.
9.
Denton, D.
The consummatory act of satiation of salt appetite in sodium deficiency.
In: The Hunger for Salt. Berlin, Germany: Springer-Verlag, 1982, chapt. 4, p. 278.
10.
Francesconi, R. P.,
and
R. H. Hubbard.
Chronic low sodium diet in rats: responses to severe heat exposure.
J. Appl. Physiol.
58:
152-156,
1985
11.
Francesconi, R. P.,
R. H. Hubbard,
and
M. Mager.
Chronic low-sodium diet in rats: hormonal and physiological effect during exercise in the heat.
J. Appl. Physiol.
55:
870-874,
1983
12.
Houpt, K. A.,
P. J. Perry,
H. F. Hintz,
and
T. R. Houpt.
Effect of meal frequency on fluid balance and behavior of ponies.
Physiol. Behav.
42:
401-407,
1988[Medline].
13.
Jansson, A.,
S. Nyman,
K. Morgan,
C. Palmgren-Karlsson,
A. Lindholm,
and
K. Dahlborn.
The effect of ambient temperature and saline loading on changes in plasma and urine electrolytes (Na+ and K+) following exercise.
Equine Vet. J. Suppl.
20:
147-152,
1995.
14.
Katz, F. H.,
P. Romfh,
and
J. A. Smith.
Diurnal variation of plasma aldosterone, cortisol and renin activity in supine man.
J. Clin. Endocrinol. Metab.
40:
125-134,
1975
15.
Kay, R. N. B.
The influence of saliva on digestion in ruminants.
World Rev. Nutr. Diet.
6:
292-325,
1966[Medline].
15a.
Lantbruksstyrelsen.
Lantbruksstyrelsens Rapport 5:
9-23, 1989.
16.
Meyer, H.
Nutrition of the equine athlete.
In: Equine Exercise Physiology, edited by J. R. Gillespie,
and N. E. Robinson. Davis, CA: International Conference on Equine Exercise Physiology, 1987, p. 644-673.
17.
Meyer, H.,
L. Ahlswede,
and
H. Reinhard.
Untersuchungen uber Fesdauer, Kaufrequenz und Futterzerkleinerung beim Pferd.
Dtsch. Tierarztl. Wochenschr.
82:
54-58,
1975.
18.
Michell, A. R.
The gut: the unobtrusive regulator of sodium balance.
Perspect. Biol. Med.
29:
203-213,
1986[Medline].
19.
Nachman, M.,
and
D. A. Valentino.
The role of taste and post-ingestional factors in the satiation of sodium appetite in rats.
J. Comp. Physiol. Psychol.
62:
280-283,
1966[Medline].
20.
Persson, S. G. B.
On blood volume and working capacity in horses.
Acta Vet. Scand. Suppl.
19:
1-189,
1967.
21.
Tasker, J. B.
Fluid and electrolyte studies in the horse. III. Intake and output of water, sodium and potassium in normal horses.
Cornell Vet.
57:
649-657,
1967.
22.
Weidmann, P.,
B. Hellmueller,
D. E. Uehlinger,
R. E. Lang,
M. P. Gnaedinger,
L. Hasler,
S. Shaw,
and
C. Bachmann.
Plasma levels and cardiovascular, endocrine and excretory effects of atrial natriuretic peptide during different sodium intakes in man.
J. Clin. Endocrinol. Metab.
62:
1027-1036,
1986
This article has been cited by other articles:
![]() |
A. P. Waller, G. J. F. Heigenhauser, R. J. Geor, L. L. Spriet, and M. I. Lindinger Fluid and electrolyte supplementation after prolonged moderate-intensity exercise enhances muscle glycogen resynthesis in Standardbred horses J Appl Physiol, January 1, 2009; 106(1): 91 - 100. [Abstract] [Full Text] [PDF] |
||||
![]() |
A. Jansson, A. Lindholm, and K. Dahlborn Effects of acute intravenous aldosterone administration on Na+, K+, and water excretion in the horse J Appl Physiol, January 1, 2002; 92(1): 135 - 141. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |