Central infusion of recombinant ovine leptin normalizes plasma insulin and stimulates a novel hypersecretion of luteinizing hormone after short-term fasting in mature beef cows

The present studies tested the hypotheses that short-term fasting would reduce leptin gene expression and circulating concentrations of leptin and insulin in mature, ovariectomized, estradiol-implanted cows and that intracerebroventricular infusions of recombinant ovine leptin (oleptin) would attenuate reductions in insulin concentration and stimulate LH secretion. Ovariectomized cows were assigned to either control (normal fed; n = 6) or fasted (60 h of fasting; n = 7) groups and infused with 200 microg recombinant oleptin three times at hourly intervals on Day 2 (n = 6 per group). Fasting decreased plasma concentrations of insulin (P < 0.01) and leptin (P < 0.04) but, as expected, did not reduce plasma concentrations of glucose or any LH secretion variable. Central infusion of leptin on Day 2 increased (P < 0.01) plasma concentrations of leptin in both control and fasted groups. Concomitantly, leptin treatment increased plasma insulin (P < 0.01) and LH (P < 0.03) concentrations in fasted but not in control cows. Increases in overall mean and baseline concentrations of LH after leptin treatment were the result of an augmentation of the size of LH pulses. The effects of fasting on leptin gene expression and the potential diurnal effects on circulating leptin were examined in a group of cows (n = 12) not treated with leptin. Fasting for 60 h reduced (P < 0.001) leptin gene expression by 30%, and no diurnal effects on circulating leptin were observed. These results indicate that although short-term fasting does not reduce the frequency or amplitude of LH pulses or the concentration of LH in mature cows, this nutritional perturbation clearly sensitizes both the hypothalamic-pituitary axis and endocrine pancreas to exogenous leptin, which in these experiments resulted in heightened secretion of both LH and insulin.     

Energy balance and diabetes. The effects of cold exposure, exercise training, and diet composition on glucose tolerance and glucose metabolism in rat peripheral tissues

The effects of cold exposure, exercise training, and diet (high fat versus high carbohydrate) on glucose tolerance and glucose metabolism in rat peripheral tissues will be briefly reviewed. Stimulation of energy expenditure by cold exposure (4 degrees C) or exercise training generally leads to decreased plasma insulin levels and to an improvement in glucose tolerance, suggesting that insulin action on peripheral tissues is increased when energy expenditure is stimulated. On the contrary, feeding high-fat diets to sedentary rats living in the warm (25 degrees C) induces hyperinsulinemia and insulin resistance resulting in a marked deterioration of glucose tolerance. Nevertheless, cold exposure reverses the diabetogenic effects of high-fat feeding, demonstrating that nutrition-induced insulin resistance is amplified in sedentary animals living at temperatures close to thermoneutrality. Radioactive tracer studies of 2-deoxyglucose uptake in peripheral tissues revealed that cold exposure synergistically potentiates the effects of insulin on glucose uptake in skeletal muscles as well as in white and brown adipose tissues. However, more recent data showed that cold exposure improves glucose tolerance and stimulates glucose uptake in starved animals (ie., in the virtual absence of circulating insulin) nearly by the same order of magnitude as in fed animals. It is therefore concluded that cold exposure, and possibly also exercise, improve glucose tolerance and stimulate glucose uptake in peripheral tissues primarily by enhancing glucose oxidation via insulin-independent pathways, and secondarily by increasing the responsiveness of peripheral tissues to insulin.     

Plasma catecholamine concentrations of newborn piglets in thermoneutral and cold environments

Plasma epinephrine and norepinephrine concentrations were measured in seventeen unanaesthetized 3 to 4 days-old piglets while in a thermoneutral environment (31.3 degrees C) and 30, 45 and 60 min after induction of environmental cold stress (19.9-23.1 degrees C). Plasma epinephrine and norepinephrine concentrations in a warm environment were 142 +/- 26 pg/ml, and 456 +/- 44 pg/ml respectively. Environmental cold stress evoked significant increases in norepinephrine values after 30 (624 +/- 58 pg/ml), 45 (626 +/- 60 pg/ml) and 60 (626 +/- 54 pg/ml) min of cold stress. Plasma epinephrine concentrations did not significantly change during environmental cold stress. Post-hoc stratification of piglets into normothermic (deep rectal temperature 38.6 degrees C-38.8 degrees C, n = 9) and hypothermic (deep rectal temperature 37.1 degrees C-37.7 degrees C, n = 7) subgroups revealed significant increases in plasma norepinephrine concentrations only in the hypothermic subgroup. We conclude that plasma norepinephrine, but not epinephrine, is increased in newborn piglets during environmental cold stress and that the changes in norepinephrine concentrations are related to body core hypothermia. We speculate that hypothermia-mediated reductions in peripheral norepinephrine breakdown and re-uptake contribute to the rise in circulating levels.     

Alterations of glucose-dependent insulinotropic polypeptide (GIP) during cold acclimation

Cold acclimation is initially associated with shivering thermogenesis in skeletal muscle followed by adaptive non-shivering thermogenesis, particularly in brown adipose tissue (BAT). In response, hyperphagia occurs to meet increased metabolic demand and thermoregulation. The present study investigates the effects of cold (4 ± 1 °C) acclimation and hyperphagia on circulating and intestinal levels of gastric inhibitory polypeptide (GIP) in rats. Pair fed animals were used as additional controls in some experiments. Cold acclimation for 42 days significantly (p<0.01) increased daily food intake. There was no corresponding change in body weight. However, body weights of pair fed cold exposed rats were significantly (p<0.01) reduced compared to controls and ad libitum fed cold exposed rats. By day 42, non-fasting plasma glucose was increased (p<0.05) by chronic cold exposure regardless of food intake. Corresponding plasma insulin concentrations were significantly (p<0.01) lower in pair fed cold exposed rats. Circulating GIP levels were elevated (p<0.05) in ad libitum fed cold acclimated rats on days 18 and 24, but returned to normal levels by the end of the study. The glycaemic response to oral glucose was improved (p<0.01) in all cold exposed rats, with significantly (p<0.05) elevated GIP responses in ad libitum fed rats and significantly (p<0.05) reduced insulin responses in pair fed rats. In keeping with this, insulin sensitivity was enhanced (p<0.05) in cold exposed rats compared to controls. By the end of the study, cold acclimated rats had significantly (p<0.01) increased BAT mass and intestinal concentrations of GIP and GLP-1 compared to controls, independent of food intake. These data indicate that changes in the secretion and actions of GIP may be involved in the metabolic adaptations to cold acclimation in rats.     

Ghrelin, macronutrient intake and dietary preferences in long-evans rats

Ghrelin is a recently discovered peptide that is primarily produced by the stomach. As a ligand of the growth hormone (GH) secretagogue (GHS) receptor, it stimulates GH secretion but it also stimulates feeding and has adipogenic effects in rodents. Although its circulating levels are modulated by fasting and refeeding, its relationship with diet composition is not known. In the present paper, we measured plasma ghrelin as well as two important hormones (leptin and insulin) in Long-Evans rats placed in two different feeding situations, e.g., either with imposed diets or with food choice. In the first case, the rats were fed unbalanced diets (either high-carbohydrate (HC) or high-fat (HF) diets) for 14 weeks, whereas in the second situation, they had the choice between these 2 diets for 2 weeks and were selected for their fat or carbohydrate preference. The intake of the HF diet for 14 weeks was associated with lower levels of ghrelin (-30% vs control diet; P < 0.01). These levels increased when the percentage of carbohydrate in the diet increased (+26 to +42% vs control diet; P < 0.01 or less). Ghrelin was inversely correlated with plasma leptin (r = -0.55; P < 0.003) and blood glucose (r = -0.58; P < 0.001) as well as with body weight (r = -0.63; P < 0.0001) and body fat content estimated by the sampling of specific fat pads (r = -0.62; P < 0.0001). In the food choice experiment, fat-preferring rats had plasma ghrelin levels lower than the carbohydrate-preferring rats (-33%; P < 0.0002). Ghrelin secretion was therefore very sensitive to the diet composition. Its down-regulation by fat ingestion might serve as a counterregulatory mechanism to limit the development of dietary-induced adiposity. Ghrelin may signal when a high calorie diet is ingested.     

Divergent effects of leptin on luteinizing hormone and insulin secretion are dose dependent

We have shown recently that fasting permits leptin to modulate both luteinizing hormone (LH) and insulin secretion in cows. In rodents, leptin causes divergent effects on LH and insulin release that are dose dependent. To test the hypothesis that leptin effects on LH and insulin secretion in fasted cows are dose related, we examined the effects of various doses of recombinant ovine leptin (oleptin) in mature cows. Twenty ovariectomized beef cows, each bearing an estradiol implant to maintain basal estradiol concentrations, were used. All cows were fasted for 60 hr with free access to water and were assigned randomly to one of four groups (n = 5/group): 1) saline control; 2) leptin, 0.2 microg/kg; 3) leptin, 2.0 microg/kg; and 4) leptin, 20 microg/kg body wt. Blood samples were collected at 10-min intervals for 6 hr on Days 0 and 2, with saline or oleptin injected intravenously immediately after the first intensive sample on Day 2 (54 hr). Leptin caused a dose-related increase (P < 0.001) in mean concentrations of circulating LH. Stimulation of LH release by leptin was significant at the lowest (141% of control) and middle (122% of control) doses used, but no increase was observed for the highest dose. Increased mean concentrations of LH appeared to result from an augmentation of basal secretion, as pulse characteristics were not affected. After 54 hr of fasting, plasma insulin concentrations were lowered (P < 0.01) in all treatment groups compared to Day 0. After leptin injections, plasma insulin concentrations increased (P < 0.01) and reached highest concentrations during the first hour of sampling. However, this increase was sustained for several hours only in the intermediate (2.0 microg/kg) dose group. Collectively, our results show that leptin has potent positive effects on both LH and insulin secretion in fasted cows, but the anterior pituitary and endocrine pancreas appear to become downregulated in the presence of excess ligand.     

Secretory patterns of leptin and luteinizing hormone in food-restricted young female sheep

Leptin, the product of the ob gene, has been proposed as a metabolic signal that regulates the secretion of GnRH/LH. This may be critical during prepubertal development to synchronize information about energy stores and the secretion of GnRH/LH. This study aimed to assess the effect of food restriction on the episodic secretion of leptin and LH in young female sheep. Five 20-week-old prepubertal females were fed a low-level diet for 10 weeks to maintain the body weight. Control females of the same age received food ad libitum. Blood samples were collected at 10-min intervals for six hours at 20, 26, and 30 weeks of age, and plasma leptin, LH, insulin and cortisol concentrations were measured. In the control group, no changes were found in pulsatile LH secretion characteristics. Mean LH concentrations and LH amplitude were lower in the food-restricted group than in the control group at 26 and 30 weeks of age. In the control group, pulsatile leptin secretion did not change. When compared to control lambs of the same age, the food-restricted group showed lower mean plasma leptin concentrations, pulse amplitude and plasma insulin levels, after 6 weeks of restriction (week 26), although by week 30, plasma leptin concentrations and plasma insulin rose to those of the control group. Leptin pulse frequency did not change, nor did mean plasma levels of insulin in the control group at any age studied. Mean plasma concentration of cortisol did not change within or between groups. These data suggest that plasma leptin concentrations may not be associated with the onset of puberty under regular feeding and natural photoperiod in lambs. Prolonged food restriction, however, induces metabolic adaptations that allow an increase of leptin during the final period, probably related to the development of some degree of insulin resistance.     

Dual effect of insulin on in vitro leptin secretion by adipose tissue

Although it is widely accepted that insulin stimulates leptin secretion, a dual action was observed using a validated in vitro system, i.e., an early (less than 48 h) inhibitory action, followed later (48-96 h) by a clear-cut stimulation. While the inhibitory phase was observed at every glucose concentration tested (from 1 to 25 mM), the stimulatory phase required the presence of physiological or supraphysiological glucose concentrations. In fact, leptin secretion was virtually eliminated in the presence of glucose uptake inhibitors. This dual effect of insulin was not due to modifications of the ob mRNA levels, suggesting that it depends entirely on posttranslational mechanisms. In conclusion, insulin appears to induce an early inhibition of leptin secretion by the adipose cell, followed later by a stimulatory effect secondary to the metabolic changes triggered by the insulin-induced increase in glucose uptake.     

Exercise training decreases plasma leptin levels and the expression of hepatic leptin receptor-a, -b, and, -e in rats

In addition to acting in the central nervous system, leptin also acts on peripheral tissues such as liver to provide a protection against lipid accretion. Previous evidence from human and animal model indicates that exercise training reduces circulating leptin levels beyond the changes in adiposity levels. Because liver is one of the main peripheral organs for leptin action, this present study was designed to determine whether leptin receptors expression in liver is changed by exercise training. Female rats trained (TR) or kept sedentary (Sed) for 8 weeks were submitted either to a standard (SD) diet for 8 weeks or for 6 weeks followed by 2 weeks of high-fat (HF) or high-carbohydrate (HC) feeding. Food intake, adiposity levels, circulating plasma leptin and insulin concentrations along with the hepatic expression of leptin receptors (ObR-a, -b, and -e) and peroxisome proliferator-activated receptor α (PPARα) and peroxisome proliferator-activated receptor-gamma co-activator-1α (PGC-1α), were measured in all the animals. Intra-abdominal fat depots were increased under the HF but not under the HC diet. As expected, exercise training decreases intra-abdominal adiposity in animals fed with the SD and the HF diet, and to a lesser extent in HC-fed rats. Plasma leptin levels either expressed in absolute values or in values relative to adiposity levels were significantly (P < 0.05) increased with the HF diet and significantly decreased in TR animals, independently of the diet. Moreover, a significant (P < 0.01) reduction in hepatic gene expression of ObR-a, -b and -e was found in TR animals in all the three diet conditions. PPARα and PGC-1α mRNAs were also decreased (P < 0.05) in TR animals in two out of three diet conditions. The present findings indicate that exercise training-induced decrease in plasma leptin levels is accompanied by a reduction in gene expression of three different isoforms of leptin receptors in liver.     

Blood leptin homeostasis: sex-associated differences in circulating leptin levels in rats are independent of tissue leptin expression

Circulating leptin levels are higher in women than in men. The aim of the study has been to determine in rats the putative existence of sex-associated differences in leptin expression in different adipose tissue depots (gonadal, retroperitoneal, mesenteric and inguinal white adipose tissue and interscapular brown adipose tissue) and the relationship with circulating leptin levels. Adult male and female Wistar rats acclimated to 22 degrees C or to 28 degrees C were used. Leptin mRNA expression was assessed by northern blot and serum leptin levels by enzyme-linked immunosorbent assay (ELISA). Contrary to what happens in humans, we report here that male rats acclimated to standard animal house conditions (22 degrees C) have a higher leptin concentration in blood than female rats. This situation cannot be explained by a greater size of the fat depots in males, because the adiposity index is similar in both genders, but are rather associated to higher leptin specific mRNA expression by the white adipose tissue. Around thermoneutral conditions (28 degrees C), sex related differences in leptin mRNA expression disappear, but the gender difference in circulating leptin levels remains. In addition, leptin mRNA expression is higher in both genders in thermoneutral conditions but this is not reflected in changes in the circulating leptin levels. In conclusion, this study shows that rat circulating leptin levels are finely regulated, and not exclusively dependent on leptin mRNA expression, but other mechanisms are also involved, possibly regarding leptin rate of degradation.     

Leptin responses to glucose infusions in obesity-prone rats

The secretion of leptin is dually regulated. In fasting animals, plasma leptin concentrations reflect body fat stores, whereas the incremental leptin response to fasting or refeeding most likely reflects insulin-mediated energy flux and metabolism within adipocytes. Impaired secretion of leptin in either pathway could result in obesity. We therefore measured plasma leptin concentrations in fasted animals and plasma leptin concentrations after an intravenous glucose infusion in a rat model of obesity. Young Sprague-Dawley (S-D) and Fischer 344 (F344) rats had similar percent body fat and fasting glucose and fasting leptin concentrations. However, F344 animals had higher insulin concentrations and leptin responses to intravenous glucose than did the S-D animals. The animals were then fed a control or high-fat diet for 6 wk. High-fat fed animals gained more weight and body fat than did the control fed animals. Control and high-fat fed F344 animals gained approximately 40% (P < 0.0001) more weight and >100% (P < 0.01) more body fat than did the S-D animals. Fasting leptin concentrations and leptin concentrations after intravenous glucose infusions and feeding were more than double (P < 0.05) in F344 animals compared with S-D animals. Whether an animal is fed a control or high-fat diet had little effect on the leptin response to intravenous glucose. In conclusion, young, lean F344 animals, before the onset of obesity, demonstrated a greater acute leptin response to intravenous glucose than similarly lean S-D animals. After a 6-wk diet, F344 animals had a greater percent increase in body weight and insulin resistance and exhibited higher fasting leptin concentrations and a greater absolute leptin response to intravenous glucose compared with the S-D animals. The chronic diet (control or high fat) had little impact on the acute leptin response to intravenous glucose. F344 animals exhibit leptin resistance in young, lean animals and after aging and fat accumulation.     

Effects of fructose and glucose on plasma leptin, insulin, and insulin resistance in lean and VMH-lesioned obese rats

To determine the influence of dietary fructose and glucose on circulating leptin levels in lean and obese rats, plasma leptin concentrations were measured in ventromedial hypothalamic (VMH)-lesioned obese and sham-operated lean rats fed either normal chow or fructose- or glucose-enriched diets (60% by calories) for 2 wk. Insulin resistance was evaluated by the steady-state plasma glucose method and intravenous glucose tolerance test. In lean rats, glucose-enriched diet significantly increased plasma leptin with enlarged parametrial fat pad, whereas neither leptin nor fat-pad weight was altered by fructose. Two weeks after the lesions, the rats fed normal chow had marked greater body weight gain, enlarged fat pads, and higher insulin and leptin compared with sham-operated rats. Despite a marked adiposity and hyperinsulinemia, insulin resistance was not increased in VMH-lesioned rats. Fructose brought about substantial insulin resistance and hyperinsulinemia in both lean and obese rats, whereas glucose led to rather enhanced insulin sensitivity. Leptin, body weight, and fat pad were not significantly altered by either fructose or glucose in the obese rats. These results suggest that dietary glucose stimulates leptin production by increasing adipose tissue or stimulating glucose metabolism in lean rats. Hyperleptinemia in VMH-lesioned rats is associated with both increased adiposity and hyperinsulinemia but not with insulin resistance. Dietary fructose does not alter leptin levels, although this sugar brings about hyperinsulinemia and insulin resistance, suggesting that hyperinsulinemia compensated for insulin resistance does not stimulate leptin production.     

Characterization of a distinctive pattern of periovulatory leptin secretion and its relationship with ovulation rate and luteal function in swine with obesity/leptin resistance

Patterns of leptin secretion during the estrous cycle and the possible relationship of changes in circulating leptin during the periovulatory period with ovarian function in sows of obese (Iberian breed) and lean genotype (Large White×Landrace) were evaluated in two consecutive experiments. Plasma leptin concentrations throughout the estrous cycle in lean sows remain unchanged, but Iberian females showed a periovulatory increase in circulating leptin levels without associated changes in body condition and fatness. In these sows, plasma leptin concentrations at Days -1 and 0 of the cycle were found to be positively correlated with the ovulation rate (r=0.943 and r=0.987, respectively; P<0.05 for both), but the levels of leptin at Day 0 were negatively correlated with the progesterone release from Day 3 (r=-0.557; P<0.05) and, became more evident at Day 5 of the estrous cycle (r=-0.924; P<0.005). Such relationships were not observed in the females of the lean genotype. In conclusion, the present study indicates the existence of a distinctive pattern in the periovulatory leptin secretion in swine with obesity and leptin resistance, which is associated with the number and functionality of the corpora lutea present in the subsequent cycle.     

Leptin gene expression, circulating leptin, and luteinizing hormone pulsatility are acutely responsive to short-term fasting in prepubertal heifers: relationships to circulating insulin and insulin-like growth factor I(1)

In the present study, we tested the hypothesis that short-term fasting would reduce leptin gene expression, circulating leptin, and LH pulsatility in prepubertal heifers in association with a decrease in circulating concentrations of insulin and insulin-like growth factor I (IGF-I). Twelve prepubertal crossbred heifers (mean +/- SD = 315 +/- 5 kg body weight) were assigned randomly to one of two treatments in two replicates: 1) control; normal feed consumption (n = 6) and 2) fasted; 48 h of total feed restriction (n = 6). Blood samples were collected at 15-min intervals for 8 h on Days 0 and 2 of the experiment and twice on Day 1. Subcutaneous fat samples were collected before treatment onset (Day -1) and at the end of the intensive blood sampling on Day 2. Acute feed restriction markedly reduced leptin mRNA in adipose tissue (P < 0.01) and circulating concentrations of leptin (P < 0.05), IGF-I (P < 0.01), and insulin (P = 0.05) as compared with controls on Day 2. Moreover, the treatment x day interaction (P < 0.076) and within-day contrasts (expressed as a percentage of Day 0 values) revealed that the mean frequency of LH pulses in the fasted group was lower (P < 0.06) than in controls on Day 2. Neither mean concentrations of growth hormone (GH) nor GH secretory dynamics were affected by acute feed restriction. Fasting-mediated decreases in leptin gene expression and circulating leptin, in association with reductions in secretion of IGF-I, insulin, and LH, provide a basis for investigating leptin as a hormone signaling energy status to the central reproductive axis in cattle.     

Hepatic steatosis and plasma dyslipidemia induced by a high-sucrose diet are corrected by an acute leptin infusion.

High sucrose (HS) feeding in rats induces hepatic steatosis and plasma dyslipidemia. In previous reports (Huang W, Dedousis N, Bhatt BA, O'Doherty RM. J Biol Chem 279: 21695-21700, 2004; and Huang W, Dedousis N, Bandi A, Lopaschuk GD, O'Doherty RM. Endocrinology 147: 1480-1487, 2006), our laboratory demonstrated a rapid ( approximately 100 min) leptin-induced decrease in liver and plasma VLDL triglycerides (TG) in lean rats, effects that were abolished in obese rats fed a high-fat diet, a model that also presents with hepatic steatosis and plasma dyslipidemia. To further examine the capacity of acute leptin treatment to improve metabolic abnormalities induced by nutrient excess, hepatic leptin action was studied in rats after 5 wk of HS feeding. HS feeding induced hepatic steatosis (TG+80+/-8%; P=0.001), plasma hyperlipidemia (VLDL-TG+102+/-14%; P=0.001), hyperinsulinemia (plasma insulin +67+/-12%; P=0.04), and insulin resistance as measured by homeostasis model assessment (+125+/-20%; P=0.02), without increases in adiposity or plasma leptin concentration compared with standard chow-fed controls. A 120-min infusion of leptin (plasma leptin 13.6+/-0.7 ng/ml) corrected hepatic steatosis (liver TG-29+/-3%; P=0.003) and plasma hyperlipidemia in HS (VLDL-TG-42+/-4%; P=0.001) and increased plasma ketones (+45+/-3%; P=0.006), without altering plasma glucose, insulin, or homeostasis model assessment compared with saline-infused HS controls. In addition, leptin activated liver phosphatidylinositol 3-kinase (+70+/-18%; P=0.01) and protein kinase B (Akt; +90+/-29%; P=0.02), and inhibited acetyl-CoA carboxylase (40+/-7%; P=0.04) in HS, further demonstrating that hepatic leptin action was intact in these animals. We conclude that 1) leptin action on hepatic lipid metabolism remains intact in HS-fed rats, 2) leptin rapidly reverses hepatic steatosis and plasma dyslipidemia induced by sucrose, and 3) the preservation of hepatic leptin action after a HS diet is associated with the maintenance of low adiposity and plasma leptin concentrations.     

Leptin acts peripherally to limit meal-induced increases in plasma insulin concentrations in mice: a brief communication

Leptin inhibits food intake and lowers plasma insulin concentrations. This study was designed to determine whether leptin acts independent of food-intake regulation to affect meal-induced increases in plasma insulin concentrations. Leptin-deficient, Lep(ob)/Lep(ob) mice were administered 1 microg leptin intracerebroventricularly (ICV) or intraperitoneally. Food intake and plasma insulin concentrations of mice administered leptin ICV before a meal were lower, as expected, than were intakes and plasma insulin concentrations of mice administered vehicle ICV. However when food intake was controlled, meal-induced increases in plasma insulin were unaffected by ICV administration of leptin. Intraperitoneal administration of 1 microg leptin before a meal lowered meal-induced increases in plasma insulin concentrations without influencing the size of the meal. We conclude that plasma leptin concentrations can affect meal-induced insulin secretion independent of the central nervous system actions of leptin associated with food-intake regulation.     

Molecular and functional resistance to insulin in hypothalamus of rats exposed to cold

Insulin and leptin act in the hypothalamus, providing robust anorexigenic signals. The exposure of homeothermic animals to a cold environment leads to increased feeding, accompanied by sustained low levels of insulin and leptin. In the present study, the initial and intermediate steps of the insulin-signaling cascade were evaluated in the hypothalamus of cold-exposed Wistar rats. By immunohistochemistry, most insulin receptor (IR) and insulin receptor substrate-2 (IRS-2) immunoreactivity localized to the arcuate nucleus. Basal levels of tyrosine phosphorylation of IR and IRS-2 were increased in cold-exposed rats compared with rats maintained at room temperature. However, after an acute, peripheral infusion of exogenous insulin, significantly lower increases of IR and IRS-2 tyrosine phosphorylation were detected in the hypothalamus of cold-exposed rats. Insulin-induced association of p85/phosphatidylinositol 3-kinase with IRS-2, Ser473 phosphorylation of Akt, and tyrosine phosphorylation of ERK was significantly reduced in the hypothalamus of cold-exposed rats. To test the hypothesis of functional impairment of insulin signaling in the hypothalamus, intracerebroventricularly cannulated rats were acutely treated with insulin, and food ingestion was measured over a period of 12 h. Cold-exposed animals presented a significantly lower insulin-induced reduction in food consumption compared with animals maintained at room temperature. Hence, the present studies reveal that animals exposed to cold are resistant, both at the molecular and the functional level, to the actions of insulin in the hypothalamus.     

Peripheral leptin administration alters hormone and metabolite levels in the young pig

The present study was conducted to determine if peripheral leptin administration can alter GH secretion or feed intake in young pigs. Six, 6 kg female pigs were fasted overnight and randomly chosen to receive porcine recombinant leptin or saline injections in a crossover design. Three leptin dosages were tested over a 10 day period, 100, 200 or 500 microg/kg body mass (L100, L200 or L500). Leptin was administered in 0.2% bovine serum albumin as a bolus injection into the carotid artery. Blood samples were obtained from the jugular vein over a 24 h period. Leptin delayed feeding in pigs treated with L200 and L500 (P<0.05), while reducing overall intake in pigs treated with L100 (P<0.05). L200 or L500 depressed blood glucose (P<0.05). Plasma insulin levels were elevated by feeding in control animals, while insulin levels were depressed in pigs treated with L200 or L500 (P<0.05). L200 elevated plasma growth hormone (P<0.05) with three peaks apparent at 5, 8, and 13 h post injection. The ability for a single injection of leptin to produce significant changes in hormone and metabolite levels suggests that this peptide has a role in regulation of peripheral metabolism.     

Plasma hyperosmolality stimulates leptin secretion acutely by a vasopressin-adrenal mechanism

Glucose administration to rodents acutely stimulates leptin secretion. To investigate the mechanism, rats were infused intravenously with various concentrations of glucose, and plasma leptin concentrations were measured with time. The osmolality of the infusates was equalized with various concentrations of carbohydrates that are not metabolized. Hyperosmolar glucose stimulates leptin secretion in a dose-dependent manner, with peak plasma leptin concentrations occurring approximately 3 h after the end of the glucose infusion. Hypertonic infusions of galactose, mannitol, and sodium chloride independently stimulate leptin secretion with approximately one-half the strength of equivalent osmolar concentrations of glucose. Peak plasma leptin concentrations occur approximately 4 h after the end of the hypertonic solution infusion. Hypertonic solutions of mannitol do not stimulate leptin secretion in vasopressin-deficient or in adrenalectomized animals. In conclusion, intravenous infusions of hypertonic glucose and hypertonic mannitol independently stimulate leptin secretion. Hyperosmolality stimulates leptin secretion by a vasopressin-adrenal mechanism.     

Alterations of Glucose-Dependent Insulinotropic Polypeptide and Expression of Genes Involved in Mammary Gland and Adipose Tissue Lipid Metabolism during Pregnancy and Lactation

Gastric inhibitory polypeptide (GIP) is a gut derived peptide with multiple emerging physiological actions. Effects of pregnancy and lactation on GIP secretion and related gene expression were studied in Wistar rats. Pregnancy moderately increased feeding (p<0.05), whilst lactation substantially increased food intake (p<0.01 to p<0.001). Circulating GIP was unchanged during pregnancy, but non-fasting plasma glucose was significantly (p<0.01) decreased and insulin increased (p<0.05). Lactation was associated with elevated circulating GIP concentrations (p<0.001) without change of glucose or insulin. Oral glucose resulted in a significantly (p<0.001) decreased glycaemic excursion despite similar glucose-induced GIP and insulin concentrations in lactating rats. Pregnant rats had a similar glycaemic excursion but exhibited significantly lowered (p<0.05) GIP accompanied by elevated (p<0.001) insulin levels. Pregnant rats exhibited increased (p<0.001) islet numbers and individual islet areas were enlarged (p<0.05). There were no significant differences in islet alpha-cell areas, but all groups of rats displayed co-expression of glucagon and GIP in alpha-cells. Lactating rats exhibited significantly (p<0.01) increased intestinal weight, whereas intestinal GIP stores were significantly (p<0.01) elevated only in pregnant rats. Gene expression studies in lactating rats revealed prominent (p<0.01 to p<0.001) increases in mammary gland expression of genes involved in energy turnover, including GIP-R. GIP was present in intestines and plasma of 17 day old foetal rats, with substantially raised circulating concentrations in neonates throughout the period of lactation/suckling. These data indicate that changes in the secretion and action of GIP play an important role in metabolic adaptations during pregnancy and especially lactation.