Angiotensin II and Aldosterone Increase with Fasting in Breeding Adult Male Northern Elephant Seals ( Mirounga angustirostris )

Article (PDF Available)inPhysiological and Biochemical Zoology 79(6):1106-12 · November 2006with26 Reads
DOI: 10.1086/505996 · Source: PubMed
Abstract
The renin-angiotensin-aldosterone system (RAAS) appears to contribute significantly to osmoregulation of fasting northern elephant seal (Mirounga angustirostris) pups; however, RAAS has not been characterized in fasting adult seals. Therefore, this study examined the contribution of RAAS to water turnover rates in fasting adult male northern elephant seals. Blood samples were obtained twice during their breeding fast at an interval of 6.5 wk, and water efflux rate was estimated by isotopic dilution during the same period. Serum electrolytes (Na+, K+, Cl-) and osmolality were unaltered between the two sampling periods, indicating ionic and osmotic homeostasis during the fast. Despite the lack of an increase in vasopressin, serum angiotensin II and aldosterone were increased and were significantly and positively correlated. Changes in aldosterone concentration and water efflux rate were significantly and negatively correlated, suggesting that the greater the increase in aldosterone, the smaller the loss of water. Adult male seals maintain ionic and osmotic homeostasis similar to that of fasting weaned pups, and this homeostasis appears to be mediated, at least in part, by RAAS, which probably contributes to increased water retention as well. The hormonal mechanisms by which northern elephant seals maintain water and electrolyte balance during fasting conditions appear to be similar regardless of age.
1106
Angiotensin II and Aldosterone Increase with Fasting in Breeding
Adult Male Northern Elephant Seals (Mirounga angustirostris)
Rudy M. Ortiz
1,
*
Daniel E. Crocker
2
Dorian S. Houser
2
Paul M. Webb
3
1
Department of Biology, University of California, Santa Cruz,
California 95064;
2
Department of Biology, Sonoma State
University, Rohnert Park, California 94928;
3
Department of
Biology, Roger Williams University, Bristol, Rhode Island
02809
Accepted 3/17/2006; Electronically Published 10/2/2006
ABSTRACT
The renin-angiotensin-aldosterone system (RAAS) appears to
contribute significantly to osmoregulation of fasting northern
elephant seal (Mirounga angustirostris) pups; however, RAAS
has not been characterized in fasting adult seals. Therefore, this
study examined the contribution of RAAS to water turnover
rates in fasting adult male northern elephant seals. Blood sam-
ples were obtained twice during their breeding fast at an interval
of 6.5 wk, and water efflux rate was estimated by isotopic
dilution during the same period. Serum electrolytes (Na
,K
,
Cl
) and osmolality were unaltered between the two sampling
periods, indicating ionic and osmotic homeostasis during the
fast. Despite the lack of an increase in vasopressin, serum
angiotensin II and aldosterone were increased and were sig-
nificantly and positively correlated. Changes in aldosterone
concentration and water efflux rate were significantly and neg-
atively correlated, suggesting that the greater the increase in
aldosterone, the smaller the loss of water. Adult male seals
maintain ionic and osmotic homeostasis similar to that of fast-
ing weaned pups, and this homeostasis appears to be mediated,
at least in part, by RAAS, which probably contributes to in-
creased water retention as well. The hormonal mechanisms by
which northern elephant seals maintain water and electrolyte
balance during fasting conditions appear to be similar regardless
of age.
* Corresponding author. Present address: School of Natural Sciences, Univer-
sity of California, P.O. Box 2039, Merced, California 95344; e-mail:
rortiz@ucmerced.edu.
Physiological and Biochemical Zoology 79(6):1106–1112. 2006. 2006 by
The University of Chicago. All rights reserved. 1522-2152/2006/7906-
5122$15.00
Introduction
Few mammals have adapted to prolonged periods of fasting.
Hibernators such as the American black bear (Ursus ameri-
canus; Brown et al. 1971), the hedgehog (Erinaceus europaeus;
Clausen and Storesund 1971), the little brown bat (Myotis lu-
cifugus; Gustafson and Belt 1981), prairie dogs (Cynomys leu-
curus and Cynomys ludovicianus; Harlow and Braun 1995), and
the yellow-bellied marmot (Marmota flaviventris; Kastner et al.
1978) experience periods as long as 7 mo without food and
water, but they are essentially metabolically inactive during this
time. In contrast, camels (Camelus dromedarius) may withstand
prolonged periods (2–3 wk) of food and water deprivation in
arid and hot environments without exhibiting deleterious ef-
fects (Ben Goumi et al. 1993; Yagil 1993). As with the afore-
mentioned species, fasting for protracted periods (1–3 mo) is
a natural component of the life-history year of the northern
elephant seal (Mirounga angustirostris), occurring twice a year,
regardless of age or sex (Le Boeuf et al. 1972). Nonetheless, the
contributions of the osmoregulatory hormones arginine va-
sopressin (AVP), angiotensin II (Ang II), and aldosterone to
the maintenance of water and electrolyte homeostasis during
periods of prolonged fasting in adapted mammals remain
largely unresolved.
Pups of the northern elephant seal maintain fluid and ionic
homeostasis throughout their postweaning fast by conserving
the water derived from the oxidation of their large body fat
stores (Ortiz et al. 1978). In fasting pups, body water is con-
served by increasing urine osmolality (Ortiz et al. 1996) and
decreasing urine output (Adams and Costa 1993; Ortiz et al.
1996). By increasing the tubular reabsorption of electrolytes,
pups are able to reduce the excretion of Na
and K
and thus
are able to maintain electrolyte homeostasis (Adams and Costa
1993; Ortiz et al. 1996). In addition, pups maintain a state of
metabolic quiescence similar to that of hibernators during their
postweaning fast, which allows them to further reduce energetic
expenditures and metabolic water losses. In contrast, adult male
seals are physically more active during their fasting period as-
sociated with the breeding season, exhibiting high rates of mass
loss and energy expenditure (Deutsch et al. 1990). This increase
in activity could potentially impede their ability to maintain
ionic and osmotic homeostasis, which is observed in the rel-
atively inactive fasting pups. However, the osmoregulatory ad-
Osmoregulation in Adult Seals 1107
justments made by fasting adult elephant seals, especially during
the breeding season, when animals are physically more active,
have yet to be examined.
In mammals, the reabsorption of solute-free water from the
collecting duct is mediated by AVP (Wade et al. 1982). As part
of the renin-angiotensin-aldosterone system (RAAS), the pre-
cursor hormone, angiotensinogen, is cleaved by renin to pro-
duce angiotensin I, which is further converted to Ang II by
angiotensin-converting enzyme (Funder 1993). Ang II, in turn,
stimulates the adrenal release of aldosterone (Morris 1981;
Funder 1993). Both Ang II (Ichikawa and Harris 1991) and
aldosterone (Morris 1981; Funder 1993) have been reported to
possess an antidiuretic function in the distal tubule of the mam-
malian kidney, in addition to regulating Na
reabsorption. Un-
der normal fasting conditions in elephant seal pups, the renal
conservation of water and electrolytes appears to be regulated,
at least in part, by an increase in the response of RAAS without
an increase in AVP (Ortiz et al. 2000). Although water re-
absorption does not appear to be mediated by AVP under nat-
ural fasting conditions in elephant seal pups, other studies in
adult seals suggest that water retention may be associated with
AVP (Bradley et al. 1954; Page et al. 1954; Hong et al. 1982;
Skog and Folkow 1994). In addition, AVP concentrations are
greater in adults of other species of pinnipeds compared to
those of conspecific pups (Zenteno-Savin and Castellini 1998),
suggesting that concentrations of AVP in adult elephant seals
may also be greater than those previously measured in pups
and may increase with the breeding fast. However, concentra-
tions of AVP, Ang II, and aldosterone have not been previously
reported in adult elephant seals during fasting conditions. In
addition, data on RAAS in marine mammals, especially during
fasting conditions, are scarce, which contributes to our lack of
thorough understanding of osmoregulation in this group of
animals.
Unlike the postweaning fast observed in pups of the northern
elephant seal, in which pups minimize activity by slumbering
for a majority of the fast, the fast of adult males during the
breeding season is associated with levels of high activity, in-
cluding combat with other males to establish dominance, cop-
ulations, and frequent periods of terrestrial locomotion. Such
stark differences in physical activity between pups and actively
breeding males could potentially be associated with differences
in osmoregulatory capabilities, since increased physical activity
could burden total body water stores. For that reason, in order
for seals to adapt to increased levels of activity during fasting
periods as they mature, renal function and capabilities, as well
as hormonal content, may change with development and age
to accommodate such an adaptation. Thus, the assumption that
the osmoregulatory alterations observed in fasting pups under
natural conditions are similar to those of conspecific adults
would be inappropriate. Therefore, this study was conducted
to determine whether breeding adult male elephant seals main-
tain ionic and osmotic homeostasis and to examine the changes
in levels of aldosterone, Ang II, and AVP during their natural
prolonged fast.
Methods
All methods were reviewed and approved by the University of
California, Santa Cruz, Chancellor’s Animal Research Com-
mittee. This study was conducted in conjunction with another
study on the metabolism and behavior of breeding adult male
northern elephant seals.
Animals. Seventeen adult male seals from An˜o Nuevo State
Reserve (approximately 30 km north of Santa Cruz, CA) were
studied during the 1998 breeding season. Animals were con-
sidered adult based on body mass measurements and devel-
opment of secondary sexual characteristics including proboscis
and neck shield. Individuals were identified by marking them
with hair dye (Clairol, Stamford, CT) and by reading preexisting
flipper tags. Markings and tags facilitated identification of an-
imals for subsequent sampling. Individuals were weighed and
sampled twice during their nonmolting fast with a sampling
interval of d. The initial sampling is referred to as “early”45 1
and the subsequent sampling as “late.”
Water efflux calculations. Isotopic dilution of tritiated water
(
3
H
2
O) was used to estimate total body water pool size and
water turnover as previously described (Ortiz et al. 1978). Males
were lured onto a platform truck scale (5 kg) and weighed.
No initial radioactive enrichment was assumed, and each male
that was successfully weighed was immediately given an intra-
muscular injection of 185–296 MBq
3
H
2
Oin12mLofsterile
saline. The following day, after an equilibration period of 12–
16 h, the animal was sedated with 0.3 mg/kg body mass tile-
tamine HCl and zolazepam HCl (Telazol, Fort Dodge Animal
Health, Fort Dodge, IA) in order to obtain blood samples
(early). Blood samples from the beginning and end of the im-
mobilization procedure (20–30 min apart) were compared to
ensure complete equilibration of isotope. For the second sample
(late), males were weighed and then immediately immobilized
and sampled. After the collection of blood samples, a second
dose of
3
H
2
O was administered. The animal was allowed to
equilibrate for 12–16 h before being immobilized for a final
blood collection. The interval from the time the animals were
injected with the sedative to the time the blood sample was
obtained was
!10 min. Blood samples (20 mL) were obtained
from the extradural spinal vein using an 18-gauge needle, col-
lected into an untreated blood collection tube, and placed on
ice in a portable ice chest until they could be returned to the
lab to be centrifuged, which was typically within 6 h. Blood
samples were then centrifuged for 15 min (1,500 g at 4C), and
serum was collected and frozen at 70C for later analyses.
Daily water efflux was calculated using equation (4) of Nagy
and Costa (1980).
Electrolyte, osmolality, and hormone analyses. Hormone
concentrations were measured by radioimmunoassay using
1108 R. M. Ortiz, D. E. Crocker, D. S. Houser, and P. M. Webb
Table 1: Mean (SE) serum electrolyte concentrations, osmolality, and
hormone concentrations in fasting adult male northern elephant seals
during early and late fasting periods
N Early Period Late Period P
Na
(mM) 16 156 1 152 1NS
K
(mM) 16 4.4 .1 4.5 .1 NS
Cl
(mM) 16 109 1 107 1NS
Osmolality (mOsm/L) 17 300 3 296 2NS
Aldosterone (pg/mL) 17 145.2 10.7 254.0 38.5
!.0001
Angiotensin II (pg/mL) 16 56.6 9.0 110.8 14.9 .02
Vasopressin (pg/mL) 16 14.5 1.2 12.2 1.1 NS
Note. significant.NS p not
Figure 1. Correlation between circulating angiotensin II and aldoste-
rone during early and late fasting periods in adult male northern
elephant seals. Line and equation describe the relationship during only
the late fasting period because the relationship during the early period
was not significant. Regression was considered significant at .P
! 0.05
commercially available kits for Ang II and AVP (Phoenix Phar-
maceuticals, Belmont, CA) and aldosterone (DPC, Los Ange-
les). Each assay had been validated previously for use with
northern elephant seal samples (Zenteno-Savin and Castellini
1998; Ortiz et al. 2000, 2002a, 2002b, 2003; Houser et al. 2001).
For measurements of Ang II and AVP, separate 1.0-mL aliquots
were extracted using C18 columns (Prep-Sep, Fisher Scientific,
Fair Lawn, NJ) as previously described (Zenteno-Savin and
Castellini 1998). All samples were analyzed in duplicate and
run in a single assay with intra-assay coefficient of variability
percentage of less than 8% for all assays. Osmolality was mea-
sured using a vapor pressure osmometer (Wescor 5500, Logan,
UT), and electrolytes were measured on a Cobas Mira auto-
analyzer (Roche Diagnostic Systems, Montclair, NJ).
Statistics. Means for each parameter at early and late pe-
riods were compared by paired t-test. In the event that sufficient
sample from an animal was not available from either the early
or the late period, comparisons were made from a sample size
of 16 (i.e., paired samples were available for at least 16 animals).
Relationships between dependent and independent variables
were evaluated by simple regression, and correlations were eval-
uated using Pearson correlation coefficients. Means, regres-
sions, and correlations were considered significant at .P
! 0.05
Statistical analyses were made using Statview (SAS Institute
1998).
Results
Body mass decreased by ( ) between23.2% 1.4% P
! 0.0001
early ( kg) and late ( kg) periods. Over1,479 65 1,135 53
the 6.5-wk period, water efflux rate averaged mL/4.04 0.20
kg/d (range: 2.80–6.15 mL/kg/d). Mean Ang II increased nearly
twofold ( ), and mean aldosterone increased 75%P p 0.02
( ) between the two sampling periods (Table 1). Dur-P
! 0.0001
ing the early period, Ang II and aldosterone did not exhibit a
significant relationship or correlation; however, during the late
period, aldosterone concentration increased significantly with
increasing Ang II concentration ( Angaldosterone p 99.9 1.2
II; ; ; Fig. 1). Water efflux rate declined sig-r p 0.612 P p 0.02
nificantly with change in aldosterone concentration (Daldo-
sterone; water Daldosterone; ;eff lux p 4.3 0.003 r p 0.492
; Fig. 2). The relationships between DAng II and DAV PP p 0.04
and water efflux rate were not significant ( ). In addition,P
1 0.10
concentrations of AVP and Ang II did not exhibit a significant
correlation ( ) at either sampling period. Electrolytes,P
1 0.10
osmolality, and AVP remained unchanged between the two
measurement periods (Table 1).
Discussion
Deprivation of food and water for extended periods (months)
has the potential to induce deleterious effects on water and
electrolyte homeostasis. However, a number of hibernating and
nonhibernating mammals have adapted various physiological
mechanisms to survive such extreme conditions. For example,
pups of the northern elephant seal rely on the metabolic water
derived from the oxidation of their large fat stores (Ortiz et al.
1978) to maintain fluid and ionic homeostasis (Costa and Ortiz
Osmoregulation in Adult Seals 1109
Figure 2. Correlation between the change in aldosterone concentration
(Daldosterone) and water efflux rate in fasting adult male northern
elephant seals. Regression was considered significant at .P
! 0.05
1982; Ortiz et al. 1996, 2000). This study demonstrates that
despite the increased physical activity and high rates of energy
expenditure observed in breeding adult male elephant seals, the
seals are still able to maintain ionic and osmotic homeostasis
during their nonmolting fast similar to that observed in rela-
tively inactive conspecific pups. The lack of change in serum
osmolality and electrolytes between the early and late sampling
periods in adult male seals provides an indication of their ability
to maintain electrolyte balance, suggesting that these animals,
regardless of their activity level, possess rigorous physiological
mechanisms to tightly regulate circulating electrolyte concen-
trations. Serum osmolality and electrolyte concentrations in this
study are similar to those measured in fasting seal pups (Costa
and Ortiz 1982; Ortiz et al. 1996, 2000) and fasting lactating
females (D. E. Crocker, unpublished data). In contrast, water
deprivation for 10 or 14 d in camels increased mean plasma
Na
(3.3% or 24%, respectively) and osmolality (10.5% or
33.5%, respectively; Ben Goumi et al. 1993; Yagil 1993), sug-
gesting that camels exhibit signs of dehydration during their
bouts of water deprivation, unlike elephant seals. Hibernating
marmots have also been shown to increase mean plasma Na
3.4%–3.8% after 9 d (Zatzman and South 1972; Kastner et al.
1978), further demonstrating the impressive feat by elephant
seals of maintaining electrolyte homeostasis, especially in the
face of increased activity levels observed in breeding adults.
Previous studies have shown that RAAS in elephant seal pups
responds to various stimuli (fasting [Ortiz et al. 2000], altered
salinity [Ortiz et al. 2002a, 2002b], AVP infusion [Ortiz et al.
2003]), and in West Indian manatees (Trichechus manatus), it
responds to changes in water salinity (Ortiz et al. 1998). How-
ever, to date, no studies of mammals adapted to prolonged
fasting have demonstrated an association between changes in
some component of the RAAS and changes in some parameter
of water conservation, such as reduced water efflux rate. The
negative relationship between Daldosterone and water efflux
rate (water loss) suggests that increases in aldosterone contrib-
ute to conservation of water in fasting adult seals. This cor-
relation implies that the greater the change in circulating al-
dosterone, the smaller the rate of water efflux or loss, and thus
it supports the contention that the increase in aldosterone sig-
nificantly contributes to osmotic homeostasis in fasting adult
northern elephant seals.
The correlation between Daldosterone and water efflux also
supports the suggestion that increases in aldosterone and Ang
II play a significant role in the conservation of electrolytes
during this period in adult seals. In mammals, Na
and K
are
regulated by aldosterone via RAAS (Morris 1981; Funder 1993).
Aldosterone induces the increased reabsorption of Na
in the
distal tubule that is accompanied by the reabsorption of water
(Morris 1981; Funder 1993). Ang II has also been reported to
have antidiuretic activity in the kidney (Ichikawa and Harris
1991). Therefore, the observed increases in Ang II and aldo-
sterone between the early and late periods, coupled with a
positive correlation between the two hormones during the late
period, indicate that RAAS is active and contributing to elec-
trolyte homeostasis during the fast in adult males. Plasma renin
activity (PRA), an indicator of circulating angiotensin I gen-
eration, and aldosterone have been shown to be positively cor-
related in fasting elephant seal pups during their postweaning
fast (Ortiz et al. 2000), as well as in other marine mammals
(Malvin et al. 1978; Ortiz et al. 1998), further implicating the
presence of an active RAAS in this group. Also, hibernating
marmots exhibited an increase in PRA and a concomitant in-
crease in aldosterone after 9 d. The contribution of RAAS in
water-deprived camels is not so definitive. After 10 d of water
deprivation, PRA increased nearly fourfold, associated with a
61% increase in aldosterone (Yagil 1993). However, in a similar
study, 14 d of water deprivation stimulated a nearly 10-fold
increase in PRA without the requisite increase in plasma al-
dosterone. The authors of that study suggest that the lack of
increase in plasma aldosterone may be attributed to the hy-
pernatremia also observed in these animals during the same
period (Ben Goumi et al. 1993), which is possible because
treatment with the natriuretic furosemide alleviated the hy-
pernatremia and resulted in an increase in PRA and concom-
itant increase in aldosterone (Riad et al. 1994). Nonetheless,
the existing data on RAAS and electrolyte balance in mammals
adapted to prolonged periods of water deprivation suggest that
discrepancies exist in the response of these animals to such an
osmotic challenge.
Although urinary excretion data are not available for adult
elephant seals, pups have been shown to exhibit a decrease in
urinary Na
concentration over 10 wk of fasting (Ortiz et al.
1996), coinciding with the time in which RAAS is increased
(Ortiz et al. 2000). This evidence further suggests that the in-
crease in RAAS contributes to an increase in renal reabsorption
of Na
in elephant seals.
1110 R. M. Ortiz, D. E. Crocker, D. S. Houser, and P. M. Webb
Unfortunately, reports of aldosterone (Sangalang and Free-
man 1976; Engelhardt and Ferguson 1980; St. Aubin and Geraci
1986; Ferreira et al. 2005) and Ang II (Zenteno-Savin and Cas-
tellini 1998) concentrations in adult or juvenile pinnipeds are
scarce. Mean aldosterone concentrations during the early pe-
riod ( pg/mL) were most similar to those measured145 11
( pg/mL) in wild adult (values for males and females139 75
combined as reported) southern elephant seals (Mirounga leo-
nina) during their molting fast (Ferreira et al. 2005) and were
approximately half those reported for captive juvenile ringed
seals (Phoca hispida) during normonatremic, salt-replete con-
ditions (St. Aubin and Geraci 1986). Samples obtained from
wild adult male gray seals (Halichoerus grypus) ranged from
1,390 to 3,300 pg/mL; however, it should be noted that these
samples were obtained from animals that were shot to death,
which may explain the comparatively high concentrations
(Sangalang and Freeman 1976). A single sample from a wild
adult male harp seal (Phoca groenlandica; physically restrained
for sampling) had an aldosterone concentration (1,200 pg/mL)
similar to that reported for gray seals. These relatively higher
concentrations of aldosterone may reflect the impact of the
mode of collection rather than the actual physiological status,
especially when compared to the values in our study, in which
animals were sedated during sample collection. Mean aldoste-
rone concentrations from nursing and postnursing adult female
harp seals (300 pg/mL) were similar to those reported here
during the late period ( pg/mL) and for lactating adult254 39
female northern elephant seals (380 pg/mL; D. E. Crocker,
unpublished data). In elephant seal pups, mean aldosterone
concentrations increased from 200 to 1,500 pg/mL during the
first 7 wk of the postweaning fast (Ortiz et al. 2000, 2002a,
2002b, 2003; Houser et al. 2001), representing a 7.5-fold in-
crease, whereas the increase in fasting adult males over a com-
parable duration was only 1.7-fold. An age-related change in
the sensitivity of aldosterone response to fasting may be but
one potential explanation for the discrepancy in the degree of
change of aldosterone between pups and adults. Interestingly,
aldosterone concentrations in hibernating marmots and water-
deprived camels remain below 100 pg/mL (Kastner et al. 1978;
Ben Goumi et al. 1993), which is two- to 15-fold lower than
concentrations observed in pinnipeds, providing an indication
of aldosterone’s importance in osmoregulation of marine
mammals.
Mean Ang II concentrations during the early period (57
pg/mL) are consistent with those measured ( pg/mL)956 12
in adult Steller sea lions (Eumetopias jubatus; Zenteno-Savin
and Castellini 1998). These mean values are two- to 6.5-fold
greater than those measured in other adult pinniped species
(Zenteno-Savin and Castellini 1998), suggesting that distinct
physiological differences with respect to RAAS, possibly dic-
tated by their environment or breeding cycles, may exist among
various species of pinnipeds.
The contributions of AVP to water and electrolyte balance
in species adapted to prolonged food deprivation remain ill
defined. For example, neurohypophyseal content of AVP in the
hibernating garden dormouse (Eliomys quercinus) increased
fourfold; however, AVP content remained unchanged in the
hibernating mouse-eared bat (Myotis myotis; Hudson and Wang
1979). In water-deprived camels, plasma AVP increased 4.4-
fold and sixfold after 10 and 14 d, respectively (Ben Goumi et
al. 1993; Yagil 1993). Only a few studies in pinnipeds have
demonstrated an association between AVP (either infused as
pitressin or quantified by changes in concentration) and re-
duced urine output or calculated free water clearance (Bradley
et al. 1954; Page et al. 1954; Hong et al. 1982; Ortiz et al.
2002b). If the observed concentrations of AVP are physiolog-
ically significant, then the relatively higher concentrations of
AVP in adults versus pups (Ortiz et al. 1996, 2000, 2002a, 2002b,
2003; Zenteno-Savin and Castellini 1998) suggest that the renal
collecting ducts may be hyposensitive to AVP during the fasting
period and thus necessitate higher circulating concentrations
to elicit a physiological response. Thus, the lack of an increase
in AVP does not necessarily preclude AVP from contributing
to water and osmotic homeostasis in fasting adult seals. In
addition, mean AVP concentrations in this study are still two-
fold higher than in water-deprived camels (Ben Goumi et al.
1993; Yagil 1993). Alternatively, the sampling schedule may not
have captured the initial increase in circulating concentrations
of AVP, and thus, the observed levels would be indicative of
maximally increased concentrations. Nonetheless, this trend to-
ward higher concentrations of AVP in adults versus pups is
consistent with that reported for other pinnipeds, including
harbor (Phoca vitulina) and Weddell (Leptonychotes weddellii)
seals (Zenteno-Savin and Castellini 1998). Mean AVP concen-
trations in our study are similar to those measured in adult
Steller sea lions ( pg/mL), harbor seals (14.2 1.5 15.9 2.5
pg/mL), and Weddell seals ( pg/mL) and are in the12.0 0.1
range of adult California sea lions (Zalophus californianus; 10.2
pg/mL; gender not reported; Zenteno-Savin and Castellini
1998).
We have previously shown that an acute intravenous infusion
of AVP induced a rapid (within 15 min) increase in circulating
cortisol in fasting pups, suggesting that AVP stimulated a neu-
roendocrine response in fasting seals (Ortiz et al. 2003). In the
current study, cortisol was measured simultaneously (data not
reported here) to address the potential concern that the ob-
served concentrations of AVP were the product of a neuroen-
docrine response induced by the sampling protocol and did
not necessarily reflect an osmotic response. The relatively rapid
acquisition of blood and the lack of a positive correlation be-
tween AVP and cortisol concentrations at either of the sampling
periods suggest that the concentrations of AVP measured here
were not the product of neuroendocrine stimulation and prob-
ably reflected circulating concentrations at that time.
In summary, the negative correlation between Daldosterone
and water efflux (water loss) indicates that water loss is abated
Osmoregulation in Adult Seals 1111
by increasing aldosterone. This provides some of the most de-
finitive evidence to date that RAAS contributes significantly to
water as well as electrolyte conservation in this species and
probably in most mammals adapted to prolonged periods of
fasting. Increases in Ang II and aldosterone in adult elephant
seals during their breeding fast, when animals are active, are
similar to those observed in pups during their postweaning
fast, when animals are relatively inactive. The positive corre-
lation between Ang II and aldosterone is consistent with the
positive correlation between PRA and aldosterone observed in
fasting pups (Ortiz et al. 2000), suggesting that RAAS is active
and probably contributing to the maintenance of ionic and
osmotic homeostasis. The discrepancy in the degree of change
in aldosterone concentration during equivalent fasting dura-
tions between adult males and pups suggests that the response
of RAAS to fasting varies with age in elephant seals. Despite
the lack of a change in AVP, the relatively higher concentrations
in adults versus those in pups suggest that an age-dependent
shift in the dynamics of AVP metabolism and function may
exist in elephant seals, similar to that observed with RAAS.
Alternatively, these concentrations of AVP may reflect maxi-
mally increased levels. The contribution of AVP to osmoreg-
ulation in marine mammals continues to be ambiguous. How-
ever, this study implies the importance of RAAS in regulating
water and electrolyte balance in mammals adapted to prolonged
fasting, especially marine mammals.
Acknowledgments
We would like to thank L. Pagarigan and B. Sherman for their
assistance with the lab analyses. We appreciate the efforts of
many student volunteers from the University of California,
Santa Cruz (UCSC), for helping in the field. This work was
performed at the University of California Natural Reserve Sys-
tem, An˜o Nuevo Reserve. We thank G. Strachan and the rangers
of the An˜o Nuevo Reserve for allowing us access to the animals
and for their assistance in the field. D.E.C. was supported by
National Oceanic and Atmospheric Administration grant
NA77RJ0453. Research was conducted under National Marine
Fisheries Service marine mammal permit 836. All procedures
were approved by the UCSC Chancellor’s Animal Research
Committee.
Literature Cited
Adams S.H. and D.P. Costa. 1993. Water conservation and
protein metabolism in northern elephant seal pups during
the postweaning fast. J Comp Physiol B 163:367–373.
Ben Goumi M., F. Riad, J. Giry, F. de la Farge, A. Safwate,
M.-J. Davicco, and J.-P. Barlet. 1993. Hormonal control of
water and sodium in plasma and urine of camels during
dehydration and rehydration. Gen Comp Endocrinol 89:
378–386.
Bradley S.E., G.H. Mudge, and W.D. Blake. 1954. The renal
excretion of sodium, potassium, and water by the harbor
seal (Phoca vitulina L.): effect of apnea; sodium, potassium,
and water loading; pitressin; and mercurial diuresis. J Cell
Comp Physiol 43:1–22.
Brown D.C., R.O. Mulhausen, D.J. Andrew, and U.S. Seal. 1971.
Renal function in anesthetized dormant and active bears.
Am J Physiol 220:293–298.
Clausen G. and A. Storesund. 1971. Electrolyte distribution and
renal function in the hibernating hedgehog. Acta Physiol
Scand 83:4–12.
Costa D.P. and C.L. Ortiz. 1982. Blood chemistry homeostasis
during prolonged fasting in the northern elephant seal. Am
J Physiol 242:R591–R595.
Deutsch C.J., M.P. Haley, and B.J. Le Boeuf. 1990. Reproductive
effort of male northern elephant seals: estimates from mass
loss. Can J Zool 68:2580–2593.
Engelhardt F.R. and J.M. Ferguson. 1980. Adaptive hormone
changes in harp seals, Phoca groenlandica, and gray seals,
Halichoerus grypus, during the postnatal period. Gen Comp
Endocrinol 40:434–445.
Ferreira A.P.S., P.E. Martı´nez, E.P Colares, R.B. Robaldo, M.E.A.
Berne, K.C. Miranda Filho, and A. Bianchini. 2005. Serum
immunoglobulin G concentration in southern elephant seal,
Mirounga leonina (Linnaeus, 1758), from Elephant Island
(Antarctica): sexual and adrenal steroid hormone effects. Vet
Immunol Immunopathol 106:239–245.
Funder J.W. 1993. Aldosterone action. Annu Rev Physiol 55:
115–130.
Gustafson A.W. and W.D. Belt. 1981. The adrenal cortex during
activity and hibernation in the male little brown bat, Myotis
lucifugus lucifugus: annual rhythm of plasma cortisol levels.
Gen Comp Endocrinol 44:269–278.
Harlow H.J. and E.J. Braun. 1995. Kidney structure and func-
tion of obligate and facultative hibernators: the white-tailed
prairie dog (Cynomys leucurus) and the black-tailed prairie
dog (Cynomys ludovicianus). J Comp Physiol B 165:320–328.
Hong S.K., R. Elsner, J.R. Claybaugh, and K. Ronald. 1982.
Renal functions of the Baikal seal Pusa sibirica and ringed
seal Pusa hispida. Physiol Zool 55:289–299.
Houser D.S., D.E. Crocker, P.M. Webb, and D.P. Costa. 2001.
Renal function in suckling and fasting pups of the northern
elephant seal. Comp Biochem Physiol A 129:405–415.
Hudson J.W. and L.C.H. Wang. 1979. Hibernation: endocri-
nological aspects. Annu Rev Physiol 41:287–303.
Ichikawa I. and R.C. Harris. 1991. Angiotensin actions in the
kidney: renewed insight into the old hormone. Kidney Int
40:583–596.
Kastner P.R., M.L. Zatzman, F.E. South, and J.A. Johnson. 1978.
Renin-angiotensin-aldosterone system of the hibernating
marmot. Am J Physiol 234:R178–R182.
1112 R. M. Ortiz, D. E. Crocker, D. S. Houser, and P. M. Webb
Le Boeuf B.J., R.J. Whiting, and R.F. Gantt. 1972. Perinatal
behavior of northern elephant seal females and their young.
Behaviour 43:121–156.
Malvin R.L., J.P. Bonjour, and S. Ridgway. 1978. Renin and
aldosterone levels in dolphins and sea lions. Proc Soc Exp
Biol Med 157:665–668.
Morris D.J. 1981. The metabolism and mechanism of action
of aldosterone. Endocr Rev 2:234–247.
Nagy K.A. and D.P. Costa. 1980. Water flux in animals: analysis
of potential errors in the tritiated water method. Am J Physiol
238:R454–R465.
Ortiz C.L., D. Costa, and B.J. Le Boeuf. 1978. Water and energy
flux in elephant seal pups fasting under natural conditions.
Physiol Zool 51:166–178.
Ortiz R.M., S.H. Adams, D.P. Costa, and C.L. Ortiz. 1996.
Plasma vasopressin levels and water conservation in fasting,
postweaned northern elephant seal pups (Mirounga angus-
tirostris). Mar Mamm Sci 12:99–106.
Ortiz R.M., C.E. Wade, D.P. Costa, and C.L. Ortiz. 2002a. Renal
effects of fresh water–induced hypo-osmolality in a marine
adapted seal. J Comp Physiol B 172:297–307.
———. 2002b. Renal responses to plasma volume expansion
and hyperosmolality in fasting seal pups. Am J Physiol 282:
R805–R817.
Ortiz R.M., C.E. Wade, and C.L. Ortiz. 2000. Prolonged fasting
increases the response of the renin-angiotensin-aldosterone
system, but not vasopressin levels, in postweaned northern
elephant seal pups. Gen Comp Endocrinol 119:217–223.
Ortiz R.M., C.E. Wade, C.L. Ortiz, and F. Talamantes. 2003.
Acutely elevated vasopressin increases circulating concentra-
tions of cortisol and aldosterone in fasting northern elephant
seal (Mirounga angustirostris) pups. J Exp Biol 206:2795–
2802.
Ortiz R.M., G.A.J. Worthy, and D.S. MacKenzie. 1998. Osmo-
regulation in wild and captive West Indian manatees (Triche-
chus manatus). Physiol Zool 71:449–457.
Page L.B., J.C. Scott-Baker, G.A. Zak, E.L. Becker, and C.F.
Baxter. 1954. The effect of variation in filtration rate on the
urinary concentrating mechanism in the seal, Phoca vitulina
L. J Cell Comp Physiol 43:257–269.
Riad F., M. Ben Goumi, J. Giry, M.-J. Davicco, A. Safwate,
and J.-P. Barlet. 1994. Renin-aldosterone axis and arginine-
vasopressin responses to sodium depletion in camels. Gen
Comp Endocrinol 95:240–247.
Sangalang G.B. and H.C. Freeman. 1976. Steroids in the plasma
of the gray seal, Halichoerus grypus. Gen Comp Endocrinol
29:419–422.
SAS Institute. 1998. Statview. SAS Institute, Cary, NC.
Skog E.B. and L.P. Folkow. 1994. Nasal heat and water exchange
is not an effector mechanism for water balance regulation
in grey seals. Acta Physiol Scand 151:233–240.
St. Aubin D.J. and J.R. Geraci. 1986. Adrenocortical function
in pinniped hyponatremia. Mar Mamm Sci 2:243–250.
Wade C.E., P. Bie, L.C. Keil, and D.J. Ramsay. 1982. Osmotic
control of plasma vasopressin in the dog. Am J Physiol 243:
E287–E291.
Yagil R. 1993. Renal function and water metabolism in the
dromedary. Pp. 161–170 in E. Bourke, N.P. Mallick, and V.E.
Pollak, eds. Moving Points in Nephrology. Karger, Basel.
Zatzman M.L. and F.E. South. 1972. Renal function of the
awake and hibernating marmot Marmota flaviventris.AmJ
Physiol 222:1035–1039.
Zenteno-Savin T. and M.A. Castellini. 1998. Plasma angiotensin
II, arginine vasopressin and atrial natriuretic peptide in free
ranging and captive seals and sea lions. Comp Biochem Phys-
iol C 119:1–6.
    • "NES have become an important comparative model system for understanding metabolic adaptations for extended fasting (reviewed in Crocker et al., 2014). Fasting NES exhibit many characteristics associated with increased oxidative stress, including renin–angiotensin system (RAS) activation (Ortiz et al., 2000; Ortiz et al., 2006), chronic hypothalamic–pituitary–adrenal axis (HPA) activation (Ortiz et al., 2001; Champagne et al., 2015; Fowler et al., 2015) and insulin resistance (Fowler et al., 2008; Viscarra et al., 2011a Viscarra et al., , 2011b). In model systems these features are associated with increased oxidative stress through activation of Nox proteins, increases in mitochondrial oxidant generation and depletion of antioxidants (Ceriello and Motz, 2004; Costantini et al., 2011; Romero and Reckelhoff, 1999; Sorensen et al., 2006; Di Simplicio et al., 1997; Sowers, 2002). "
    [Show abstract] [Hide abstract] ABSTRACT: Northern elephant seals experience conditions that increase oxidative stress (OS), including extended fasting, ischemia and hypoxia during breath-holds, and immune responses during colonial breeding. Increased OS is suggested by increases in tissue and plasma concentrations of pro-oxidant enzymes NADPH oxidase and xanthine oxidase (XO). Serum cortisol concentrations were positively associated with XO concentrations and damage markers. Elephant seals exhibit robust anti-oxidant responses as evidenced by increases in anti-oxidant enzymes in plasma and tissues. These responses were sufficient to prevent oxidative damage during breath-holds and extended fasts in juveniles. However, high rates of energy expenditure during breeding were associated with increased evidence for oxidative damage to lipids, proteins and DNA in adults. We integrated investigations of the fasting metabolome and muscle and blubber transcriptomes into our oxidative stress studies. Non-targeted metabolomics analysis of fasting seals identified 227 known metabolites in plasma, including those related to glutathione and purine metabolism. Changes in plasma metabolites suggested that glutathione biosynthesis increased during fasting in weaned pups but not in lactating females. We produced the first reference sequence for elephant seals by RNA sequencing of skeletal muscle and adipose tissue transcriptomes and de novo transcriptome assembly. We annotated muscle and adipose transcripts and identified thousands of genes, including potential mediators of OS. This resource provides elephant seal-specific gene sequences, complements existing metabolite and protein expression studies and provides tools for examining cellular responses to OS in a variety of contexts. We examined changes in tissue gene expression in response to experimental elevation of plasma cortisol. Responses included downregulation of Negative Regulator of Reactive Oxygen Species (NRROS) in muscle, a regulator that limits reactive oxygen species production by tissues. These tools provide novel views of the cellular and systemic mechanisms that enable seals to tolerate high levels of OS.
    Full-text · Article · Feb 2016
    • "In phocids, cortisol has been suggested to play a role during molting (Riviere et al., 1977; Ashwell-Erickson et al., 1986; Boily, 1996), lactation (Engelhard et al., 2002), fasting (Ortiz et al., 2001aOrtiz et al., , 2001bOrtiz et al., , 2003a Rosen and Kumagai, 2008) and potentially during diving (Zapol et al., 1979 ). While aldosterone secretion is influenced by the HPA axis during a stress response, it has been thought to be predominately under regulation by the renin– angiotensin system in pinnipeds (Malvin et al., 1975; Ortiz et al., , 2003b Ortiz et al., , 2006 Houser et al., 2001 ) with secretion stimulated by angiotensin II in order to maintain water and electrolyte balance. Previous studies have demonstrated an increase in cortisol and aldosterone following a natural (chase and capture) and simulated stressor in phocids (Gulland et al., 1999; Engelhard et al., 2002). "
    [Show abstract] [Hide abstract] ABSTRACT: There is increasing interest in measuring endocrine and immune parameters in free-ranging seals and sea lions, but there lacks an understanding of how an acute stress response, often associated with capture and handling , influences these parameters of interest. The main objective of this study was to assess the impact of a simulated stressor on both endocrine and immune parameters. During two seasons, exogenous adrenocorticotrophic hormone (ACTH) was administered to seven female juvenile harbor seals and the response of several hormones (cortisol, aldosterone, total and free thyroxine and total triiodothyronine) and immunological parameters (total and differential leukocyte counts and peripheral blood mononuclear cells (PBMC) proliferation) were assessed. Cortisol peaked at 165min (winter 203.1±84.7ng/ml; summer 205.3±65.7ng/ml) and remained significantly elevated 240min after ACTH infusion in both seasons. Aldosterone peaked at 90min (winter 359.3±249.3pg/ml; summer 294.1±83.7pg/ml) and remained elevated 240min after administration of ACTH in both seasons. An increase in circulating total white blood cells driven primarily by the increase in neutrophils which occurred simultaneously with a decrease in lymphocytes leading to an overall increase in neutrophil to lymphocyte ratio. These findings demonstrate that a simulated stress response in juvenile harbor seals results in a predictable increase in both cortisol and aldosterone concentrations, and were associated with altered immunological parameters. Copyright © 2015. Published by Elsevier Inc.
    Full-text · Article · Jun 2015
    • "The difference may reflect the high nutrient requirements of lactation in females, where nutrient delivery to the mammary gland may be prioritized over other processes. In addition to XO activity, it has been shown that fasting is associated with strong up-regulation of the renin–angiotensin system (RAS) (Ortiz et al. 2006) with resultant angiotensin-II mediated up-regulation of NADPH oxidase 4 (Nox4) (V azquez-Medina et al. 2010V azquez-Medina et al. , 2013). This together with XO may contribute to increased oxidant production over the breeding fast. "
    [Show abstract] [Hide abstract] ABSTRACT: 1.The trade-off between current reproductive effort and survival is a key concept of life history theory. A variety of studies support the existence of this trade-off but the underlying physiological mechanisms are not well-understood. Oxidative stress has been proposed as a potential mechanism underlying the observed inverse relationship between reproductive investment and lifespan. Prolonged fasting is associated with oxidative stress including increases in the production of reactive oxygen species, oxidative damage and inflammation.2.Northern elephant seals (NES) undergo prolonged fasts while maintaining high metabolic rates during breeding. We investigated NES of both sexes to assess oxidative stress associated with extended breeding fasts. We measured changes in the plasma activity or concentrations of markers for oxidative stress in 30 adult male and 33 adult female northern elephant seals across their 1-3 month breeding fasts. Markers assessed included a pro-oxidant enzyme, several antioxidant enzymes, markers for oxidative damage to lipids, proteins and DNA, and markers for systemic inflammation.3.Plasma xanthine oxidase (XO), a pro-oxidant enzyme that increases production of oxidative radicals, and several protective antioxidant enzymes increased over breeding in both sexes. Males showed increased oxidative damage to lipids and DNA and increased systemic inflammation, while oxidative damage to proteins declined across breeding. In contrast, females showed no oxidative damage to lipids or DNA or changes in inflammation, but showed increases in oxidative damage to proteins. XO activity, antioxidant enzymes, oxidative damage markers, and inflammatory markers were strongly correlated in males but these relationships were weaker or non-existent in females.4.NES provide evidence for oxidative stress as a physiological cost of reproduction in a capital breeding mammal. Both sexes strongly up-regulated antioxidant defenses during breeding. Despite this response, and in contrast to similar duration non-breeding fasts in previous studies on conspecifics, there was evidence of oxidative damage to tissues. These data demonstrate the utility of using plasma markers to examine oxidative stress but also suggest the necessity of measuring a broad suite of plasma markers to assess systemic oxidative stress.This article is protected by copyright. All rights reserved.
    Full-text · Article · Sep 2014
Show more

We use cookies to give you the best possible experience on ResearchGate. Read our cookies policy to learn more.