Glucose metabolism in the newborn rat: the role of insulin

Selenomethionine alleviates environmental heat stress induced hepatic lipid accumulation and glycogen infiltration of broilers via maintaining mitochondrial and endoplasmic reticulum homeostasis

With the increasing of global mean surface air temperature, heat stress (HS) induced by extreme high temperature has become a key factor restricting the poultry industry. Liver is the main metabolic organ of broilers, HS induces liver damage and metabolic disorders, which impairs the health of broilers and affects food safety. As an essential trace element for animals, selenium (Se) involves in the formation of antioxidant system, and its biological functions are generally mediated by selenoproteins. However, the mechanism of Se against HS induced liver damage and metabolic disorders in broilers is inadequate. Therefore, we developed the chronic heat stress (CHS) broiler model and investigated the potential protection mechanism of organic Se (selenomethionine, SeMet) on CHS induced liver damage and metabolic disorders. In present study, CHS caused liver oxidative damage, and induced hepatic lipid accumulation and glycogen infiltration of broilers, which are accompanied by mitochondrial dysfunction, abnormal mitochondrial tricarboxylic acid (TCA) cycle and endoplasmic reticulum (ER) stress. Dietary SeMet supplementation increased the hepatic Se concentration and exhibited protective effects via promoting the expression of selenotranscriptome and several key selenoproteins (GPX4, TXNRD2, SELENOK, SELENOM, SELENOS, SELENOT, GPX1, DIO1, SELENOH, SELENOU and SELENOW). These key selenoproteins synergistically improved the antioxidant capacity, and mitigated the mitochondrial dysfunction, abnormal mitochondrial TCA cycle and ER stress, thus recovered the hepatic triglyceride and glycogen concentration. What's more, SeMet supplementation suppressed lipid and glycogen biosynthesis and promoted lipid and glycogen breakdown in liver of broilers exposed to CHS though regulating the AMPK signals. Overall, our present study reveals a potential mechanism that Se alleviates environment HS induced liver damage and glycogen and lipid metabolism disorders in broilers, which provides a preventive and/or treatment measure for environment HS-dependent hepatic metabolic disorders in poultry industry.

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.

Repetitive exposures to a surrogate nipple providing nutritive and non-nutritive fluids: effects on suckling behavior of the newborn rat

Responsiveness to a surrogate nipple providing water, 0.1% saccharin, 10% sucrose, pedialyte, or milk was tested in naïve-to-suckling newborn rats during six 10-min exposures, one every 1.5 h over a 7.5 h period. Across a succession of exposures, newborn rats repeatedly attached to and ingested milk from a surrogate nipple, yielding significant body weight gain and increased concentration of blood plasma glucose. Initially, pups ingested considerable amounts of saccharin and sucrose, but then dramatically decreased their consumption of these fluids across the experimental sessions. Intake of milk was significantly higher than that of all other substances. Blood glucose concentration in pups treated with water, saccharin, sucrose, and pedialyte did not differ significantly from that of non-treated pups. The present data suggest a potential contribution of a fluid's palatability and nutritive value in the persistence and efficacy of diet intake for neonatal rats in the context of suckling behavior.

Postnatal profiles of glycogenolysis and gluconeogenesis are modified in rat pups by maternal dietary glucose restriction

Because glucose is an important metabolic fuel during perinatal development, the effect of restriction of maternal dietary glucose on the developmental profile of neonatal glucoregulatory pathways was investigated. Pregnant rats were fed isoenergetic diets (0, 12, 24 or 60% glucose) and offspring were killed at seven postpartum time periods: 0-2, 4-6, 12-16 and 24 h, and 3, 6 and 15 d. Failure of the most restricted pups (0%) to survive 24 h was explained by persistent hypoglycemia resulting from the following: 1) insufficient tissue glycogen reserves at birth; 2) lower liver glycogen mobilization; 3) delayed phosphorylase a induction; and 4) low phosphoenolpyruvate carboxykinase (PEPCK) gene expression, all of which occurred despite the lower insulin:glucagon ratio. Differences in liver glycogen stores, which had been exhausted in all dietary groups by 16 h, could not account for the high d 1 pup mortality in the moderately restricted (12 and 24% glucose) groups. However, a certain metabolic distress was suggested because these moderately restricted neonates had significantly higher liver PEPCK gene expression at 12-16 h but significantly lower plasma glucose at 24 h. The high d 3 mortality, confirmed by analysis of deviance, was not supported by significant differences in any of the measured glucoregulatory indices. We conclude that dietary glucose during pregnancy is required for neonatal survival; its restriction not only lowers tissue glycogen reserves, but can disrupt the normal gene expression of liver PEPCK and the neonatal profile of phosphorylase a activity. Importantly, these observations show that the development of neonatal glucoregulatory mechanisms is modified by the availability of maternal dietary glucose.

Ontogeny of glucagon messenger RNA in the rat pancreas

The synthesis of proglucagon mRNA was studied in rat pancreas from day 11 of fetal gestation (E11) to maturity. Proglucagon mRNA was first detected on E11, the time that the pancreatic bud forms in developing rats. The synthesis of proglucagon mRNA and its translation product at this early time point in pancreatic development suggests an early differentiation of A cell function. Between E17 prenatally and day 10-14 postnatally, pancreatic proglucagon mRNA abundance was higher than in adult pancreas. Regulation of the abundance of pancreatic proglucagon mRNA therefore appears to underlie the previously documented increases in serum and pancreatic glucagon immunoreactivity in the late fetal and perinatal periods. By day 20 postnatally, pancreatic proglucagon mRNA declined to levels found in adult pancreas. Prenatally between E17 and E21, changes in proglucagon mRNA abundance did not parallel previously reported developmental changes in relative mass of proglucagon-producing pancreatic A cells. This suggests that changes in proglucagon mRNA abundance during these times may be attributed to changes in proglucagon gene transcription or proglucagon mRNA stability per cell. In contrast between E21 and maturity, changes in proglucagon mRNA abundance paralleled previously reported changes in relative A cell mass, suggesting no major changes in proglucagon gene transcription or mRNA stability per cell during these times.

Variations in the antagonistic effects of insulin and glucagon on glycogen metabolism in cultured foetal hepatocytes

The antagonistic effects of insulin and glucagon on glycogen formation and mobilization were studied in cultured 18-day foetal rat hepatocytes with regard to different modes of exposure. Hormone combinations were achieved with a constant dose of 10 nM-insulin (maximal for the glycogenic effect of this hormone) and increasing doses of glucagon [from 0.03 to 10 nM: concn. causing half-maximal response (ED50) = 0.3 nM)]. When insulin and glucagon were added simultaneously, increasing glucagon concentrations progressively depressed the glycogenic effect of insulin and 0.3 nM-glucagon antagonized the insulin effect completely. The maximal glycogenolytic effect of glucagon was observed at concentrations greater than 1 nM. When the two hormones were introduced successively, with an interval of 4 h between additions, the effect of the second hormone was always fully expressed between 4 and 8 h. at which time the effect of the first hormone had ceased; the dominance of glucagon over insulin was also lost, due to cell desensitization to glucagon. Both continuous or intermittent (10 min on/10 min off periods) exposure to insulin and/or glucagon gave similar antagonistic effects, while in cells exposed to insulin plus glucagon alternating with exposure to insulin or glucagon alone, the glycogenic effect of insulin was less or more antagonized respectively by glucagon. Whatever the situation, the results obtained could not be related to antagonism by a glucagon-induced rise in either cyclic AMP levels (ED50 = 3 nM) or cell-surface hormone binding. Thus, depending on the hormonal state and the mode of hormone administration, regulation of glycogenesis in cultured foetal hepatocytes appears to be different from that predicted by the insulin/glucagon molar ratio, which is strikingly altered in the perinatal period.

Characterization of rat liver beta-adrenoceptors during perinatal development as determined by [125I]-iodopindolol radioligand binding assays

1. The subtype specificity of beta-adrenoceptors in foetal (20 days post coitum) rat liver membrane preparations has been determined by use of [125I]-iodopindolol binding assays and the characteristics of radioligand binding have been resolved. 2. The kinetics of radioligand association and dissociation (in the presence of 5 x 10(-4) M isoprenaline) showed an association rate constant of 1.5 x 10(7) M-1 S-1 and dissociation rate constant of 9.1 x 10(-4) S-1, corresponding to a dissociation constant for [125I]-iodopindolol of 60.7 pM. A similar dissociation constant (75 pM) was determined by saturation binding assays. 3. The rank order of potency for displacement of [125I]-iodopindolol binding was consistent with binding to a predominantly beta 2-adrenoceptor population (i.e. ICI 118551 greater than isoprenaline greater than adrenaline greater than noradrenaline greater than atenolol). Computer analysis of displacement curves in the presence of a beta 1-subtype selective agent (atenolol) or a beta 2-subtype selective agent (ICI 118551) revealed the presence of beta 2- and beta 1-adrenoceptor subtypes in a ratio of about 80:20%. 4. Saturation binding assays by use of [125I]-iodopindolol were carried out at different perinatal ages to determine total beta-adrenoceptor concentrations and beta 2-subtype (in the presence of 5 x 10(-7) M atenolol) adrenoceptor concentrations. Competition binding assays with atenolol confirmed that at all ages apparent beta 2-adrenoceptor binding accounted for 84-95% of the total beta-adrenoceptor binding. The total beta- and beta 2-adrenoceptor binding capacity increased by 2.3 fold from 20 days post coitum to birth, and then decreased postnatally at 1 and 2 days post partum. The dissociation constant for [125I]-iodopindolol binding did not show any change with age. 5. The change in beta 2-adrenoceptor concentration with age is discussed in relation to the changing beta-adrenoceptor-mediated responsiveness of glucose production by rat liver during perinatal development.

Differences between glucose and insulin stimulation of glycogenesis in cultured fetal hepatocytes

The glycogenic effects of a glucose load (15 mM) and/or insulin (10 nM) were studied in 18-day-old fetal rat hepatocytes after 2 days of culture when medium contained 4 mM glucose. A glucose load led to a stimulation of [14C]glucose glycogen labelling (20 min) earlier than with insulin (30-40 min); maximal stimulations were 3-fold after 1 h for the glucose load and 5-fold after 2-3 h for insulin. Simultaneous addition of the two agents produced synergic effects. When insulin was added 4 h after a glucose load (or vice versa), a second glycogenic response was elicited: a further addition of the same glycogenic agent was ineffective. The early glycogenic effects (up to 2 h) also occurred in the presence of 10 microM cycloheximide, with, however, some decrease of insulin stimulation. The contribution of medium glucose to the glycogen formed for 2 days (67% in the absence of glycogenic agent) was clearly enhanced by a glucose load and to a lesser degree by insulin after a 4-h exposure (83 and 71%, respectively). This was accompanied by a related modification of the participation of glucogenic precursors such as fructose and galactose. Thus, acute glycogenic response to glucose and insulin appeared both synergic and independent, and quite different in several aspects in cultured fetal hepatocytes.

An estimate of the effect of acute exsanguination on catecholamine and glucoregulatory hormone concentrations in the newborn rat

We have determined the effect of acute exsanguination on plasma concentrations of glucose, insulin, glucagon, dopamine, epinephrine and norepinephrine in neonatal Sprague-Dawley rats. This was done by comparing concentrations of these substances in aliquots of blood obtained within 10 s of the initial bleeding to those in blood obtained over the next 50 s of the blood drawing process. Concentrations of glucose and insulin showed no change between early and late samples. Glucagon concentrations showed variable responses dependent upon the age of the animal. Concentrations were unchanged at birth, but increased 22% and 58% at 1 and 6 h of age respectively. Catecholamine concentrations increased greatly (57 to 215%) between blood aliquots regardless of age at the time of sampling. These findings indicate that hormonal responses occur during the relatively brief period of blood drawing in Sprague-Dawley rats, pointing out further limitations inherent in the use of small animals for acute metabolic and hormonal studies.

Glucose metabolism, insulin effects, and developmental age of cultured neonatal rat heart cells

Cultured heart cells from 2-3 day old and 5-6 day old neonatal rats have been used as a model system for the characterization of carbohydrate metabolism in developing cardiac tissue. The rate of depletion of glucose from the growth medium was dependent on 1) the age of the animals from which the cultured cells were obtained, and 2) the presence and absence of serum and/or insulin in the growth medium. The glucose depletion rate in insulin and serum-containing medium was 9.63 +/- 0.96 nmol/min/mg protein for heart cell cultures from 2 day old rats and 3.51 +/- 0.68 nmol/min/mg protein in heart cell cultures from 5 day old rats. Appearance of lactate in the medium during these experiments occurred at the rates of 18.6 +/- 7.9 nmol/min/mg and 6.4 +/- 1.2 nmol/min/mg, respectively. In the absence of serum and insulin, the medium glucose depletion rates were 5.7 +/- 1.6 and 2.2 +/- 0.5 nmol/min/mg for cells derived from 2-day-old and 5-day-old rats, respectively. It is apparent from these data that immature cardiac cells depend upon glucose as a primary source of energy for muscle contraction and cellular growth, and that less-efficient energy-yielding metabolic pathways are used to obtain ATP.

Glucose homeostasis during the perinatal period in normal rats and rats with a glycogen storage disorder

The fetal rat mobilizes liver glycogen during parturition for use as a glucose source until the onset of gluconeogenesis at 2 h after birth. A rat strain (NZR/Mh) unable to mobilize liver glycogen because of a phosphorylase b kinase deficiency has been used to assess the importance of liver glycogen in glucose homeostasis of the newborn. In normal rats the mean blood glucose concentration of the fetus measured at various times up to 24 h after natural birth ranged between 3.7 and 5.4 mM. In contrast, fetuses of the affected rats were hypoglycemic before birth (2.02 +/- 0.15 mM), and by 1 h after birth the blood glucose had decreased to 0.74 +/- 0.14 mM. Concentrations increased by 4 h to 1.48 +/- 0.17 mM and by 24 h reached values not significantly different from the normal newborn rats. Changes in plasma insulin over the perinatal period were similar in both groups although concentrations were always significantly lower in the affected rts. The findings demonstrate the crucial role of the fetal liver glycogen store in the maintenance of normoglycemia in the newborn. The normal rat does not develop hypoglycemia when born naturally and left with the mother after birth (in contrast to other studies in which the newborn were taken by cesarian delivery 1 d prematurely and kept in an artificial environment without food). The rats with the glycogen storage disorder experienced severe hypoglycemia without any apparent effects, which raises questions concerning alternative fuels available to and utilized by the newborn.

Isoxsuprine infusion in the rat: alterations in maternal, fetal and neonatal glucose homeostasis

To determine the mechanism of alteration in glucose homeostasis associated with maternal isoxsuprine administration, isoxsuprine or 0.04 M saline was administered intravenously for 3 hours to term pregnant and age-matched virgin rats. Isoxsuprine infusion significantly increased plasma glucose and insulin concentrations and decreased hepatic glycogen stores in both. Compared to rat pups of saline infused mothers, pups of isoxsuprine infused mothers had significantly elevated plasma glucose concentrations for the first 4 hours of life and plasma insulin concentrations for the first two. Plasma glucose concentrations for the offspring of isoxsuprine treated mothers then decreased significantly and remained so until 16 hours of age. Hepatic glycogen concentrations were significantly less in rat pups of isoxsuprine treated mothers at birth and for the first 4 hours of life. In a limited number of studies, isoxsuprine was present at birth in substantial quantities (80-85% of maternal levels) in the plasma of rat pups of isoxsuprine infused mothers. These data suggest that maternal isoxsuprine therapy mobilizes hepatic glycogen and results in maternal hyperlgycemia. Maternal isoxsuprine infusion may directly deplete fetal hepatic glycogen and result in transient fetal and neonatal hyperglycemia. the in utero depletion of glycogen and possibly, the early stimulation of insulin production may be responsible for the later significant decreases in plasma glucose in the offspring of isoxsuprine treated mothers.