bluuebunny/crossref_metadata_2025 huggingface dataset:
Muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase in human skeletal muscle
Authors: Del Aguila Luis F. , Krishnan Raj K. , Ulbrecht Jan S. , Farrell Peter A. , Correll Pamela H. , Lang Charles H. , Zierath Juleen R. , Kirwan John P.
Published: December 2017

Physiological stress associated with muscle damage results in systemic insulin resistance. However, the mechanisms responsible for the insulin resistance are not known; therefore, the present study was conducted to elucidate the molecular mechanisms associated with insulin resistance after muscle damage. Muscle biopsies were obtained before (base) and at 1 h during a hyperinsulinemic-euglycemic clamp (40 mU · kg−1 · min−1) in eight young (age 24 ± 1 yr) healthy sedentary (maximal O2 consumption, 49.7 ± 2.4 ml · kg−1 · min−1) males before and 24 h after eccentric exercise (ECC)-induced muscle damage. To determine the role of cytokines in ECC-induced insulin resistance, venous blood samples were obtained before (control) and 24 h after ECC to evaluate ex vivo endotoxin-induced mononuclear cell secretion of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1β. Glucose disposal was 19% lower after ECC ( P < 0.05). Insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation was 45% lower after ECC ( P < 0.05). Insulin-stimulated phosphatidylinositol (PI) 3-kinase, Akt (protein kinase B) serine phosphorylation, and Akt activity were reduced 34, 65, and 20%, respectively, after ECC ( P < 0.05). TNF-α, but not IL-6 or IL-1β production, increased 2.4-fold 24 h after ECC ( P < 0.05). TNF-α production was positively correlated with reduced insulin action on PI 3-kinase ( r = 0.77, P = 0.04). In summary, the physiological stress associated with muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase, presumably leading to decreased insulin-mediated glucose uptake. Although more research is needed on the potential role for TNF-α inhibition of insulin action, elevated TNF-α production after muscle damage may impair insulin signal transduction.
Induction of Ca-independent PLA2and conservation of plasmalogen polyunsaturated fatty acids in diabetic heart
Authors: McHowat Jane , Creer Michael H. , Hicks Kristine K. , Jones Janet H. , McCrory Raetreal , Kennedy Richard H.
Published: December 2017

Diabetes-induced changes in phospholipase A2(PLA2) activity have been measured in several tissues but are undefined in diabetic myocardium. We measured ventricular PLA2activity in control, streptozotocin-induced diabetic, and insulin-treated diabetic rats and characterized myocardial phospholipids to determine whether diabetes altered myocardial phospholipid metabolism. Increased membrane-associated Ca2+-independent PLA2(iPLA2) activity was observed in diabetes that was selective for arachidonylated phospholipids. Increased iPLA2activity was accompanied by an increase in choline lysophospholipids. Diabetes was associated with marked alterations in the phospholipid composition of the myocardium, characterized by decreases in esterified arachidonic and docosahexaenoic acids and increases in linoleic acid. The decrease in polyunsaturated fatty acids was confined to diacylphospholipids, whereas the relative amount of these fatty acids in plasmalogens was increased. Diabetes-induced changes in PLA2activity, lysophospholipid production, and alterations in phospholipid composition were all reversed by insulin treatment of diabetic animals. Diabetes-induced changes in membrane phospholipid content and phospholipid hydrolysis may contribute to some of the alterations in myocardial function that are observed in diabetic patients.
Association of phosphatidylinositol 3-kinase with the insulin receptor: compartmentation in rat liver
Authors: Drake Paul G. , Balbis Alejandro , Wu Jiong , Bergeron John J. M. , Posner Barry I.
Published: December 2017

Phosphatidylinositol 3-kinase (PI 3-kinase) plays an important role in a variety of hormone and growth factor-mediated intracellular signaling cascades and has been implicated in the regulation of a number of metabolic effects of insulin, including glucose transport and glycogen synthase activation. In the present study we have examined 1) the association of PI 3-kinase with the insulin receptor kinase (IRK) in rat liver and 2) the subcellular distribution of PI 3-kinase-IRK interaction. Insulin treatment promoted a rapid and pronounced recruitment of PI 3-kinase to IRKs located at the plasma membrane, whereas no increase in association with endosomal IRKs was observed. In contrast to IRS-1-associated PI 3-kinase activity, association of PI 3-kinase with the plasma membrane IRK did not augment the specific activity of the lipid kinase. With use of the selective PI 3-kinase inhibitor wortmannin, our data suggest that the cell surface IRK β-subunit is not a substrate for the serine kinase activity of PI 3-kinase. The functional significance for the insulin-stimulated selective recruitment of PI 3-kinase to cell surface IRKs remains to be elucidated.
Loss of regulation of lipogenesis in the Zucker diabetic (ZDF) rat
Authors: Lee W.-N. Paul , Bassilian Sara , Lim Shu , Boros Laszlo G.
Published: December 2017

We present here a study on the role of leptin in the regulation of lipogenesis by examining the effect of dietary macronutrient composition on lipogenesis in the leptin receptor-defective Zucker diabetic fatty rat (ZDF) and its lean litter mate (ZL). Animals were pair fed two isocaloric diets differing in their fat-to-carbohydrate ratio providing 10 and 30% energy as fat. Lipogenesis was measured in the rats using deuterated water and isotopomer analysis. From the deuterium incorporation into plasma palmitate, stearate, and oleate, we determined de novo synthesis of palmitate and synthesis of stearate by chain elongation and of oleate by desaturation. Because the macronutrient composition and the caloric density were controlled, changes in de novo lipogenesis under these dietary conditions represent adaptation to changes in the fat-to-carbohydrate ratio of the diet. De novo lipogenesis was normally suppressed in response to the high-fat diet in the ZL rat to maintain a relatively constant amount of lipids transported. The ZDF rat had a higher rate of lipogenesis, which was not suppressed by the high-fat diet. The results suggest an important hormonal role of leptin in the feedback regulation of lipogenesis.
Glucose transporter expression in human skeletal muscle fibers
Authors: Gaster M. , Handberg A. , Beck-Nielsen H. , Schrøder H. D.
Published: December 2017

The present study was initiated to investigate GLUT-1 through -5 expression in developing and mature human skeletal muscle. To bypass the problems inherent in techniques using tissue homogenates, we applied an immunocytochemical approach, employing the sensitive enhanced tyramide signal amplification (TSA) technique to detect the localization of glucose transporter expression in human skeletal muscle. We found expression of GLUT-1, GLUT-3, and GLUT-4 in developing human muscle fibers showing a distinct expression pattern. 1) GLUT-1 is expressed in human skeletal muscle cells during gestation, but its expression is markedly reduced around birth and is further reduced to undetectable levels within the first year of life; 2) GLUT-3 protein expression appears at 18 wk of gestation and disappears after birth; and 3) GLUT-4 protein is diffusely expressed in muscle cells throughout gestation, whereas after birth, the characteristic subcellular localization is as seen in adult muscle fibers. Our results show that GLUT-1, GLUT-3, and GLUT-4 seem to be of importance during muscle fiber growth and development. GLUT-5 protein was undetectable in fetal and adult skeletal muscle fibers. In adult muscle fibers, only GLUT-4 was expressed at significant levels. GLUT-1 immunoreactivity was below the detection limit in muscle fibers, indicating that this glucose transporter is of minor importance for muscle glucose supply. Thus we hypothesize that GLUT-4 also mediates basal glucose transport in muscle fibers, possibly through constant exposure to tonal contraction and basal insulin levels.
Effects of individual fatty acids on glucose uptake and glycogen synthesis in soleus muscle in vitro
Authors: Thompson Alison L. , Lim-Fraser Megan Y.-C. , Kraegen Edward W. , Cooney Gregory J.
Published: December 2017

Soleus muscle strips from Wistar rats were preincubated with palmitate in vitro before the determination of insulin-mediated glucose metabolism in fatty acid-free medium. Palmitate decreased insulin-stimulated glycogen synthesis to 51% of control in a time- (0–6 h) and concentration-dependent (0–2 mM) manner. Basal and insulin-stimulated glucose transport/phosphorylation also decreased with time, but the decrease occurred after the effect on glycogen synthesis. Preincubation with 1 mM palmitate, oleate, linoleate, or linolenate for 4 h impaired glycogen synthesis stimulated with a submaximal physiological insulin concentration (300 μU/ml) to 50–60% of the control response, and this reduction was associated with impaired insulin-stimulated phosphorylation of protein kinase B (PKB). Preincubation with different fatty acids (all 1 mM for 4 h) had varying effects on insulin-stimulated glucose transport/phosphorylation, which was decreased by oleate and linoleate, whereas palmitate and linolenate had little effect. Across groups, the rates of glucose transport/phosphorylation correlated with the intramuscular long-chain acyl-CoA content. The similar effects of individual fatty acids on glycogen synthesis but different effects on insulin-stimulated glucose transport/phosphorylation provide evidence that lipids may interact with these two pathways via different mechanisms.
Masoprocol decreases rat lipolytic activity by decreasing the phosphorylation of HSL
Authors: Gowri Maya S. , Azhar Rakia K. , Kraemer Fredric B. , Reaven Gerald M. , Azhar Salman
Published: December 2017

Masoprocol (nordihydroguaiaretic acid), a lipoxygenase inhibitor isolated from the creosote bush, has been shown to decrease adipose tissue lipolytic activity both in vivo and in vitro. The present study was initiated to test the hypothesis that the decrease in lipolytic activity by masoprocol resulted from modulation of adipose tissue hormone-sensitive lipase (HSL) activity. The results indicate that oral administration of masoprocol to rats with fructose-induced hypertriglyceridemia significantly decreased their serum free fatty acid (FFA; P < 0.05), triglyceride (TG; P < 0.001), and insulin ( P < 0.05) concentrations. In addition, isoproterenol-induced lipolytic rate and HSL activity were significantly lower ( P < 0.001) in adipocytes isolated from masoprocol compared with vehicle-treated rats and was associated with a decrease in HSL protein. Incubation of masoprocol with adipocytes from chow-fed rats significantly inhibited isoproterenol-induced lipolytic activity and HSL activity, associated with a decrease in the ability of isoproterenol to phosphorylate HSL. Masoprocol had no apparent effect on adipose tissue phosphatidylinositol 3-kinase activity, but okadaic acid, a serine/threonine phosphatase inhibitor, blocked the antilipolytic effect of masoprocol. The results of these in vitro and in vivo experiments suggest that the antilipolytic activity of masoprocol is secondary to its ability to inhibit HSL phosphorylation, possibly by increasing phosphatase activity. As a consequence, masoprocol administration results in lower serum FFA and TG concentrations in hypertriglyceridemic rodents.
Effects of high-dose estrogen on murine hematopoietic bone marrow precede those on osteogenesis
Authors: Perry Mark J. , Samuels Abigail , Bird Deborah , Tobias Jonathan H.
Published: December 2017

High-dose estrogen both stimulates new medullary bone formation and suppresses hematopoiesis in mouse long bones. To determine whether the latter response is a direct consequence of the former, we compared the time course of estrogen's effects on osteogenesis and hematopoietic bone marrow. Flow cytometry was employed to measure hematopoietic subpopulations in bone marrow from femurs of female mice killed at different times after commencing 0.5 mg estradiol/wk to each animal. Estrogen markedly reduced the number of leucocytes (CD11a positive), which had already diminished by 75% after 4 days and had virtually disappeared by 18 days. Specific populations showed a similar pattern of decline after estrogen, including B lymphocytes, monocytes, and endothelial cells. In contrast, the osteogenic precursor population showed a marked increase after estrogen treatment, as assessed by assaying alkaline phosphatase-positive colony-forming units (fibroblastic) ex vivo. However, this rise did not reach significance until 8 days after estrogen administration, suggesting that it follows rather than precedes estrogen's effects on hematopoiesis. We conclude that estrogen does not suppress hematopoiesis in mouse long bones as a direct consequence of its effects on osteogenesis.
Reduced GLP-1 and insulin responses and glucose intolerance after gastric glucose in GRP receptor-deleted mice
Authors: Persson Kristin , Gingerich Ronald L. , Nayak Sonali , Wada Keiji , Wada Etsuko , Ahrén Bo
Published: December 2017

By applying a newly developed ELISA technique for determining biologically active intact glucagon-like peptide [GLP-1, GLP-1-(7–36)amide] in mouse, plasma baseline GLP-1 in normal NMRI mice was found to be normally distributed (4.5 ± 0.3 pmol/l; n = 72). In anesthetized mice, gastric glucose (50 or 150 mg) increased plasma GLP-1 levels two- to threefold ( P < 0.01). The simultaneous increase in plasma insulin correlated to the 10-min GLP-1 levels ( r = 0.36, P < 0.001; n = 12). C57BL/6J mice deleted of the gastrin-releasing peptide (GRP) receptor by genetic targeting had impaired glucose tolerance ( P = 0.030) and reduced early (10 min) insulin response ( P = 0.044) to gastric glucose compared with wild-type controls. Also, the GLP-1 response to gastric glucose was significantly lower in the GRP receptor-deleted mice than in the controls ( P = 0.045). In conclusion, this study has shown that 1) plasma levels of intact GLP-1 increase dose dependently on gastric glucose challenge in correlation with increased insulin levels in mice, and 2) intact GRP receptors are required for normal GLP-1 and insulin responses and glucose tolerance after gastric glucose in mice.
Developmental changes in the feeding-induced stimulation of translation initiation in muscle of neonatal pigs
Authors: Davis Teresa A. , Nguyen Hanh V. , Suryawan Agus , Bush Jill A. , Jefferson Leonard S. , Kimball Scot R.
Published: December 2017

The rapid gain in skeletal muscle mass in the neonate is associated with a marked elevation in skeletal muscle protein synthesis in response to feeding. The feeding-induced response decreases with development. To determine whether the response to feeding is regulated at the level of translation initiation, the expression, phosphorylation, and function of a number of eukaryotic initiation factors (eIF) were examined. Pigs at 7 and 26 days of age were either fasted overnight or fed porcine milk after an overnight fast. In muscle of 7-day-old pigs, the hyperphosphorylated form of the eIF4E repressor protein, 4E-binding protein 1 (4E-BP1), was undetectable in the fasting state but rose to 60% of total 4E-BP1 after feeding; eIF4E phosphorylation was unaffected by feeding status. The amount of eIF4E in the inactive 4E-BP1 · eIF4E complex was reduced by 80%, and the amount of eIF4E in the active eIF4E · eIF4G complex was increased 14-fold in muscle of 7-day-old pigs after feeding. The amount of 70-kDa ribosomal protein S6 (p70S6) kinase in the hyperphosphorylated form rose 2.5-fold in muscle of 7-day-old pigs after feeding. Each of these feeding-induced responses was blunted in muscle of 26-day-old pigs. eIF2B activity in muscle was unaffected by feeding status but decreased with development. Feeding produced similar changes in eIF characteristics in liver and muscle; however, the developmental changes in liver were not as apparent as in skeletal muscle. Thus the results demonstrate that the developmental change in the acute stimulation of skeletal muscle protein synthesis by feeding is regulated by the availability of eIF4E for 48S ribosomal complex formation. The results further suggest that the overall developmental decline in skeletal muscle protein synthesis involves regulation by eIF2B.
Effect of hepatic denervation on the counterregulatory response to insulin-induced hypoglycemia in the dog
Authors: Jackson Patricia A. , Cardin Sylvain , Coffey Christopher S. , Neal Doss W. , Allen Eric J. , Penaloza Angelina R. , Snead Wanda L. , Cherrington Alan D.
Published: December 2017

Our aim was to determine whether complete hepatic denervation would affect the hormonal response to insulin-induced hypoglycemia in dogs. Two weeks before study, dogs underwent either hepatic denervation (DN) or sham denervation (CONT). In addition, all dogs had hollow steel coils placed around their vagus nerves. The CONT dogs were used for a single study in which their coils were perfused with 37°C ethanol. The DN dogs were used for two studies in a random manner, one in which their coils were perfused with −20°C ethanol (DN + COOL) and one in which they were perfused with 37°C ethanol (DN). Insulin was infused to create hypoglycemia (51 ± 3 mg/dl). In response to hypoglycemia in CONT, glucagon, cortisol, epinephrine, norepinephrine, pancreatic polypeptide, glycerol, and hepatic glucose production increased significantly. DN alone had no inhibitory effect on any hormonal or metabolic counterregulatory response to hypoglycemia. Likewise, DN in combination with vagal cooling also had no inhibitory effect on any counterregulatory response except to reduce the arterial plasma pancreatic polypeptide response. These data suggest that afferent signaling from the liver is not required for the normal counterregulatory response to insulin-induced hypoglycemia.
Effect of hyperinsulinemia on amino acid utilization in the ovine fetus
Authors: Thureen Patti J. , Scheer Bryan , Anderson Susan M. , Tooze Janet A. , Young David A. , Hay William W.
Published: December 2017

We studied the effect of an acute 4-h period of hyperinsulinemia (H) on net utilization rates (AAURnet) of 21 amino acids (AA) in 17 studies performed in 13 late-gestation fetal sheep by use of a novel fetal hyperinsulinemic-euglycemic-euaminoacidemic clamp. During H [84 ± 12 (SE) μU/ml H, 15 ± 2 μU/ml control (C), P < 0.00001], euglycemia was maintained by glucose clamp (19 ± 0.05 μmol/ml H, 1.19 ± 0.04 μmol/ml C), and euaminoacidemia (mean 4.1 ± 3.3% increase for all amino acid concentrations [AA], nonsignificantly different from zero) was maintained with a mixed amino acid solution adjusted to keep lysine concentration constant and other [AA] near C values. H produced a 63.7% increase in AAURnet (3.29 ± 0.66 μmol · min−1 · kg−1 H, 2.01 ± 0.55 μmol · min−1 · kg−1 C, P < 0.001), accounting for a 60.1% increase in fetal nitrogen uptake rate (2,064 ± 108 mg · day−1 · kg−1 H, 1,289 ± 73 mg · day−1 · kg−1 C, P < 0.001). Mean AA clearance rate (AAURnet/[AA]) increased by 64.5 ± 18.9% ( P < 0.001). Thus acute physiological H increases net amino acid and nitrogen utilization rates in the ovine fetus independent of plasma glucose and [AA].
Mechanisms underlying impaired GLUT-4 translocation in glycogen-supercompensated muscles of exercised rats
Authors: Kawanaka Kentaro , Nolte Lorraine A. , Han Dong-Ho , Hansen Polly A. , Holloszy John O.
Published: December 2017

Exercise training induces an increase in GLUT-4 in muscle. We previously found that feeding rats a high-carbohydrate diet after exercise, with muscle glycogen supercompensation, results in a decrease in insulin responsiveness so severe that it masks the effect of a training-induced twofold increase in GLUT-4 on insulin-stimulated muscle glucose transport. One purpose of this study was to determine whether insulin signaling is impaired. Maximally insulin-stimulated phosphatidylinositol (PI) 3-kinase activity was not significantly reduced, whereas protein kinase B (PKB) phosphorylation was ∼50% lower ( P < 0.01) in muscles of chow-fed, than in those of fasted, exercise-trained rats. Our second purpose was to determine whether contraction-stimulated glucose transport is also impaired. The stimulation of glucose transport and the increase in cell surface GLUT-4 induced by contractions were both decreased by ∼65% in glycogen-supercompensated muscles of trained rats. The contraction-stimulated increase in AMP kinase activity, which has been implicated in the activation of glucose transport by contractions, was ∼80% lower in the muscles of the fed compared with the fasted rats 18 h after exercise. These results show that both the insulin- and contraction-stimulated pathways for muscle glucose transport activation are impaired in glycogen-supercompensated muscles and provide insight regarding possible mechanisms.
Thyroid hormone metabolism and cardiac gene expression after acute myocardial infarction in the rat
Authors: Ojamaa Kaie , Kenessey Agnes , Shenoy Rajesh , Klein Irwin
Published: December 2017

In a rat model of acute myocardial infarction (MI) produced by coronary artery ligation, thyroid hormone metabolism was altered with significant reductions (54%) in serum triiodo-l-thyronine (T3), the cellular active hormone metabolite. T3 has profound effects on the heart; therefore, rats were treated with T3 after acute MI for 2 or 3 wk, at either replacement or elevated doses, to determine whether cardiac function and gene expression could be normalized. Acute MI resulted in a 50% ( P < 0.001) decrease in percent ejection fraction (%EF) with a 32–35% increase ( P < 0.01) in compensatory left ventricle (LV) hypertrophy. Treatment of the MI animals with either replacement or elevated doses of T3 significantly increased %EF to 64 and 73% of control, respectively. Expression levels of several T3-responsive genes were altered in the hypertrophied LV after MI, including significant decreases in α-myosin heavy chain (MHC), sarcoplasmic reticulum calcium-activated ATPase (SERCA2), and Kv1.5 mRNA, whereas β-MHC and phospholamban (PLB) mRNA were significantly increased. Normalization of serum T3 did not restore expression of all T3-regulated genes, indicating altered T3 responsiveness in the postinfarcted myocardium. Although β-MHC and Kv1.5 mRNA content was returned to control levels, α-MHC and SERCA2 were unresponsive to T3 at replacement doses, and only at higher doses of T3 was α-MHC mRNA returned to control values. The present study showed that acute MI in the rat was associated with a fall in serum T3 levels, LV dysfunction, and altered expression of T3-responsive genes and that T3 treatment significantly improved cardiac function, with normalization of some, but not all, of the changes in gene expression.
Reference intervals for glucose, β-cell polypeptides, and counterregulatory factors during prolonged fasting
Authors: Højlund Kurt , Wildner-Christensen Mette , Eshøj Ole , Skjærbæk Christian , Holst Jens Juul , Koldkjær Ole , Møller Jensen Dorte , Beck-Nielsen Henning
Published: December 2017

To establish reference intervals for the pancreatic β-cell response and the counterregulatory hormone response to prolonged fasting, we studied 33 healthy subjects (16 males, 17 females) during a 72-h fast. Glucose, insulin, C-peptide, and proinsulin levels decreased (P < 0.001) , and the levels of counterregulatory factors increased during the fast [P < 0.05; glucagon and free fatty acids (FFA) with a linear increase and epinephrine, norepinephrine, and cortisol with a clear underlying circadian rhythm]. Growth hormone secretion increased from the first to third day of fasting (P < 0.05) but actually decreased from the second to third day of fasting (P = 0.03). Males had higher glucose and glucagon levels and lower FFA levels during the fast (P < 0.05), whereas no effect of gender on β-cell polypeptides was observed. A high body mass index resulted in higher insulin and C-peptide levels during the fast (P < 0.05). In conclusion, we have provided reference intervals for glucoregulatory factors during a 72-h fast. We observed a diminished β-cell response concomitant with an increased secretion of counterregulatory hormones. These results should be of clinical and scientific value in the investigation of hypoglycemic disorders.
Lack of effect of incretin hormones on insulin release from pancreatic islets in the bile duct-ligated rats
Authors: Niwa Tae , Nimura Yuji , Niki Ichiro
Published: December 2017

Hyperglycemia associated with obstructive jaundice seriously affects the prognosis of patients with hepatobiliary diseases. We investigated secretory properties of isolated islets from bile duct-ligated (BDL) rats. Pancreatic islets from BDL rats lost their secretory responses to glucagon-like peptide-1 (GLP-1), although their responses to glucose were normal. Loss of potentiation of insulin release was also observed in glucagon and glucose-dependent insulinotropic peptide (GIP), whereas modulation of the release by forskolin, dibutyryl cAMP, or epinephrine remained unaffected. cAMP production by BDL islets was not increased by these insulinotropic hormones. Serum levels of glucagon, but not GIP, were increased in BDL rats. GLP-1 levels were also elevated, although they did not reach statistical significance. Immunoblotting of trimeric G protein subunits demonstrated that GsαL and GsαS, but not Giα1/2 and Giα3/oα, were less expressed in BDL islets. Therefore, unresponsiveness of the β-cell to cAMP-raising hormones is involved in glucose intolerance under cholestasis. It results from diminished expression of α-subunits of the relevant G protein, Gs, and desensitization of receptors of these hormones.
Diversification of cardiac insulin signaling involves the p85α/β subunits of phosphatidylinositol 3-kinase
Authors: Kessler Alexandra , Uphues Ingo , Ouwens D. Margriet , Till Martin , Eckel Jürgen
Published: December 2017

Ventricular cardiomyocytes and cardiac tissue of lean and genetically obese ( fa/fa) Zucker rats were used 1) to study the role of the p85 regulatory subunit isoforms p85α and p85β for insulin signaling through the phosphatidylinositol (PI) 3-kinase pathway, and 2) to elucidate the implications of these mechanisms for cardiac insulin resistance. Western blot analysis of cardiomyocyte lysates revealed expression of p85α and p85β but no detectable amounts of the splice variants of p85α. Essentially no p85α subunit of PI 3-kinase was found to be associated with insulin receptor substrate (IRS)-1 or IRS-2 in basal and insulin-stimulated (5 min) cardiomyocytes. Instead, insulin produced a twofold increase in p85β associated with IRS-1, leading to a three- to fourfold increase in p85β-associated PI 3-kinase activity. This response was significantly reduced in obese animals. Comparable results were obtained in the intact heart after in vivo stimulation. In GLUT-4-containing vesicles, an increased abundance (3.7 ± 0.7-fold over basal) of p85α was observed after insulin stimulation of lean animals, with no significant effect in the obese group. No p85β could be detected in GLUT-4-containing vesicles. Recruitment of the p110 catalytic subunit of PI 3-kinase and a twofold increase in enzyme activity in GLUT-4-containing vesicles by insulin was observed only in lean rats. We conclude that, in the heart, p85α recruits PI 3-kinase activity to GLUT-4 vesicles, whereas p85β represents the main regulator of IRS-1- and IRS-2-mediated PI 3-kinase activation. Furthermore, multiple defects of PI 3-kinase activation, involving both the p85α and the p85β adaptor subunits, may contribute to cardiac insulin resistance.
Uric acid inhibits liver phosphorylaseaactivity under simulated in vivo conditions
Authors: Ercan-Fang Nacide G. , Nuttall Frank Q. , Gannon Mary C.
Published: December 2017

We have reported that glycogen synthesis and degradation can occur in vivo without a significant change in the amount of phosphorylase a present. These data suggest the presence of a regulatable mechanism for inhibiting phosphorylase a activity in vivo. Several effectors have been described. AMP stimulates, whereas ADP, ATP, and glucose inhibit activity. Of these effectors, only the glucose concentration changes under normal conditions; thus it could regulate phosphorylase a activity in vivo. We previously have reported that, when all of these effectors were present at physiological concentrations, the net effect was no change in phosphorylase a activity. Addition of caffeine, an independent inhibitor of activity, to the above effectors not only resulted in inhibition but also restored a glucose concentration-dependent inhibition. Because uric acid is an endogenous xanthine derivative, we decided to determine whether it had an effect on phosphorylase a activity. Independently, uric acid did not affect activity; however, when added at a presumed physiological concentration in combination with AMP, ADP, ATP, and glucose, it inhibited activity. A modest but not statistically significant glucose concentration-dependent inhibition was also present. Thus uric acid may play an important role in regulating phosphorylase a activity in vivo.
A potent PPARα agonist stimulates mitochondrial fatty acid β-oxidation in liver and skeletal muscle
Authors: Minnich Anne , Tian Nian , Byan Lisa , Bilder Glenda
Published: December 2017

The proposed mechanism for the triglyceride (TG) lowering by fibrate drugs is via activation of the peroxisome proliferator-activated receptor-α (PPARα). Here we show that a PPARα agonist, ureido-fibrate-5 (UF-5), ∼200-fold more potent than fenofibric acid, exerts TG-lowering effects (37%) in fat-fed hamsters after 3 days at 30 mg/kg. In addition to lowering hepatic apolipoprotein C-III (apoC-III) gene expression by ∼60%, UF-5 induces hepatic mitochondrial carnitine palmitoyltransferase I (CPT I) expression. A 3-wk rising-dose treatment results in a greater TG-lowering effect (70%) at 15 mg/kg and a 2.3-fold elevation of muscle CPT I mRNA levels, as well as effects on hepatic gene expression. UF-5 also stimulated mitochondrial [3H]palmitate β-oxidation in vitro in human hepatic and skeletal muscle cells 2.7- and 1.6-fold, respectively, in a dose-related manner. These results suggest that, in addition to previously described effects of fibrates on apoC-III expression and on peroxisomal fatty acid (FA) β-oxidation, PPARα agonists stimulate mitochondrial FA β-oxidation in vivo in both liver and muscle. These observations suggest an important mechanism for the biological effects of PPARα agonists.
Role of Na+-K+-ATPase in insulin-induced lactate release by skeletal muscle
Authors: Novel-Chaté Valérie , Rey Valentine , Chioléro René , Schneiter Philippe , Leverve Xavier , Jéquier Eric , Tappy Luc
Published: December 2017

Hyperinsulinemia increases lactate release by various organs and tissues. Whereas it has been shown that aerobic glycolysis is linked to Na+-K+-ATPase activity, we hypothesized that stimulation by insulin of skeletal muscle Na+-K+-ATPase is responsible for increased muscle lactate production. To test this hypothesis, we assessed muscle lactate release in healthy volunteers from the [13C]lactate concentration in the effluent dialysates of microdialysis probes inserted into the tibialis anterior muscles on both sides and infused with solutions containing 5 mmol/l [U-13C]glucose. On one side, the microdialysis probe was intermittently infused with the same solution additioned with 2.10−5M ouabain. In the basal state, [13C]lactate concentration in the dialysate was not affected by ouabain. During a euglycemic-hyperinsulinemic clamp, [13C]lactate concentration increased by 135% in the dialysate without ouabain, and this stimulation was nearly entirely reversed by ouabain (56% inhibition compared with values in the dialysate collected from the contralateral probe). These data indicate that insulin stimulates muscle lactate release by activating Na+-K+-ATPase in healthy humans.
Whole body and skeletal muscle glutamine metabolism in healthy subjects
Authors: Mittendorfer B. , Volpi E. , Wolfe R. R.
Published: December 2017

We measured glutamine kinetics usingl-[5-15N]glutamine andl-[ ring-2H5]phenylalanine infusions in healthy subjects in the postabsorptive state and during ingestion of an amino acid mixture that included glutamine, alone or with additional glucose. Ingestion of the amino acid mixture increased arterial glutamine concentrations by ∼20% (not by 30%; P < 0.05), irrespective of the presence or absence of glucose. Muscle free glutamine concentrations remained unchanged during ingestion of amino acids alone but decreased from 21.0 ± 1.0 to 16.4 ± 1.6 mmol/l ( P < 0.05) during simultaneous ingestion of glucose due to a decrease in intramuscular release from protein breakdown and glutamine synthesis (0.82 ± 0.10 vs. 0.59 ± 0.06 μmol · 100 ml leg−1 · min−1; P < 0.05). In both protocols, muscle glutamine inward and outward transport and muscle glutamine utilization for protein synthesis increased during amino acid ingestion; leg glutamine net balance remained unchanged. In summary, ingestion of an amino acid mixture that includes glutamine increases glutamine availability and uptake by skeletal muscle in healthy subjects without causing an increase in the intramuscular free glutamine pool. Simultaneous ingestion of glucose diminishes the intramuscular glutamine concentration despite increased glutamine availability in the blood due to decreased glutamine production.
PVN lesions prevent the endothelin 1-induced increase in arterial pressure and vasopressin
Authors: Rossi Noreen F. , Chen Haiping
Published: December 2017

Endothelin (ET) acts within the central nervous system to increase arterial pressure and arginine vasopressin (AVP) secretion. This study assessed the role of the paraventricular nuclei (PVN) in these actions. Intracerebroventricular ET-1 (10 pmol) or the ETA antagonist BQ-123 (40 nmol) was administered in conscious intact or sinoaortic-denervated (SAD) Long-Evans rats with sham or bilateral electrolytic lesions of the magnocellular region of the PVN. Baseline values did not differ among groups, and artificial cerebrospinal fluid (CSF) induced no significant changes. In sham-lesioned rats, ET-1 increased mean arterial pressure (MAP) 15.9 ± 1.3 mmHg in intact and 22.3 ± 2.7 mmHg in SAD ( P < 0.001 ET-1 vs. CSF) rats. PVN lesions abolished the rise in MAP: −0.1 ± 2.8 mmHg in intact and 0.0 ± 2.9 mmHg in SAD. AVP increased in only in the sham-lesioned SAD group 8.6 ± 3.5 pg/ml ( P < 0.001 ET-1 vs. CSF). BQ-123 blocked the responses. Thus the integrity of the PVN is required for intracerebroventricularly administered ET-1 to exert pressor and AVP secretory effects.
Selective amylin inhibition of the glucagon response to arginine is extrinsic to the pancreas
Authors: Silvestre R. A. , Rodrı́guez-Gallardo J. , Jodka C. , Parkes D. G. , Pittner R. A. , Young A. A. , Marco J.
Published: December 2017

Amylin, a peptide hormone from pancreatic β-cells, is reported to inhibit insulin secretion in vitro and in vivo and to inhibit nutrient-stimulated glucagon secretion in vivo. However, it has been reported not to affect arginine-stimulated glucagon secretion in vitro. To resolve if the latter resulted from inactive peptide (a problem in the early literature), those experiments were repeated here with well-characterized peptide and found to be valid. In isolated perfused rat pancreas preparations, coperfusion with 1 nM amylin had no effect on arginine-, carbachol-, or vasoactive intestinal peptide-stimulated glucagon secretion. Amylin also had no effect on glucagon output stimulated by decreasing glucose concentration from 11 to 3.2 mM or on glucagon suppression caused by increasing glucose from 3.2 to 7 mM. Amylin at 100 nM had no effect in isolated islets in which glucagon secretion was stimulated by exposure to 10 mM arginine, even though glucagon secretion in the same preparation was inhibited by somatostatin. In anesthetized rats, amylin coinfusion had no effect on glucagon secretion stimulated by insulin-induced hypoglycemia. To reconcile reports of glucagon inhibition with the absence of effect in the experiments just described, anesthetized rats coinfused with rat amylin or with saline were exposed sequentially to intravenous l-arginine (during a euglycemic clamp) and then to hypoglycemia. Amylin inhibited arginine-induced, but not hypoglycemia-induced, glucagon secretion in the same animal. In conclusion, we newly identify a selective glucagonostatic effect of amylin that appears to be extrinsic to the isolated pancreas and may be centrally mediated.
Localization and expression of BCAT during pregnancy and lactation in the rat mammary gland
Authors: Tovar Armando R. , Becerril Enrique , Hernández-Pando Rogelio , López Gabriel , Suryawan Agus , Desantiago Soledad , Hutson Susan M. , Torres Nimbe
Published: December 2017

During lactation, branched-chain aminotransferase (BCAT) gene expression increases in the mammary gland. To determine the cell type and whether this induction is present only during lactation, female rats were randomly assigned to one of three experimental groups: pregnancy, lactation, or postweaning. Mammary gland BCAT activity during the first days of pregnancy was similar to that of virgin rats, increasing significantly from day 16 to the last day of pregnancy. Maximal BCAT activity occurred on day 12 of lactation. During postweaning, BCAT activity decreased rapidly to values close to those observed in virgin rats. Analyses by Western and Northern blot revealed that changes in enzyme activity were accompanied by parallel changes in the amount of enzyme and its mRNA. Immunohistochemical studies of the mammary gland showed a progressive increase in mitochondrial BCAT (mBCAT)-specific staining of the epithelial acinar cells during lactation, reaching high levels by day 12. Immunoreactivity decreased rapidly after weaning. There was a significant correlation between total BCAT activity and milk production. These results indicate that the pattern of mBCAT gene expression follows lactogenesis stages I and II and is restricted to the milk-producing epithelial acinar cells. Furthermore, BCAT activity is associated with milk production in the mammary gland during lactation.
Arginine and ornithine kinetics in severely burned patients: increased rate of arginine disposal
Authors: Yu Yong-Ming , Ryan Colleen M. , Castillo Leticia , Lu Xiao-Ming , Beaumier Louis , Tompkins Ronald G. , Young Vernon R.
Published: December 2017

Arginine serves multiple roles in the pathophysiological response to burn injury. Our previous studies in burn patients demonstrated a limited net rate of arginine de novo synthesis despite a significantly increased arginine turnover (flux), suggesting that this amino acid is a conditionally indispensable amino acid after major burns. This study used [15N2-guanidino-5,5-2H2]arginine and [5-13C]ornithine as tracers to assess the rate of arginine disposal via its conversion to and subsequent oxidation of ornithine; [5,5-2H2]proline and [5,5,5-2H3]leucine were also used to assess proline and protein kinetics. Nine severely burned patients were studied during a protein-free fast (“basal” or fast) and total parenteral nutrition (TPN) feedings. Compared with values from healthy volunteers, burn injury significantly increased 1) fluxes of arginine, ornithine, leucine, and proline; 2) arginine-to-ornithine conversion; 3) ornithine oxidation; and 4) arginine oxidation. TPN increased arginine-to-ornithine conversion and proportionally increased irreversible arginine oxidation. The elevated arginine oxidation, with limited net de novo synthesis from its immediate precursors, further implies that arginine is a conditionally indispensable amino acid in severely burned patients receiving TPN.
Plasma protein synthesis in patients with low-grade nephrotic proteinuria
Authors: Zanetti Michela , Barazzoni Rocco , Garibotto Giacomo , Davanzo Gloria , Gabelli Carlo , Kiwanuka Edward , Piccoli Antonio , Tosolini Marina , Tessari Paolo
Published: December 2017

Overt nephrotic syndrome is characterized by albumin and fibrinogen hyperproduction and reduced very low density lipoprotein apolipoprotein B-100 (VLDL apoB-100) clearance. Whether similar changes also occur in low-grade proteinuria is not known. Thus we measured albumin, fibrinogen, and VLDL apoB-100 kinetics in six patients with modest proteinuria and normal creatinine clearance (P) and in ten control subjects (C) by leucine tracer infusion and precursor-product relationships. In P, plasma albumin concentration was decreased ( P < 0.003), whereas concentrations of fibrinogen and VLDL apoB-100 were increased ( P < 0.001). In P, albumin fractional secretion rate (FSR) was increased ( P < 0.01), fibrinogen FSR was normal, and VLDL apoB-100 FSR was decreased ( P < 0.03). As a result, in P, absolute secretion rates (ASR) of albumin and fibrinogen were increased ( P < 0.03), whereas VLDL apoB-100 ASR was normal. Albumin FSR was inversely correlated to oncotic pressure in P but not in C. These findings suggest that low-grade nephrotic proteinuria is characterized by simultaneous multiple alterations in turnover rates of albumin, fibrinogen, and VLDL apoB-100. Their pathogenesis, however, appears to be multifactorial.
Differential GH-releasing hormone regulation of GHRH receptor mRNA expression in the rat pituitary
Authors: Lasko Catherine M. , Korytko Andrew I. , Wehrenberg William B. , Cuttler Leona
Published: December 2017

To understand the capacity of growth hormone-releasing hormone (GHRH) to regulate expression of the GHRH receptor, we studied the effects of GHRH on GHRH receptor mRNA expression in immature and adult rats by use of pituitary cell culture and immunoneutralization approaches. Pituitary cell cultures from neonatal (2-day-old) and adult (70-day-old) rats were treated with GHRH for 4, 24, or 72 h. The effect of GHRH on GHRH receptor mRNA expression depended on the duration of GHRH exposure in both age groups; short-term (4 h) GHRH treatment significantly reduced GHRH receptor mRNA expression ( P < 0.05), whereas intermediate treatment (24 h) restored GHRH receptor mRNA to basal levels, and long-term treatment (72 h) stimulated GHRH receptor mRNA expression ( P < 0.02). The long-term stimulatory effect of GHRH on GHRH receptor mRNA expression required the presence of serum in the culture medium, and, in the absence of serum, the stimulatory effect was completely abolished. Moreover, the capacity of the pituitary to increase GHRH receptor mRNA expression in response to 72-h GHRH treatment was age dependent, with neonatal pituitaries exhibiting a much greater stimulatory effect than adult pituitaries ( P < 0.025). Immunoneutralization of endogenous GHRH significantly reduced GHRH receptor mRNA expression in neonatal ( P < 0.004), juvenile ( P < 0.003), and mature ( P < 0.004) pituitaries compared with age-matched controls. Taken together, these results indicate that GHRH is a potent regulator of GHRH receptor gene expression in immature and mature pituitaries; however, the nature and direction of GHRH regulation of its receptor depend significantly on several variables, including the duration of GHRH exposure, the presence of permissive components in serum, and the developmental stage of the pituitary.
Elevated intramyocellular lipid concentration in obese subjects is not reduced after diet and exercise training
Authors: Malenfant Patrick , Tremblay Angelo , Doucet Éric , Imbeault Pascal , Simoneau Jean-Aimé , Joanisse Denis R.
Published: December 2017

To determine the effects of weight loss on intramyocellular energy substrates, vastus lateralis muscle biopsies were taken from six obese subjects (body mass index 34 ± 5 kg/m2) before, after 15 wk of energy restriction (ER; −700 kcal/day), and after a further average 20.7 ± 1.6 wk of endurance training plus low-fat diet (ET-LFD). Body weight fell from 100 ± 6 to 89 ± 6 kg during ER and to 84 ± 4 kg after ET-LFD. Lipids and glycogen were histochemically measured in type I, IIA, and IIB fibers. Total muscle glycogen content (MGC; per 100 fibers) decreased after ER [from 72 ± 13 to 55 ± 8 arbitrary units (AU)]. A similar but not significant decrease was seen in total muscle lipid content (MLC; 14 ± 5 to 9 ± 1 AU). After ET-LFD, MGC returned to initial values (74 ± 8 AU), and MLC approached near-initial values (12 ± 3 AU). Individual fiber lipid concentration did not change throughout the protocol in all fiber types, whereas glycogen concentration increased after ET-LFD. The training effects of ET-LFD were measured as increasing activities of key mitochondrial enzymes. Although total muscle energy reserves can be reduced after weight loss, their concentration within individual myofibers remains elevated. Weight loss does not appear sufficient to correct the potential detrimental effects of high intracellular lipid concentrations.
Involvement of upstream open reading frames in regulation of rat V1bvasopressin receptor expression
Authors: Nomura Atsushi , Iwasaki Yasumasa , Saito Masayuki , Aoki Yoshiaki , Yamamori Etsuko , Ozaki Nobuaki , Tachikawa Kazushige , Mutsuga Noriko , Morishita Minako , Yoshida Masanori , Asai Masato , Oiso Yutaka , Saito Hidehiko
Published: December 2017

The V1bvasopressin receptor, expressed mainly in the corticotroph of the anterior pituitary, mediates the stimulatory effect of vasopressin on ACTH release. To clarify the regulation of receptor expression, we cloned, sequenced (up to ∼5 kb from the translation start site), and characterized the 5′-flanking region of the rat V1breceptor gene. We identified the transcription start site by amplification of cDNA ends and found a new intron within the 5′-untranslated region (5′-UTR) by comparing the sequence with that of cDNA. We then confirmed that the obtained promoter indeed has transcriptional activity by use of the luciferase reporter in AtT-20 mouse corticotroph cells. Interestingly, there were five short upstream open reading frames (uORFs) located within the 5′-UTR that were found to suppress V1bexpression. Subsequent mutational analyses showed that the two downstream uORFs have an inhibitory effect on expression in both homologous and heterologous contexts. Furthermore, the inhibition did not accompany a parallel decrease in mRNA, suggesting that the suppressive effect occurs at a level downstream of transcription. Taken together, our data strongly suggest that the expression of the V1breceptor is regulated at the posttranscriptional as well as transcriptional level through uORFs within the 5′-UTR region of the mRNA. Whether the uORF-mediated regulation of V1bexpression is functionally linked to any intracellular and/or extracellular factor(s) awaits further research.
Stimulation of splanchnic glucose production during exercise in humans contains a glucagon-independent component
Authors: Coker Robert H. , Simonsen Lene , Bülow Jens , Wasserman David H. , Kjær Michael
Published: December 2017

To determine the importance of basal glucagon to the stimulation of net splanchnic glucose output (NSGO) during exercise, seven healthy males performed cycle exercise during a pancreatic islet cell clamp. In one group (BG), glucagon was replaced at basal levels and insulin was adjusted to achieve euglycemia. In another group (GD), only insulin was replaced at the identical rate used in BG, and basal glucagon was not replaced. Exogenous glucose infusion was necessary to maintain euglycemia during exercise in BG and during rest and exercise in GD. Arterial glucagon was at least twofold greater in BG than in GD throughout the pancreatic islet cell clamp. Although basal NSGO remained stable in BG (2.5 ± 0.5 mg · kg−1 · min−1), basal NSGO dropped by 70% in GD (0.7 ± 0.3 mg · kg−1 · min−1). NSGO was also greater in BG than in GD at 10 min of moderate exercise, most likely due to the residual effect of basal glucagon replacement. However, NSGO increased slightly and remained similar throughout the remainder of moderate and heavy exercise in BG and GD. Therefore, a mechanism independent of changes in pancreatic hormones and/or the level of glycemia contributes toward modest stimulation of NSGO during moderate and heavy exercise.