Postweaning Fast Research Articles

Gluconeogenesis is associated with high rates of tricarboxylic acid and pyruvate cycling in fasting northern elephant seals

Animals that endure prolonged periods of food deprivation preserve vital organ function by sparing protein from catabolism. Much of this protein sparing is achieved by reducing metabolic rate and suppressing gluconeogenesis while fasting. Northern elephant seals (Mirounga angustirostris) endure prolonged fasts of up to 3 mo at multiple life stages. During these fasts, elephant seals maintain high levels of activity and energy expenditure associated with breeding, reproduction, lactation, and development while maintaining rates of glucose production typical of a postabsorptive mammal. Therefore, we investigated how fasting elephant seals meet the requirements of glucose-dependent tissues while suppressing protein catabolism by measuring the contribution of glycogenolysis, glycerol, and phosphoenolpyruvate (PEP) to endogenous glucose production (EGP) during their natural 2-mo postweaning fast. Additionally, pathway flux rates associated with the tricarboxylic acid (TCA) cycle were measured specifically, flux through phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate cycling. The rate of glucose production decreased during the fast (F(1,13) = 5.7, P = 0.04) but remained similar to that of postabsorptive mammals. The fractional contributions of glycogen, glycerol, and PEP did not change with fasting; PEP was the primary gluconeogenic precursor and accounted for ∼95% of EGP. This large contribution of PEP to glucose production occurred without substantial protein loss. Fluxes through the TCA cycle, PEPCK, and pyruvate cycling were higher than reported in other species and were the most energetically costly component of hepatic carbohydrate metabolism. The active pyruvate recycling fluxes detected in elephant seals may serve to rectify gluconeogeneic PEP production during restricted anaplerotic inflow in these fasting-adapted animals.

Prolonged fasting increases purine recycling in post-weaned northern elephant seals

Northern elephant seals are naturally adapted to prolonged periods (1-2 months) of absolute food and water deprivation (fasting). In terrestrial mammals, food deprivation stimulates ATP degradation and decreases ATP synthesis, resulting in the accumulation of purines (ATP degradation byproducts). Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) salvages ATP by recycling the purine degradation products derived from xanthine oxidase (XO) metabolism, which also promotes oxidant production. The contributions of HGPRT to purine recycling during prolonged food deprivation in marine mammals are not well defined. In the present study we cloned and characterized the complete and partial cDNA sequences that encode for HGPRT and xanthine oxidoreductase (XOR) in northern elephant seals. We also measured XO protein expression and circulating activity, along with xanthine and hypoxanthine plasma content in fasting northern elephant seal pups. Blood, adipose and muscle tissue samples were collected from animals after 1, 3, 5 and 7 weeks of their natural post-weaning fast. The complete HGPRT and partial XOR cDNA sequences are 771 and 345 bp long and encode proteins of 218 and 115 amino acids, respectively, with conserved domains important for their function and regulation. XOR mRNA and XO protein expression increased 3-fold and 1.7-fold with fasting, respectively, whereas HGPRT mRNA (4-fold) and protein (2-fold) expression increased after 7 weeks in adipose tissue and muscle. Plasma xanthine (3-fold) and hypoxanthine (2.5-fold) levels, and XO (1.7- to 20-fold) and HGPRT (1.5- to 1.7-fold) activities increased during the last 2 weeks of fasting. Results suggest that prolonged fasting in elephant seal pups is associated with increased capacity to recycle purines, which may contribute to ameliorating oxidant production and enhancing the supply of ATP, both of which would be beneficial during prolonged food deprivation and appear to be adaptive in this species.

Prolonged fasting increases purine recycling in postweaned northern elephant seals

Northern elephant seals are naturally adapted to prolonged periods (1–2 months) of absolute food and water deprivation (fasting). In terrestrial mammals, fasting stimulates ATP degradation and decreases ATP synthesis. Hypoxanthine‐guanine phosphoribosyl transferase (HGPRT) salvages ATP by recycling the purine degradation products derived from xanthine oxidase (XO) metabolism. The contributions of HGPRT to purine recycling during prolonged fasting in marine mammals are not well defined. In the present study, complete and partial HGPRT and XO cDNA sequences were obtained. Also, HGPRT and XO mRNA and protein expression was quantified in tissue from fasting elephant seals, along with the plasma levels of hypoxanthine (HX) and xanthine and activities of HGPRT and XO. Blood and tissue samples were collected from animals during the 1, 3, 5 and 7 week of their natural post‐weaning fast. The HGPRT and XO cDNA sequences encode proteins with conserved domains important for their function and regulation. HGPRT and XO mRNA and protein expression increased during the 7 wk in adipose tissue and muscle, respectively. Plasma xanthine and HX levels, and XO and HGPRT activities increased during the last two weeks of fasting. Results suggest that prolonged fasting in elephant seal pups is associated with increased capacity to recycle purines, which may contribute to enhancing the supply of ATP.

Geography and stage of development affect persistent organic pollutants in stranded and wild-caught harbor seal pups from central California

Persistent organic pollutants have been associated with disease susceptibility and decreased immunity in marine mammals. Concentrations of polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane and its metabolites (DDTs), polybrominated diphenyl ethers (PBDEs), chlordanes (CHLDs), and hexachlorocyclohexane isomers (HCHs) were evaluated in terms of stage of development and likely exposure routes (in utero, suckling, fasting) in the blubber of 202 stranded and wild-caught, primarily young of the year (n=177), harbor seals (Phoca vitulina) in the central California coast. This is the first report of HCH concentrations in the blubber of California seals. Lipid normalized concentrations ranged from 200 to 330,000ng/g for sum PCBs, 320–1,500,000ng/g for sum DDTs, 23–63,000ng/g for sum PBDEs, 29–29,000ng/g for sum CHLDs, and 2–780ng/g for sum HCHs. The highest concentrations were observed in harbor seal pups that suckled in the wild and then lost mass during the post-weaning fast. Among the pups sampled in the wild and those released from rehabilitation, there were no differences in mass, blubber depth, or percent lipid although contaminant concentrations were significantly higher in the pups which nursed in the wild. When geographic differences were evaluated in a subset of newborn animals collected near their birth locations, the ratio of sum DDTs to sum PCBs was significantly greater in samples from an area with agricultural inputs (Monterey), than one with industrial inputs (San Francisco Bay). A principal components analysis distinguished between seals from San Francisco Bay and Monterey Bay based on specific PCB and PBDE congeners and DDT metabolites. These data illustrate the important influence of life stage, nutritional status, and location on blubber contaminant levels, and thus the need to consider these factors when interpreting single sample measurements in marine mammals.

Prolonged fasting increases glutathione biosynthesis in postweaned northern elephant seals

Northern elephant seals experience prolonged periods of absolute food and water deprivation (fasting) while breeding, molting or weaning. The postweaning fast in elephant seals is characterized by increases in the renin-angiotensin system, expression of the oxidant-producing protein Nox4, and NADPH oxidase activity; however, these increases are not correlated with increased oxidative damage or inflammation. Glutathione (GSH) is a potent reductant and a cofactor for glutathione peroxidases (GPx), glutathione-S transferases (GST) and 1-cys peroxiredoxin (PrxVI) and thus contributes to the removal of hydroperoxides, preventing oxidative damage. The effects of prolonged food deprivation on the GSH system are not well described in mammals. To test our hypothesis that GSH biosynthesis increases with fasting in postweaned elephant seals, we measured circulating and muscle GSH content at the early and late phases of the postweaning fast in elephant seals along with the activity/protein content of glutamate-cysteine ligase [GCL; catalytic (GCLc) and modulatory (GCLm) subunits], γ-glutamyl transpeptidase (GGT), glutathione disulphide reductase (GR), glucose-6-phosphate dehydrogenase (G6PDH), GST and PrxVI, as well as plasma changes in γ-glutamyl amino acids, glutamate and glutamine. GSH increased two- to four-fold with fasting along with a 40-50% increase in the content of GCLm and GCLc, a 75% increase in GGT activity, a two- to 2.5-fold increase in GR, G6PDH and GST activities and a 30% increase in PrxVI content. Plasma γ-glutamyl glutamine, γ-glutamyl isoleucine and γ-glutamyl methionine also increased with fasting whereas glutamate and glutamine decreased. Results indicate that GSH biosynthesis increases with fasting and that GSH contributes to counteracting hydroperoxide production, preventing oxidative damage in fasting seals.

Prolonged fasting suppresses cellular insulin‐dependent activity in adipose tissue of the northern elephant seal

Throughout its life, the northern elephant seal (Mirounga angustirostris) experiences periods of prolonged fasting during which plasma glucose reaches hyperglycemic levels. As the fast progresses, the levels of circulating glucose decrease, despite relatively low or decreasing plasma insulin levels. Cellular glucose uptake is dependent on an insulin‐activated signaling cascade resulting in the translocation of the glucose transport protein, Glut4, into the plasma membrane. In order to understand how this pathway is affected by prolonged fasting PI3K, AKT and VAMP2 content, and plasma insulin were examined. Blubber biopsies as well as plasma samples were obtained from two groups of northern elephant seal pups (n = 10/group) sampled during their post weaning fast: early (2 weeks) and late (6 to 8 weeks). Tissue content of PI3K, AKT, and VAMP2 proteins was quantified by western blot. Plasma insulin did not change (1.5 ± 0.4 vs. 1.2 ± 0.2 uU/mL) between early and late fasting. While PI3K and VAMP2 also did not change, AKT decreased 29% with the fast. We have previously shown that fasting is associated with a decrease in insulin receptor (IR), but an increase in Glut 4. Because insulin stimulates PI3K and AKT, the decreases in insulin and IR may explain the lack of change in PI3K and the decrease in AKT suggesting that fasting suppresses cellular insulin‐dependent activity. Funded by NIH HL91767 and UC Startup funds.

Glut4 Increases with Prolonged Fasting in Northern Elephant Seals

The northern elephant seal (Mirounga angustirostris) experiences periods of prolonged fasting (2‐3 months) as part of its natural life history during which plasma glucose concentrations decrease despite the maintenance of relatively low plasma insulin. Cellular glucose metabolism is regulated by the insulin‐dependent glucose transport protein, Glut 4. Mechanisms of cellular glucose metabolism regulation have not been examined in fasting seals. Therefore, the effects of prolonged fasting on Glut4 and insulin receptor protein content, and plasma glucose were examined. Muscle and blubber biopsies as well as plasma samples were obtained from two groups of northern elephant seal pups (n = 10/group) sampled during their post weaning fasting period: early (2 weeks) and late (6 to 8 weeks). Tissue samples were analyzed by Western blot for Glut4 and insulin receptor protein content. Circulating glucose levels decreased during the fast (9.8 ± 0.5 vs 8.0 ± 0.3 mM). In contrast, mean Glut4 levels increased in both muscle (31%) and blubber (26%) during the fast. Mean blubber insulin receptor content decreased 14% with the fast suggesting that the increase in Glut4 protein content is not insulin‐dependent during the fast in seals, which is consistent with the reduced circulating levels of insulin that we have reported for this species. Furthermore, the increase in Glut4 protein content may represent increased tissue utilization of glucose, which may partially explain the decrease in circulating levels. Funded by UC startup funds.

Multivitamin supplementation of Wistar rats during pregnancy accelerates the development of obesity in offspring fed an obesogenic diet

The effect of gestational multivitamin supplementation on the development of obesity in rat offspring fed an obesogenic diet was investigated. Pregnant Wistar rats (n=10 per group) were fed the AIN-93G diet with the recommended vitamin (RV) content or a 10-fold increase (high vitamin, HV). At weaning, 10 males and 10 females, from separate dams, and from each gestational diet group were weaned to the liquid obesogenic diet for 48 weeks post-weaning. Body weight (BW) was measured weekly, and food intake over 24 h was measured once every 3 weeks for 24 weeks. Every 4 weeks, after an overnight fast, food intake over 1 h was measured 30 min after a gavage of water or glucose. An oral glucose tolerance test (OGTT) was carried out every 3-5 weeks. Post-weaning fasting glucose, insulin, ghrelin, glucagon-like peptide 1 (GLP-1), and systolic blood pressure (SBP) were measured. No difference in BW at birth or litter size was observed. Males and females from HV dams gained 17% (P<0.05) and 37% (P<0.001) more BW at 48 weeks post-weaning, and consumed 18% (P=0.07) and 20% (P<0.05) more food. One-hour food intake after water and glucose preloads was 27% (P<0.01) and 34% (P<0.05) higher in males from HV dams. Fasting ghrelin and GLP-1 were 27 and 32% higher in males from HV dams at weaning (P<0.05). Blood glucose response to the OGTT was greater in both males and females from HV dams at 13 weeks post-weaning (P<0.05), and the insulin resistance index was 76 and 43% higher in females from HV dams at 14 and 28 weeks post-weaning (P<0.05). SBP was 23 and 16% higher at 44 weeks post-weaning in male and females (P<0.01). High multivitamin intake during pregnancy increases the phenotypic expression of obesity and components of the metabolic syndrome in both female and male rats fed an obesogenic diet.

Morphological Features and Blood Parameters of Weddell Seal (Leptonychotes weddellii) Mothers and Pups during the Breeding Season

We investigated the blood status of 9 Weddell seal mothers and 9 pups during the breeding season in a field study conducted from November to December 2004 at a breeding colony in Antarctica. The blood glucose and total cholesterol concentrations were higher in the pups than in the mothers. On the other hand, the blood urea nitrogen concentration was lower in the pups than in the mothers. Growth-associated depletion of blood triglyceride was observed in the pups and may have been due to the post-weaning fast. The results characterize the blood status of Weddell seals in relation to physiological adaptations for breeding.

Body Condition at Weaning Affects the Duration of the Postweaning Fast in Gray Seal Pups (Halichoerus grypus)

Gray seals (Halichoerus grypus) undergo a terrestrial postweaning fast (PWF) that depletes energy reserves acquired during the suckling interval. Plasticity in PWF duration may ensure that pups of variable body condition depart for sea with adequate energy reserves. To test this hypothesis, we examined body condition of 30 gray seal pups at weaning and monitored their PWF duration. On average, fat accounted for 47.3% +/- 0.7% of their 53.2 +/- 1.3-kg weaning mass. Although fasting duration averaged 21 +/- 1.1 d (n = p28), there was considerable variation in fasting duration (9 to > 31 d) and the resulting age when pups departed to sea (26 to > 49 d). Percent fat at weaning(38.6%-54.6%) was positively correlated with fasting duration(n = 28, r = 0.376, P = 0.0489). In contrast, total body gross energy (735.3-1,447.4 MJ) and body mass (39.0-66.0 kg) were not correlated with fasting duration. Thus, body composition,not overall body reserves, predicted fasting duration, but the effect was weak, indicating that other factors also account for the observed variation in fasting duration. We speculate that pups with greater percent fat more effectively utilized lipid and conserved protein while meeting metabolic costs throughout the PWF. As a result, fatter pups extended the PWF duration,which may be critical for development of diving physiology and may have facilitated their survivorship to age 1.

Effects of mass and body composition on fasting fuel utilisation in grey seal pups (Halichoerus grypusFabricius): an experimental study using supplementary feeding

This study used supplementary feeding to test the hypothesis that fuel partitioning during the postweaning fast in grey seal pups is affected by size and composition of energy reserves at weaning, and by extra provisioning. Mass and body composition changes were measured during suckling and fasting to investigate the effect of natural differences in energy reserves at weaning on subsequent allocation of fat and protein to energy use. We fed seven pups for 5 days after weaning, to investigate the effect of increased fuel availability, and particularly protein, on fuel utilisation. After correcting for protein used during the moult, the proportional contribution of fat was 86-99% of total energy use. Pups with greater energy reserves, i.e. those that were heavier and fatter at weaning, had higher rates of fat and energy use. There was no significant relationship between adiposity at weaning and proportional contribution of fat to energy use, perhaps due to a limited sample size or range of body masses and adiposity. Supplemented individuals used energy, specifically fat, much faster and utilised proportionally less of their endogenous protein by departure than non-supplemented individuals. Fat metabolism contributed a similar percentage to daily energy use in both groups. These findings show that pups spare protein, even when energy use is dramatically increased. Pups that receive greater maternal provisioning and lay down more protein may have increased survival chances at sea. This study highlights the importance of protein reserves in first year survival of grey seal pups.

Size and distribution of oxygen stores in harp and hooded seals from birth to maturity

Pinnipeds rely primarily on oxygen stores in blood and muscles to support aerobic diving; therefore rapid development of body oxygen stores (TBO(2)) is crucial for pups to transition from nursing to independent foraging. Here, we investigate TBO(2) development in 45 harp (Pagophilus groenlandicus) and 46 hooded (Cystophora cristata) seals ranging in age from neonates to adult females. We found that hooded seal adults have the largest TBO(2) stores yet reported (89.5 ml kg(-1)), while harp seal adults have values more similar to other phocids (71.6 ml kg(-1)). In adults, large TBO(2) stores resulted from large blood volume (harp169, hood 194 ml kg(-1)) and high muscle Mb content (harp 86.0, hood 94.8 mg g(-1)). In contrast, pups of both species had significantly lower mass-specific TBO(2 )stores than adults, and stores declined rather than increased during the nursing period. This decline was due to a reduction in mass-specific blood volume and the absence of an increase in the low Mb levels (harp 21.0, hood 31.5 mg g(-1)). Comparisons with other phocid species suggests that the pattern of blood and muscle development in the pre- and post-natal periods varies with terrestrial period, and that muscle maturation rates may influence the length of the postweaning fast. However, final maturation of TBO(2) stores does not take place until after foraging begins.

Shifts in specific serum protein concentrations: apolipoprotein-B and albumin in the southern elephant seal during the breeding and molting periods in Antarctica

Serum protein values in Southern elephant seals (Mirounga leonina) were analyzed during the breeding and molting periods at the 25 de Mayo Island (King George Island), Antarctica, during the 1999/2000 and 2000/2001 field seasons, in order to study the reaction of adults to fasting, and of pups to nursing and fasting. The following analyses were carried out: Total Proteins (TP) and Albumin (Alb) were analyzed by a colorimetric technique, and Apolipoprotein B (Apo-B) was determined by the immunodiffusion technique on agarose gel plates. The general ranges of average values for total proteins and specific fractions during the breeding period were: TP (g/dl) = 5.12–9.83; Alb (g/dl) = 1.72–5.71 and Apo-B (mg/dl) = 10–266, and during the post-breeding period: TP = 4.85–9.45; Alb = 2.06–4.20 and Apo-B = 13–232. The essential obtained data were: (a) fasting does not impact adult males, except for a significant decrease of Apo-B during breeding; (b) fasting does not impact adult females, except for a significant decrease in TP during molting; (c) suckling increases significantly TP and Apo-B in pups; (d) post-weaning fast decreases significantly all measured serum components in pups. We can conclude that the adults are adapted to long term fasting, without any metabolic downside they maintain homeostasis during this period, as shown by the serum data. The pups clearly react to both suckling and the post-weaning fast, which are periods driven almost exclusively by lipid chemistry. These impacts may be seen in the chemistry of serum proteins.

Mobilization of PCBs from blubber to blood in northern elephant seals ( Mirounga angustirostris) during the post-weaning fast

Northern elephant seals ( Mirounga angustirostris) are characterized by extended fasting during which they rely entirely on their own body reserves. During fasts, lipids are mobilized from blubber to match the energy requirements of the animal. This transfer frees toxic fat-soluble pollutants into the blood circulation, which may exert adverse health effects, especially in young and developing animals. We investigated the dynamics of mobilization of polychlorinated biphenyls (PCBs) from the blubber of northern elephant seal pups during the post-weaning fast. Longitudinal samples of blubber and serum were collected from free-ranging animals throughout the fast at Año Nuevo, California. Blubber biopsies were separated into inner and outer layers. The PCB profiles of blubber and serum consisted mainly of penta- (PCB-101, -110, -118), hexa- (PCB-138, -153) and hepta- (PCB-180, -183, -187) chlorobiphenyls, which accounted for almost 90% of the total PCB burden. Total PCB concentrations in inner blubber increased significantly between early and late fasting (563.6 ± 162.0 μg/kg lipids at early versus 911.6 ± 513.1 μg/kg lipids at late fasting) whereas they remained fairly constant in outer blubber (572.6 ± 134.8 μg/kg lipids at early versus 659.2 ± 158.8 μg/kg lipids at late fasting). A corresponding rise of PCB concentrations was observed in serum during the second half of the fast (3.8 ± 1.1 μg/l serum at early versus 7.2 ± 0.9 μg/l at late fasting). The longitudinal changes in circulating total PCBs could not be explained by the changes in serum lipid fractions (cholesterol, phospholipids, triacylglycerols and free fatty acids). The increases in total PCB concentrations in inner blubber and serum were more pronounced in leaner animals, which suggests that they might be more at risk to potential toxic effects.

Development of the Blood and Muscle Oxygen Stores in Gray Seals (Halichoerus grypus): Implications for Juvenile Diving Capacity and the Necessity of a Terrestrial Postweaning Fast

To successfully transition from nursing to foraging, phocid seal pups must develop adequate diving physiology within the limited time between birth and their first independent foraging trip to sea. We studied the postpartum development of oxygen stores in gray seals (Halichoerus grypus, n=40) to better understand the ontogeny of diving capacity in phocids. Hemoglobin (Hb), hematocrit (Hct), blood volume (BV), and myoglobin (Mb) levels in newborn (3 d postpartum [DPP]) and newly weaned (17+/-0.4 DPP) pups were among the lowest measured across age classes. During the pups' terrestrial postweaning fast (PWF), Hb, Hct, mass-specific BV, and Mb increased by 28%, 21%, 13%, and 29%, respectively, resulting in a 35% increase in total body mass-specific oxygen stores and a 23% increase in calculated aerobic dive limit (CADL). Although Hb and Hct levels at the end of the PWF were nearly identical to those of yearlings, total body mass-specific oxygen stores and CADL of weaned pups departing for sea were only 66%-67% and 32%-62%, respectively, of those for yearlings and adult females. The PWF represents an integral component of the physiological development of diving capacity in phocids; however, newly independent phocids still appear to have limited diving capabilities at the onset of foraging.

Glucose production and substrate cycle activity in a fasting adapted animal, the northern elephant seal

During prolonged fasting physiological mechanisms defend lean tissue from catabolism. In the fasting state, glucose is derived solely from gluconeogenesis, requiring some catabolism of amino acids for gluconeogenic substrates. This creates a conflict in animals undergoing fasts concurrently with metabolically challenging activities. This study investigated glucose metabolism in fasting and developing neonatal elephant seals. Glucose production and glucose cycle activity were measured early (2 weeks) and late (6 weeks) in the postweaning fasting period. Additionally the role of regulatory hormones on glucose production and glucose cycle activity were investigated. Glucose cycle activity was highly variable throughout the study period, did not change over the fasting period, and was not correlated with insulin or glucagon level. Endogenous glucose production (EGP) was 2.80+/-0.65 mg kg(-1) min(-1) early and 2.21+/-0.12 during late fasting. Insulin to glucagon molar ratio decreased while cortisol levels increased over the fast (t=5.27, 2.84; P=0.003, 0.04; respectively). There was no relationship between EGP and hormone levels. The glucose production values measured in this study were high and exceeded the estimated gluconeogenic substrate available. These data suggest extensive glucose recycling via Cori cycle activity occurring in northern elephant seals, and we propose a possible justification for this recycling.

Estradiol and Testosterone Concentrations Increase with Fasting in Weaned Pups of the Northern Elephant Seal (Mirounga angustirostris)

Although neonatal development is generally associated with increased levels of circulating testosterone (T) and estradiol (E2), food deprivation may inhibit steroidogenesis. Therefore, these potentially conflicting stimuli were examined in fasting weaned northern elephant seal (Mirounga angustirostris) pups by measuring serum concentrations of T, E2, progesterone (P4), and luteinizing hormone (LH) by either radioimmunoassay (P4, LH) or enzymeimmunoassay (T, E2). Blood samples were obtained from 20 male and 20 female pups at both early (<1 wk postweaning) and late (6-8 wk postweaning) periods during their natural postweaning fast. T in males (early: 2.9 +/- 0.4 ng/mL; late: 16 +/- 2 ng/mL; P < 0.0001) and E2 in females (early: 42 +/- 6 pg/mL; late: 67 +/- 5 pg/mL; P < 0.01) increased between the two measurement periods, while P4 (early: 2.5 +/- 0.3 ng/mL; late: 2.1 +/- 0.3 ng/mL; P > 0.05) did not. LH increased (early: 46 +/- 4 pg/mL; late: 65 +/- 6 pg/mL; P < 0.05) in males but not in females (early: 69 +/- 9 pg/mL; late: 65 +/- 6 pg/mL; P > 0.05). Increases in LH and T suggest that LH may stimulate T secretion. Alternatively, relatively low concentrations of LH in both males and females may reflect negative feedback inhibition imposed by elevated T and E2 concentrations. Despite the inherent postweaning fast, concentrations of T and E2 increased, suggesting that they may be critical for the continued development of pups. Therefore, compensatory mechanisms may exist that alleviate the fasting-induced inhibition of gonadal steroidogenesis during neonatal development in elephant seal pups.

Age, Sex, and Reproductive State Influence Free Amino Acid Concentrations in the Fasting Elephant Seal

Long-term fasting is a component of northern elephant seal (Mirounga angustirostris) life history requiring physiological adaptations to nitrogen conservation. Plasma free amino acids (FAAs) were determined for five elephant seal pups during the second and eighth weeks of the postweaning fast, six lactating female seals at 4-6 and 25 d postpartum, and seven sexually competitive adult male seals taken midway through the breeding season. Total FAAs declined in lactating females (11%) and pups (30%) with time fasting, but cystine concentration more than doubled in pups while decreasing by approximately 43% in lactating females. Methionine concentration significantly increased (approximately 68%) across lactation in adult females but was low for all classes of seal. Alanine was the most abundant FAA in adult males, and glycine became the dominant FAA in adult females late in lactation. Glutamine dominated the FAAs of weaned pups across the fast. Reductions in total FAAs of weanlings mirrored reductions in protein catabolism, but reductions in total FAAs also occurred in lactating females concomitant with an increase in protein catabolism. Observed variation in FAA concentrations may reflect ontogenetic requirements for certain amino acids in fasting weanlings. Similarly, increases in specific FAA concentrations across lactation may reflect variations in FAA flux resulting from the nutrient demands of lactogenesis.

Energy reserve allocation in fasting Northern Elephant Seal Pups: inter‐relationships between body condition and fasting duration

1 Organisms are forced to make trade-offs when allocating energy reserves during sustained periods of fasting. 2 For most animals, lipid catabolism is the preferred source of energy to safeguard important protein sources. However, marine mammals also have a compounding pressure to conserve some lipid stores. The main site of lipid storage in these animals is the blubber layer, which is not only an important energy source during fasting, but is also the primary thermal barrier when at sea. 3 To explain how the allocation of protein and lipid reserves during fasting are influenced by body condition (body mass and percentage lipid of total body mass), a dynamic state variable model that takes into account fitness consequences of different allocation strategies was developed. 4 This model was parameterized with respect to conditions faced by weaned Northern Elephant Seal (Mirounga angustirostris Gill) pups. It incorporates the independent effects of body mass and protein mass (the two state variables) on survival and the costs of utilizing either lipid or protein during the postweaning fast. 5 Predictions of lipid and protein allocation by the model were not significantly different from measurements on wild seals. Finally, the model showed that body lipid content and fasting duration both influence allocation of energy reserves.

GH and ghrelin increase with fasting in a naturally adapted species, the northern elephant seal (Mirounga angustirostris).

After nursing, pups of the northern elephant seal (Mirounga angustirostris) are approximately 46% body fat and rely almost entirely on the oxidation of their large fat stores to sustain their metabolism for the ensuing 8-12 week postweaning fast, which is a natural component of their life history. Thus, fasting pups provide an ideal opportunity to examine the hormonal alterations associated with prolonged food deprivation in a naturally adapted model. Cortisol, ghrelin, glucagon, growth hormone (GH), insulin-like growth factor-I (IGF-I), insulin, blood urea nitrogen (BUN), glucose and non-esterified fatty acids (NEFA) were examined in 20 male and 20 female pups blood sampled early (<1 week postweaning) and late (6-8 weeks postweaning) during the fast. Mean cortisol, ghrelin, GH, and glucagon increased 1.8-, 1.8-, 1.4-, and 2.3-fold between early and late periods, while mean IGF-I and insulin decreased 97% and 38%, respectively. NEFA increased 2.3-fold, while BUN and glucose decreased 46% and 11%, respectively. NEFA was significantly and positively correlated with cortisol and GH; individually; however, when the relationship was examined as a multiple regression the correlation improved suggesting that cortisol and GH act synergistically to promote lipolysis during the fast. GH and BUN were negatively and significantly correlated between early and late fasting suggesting that GH may promote protein sparing as well. The decrease in glucose may be responsible for stimulating glucagon, resulting in the maintenance of relative hyperglycemia. The increases in cortisol, ghrelin, glucagon, and GH suggest that these hormones may be integral in mediating the metabolism of seal pups during prolonged fasting.