Plasma leptin and thyroxine of mink (Mustela vison) vary with gender, diet and subchronic exposure to PCBs

Dam characteristics associated with pre-weaning diarrhea in mink (Neovison vison)

BACKGROUND

Pre-weaning diarrhea (PWD) in mink, also known as "sticky kits", is a frequently occurring syndrome in suckling mink kits on commercial mink farms. Outbreaks of PWD result in weakened kits, increased mortality and reduced growth and welfare as well as considerable economic losses for the farmers. The syndrome is regarded as multifactorial with a complex etiology, and studies have focused on associations with environment, management and dam characteristics. The present study was conducted from May to June 2015 and included 70 dams with mink litters with and without PWD. The aims were to examine associations between PWD and mastitis (bacterial infection and histological signs of inflammation or other lesions in the mammary gland), and to examine associations between PWD and other dam-related characteristics (age, litter size, body mass index, and weight and number of active mammary glands of the dam).

RESULTS

Using multivariable mixed logistic regression analyses with farm id as a random intercept, we found that the odds for PWD in the litter were significantly higher in 1 year old dams versus > 1 year old (OR = 13.3, CI 2.0-90.2, P = 0.01), higher if litter size observed after birth was > 5 kits versus ≤ 5 kits (OR = 16.5, CI 2.2-123.7, P = 0.01), higher if the number of active mammary glands per kit was ≤ 1.5 versus > 1.5 glands per kit (OR = 6.5, CI 1.2-36.0), P = 0.03), and higher in farms with high prevalence of PWD versus low prevalence (OR = 16.8, CI 2.9-97.6, P = 0.002). There were no significant associations between PWD and bacterial infection, histological signs of inflammation or other lesions of the mammary gland, body mass index or weight of mammary gland per kit.

CONCLUSION

Pre-weaning diarrhea had a statistically significant association with age of the dam, litter size and the number of active mammary glands per kit. However, PWD was not associated with mastitis, body mass index and weight of mammary gland tissue per kit.

Endocrinologic Adaptations to Wintertime Fasting in the Male American Mink (Mustela vison)

The aim of this study was to investigate the endocrine response to wintertime starvation in the male American mink (Mustela vison) fasted for 16 hrs, 2 days, 3 days, 5 days, or 7 days (n–10 per group). After 2 days of fasting, the plasma leptin concentrations decreased, along with the triiodothyronine, testosterone, and progesterone levels, and the blood monocyte counts. Leptin also seems to trigger the response to fasting in mustelids by inducing immunosuppression and downregulation of the reproductive and thyroid axes. The dramatic increase in the peptide YY concentrations after 3 days of fasting may be required to suppress gastrointestinal processes during food scarcity. The plasma insulin levels decreased, and those of glucagon increased after 5 days of fasting in association with efficient glucose sparing and lipid mobilization. Body energy stores cannot be wasted for growth during nutritional scarcity and, thus, the growth hormone levels of the minks decreased after 5 days of fasting. The plasma noradrenaline and cortisol concentrations also decreased after 3 and 7 days without food, respectively. The plasma ghrelin, adiponectin, resistin, thyroxine, adrenaline, or estradiol levels did not respond to fasting. The endocrine response to food deprivation is remarkably similar in divergent mammalian orders, indicating that the hormonal signals enhancing survival during nutritional scarcity must be evolutionarily old and well conserved.

Energy stores, lipid mobilization and leptin endocrinology of rainbow trout

The physiological role of leptin in fish is not fully elucidated. In the present study, the involvement of the leptin system in lipid deposition and mobilization in rainbow trout during feeding and 1, 2 and 4 weeks of fasting was investigated in two lines of rainbow trout with different muscle and visceral adiposity: a fat line (FL) with high total energy reserves, high muscle adiposity, but low visceral adiposity and a lean line (LL) with lower total energy reserves and lower muscle adiposity, but higher visceral adiposity. During 4 weeks of fasting, muscle lipids decreased by 63 % in the FL fish, while no such energy mobilization from muscle occurred in the LL fish. On the other hand, lipid stores in liver and visceral adipose tissue was utilized to a similar extent by the two fish lines during fasting. Under normal feeding conditions, plasma leptin levels were higher in the LL than the FL fish, suggesting a possible contribution of visceral adipocytes to plasma leptin levels. Plasma leptin-binding protein levels did not differ between the lines and were not affected by fasting. After 4 weeks of fasting, the long leptin receptor and the leptin-binding protein isoforms 1 and 3 muscle expression increased in the LL fish, as well as hepatic expression of leptin A1 and the two binding protein isoforms. These responses were not seen in the FL fish. The data suggest that the Lep system in rainbow trout is involved in regulation of energy stores and their mobilization.

Elevated plasma leptin levels of fasted rainbow trout decrease rapidly in response to feed intake

Leptin has an anorexigenic effect in fish, indicating a role in regulation of growth and energy homeostasis. The study aimed to further clarify the physiological role of leptin in rainbow trout, specifically its short-term response to feed intake after a period of fasting. Utilizing a salmonid leptin radioimmunoassay, the study demonstrates differences in plasma leptin levels in fishes with different nutritional status and at the onset of feeding. Some of the fasted fish were clearly in a state of anorexia, and did not initiate feeding during the 72h refeeding period. For those fish that did initiate feeding, both previously fed and fasted, plasma leptin levels rapidly decreased during the first 24h in correlation with increased amount of food reaching the gastrointestinal tract, while non-feeding individuals retained a high plasma leptin levels. The data indicate that the leptin-induced anorexic state is broken after onset of feeding and that the regulatory mechanisms leading to decreased plasma leptin levels are linked to nutrient levels.

Effects of nutritional status on plasma leptin levels and in vitro regulation of adipocyte leptin expression and secretion in rainbow trout

As leptin has a key role on appetite, knowledge about leptin regulation is important in order to understand the control of energy balance. We aimed to explore the modulatory effects of adiposity on plasma leptin levels in vivo and the role of potential regulators on leptin expression and secretion in rainbow trout adipocytes in vitro. Fish were fed a regular diet twice daily ad libitum or a high-energy diet once daily at two ration levels; satiation (SA group) or restricted (RE group) to 25% of satiation, for 8weeks. RE fish had significantly reduced growth (p<0.001) and adipose tissue weight (p<0.001), and higher plasma leptin levels (p=0.022) compared with SA fish. Moreover, plasma leptin levels negatively correlated with mesenteric fat index (p=0.009). Adipocytes isolated from the different fish were treated with insulin, ghrelin, leucine, eicosapentaenoic acid or left untreated (control). In adipocytes from fish fed regular diet, insulin and ghrelin increased leptin secretion dose-dependently (p=0.002; p=0.033, respectively). Leptin secretion in control adipocytes was significantly higher in RE than in SA fish (p=0.022) in agreement with the in vivo findings, indicating that adipose tissue may contribute to the circulating leptin levels. No treatment effects were observed in adipocytes from the high-energy diet groups, neither in leptin expression nor secretion, except that leptin secretion was significantly reduced by leucine in RE fish adipocytes (p=0.025). Overall, these data show that the regulation of leptin in rainbow trout adipocytes by hormones and nutrients seems to be on secretion, rather than at the transcriptional level.

Early weaning PCB 95 exposure alters the neonatal endocrine system: thyroid adipokine dysfunction

Polychlorinated biphenyls (PCBs) are persistent environmental pollutants that can severely disrupt the endocrine system. In the present study, early-weaned male rats were administered a single dose of 2,3,6-2',5'-pentachlorinated biphenyl (PCB 95; 32 mg/kg per day, by i.p. injection) for two consecutive days (postnatal days (PNDs) 15 and 16) and killed 24 and 48 h after the administration of the last dose. Compared with the control group, administration of PCB 95 induced a reduction (P<0.01) in serum concentrations of thyroxine, triiodothyronine, and GH and an increase (P<0.01) in the serum concentration of TSH at PNDs 17 and 18. These conspicuous perturbations led to some histopathological deterioration in the thyroid gland characterized by follicular degeneration, edema, fibrosis, hemorrhage, luminal obliteration, and hypertrophy with reduced colloidal contents at PND 18. The dyshormonogenesis and thyroid dysgenesis may be attributed to the elevation of DNA fragmentation at PNDs 17 and 18. Furthermore, this hypothyroid state revealed higher (P<0.01) serum concentrations of leptin, adiponectin, and tumor necrosis factor and lower (P<0.01) serum concentrations of IGF1 and insulin at both PNDs compared with the control group. Interestingly, the body weight of the neonates in the PCB 95 group exhibited severe decreases throughout the experimental period in relation to that of the control group. These results imply that PCB 95 may act as a disruptor of the developmental hypothalamic-pituitary-thyroid axis. Hypothyroidism caused by PCB 95 may impair the adipokine axis, fat metabolism, and in general postnatal development. Thus, further studies need to be carried out to understand this concept.

Chronic dietary exposure to environmental organochlorine contaminants induces thyroid gland lesions in Arctic foxes (Vulpes lagopus)

The impact of dietary organochlorine (OC) exposure on thyroid gland pathology was studied in farmed male Arctic foxes (Vulpes lagopus). The exposed group (n=16) was fed a diet based on wild minke whale (Balaenoptera acutorostrata) blubber as a main fat source in order to mimic the exposure to OC cocktails in the Artic environment. This resulted in an exposure of approximately 17 microg Sigma OC/kg day and a Sigma OC residue adipose tissue and liver concentration of 1700 and 4470 ng/gl.w., respectively, after 16 months of exposure. Control foxes (n=13) were fed a diet with pork (Sus scrofa) fat as a main fat source containing significantly lower OC concentrations. The food composition fed to the control and exposed group was standardized for nutrient contents. Four OC-related histopathological changes were found: (1) flat-epithelial-cell true thyroid cysts (TC) characterized by neutral content; (2) remnants of simple squamous epithelial-cell embryonic ducts containing neutral debris (EDN); (3) remnants of stratified squamous epithelial-cell embryonic ducts containing acid mucins often accompanied with debris of leukocyte inflammatory nature (EDM) and (4) disseminated thyroid C-cell hyperplasia (HPC). Of these, the prevalence of TC, EDN and HPC was significantly highest in the exposed group (chi(2) test: all p<0.04). The study shows that the OC mixture in minke whale blubber may cause development of thyroid gland cysts, C-cell hyperplasia and increase the prevalence of cystic remnants of embryonic ducts. The mechanism causing these effects could include endocrine disruption of the hypothalamus-pituitary-thyroid (HPT) axis, a disturbance of the calcium homeostasis/metabolism or energy metabolism or immune suppression. Because concentrations of OCs are higher in wild Arctic foxes, it is likely that these animals could suffer from similar OC-induced thyroid gland pathological and functional changes.

Influence of Fasting and Exercise on the Daily Rhythm of Serum Leptin in the Horse

The hormone leptin is secreted by white adipocytes and regulates food intake and energy expenditure in rodents and humans. The goal of the present study was to investigate the existence of a daily rhythm of serum leptin in horses and its dependence on fasting and physical exercise. A robust daily rhythm of leptin was found in both athletic and sedentary horses, with a daytime trough and a peak in the dark phase. While physical exercise never induced changes in circulating leptin, fasting reliably affected serum leptin levels. Food deprivation did not abolish the daily rhythm of serum leptin, but daily mean leptin levels in fasted horses were significantly lower than in regularly fed horses. This result indicates that leptin production is not a mere consequence of feeding behavior. The fact that in a large animal such as the horse a short fast decreases leptin without significantly changing the body weight demonstrates that changes in levels of circulating leptin associated with food restriction do not solely reflect changes in amount of body fat.

Adaptations to fasting in a terrestrial mustelid, the sable (Martes zibellina)

The aim of this study was to investigate whether the actively wintering Palearctic sable Martes zibellina has evolved physiological adaptations to tolerate nutritional scarcity. Sixteen farm-bred male sables were divided into a fed control group and an experimental group fasted for 4 days. The rate of weight loss in the sable was similar to other medium-sized mustelids. Fasting led to hypoglycaemia and to a decreased lymphocyte percentage. The sable derived metabolic energy from both subcutaneous and intraabdominal white adipose tissues and the relative decrease in fat mass was the largest for the retroperitoneal and subcutaneous depots. Metabolic energy derived partly from body proteins indicated by the increased plasma levels of urea, uric acid and total essential amino acids. Triacylglycerols accumulated in the livers of the fasted sables and the increased plasma aminotransferase activities suggested hepatic dysfunction. The decreased plasma insulin concentrations and the elevated cortisol levels probably contributed to stimulated lipolysis and protein catabolism. Moreover, fasting increased the plasma ghrelin concentrations of the sables and down-regulated the thyroid activity.

Molecular cloning and expression of leptin in gray and harbor seal blubber, bone marrow, and lung and its potential role in marine mammal respiratory physiology

Leptin is a multifunctional hormone, produced predominantly in adipocytes. It regulates energy balance through its impact on appetite and fat metabolism, and its concentration indicates the size of body fat reserves. Leptin also plays a vital role in stretch-induced surfactant production during alveolar development in the fetus. The structure, expression pattern, and role of leptin have not previously been explored in marine mammals. Phocid seals undergo cyclical changes in body composition as a result of prolonged fasting and intensive foraging bouts and experience rapid, dramatic, and repeated changes in lung volume during diving. Here, we report the tissue-specific expression pattern of leptin in these animals. This is the first demonstration of leptin expression in the lung tissue of a mature mammal, in addition to its expression in the blubber and bone marrow, in common with other animals. We propose a role for leptin in seal pulmonary surfactant production, in addition to its likely role in long-term energy balance. We identify substitutions in the phocine leptin sequence in regions normally highly conserved between widely distinct vertebrate groups, and, using a purified seal leptin antiserum, we confirm the presence of the leptin protein in gray seal lung and serum fractions. Finally, we report the substantial inadequacies of using heterologous antibodies to measure leptin in unextracted gray seal serum.

Adaptations to fasting in the American mink (Mustela vison): carbohydrate and lipid metabolism

The aim of this study was to investigate whether the actively wintering American mink Mustela vison is strictly dependent on continuous food availability or if it has evolved physiological adaptations to tolerate nutritional scarcity. Fifty farm-bred male minks were divided into a fed control group and four experimental groups fasted for 2, 3, 5 or 7 days. The rate of weight loss was several-fold higher (1.5-3.2% day(-1)) in the mink than recorded previously in larger carnivores utilizing passive wintering strategies. The minks remained normoglycaemic, although their liver glycogen stores and glucose-6-phosphatase activities decreased during fasting. Adipose tissue constituted approximately 36% of their body mass after 7 days of food deprivation. Intra-abdominal fat, especially retroperitoneal but also mesenteric adipose tissue, were the most important fat depots to be hydrolyzed, but the ability of the mink to utilize its body lipids during fasting may be limited. The increased liver size, hepatic triacylglycerol accumulation and increases in the activities of plasma aminotransferases indicated liver dysfunction. Food deprivation also affected the red blood cell indices, and the blood monocyte and lymphocyte counts decreased suggesting immunosuppression during fasting. The results of the present study suggest that the mink has not evolved sophisticated adaptations to wintertime fasting.

Postmetamorphic Xenopus laevis shows decreased plasma triiodothyronine concentrations and phosphorylase activity due to subacute phytosterol exposure

Postmetamorphic South African clawed frogs (Xenopus laevis) were exposed to a phytosterol mixture (ca. 80% beta-sitosterol and less sitostanol, campesterol, and campestanol) for 14 days at 30 mugl(-1) in a flow-through system. The effects of phytosterols (PS) on the plasma thyroid hormone (T(3) and T(4)), testosterone, leptin-immunoreactive peptide and tissue glycogen concentrations were determined. The following enzyme activities were also analyzed from the liver and muscle: glycogen phosphorylase and lipase, and from the liver only: glucose-6-phosphatase. The plasma T(3) concentration was lower in the PS-exposed female frogs. Both muscle lipase and glycogen phosphorylase activities were also lower in the PS-exposed animals. These results could indicate that the basal metabolic rate and locomotion activity of the frogs were decreased. The effects could not be attributed to the possible estrogenicity of the PS mixture. Further studies will be needed to evaluate the possible significance of these effects.

In vivo effects of bisphenol A on the polecat (mustela putorius)

Bisphenol A (BPA), an environmental estrogen derived from the plastic industry, was given orally via incorporation into the food of 30 male and female polecats at 3 different doses (10, 50, or 250 mg/kg body weight/day) for 2 wk with 10 animals acting as controls. Several hormone levels in the plasma were determined as well as the activities of the phase I and II biotransformation enzymes 7-ethoxyresorufin O-deethylase (EROD), cytosolic glutathione S-transferase (GST), and UDP-glucuronosyltransferase (UDPGT). BPA did not cause any macroscopic effects on body mass or the health of the animals. UDPGT and GST activities increased significantly in direct correlation with increasing BPA exposure in females and UDPGT increased in a dose-related manner in males. There was no change in the plasma T4 and testosterone concentrations of the males with increasing BPA exposure. Discriminant analysis indicated that the group receiving 10 mg BPA/kg body weight/d was not different from the control group but the groups receiving 50 and 250 mg/kg body weight/d were different from the control group. This suggests physiological changes in the groups receiving 50 or 250 mg BPA/kg body weight/d.

Leptin, ghrelin, and energy metabolism of the spawning burbot (Lota lota, L.)

The aim of this study was to investigate the energy metabolism of the burbot (Lota lota, n=38) before, during, and after spawning, which represents the greatest annual metabolic demand for the species. A decrease in body mass, relative weight of the livers, and glycogen concentration of the livers was observed toward the end of spawning. The prespawning period was characterized by high rates of liver glycogenolysis and lipid mobilization. Also, plasma triiodothyronine and sex steroid levels were high before reproduction. During spawning, liver lipolysis was reduced and muscle glycogenolysis stimulated. The levels of triiodothyronine and sex steroids decreased. After reproduction, liver glycogenolysis was suppressed and the rate of gluconeogenesis increased. Thyroid hormone levels were elevated after spawning. Leptin protein and a ghrelin-immunoreactive peptide were detected in burbot plasma. Their concentrations were relatively low before and during reproduction but increased after spawning. The functions of leptin and the ghrelin-immunoreactive peptide in the physiology of the burbot are not consistent with the models of their function in mammals.

Body-weight changes are clearly reflected in plasma concentrations of leptin in female mink (Mustela vison)

The mink is a seasonal breeder with a propensity for seasonal fatness, and it is very responsive to changes in energy supply. The objectives of the present study were first, to validate a multi-species leptin assay for mink (Mustela vison) plasma, and second, to evaluate how plasma leptin and insulin concentrations responded to energy restriction and body-weight loss and refeeding with restoration of body reserves. The study was performed with six very fat yearling females (initial mean body weight 1,451 (SD 119) g, i.e. approximately 300 g more than for a female in normal body condition). The animals were fed in restricted amounts (about 35 % metabolizable energy requirement for maintenance) in order to reach a very lean body condition. The target weight of 800 g was reached after about 1 month of restriction. The animals were then refed ad libitum until almost complete weight recovery. Blood samples were taken on days 1, 24, 34 (end of restriction), 44, 55 and 71 (end of experiment) and analysed for plasma concentrations of leptin and insulin. Three females were mated on day 44. Leptin and insulin concentrations mirrored each other and clearly reflected changes in body weight. Significant (P<0.001) Pearson correlation coefficients of 0.75 (leptin-insulin), 0.72 (leptin-body weight) and 0.59 (insulin-body weight) were found. Two of the three females that were mated gave birth to normal litters. It was concluded that the leptin assay yielded acceptable results for animals with body weight:fat content within the range investigated here, and that plasma leptin reflected body fat mass.