bluuebunny/crossref_metadata_2025 huggingface dataset:
Ascorbic acid, a vitamin, is observed by in vivo13C nuclear magnetic resonance spectroscopy of rat liver
Authors: Küstermann Ekkehard , Seelig Joachim , Künnecke Basil
Published: December 2017

The first in vivo detection of a vitamin with nuclear magnetic resonance (NMR) is reported for mammalian liver. Vitamin C, also known as ascorbic acid, was monitored noninvasively in rat liver by “whole body”13C NMR spectroscopy at high field after infusion of [1,2-13C2]glucose into anesthetized rats. Generally, the carbon resonances of ascorbic acid overlap with those of other highly abundant cellular metabolites, thus precluding their observation in situ. This problem was resolved by taking advantage of the13C-13C spin couplings introduced by the two covalently bound13C nuclei in [1,2-13C2]glucose. During glucose metabolism, [5,6-13C2]ascorbic acid was synthesized, which also exhibited characteristic13C homonuclear spin couplings. This feature enabled the spectral discrimination of ascorbic acid from overlapping singlet resonances of other metabolites. Quantitative analysis of the spin-coupling patterns provided an estimate of the turnover rate of hepatic ascorbic acid in vivo (1.9 ± 0.4 nmol ⋅ min−1⋅ g−1) and a novel approach toward a better understanding of optimal ascorbic acid requirements in humans. The results obtained in vivo were confirmed with high-resolution proton and13C NMR spectroscopy of liver extracts.
New insights into interactions between the human PTH/PTHrP receptor and agonist/antagonist binding
Authors: Fukayama Shoichi , Royo Miryam , Sugita Masahiko , Imrich Amy , Chorev Michael , Suva Larry J. , Rosenblatt Michael , Tashjian Armen H.
Published: December 2017

We prepared a polyclonal antiserum [Ab-(88—97)] against residues 88–97 of the NH2-terminal tail of the human (h) parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor. Ab-(88—97) bound specifically to the receptor, as assessed by fluorescence-activated cell sorter analysis of HEK C21 cells, which stably express ∼400,000 hPTH/PTHrP receptors per cell. Unlike PTH, Ab-(88—97) binding did not elicit either adenosine 3′,5′-cyclic monophosphate or intracellular calcium concentration signaling responses in these cells. Incubation of C21 cells for 90 min at 4°C with hPTH-(1—34) plus antiserum reduced the Ab-(88—97) binding to the cells by up to 40–50% of control values in a PTH concentration-dependent fashion with a half-maximal effective concentration of ∼5 nM. The decrease in Ab-(88—97) binding caused by hPTH-(1—34) was completely reversed by coincubation with hPTHrP-(7—34). We conclude that residues 88–97 of the hPTH/PTHrPR are involved, either directly or indirectly, in agonist but not antagonist binding to the receptor.
A noninternalized nondesensitized truncated AT1Areceptor transduces an amplified ANG II signal
Authors: Conchon Sophie , Peltier Nicolas , Corvol Pierre , Clauser Eric
Published: December 2017

The structural determinants of the rat angiotensin (ANG) II AT1Areceptor involved in receptor internalization, desensitization, and activation are investigated by producing six mutants that had progessively larger deletions of the cytoplasmic tail (−13, −19, −24, −31, −46, and −56 residues, respectively). After stable transfection of the cDNAs into Chinese hamster ovary cells, all mutants, except the most truncated, exhibit normal [Sar1]ANG II affinities [dissociation constant ( Kd) = 0.19–0.70 nM] compared with the wild-type (WT) receptor ( Kd= 0.62 nM) and are able to activate a Gq/11protein and a phospholipase C as measured by the ANG II-induced inositol phosphate (IP) turnover in the different clones. However, one of these mutants, Δ329 (deletion of 31 residues), exhibits a peculiar phenotype. This mutant shows a reduced ligand-induced internalization as measured by the acid-washing procedure (only 32% of receptors are internalized vs. 83% for WT). Moreover, the Δ329 mutant is less desensitized by a pretreatment with either ANG II (15% desensitization of ANG II-stimulated IP turnover vs. 60% for WT receptor) or the phorbol ester phorbol 12-myristate 13-acetate (no desensitization vs. 29% for WT receptor). These functional modifications of the Δ329 mutant are associated with the transduction of an amplified signal as demonstrated on both IP turnover and an integrated physiological effect of ANG II. Taken together, these data indicate that the sequence329SLSTKMS335of the rat AT1Areceptor is involved in both receptor internalization and desensitization. This is the first demonstration that a desensitization- and internalization-defective AT1Areceptor mutant is also hyperreactive and mediates augmented cellular responses.
Minimal influence of blood flow on interstitial glucose and lactate-normal and insulin-resistant muscle
Authors: Holmäng A. , Müller M. , Andersson O. K. , Lönnroth P.
Published: December 2017

To study the regulation of the interstitial glucose concentration in skeletal muscle, nine control subjects and nine older and overweight non-insulin-dependent diabetes mellitus (NIDDM) subjects with extreme insulin resistance were investigated with microdialysis in the medial femoral muscle before and during a euglycemic insulin clamp. After an overnight fast, arterial plasma glucose concentration was 4.9 ± 0.1 and 8.5 ± 0.6 mmol/l ( P < 0.001), respectively. The arterial-interstitial concentration ([a-i]) differences of glucose and lactate were 0.43 ± 0.16 ( P < 0.05) and −0.13 ± 0.05 mmol/l, respectively, in normal subjects. In NIDDM subjects, [a-i] differences for glucose and lactate were nonsignificant. Muscle blood flow was similar in controls and NIDDM subjects. During the glucose clamp, the glucose [a-i] differences increased and the lactate [a-i] differences decreased significantly in both groups. The glucose 170 infusion rate was 8.0 ± 0.77 vs. 3.2 ± 0.51 mg ⋅ kg−1 ⋅ min−1( P < 0.001), and blood flow was 9.9 ± 1.6 vs. 6.7 ± 0.9 ml ⋅ 100 g−1 ⋅ min−1( P < 0.05) in controls and NIDDM subjects, respectively. These results show that 1) the capillary wall is rate limiting for muscle glucose uptake and lactate release in control subjects but not in postabsorptive hyperglycemic insulin-resistant subjects, 2) vasodilation during insulin infusion does not prevent the increase in [a-i] difference of glucose in normal subjects, and 3) in severely insulin-resistant muscle, the [a-i] difference of glucose is not extended despite lack of vasodilation.
Insulin release transduction mechanism through acid glucan 1,4-α-glucosidase activation is Ca2+regulated
Authors: Salehi Albert , Mosén Henrik , Lundquist Ingmar
Published: December 2017

An important signal involved in glucose-stimulated insulin secretion is transduced through the action of a lysosomal acid, glucan 1,4-α-glucosidase. We investigated the Ca2+dependency of this enzyme activity in relation to insulin release. In isolated islets, increased levels of extracellular Ca2+induced a large increase in acid glucan 1,4-α-glucosidase activity accompanied by a similar increase in insulin release at both substimulatory and stimulatory concentrations of glucose. At low glucose the Ca2+“inflow” blocker nifedipine unexpectedly stimulated enzyme activity without affecting insulin release. However, nifedipine suppressed45Ca2+outflow from perifused islets at low glucose and at Ca2+deficiency when intracellular Ca2+was mobilized by carbachol. This nifedepine-induced retention of Ca2+was reflected in increased acid glucan 1,4-α-glucosidase activity. Adding different physiological Ca2+concentrations or nifedipine to islet homogenates did not increase enzyme activity. Neither selective glucan 1,4-α-glucosidase inhibition nor the ensuing suppression of glucose-induced insulin release was overcome by a maximal Ca2+concentration. Hence, Ca2+-induced changes in acid glucan 1,4-α-glucosidase activity were intimately coupled to similar changes in Ca2+-glucose-induced insulin release. Ca2+did not affect the enzyme itself but presumably activated either glucan 1,4-α-glucosidase-containing organelles or closely interconnected messengers.
Metabolism of skin and muscle protein is regulated differently in response to nutrition
Authors: Zhang Xiao-Jun , Chinkes David L. , Doyle David , Wolfe Robert R.
Published: December 2017

We have measured skin and muscle protein kinetics and amino acid (AA) transport in anesthetized rabbits during 1) 64-h fast, 2) AA infusion, 3) AA plus fat emulsion infusion, and 4) AA plus hyperinsulinemia.l-[ ring-13C6]phenylalanine was infused as the tracer, and the ear and hindlimb were used as arteriovenous units to reflect skin and muscle protein kinetics, respectively. Skin protein net balance was not different from zero in all groups, indicating a maintenance of protein mass. In contrast, the muscle net balance differed over a range from −1.6 ± 0.6 after fasting to 0.2 ± 0.2 μmol ⋅ 100 g−1⋅ h−1during hyperinsulinemia. In the skin, 59–66% of intracellular free phenylalanine came from proteolysis, and phenylalanine availability from proteolysis was positively correlated to the protein synthesis rate. In conclusion, normal skin maintains its constant protein mass by efficient reutilization of AAs from proteolysis. In contrast to muscle, skin protein is relatively insensitive to control by nutritional and hormonal factors. Because of the metabolic differences, when limb models are used for muscle protein metabolism, the potential contribution by limb skin should be considered.
Gut mucosal protein synthesis in fed and fasted humans
Authors: Bouteloup-Demange Corinne , Boirie Yves , Déchelotte Pierre , Gachon Pierre , Beaufrère Bernard
Published: December 2017

Fractional protein synthesis rate (FSR) of duodenal mucosa was measured in two groups of six healthy young men, either in the fed state at the end of a 10-day standardized diet or after a 36-h fast. Protein synthesis rate was measured during a 9-h intravenous infusion of [13C]leucine and [2H5]phenylalanine. The fed group also received an intragastric tracer, [2H3]leucine, mixed with the liquid diet. At the end of the tracer infusion, an endoscopy was performed to take duodenal mucosal biopsies. The major results were that 1) duodenal mucosal protein synthesis was high, 48.0 ± 8.5% (SE)/day by use of intravenous leucine tracer and intracellular leucine enrichment; 2) it was not affected by feeding whatever the tracer or the precursor pool used for the calculations; 3) the two intravenous tracers gave different FSR values; and 4) with the intragastric tracer, FSR was 25–220% of the rate calculated with the intravenous tracer, depending on the precursor pool used for the calculation. Thus absolute values of FSR should be taken with caution, because they depend on the precursor pool chosen, the route of tracer administration, and the tracer itself. However, gut mucosal protein synthesis as assessed by an intravenous tracer is not affected by feeding in humans.
Effect of prior eccentric contractions on lactate/H+ transport in rat skeletal muscle
Authors: Pilegaard Henriette , Asp Sven
Published: December 2017

The effect of prior eccentric contractions on skeletal muscle lactate/H+transport was investigated in rats. Lactate transport was measured in sarcolemmal giant vesicles obtained from soleus and red (RG) and white gastrocnemii (WG) muscles 2 days after intense eccentric contractions (ECC) and from the corresponding contralateral control (CON) muscles. The physiochemical buffer capacity was determined in the three muscle types from both ECC and CON legs. Furthermore, the effect of prior eccentric contractions on release and muscle content of lactate and H+ during and after supramaximal stimulation was examined using the perfused rat hindlimb preparation. The lactate transport rate was lower ( P < 0.05) in vesicles obtained from ECC-WG (29%) and ECC-RG (13%) than in vesicles from the CON muscles. The physiochemical buffer capacity was reduced ( P < 0.05) in ECC-WG (13%) and ECC-RG (9%) compared with the corresponding CON muscles. There were only marginal effects on the soleus muscle. Muscle lactate concentrations and release of lactate during recovery from intense isometric contractions were lower ( P< 0.05) in ECC than in CON hindlimbs, indicating decreased anaerobic glycogenolysis. In conclusion, the sarcolemmal lactate/H+ transport capacity and the physiochemical buffer capacity were reduced in prior eccentrically stimulated WG and RG in rats, suggesting that muscle pH regulation may be impaired after unaccustomed eccentric exercise. In addition, the data indicate that the glycogenolytic potential is decreased in muscles exposed to prior eccentric contractions.
Letters to the Editor
Authors: Caumo Andrea , Cobelli Claudio
Published: December 2017

The following is an abstract of the article discussed in the subsequent letter:

Finegood, Diane T., and Dan Tzur. Reduced glucose effectiveness associated with reduced insulin release: an artifact of the minimal-model method. Am. J. Physiol. 271 ( Endocrinol. Metab. 34): E485–E495, 1996.—We previously demonstrated that minimal model-derived estimates of glucose effectiveness (SG), based on the frequently sampled intravenous glucose tolerance test (SG FSIGT), were reduced in islet-transplanted or streptozotocin-treated dogs and in patients with insulin-dependent diabetes mellitus. To ascertain the validity of our observations, we compared SG FSIGT with estimates based on a basal hormone replacement glucose clamp (SG BRCLAMP) and a basal hormone replacement glucose tolerance test (SG BRGTT) in normal control (CNTL, n = 12) and streptozotocin-treated dogs with normal fasting plasma glucose (STZ-Rx, n = 9). SG FSIGT was reduced in STZ-Rx compared with CNTL ( P < 0.05). However, neither SG BRCLAMP nor SG BRGTT was reduced in the STZ-Rx group ( P > 0.05). Comparison of protocols for each subject indicated that SG FSIGT was greater than either SG BRCLAMP or SG BRGTT in control ( P < 0.002) but not in STZ-Rx dogs ( P > 0.1). The relationship of SG FSIGT to insulin secretory function suggests that our previous conclusion that SG FSIGT was reduced in subjects with limited insulin release may be an artifact of the minimal-model method. Our results suggest that caution must be exercised in the interpretation of differences in minimal-model estimates of SG between subject groups with significantly different levels of insulin secretory function.
Adipose tissue metabolism in the postprandial period: microdialysis and arteriovenous techniques compared
Authors: Summers Lucinda K. M. , Arner Peter , Ilic Vera , Clark Mo L. , Humphreys Sandy M. , Frayn Keith N.
Published: December 2017

We investigated whether two different methods of studying metabolism in adipose tissue, microdialysis and the arteriovenous technique, produced comparable results during the postprandial period. Interstitial glycerol concentrations measured by microdialysis are usually used as an index of intracellular lipolysis, and it is not known whether they also reflect the intravascular action of lipoprotein lipase in the postprandial period. The two techniques were compared in 10 healthy subjects fed mixed meals. Interstitial glycerol concentrations reflected those measured in adipose tissue venous plasma. However, the calculation of the rate of glycerol release from adipose tissue using the microdialysis data differed systematically from that using arteriovenous difference measurement. The former method gave, on average, 40% lower values than the latter one. The difference is probably due to the assumptions that had to be made for the calculation of glycerol release. The two techniques have complementary places in the study of postprandial adipose tissue metabolism, with microdialysis reflecting intracellular hormone-sensitive lipase action rather than intravascular lipoprotein lipase.
Adiposity and serum leptin increase in fatty (fa/fa) BNZ neonates without decreased VMH serotonergic activity
Authors: Horwitz Barbara A. , Hamilton Jock S. , Routh Vanessa H. , Green Kerri , Havel Peter , Chan Albert
Published: December 2017

Decreased ventromedial hypothalamic (VMH) serotonergic activity occurs in genetic and diet-induced animal models of obesity. We previously found that this activity was lower in adult and in 12-day-old Zucker fa/ favs. Fa/ Fapups, the fa/ faanimals being identified by their greater adiposity. In the present study, we evaluated fa/ farats (Brown Norway-Zucker hybrids) at ages 2, 4, 7, and 12 days to test the hypothesis that lower VMH serotonergic activity occurs before increased adiposity and/or attenuated energy expenditure. Our results negate this hypothesis. VMH serotonergic activity showed no consistent genotype differences even at 12 days of age. In contrast, by day 7, fa/ favs. Fa/ Fapups had higher serum leptin concentrations, greater percent body fat, lower resting and cold-induced energy expenditure, and lower activity of brown fat thyroxine 5′-deiodinase, an enzyme that converts thyroxine to triiodothyronine. We conclude that the onset of increased adiposity induced by the fa gene does not require decreased VMH serotonergic activity and that the lower serotonergic activity seen in older fa/ fapups may be secondary to metabolic consequences of the disruption of the leptin regulatory pathway.
Thyroid receptor plasticity in striated muscle types: effects of altered thyroid state
Authors: Haddad Fadia , Qin Anqi X. , McCue Samuel A. , Baldwin Kenneth M.
Published: December 2017

This study examined nuclear thyroid receptor (TR) maximum binding capacity (Bmax), dissociation constant ( Kd), and TR isoform (α1, α2, β1) mRNA expression in rodent cardiac, “fast-twitch white,” “fast-twitch red,” and “slow-twitch red” muscle types as a function of thyroid state. These analyses were performed in the context of slow-twitch type I myosin heavy-chain (MHC) expression, a 3,5,3′-triiodothyronine (T3)-regulated gene that displays varying responsiveness to T3in the above tissues. Nuclear T3binding analyses show that the skeletal muscle types express more TRs per unit DNA than cardiac muscle, whereas the latter has a lower Kdthan the former. Altered thyroid state had little effect on either cardiac Bmaxor Kd, whereas hypothyroidism increased Bmaxin the skeletal muscle types without affecting its Kd. Cardiac muscle demonstrated the greatest mRNA signal of TR-β1compared with the other muscle types, whereas the TR-α1mRNA signals were more abundant in the skeletal muscle types, especially fast-twitch red. Hyperthyroidism increased the ratio of β1to α1and decreased the ratio of α2- to α1+β1-mRNA signal across the muscle types, whereas hypothyroidism caused the opposite effects. The nuclear T3affinity correlated significantly with the TR-β1mRNA expression but not with TR-α1mRNA expression. Collectively, these findings suggest that, despite a divergent pattern of TR mRNA expression in the different muscle types, these patterns follow similar qualitative changes under altered thyroid state. Furthermore, TR expression pattern cannot account for the quantitative and qualitative changes in type I MHC expression that occur in the different muscle types.
Extreme hyperinsulinemia unmasks insulin’s effect to stimulate protein synthesis in the human forearm
Authors: Hillier Teresa A. , Fryburg David A. , Jahn Linda A. , Barrett Eugene J.
Published: December 2017

Insulin clearly stimulates skeletal muscle protein synthesis in vitro. Surprisingly, this effect has been difficult to reproduce in vivo. As in vitro studies have typically used much higher insulin concentrations than in vivo studies, we examined whether these concentration differences could explain the discrepancy between in vitro and in vivo observations. In 14 healthy volunteers, we raised forearm insulin concentrations 1,000-fold above basal levels while maintaining euglycemia for 4 h. Amino acids (AA) were given to either maintain basal arterial ( n = 4) or venous plasma ( n = 6) AA or increment arterial plasma AA by 100% ( n = 4) in the forearm. We measured forearm muscle glucose, lactate, oxygen, phenylalanine balance, and [3H]phenylalanine kinetics at baseline and at 4 h of insulin infusion. Extreme hyperinsulinemia strongly reversed postabsorptive muscle’s phenylalanine balance from a net release to an uptake ( P < 0.001). This marked anabolic effect resulted from a dramatic stimulation of protein synthesis ( P < 0.01) and a modest decline in protein degradation. Furthermore, this effect was seen even when basal arterial or venous aminoacidemia was maintained. With marked hyperinsulinemia, protein synthesis increased further when plasma AA concentrations were also increased ( P< 0.05). Forearm blood flow rose at least twofold with the combined insulin and AA infusion ( P< 0.01), and this was consistent in all groups. These results demonstrate an effect of high concentrations of insulin to markedly stimulate muscle protein synthesis in vivo in adults, even when AA concentrations are not increased. This is similar to prior in vitro reports but distinct from physiological hyperinsulinemia in vivo where stimulation of protein synthesis does not occur. Therefore, the current findings suggest that the differences in insulin concentrations used in prior studies may largely explain the previously reported discrepancy between insulin action on protein synthesis in adult muscle in vivo vs. in vitro.
β-Adrenergic receptor number in human lymphocytes is inversely correlated with aerobic capacity
Authors: Fujii Nobuharu , Homma Sachiko , Yamazaki Fumio , Sone Ryoko , Shibata Takeshi , Ikegami Haruo , Murakami Kazuo , Miyazaki Hitoshi
Published: December 2017

In the present study, the relationships between β-adrenergic receptor (β-AR) expression and aerobic capacity evaluated by maximal oxygen consumption ([Formula: see text]) and oxygen consumption level at ventilatory threshold (V˙o2@VT) were investigated. Seventeen physically untrained and 25 trained men participated in the study. After supine resting, the peripheral blood was sampled for preparation of lymphocytes, the model cell used to analyze the β-AR state. The total number of β-AR in lymphocytes (β-ARtotal) was inversely correlated with theV˙o2 max( r = −0.368; P < 0.05) and theV˙o2@VT ( r = −0.359; P < 0.05). Similar relationships were also observed between the number of β-AR in cell surface and both V˙o2 max( r = −0.491; P < 0.05) andV˙o2@VT ( r = −0.498; P < 0.05). However, no correlation was obtained between the number of β-AR in intracellular compartments and eitherV˙o2 maxorV˙o2@VT. The β2-AR mRNA level quantified by the use of competitive reverse transcription-polymerase chain reaction was inversely correlated withV˙o2@VT ( r = −0.567; P < 0.05) and positively correlated with β-ARtotal( r = 0.521; P < 0.05). These findings suggest that the β-AR number in lymphocytes is inversely correlated with aerobic capacity. This relationship may be explained by downregulation of β-AR, including internalization with subsequent degradation of the receptors and inhibition of the β-AR biosynthesis.
Glycemia-lowering effect of cobalt chloride in the diabetic rat: role of decreased gluconeogenesis
Authors: Saker Firas , Ybarra Juan , Leahy Patrick , Hanson Richard W. , Kalhan Satish C. , Ismail-Beigi Faramarz
Published: December 2017

Results of previous studies indicated that treatment of diabetic rats (induced by streptozotocin) with cobalt chloride (CoCl2) resulted in a significant decrement in serum glucose concentration. The present study was designed to determine the potential role of enhanced glucose uptake vs. decreased glucose production in the above response. The rate of systemic appearance of glucose, measured under fasting conditions using [3-3H]glucose tracer, was reduced from 35.5 ± 2.5 to 17.5 ± 1.8 μmol ⋅ kg−1⋅ min−1in diabetic rats treated with 2 mM CoCl2added to the drinking water for 10–14 days ( P < 0.01). Tissue accumulation of intravenously administered 2-deoxy-[14C]glucose was significantly reduced in kidney and eye of diabetic rats treated with CoCl2, whereas the uptake remained unchanged in several other tissues including cerebrum, red and white skeletal muscle, heart, and liver. The relative content of phospho enolpyruvate carboxykinase (PEPCK) mRNA was increased 3.1-fold in livers of diabetic compared with normal rats ( P < 0.001), and treatment of diabetic rats with CoCl2decreased hepatic PEPCK mRNA levels to normal. The content of PEPCK mRNA in the liver was decreased by 33% in CoCl2-treated normal rats ( P < 0.05). Treatment with CoCl2resulted in no change in cAMP levels in the livers of either diabetic or normal rats. These results suggest that the glycemia-lowering effect of CoCl2is mediated by reductions in the rate of systemic appearance of glucose and hepatic gluconeogenesis.
Dietary carbohydrate and postexercise synthesis of proglycogen and macroglycogen in human skeletal muscle
Authors: Adamo K. B. , Tarnopolsky M. A. , Graham T. E.
Published: December 2017

This study examined the role of carbohydrate (CHO) ingestion on the resynthesis of two pools of glycogen, proglycogen (PG) and macroglycogen (MG), in human skeletal muscle. Nine males completed an exhaustive glycogen depletion exercise bout at 70% maximal O2 consumption on two occasions. Subsequent 48-h dietary interventions consisted of either high (HC, 75% of energy intake) or low (LC, 32% of energy intake) CHO diets. Muscle biopsies were taken at exhaustion (EXH) and 4, 24, and 48 h later. The total muscle glycogen (Gt) at EXH for the HC and LC conditions was not significantly different, and the MG represented ∼12% of the Gt. From EXH to 4 h, there was an increase in the PG only for HC and no change in MG in either diet ( P < 0.05). From 4 to 24 h, the concentration of PG increased in both conditions ( P < 0.05). Between 24 and 48 h, in HC the majority of the increase in Gt was due to the MG pool ( P < 0.05). The MG and PG concentrations for HC were significantly greater than for LC at 24 and 48 h ( P < 0.05). At 48 h the MG represented 40% of the Gt for the HC diet and only 21% for the LC diet. There was no change in the net rates of synthesis of PG or MG over 48 h for LC ( P < 0.05). The net rate of PG synthesis from 0 to 4 h for HC was 16 ± 1.68 mmol glucosyl units ⋅ kg dry wt−1 ⋅ h−1, which was threefold greater than for LC ( P < 0.05). The net rate of PG synthesis decreased significantly from 4 to 24 h for HC, whereas the net rate of MG synthesis was not different over 48 h but was significantly greater than in LC ( P< 0.05). The two pools are synthesized at very different rates; both are sensitive to CHO, and the supercompensation associated with HC is due to a greater synthesis in the MG pool.
Organ-tissue mass measurement allows modeling of REE and metabolically active tissue mass
Authors: Gallagher Dympna , Belmonte Daniel , Deurenberg Paul , Wang Zimian , Krasnow Norman , Pi-Sunyer F. Xavier , Heymsfield Steven B.
Published: December 2017

Investigators have expressed interest in the associations between resting energy expenditure (REE) and body mass for over a century. Traditionally, descriptive models using regression analysis are applied, linking REE with metabolically active compartments such as body cell mass (BCM) and fat-free body mass (FFM). Recently developed whole body magnetic resonance imaging (MRI) and echocardiography methods now allow estimation of all major organs and tissue volumes in vivo. Because measured values are available for REE, BCM, and FFM content of individual organs and tissues, it should now be possible to develop energy expenditure-body composition estimation models based on MRI-measured organ-tissue volumes. Specifically, the present investigation tested the hypothesis that in vivo estimation of whole body REE, BCM, and FFM is possible using MRI- and echocardiography-derived organ volumes combined with previously reported organ-tissue metabolic rates and chemical composition. Thirteen subjects (5 females, 8 males) had REE, BCM, and FFM measured by indirect calorimetry, whole body40K counting, and dual-energy X-ray absorptiometry, respectively. Models developed from estimated and measured variables were highly correlated, with no significant differences between those estimated and measured [e.g., calculated vs. measured REE: r = 0.92, P < 0.001; (mean ± SD) 6,962 ± 1,455 and 7,045 ± 1,450 kJ/day, respectively ( P = not significant)]. Strong associations were observed between REE, individual or combined organ weights, BCM, and FFM that provide new insights into earlier observed metabolic phenomona. The present approach, the first to establish an energy expenditure-body composition link with a mechanistic model in vivo, has the potential to greatly expand our knowledge of energy expenditure-body size relationships in humans.
Aging and fasting regulation of leptin and hypothalamic neuropeptide Y gene expression
Authors: Li Hua , Matheny Michael , Tümer Nihal , Scarpace Philip J.
Published: December 2017

To investigate the role of aging on the fasting-induced suppression of leptin gene expression and increase in hypothalamic neuropeptide Y (NPY) gene expression, we fasted or fed ad libitum male F-344xBN rats aged 3, 24, and 31 mo for 2 days. We examined leptin mRNA levels in retroperitoneal, inguinal, and epididymal white adipose tissue (WAT); serum leptin levels; and NPY mRNA levels in the hypothalamus. We found that leptin mRNA levels were increased from 3 to 24 mo and leveled off between 24 and 31 mo in both retroperitoneal WAT and inguinal WAT but were unchanged with age in epididymal WAT. Serum leptin levels increased with age, whereas hypothalamic NPY mRNA levels did not change with age. Fasting suppressed leptin gene expression in all three WATs and serum leptin. Moreover, this suppression of serum leptin and of leptin message in retroperitoneal WAT was less in aged rats. Conversely, fasting increased hypothalamic NPY message, again to a lesser extent in aged rats. In both fed (ad libitum) and fasted rats, there was a strong correlation between serum leptin and hypothalamic NPY mRNA levels in the young but not in either of the two aged groups. These data suggest that aged F-344xBN rats are leptin resistant and that the fasting regulation of serum leptin, leptin mRNA, and hypothalamic NPY mRNA is impaired in aged rats.
Relation between transamination of branched-chain amino acids and urea synthesis: evidence from human pregnancy
Authors: Kalhan Satish C. , Rossi Karen Q. , Gruca Lourdes L. , Super Dennis M. , Savin Samuel M.
Published: December 2017

Protein and nitrogen (N) accretion by the mother is a major adaptive response to pregnancy in humans and animals to meet the demands of the growing conceptus. Quantitative changes in whole body N metabolism were examined during normal pregnancy by measuring the rates of leucine N ( QN) and carbon ( QC) kinetics with the use of [1-13C,15N]leucine. Rate of synthesis of urea was measured by [15N2]urea tracer. Pregnancy-related change in total body water was quantified by H2[18O] dilution, and respiratory calorimetry was performed to quantify substrate oxidation. A significant decrease in the rate of urea synthesis was evident in the 1st trimester (nonpregnant 4.69 ± 1.14 vs. pregnant 3.44 ± 1.11 μmol ⋅ kg−1⋅ min−1; means ± SD, P < 0.05). The lower rate of urea synthesis was sustained through the 2nd and 3rd trimesters. QNwas also lower in the 1st trimester during fasting; however, it reached a significant level only in the 3rd trimester (nonpregnant 166 ± 35 vs. 3rd trimester 135 ± 16 μmol ⋅ kg−1⋅ h−1; P < 0.05). There was no significant change in QCduring pregnancy. A significant decrease in the rate of transamination of leucine was evident in the 3rd trimester both during fasting and in response to nutrient administration ( P< 0.05). The rate of deamination of leucine was correlated with the rate of urea synthesis during fasting ( r = 0.59, P = 0.001) and during feeding ( r = 0.407, P = 0.01). These data show that pregnancy-related adaptations in maternal N metabolism are evident early in gestation before any significant increase in fetal N accretion. It is speculated that the lower transamination of branched-chain amino acids may be due to decreased availability of N acceptors such as α-ketoglutarate as a consequence of resistance to insulin action evident in pregnancy.
Age-related differences in the pancreatic β-cell response to hyperglycemia after eccentric exercise
Authors: Krishnan Raj K. , Hernandez Jazmir M. , Williamson David L. , O’Gorman Donal J. , Evans William J. , Kirwan John P.
Published: December 2017

Eccentric exercise (ECC) causes muscle damage, insulin resistance, and increased pancreatic β-cell secretion in young individuals. However, the effects of age on the pancreatic β-cell response to glucose after ECC are unknown. Hyperglycemic clamps (180 min, 10.0 mM) were performed on eight young (age 22 ± 1 yr) and eight older (age 66 ± 2 yr) healthy sedentary males without exercise (CONT) and 48 h after ECC. ECC increased ( P < 0.02) muscle soreness ratings and plasma creatine kinase concentrations in both groups. Insulin and C-peptide secretions were similar between young and older subjects during CONT clamps. ECC increased ( P < 0.05) first-phase (0–10 min) C-peptide area under the curve in young (4.2 ± 0.4 vs. 3.7 ± 0.6 nM ⋅ min; ECC vs. CONT, respectively) but not in older subjects (3.2 ± 0.7 vs. 3.5 ± 0.7 nM ⋅ min; ECC vs. CONT), with significant group differences ( P < 0.02). Indeed, ECC repressed ( P < 0.05) first-phase peak C-peptide concentrations in older subjects (0.93 ± 0.16 vs. 1.12 ± 0.11 nM; ECC vs. CONT). Moreover, first-phase C-peptide-to-insulin molar ratios suggest age-related differences ( P < 0.05) in insulin/C-peptide clearance after ECC. Furthermore, the observed C-peptide response after ECC was related to abdominal adiposity [ r = −0.62, P < 0.02, and r = −0.66, P < 0.006, for first and second (10–180 min) phases, respectively]. In conclusion, older individuals did not exhibit the compensatory increase in β-cell secretion observed among young individuals after ECC. Thus, with increasing age, the pancreatic β-cell may be less responsive to the physiological stress associated with ECC.
Insulin does not change the intracellular distribution of hexokinase in rat heart
Authors: Doenst Torsten , Han Qiuying , Goodwin Gary W. , Guthrie Patrick H. , Taegtmeyer Heinrich
Published: December 2017

Preliminary evidence has suggested that hexokinase in rat heart changes its kinetic properties in response to insulin through translocation to the outer mitochondrial membrane. We reexamined this hypothesis in light of tracer kinetic evidence to the contrary. Our methods were as follows. Working rat hearts were perfused with Krebs-Henseleit buffer containing glucose (5 mmol/l) and sodium oleate (0.4 mmol/l). Dynamic glucose uptake was measured with [2-3H]glucose and with 2-deoxy-2-[18F]fluoroglucose (2-[18F]DG). Hexokinase activity was determined in the cytosolic and mitochondrial fractions. Our results are as follows. Uptake of glucose and uptake of 2-[18F]DG were parallel. Insulin (1 mU/ml) increased glucose uptake threefold but had no effect on 2-[18F]DG uptake. The tracer-to-tracee ratio decreased significantly. The Michaelis-Menten constant of hexokinase for 2-deoxyglucose was up to 10 times higher than for glucose. There was no difference in maximal reaction velocity. Two-thirds of hexokinase was bound to mitochondria. Insulin neither caused translocation nor changed Michaelis-Menten constant or maximal reaction velocity. In conclusion, the insulin-induced changes in the tracer-to-tracee ratio are due to a shift of the rate-limiting step for glucose uptake from transport to phosphorylation by hexokinase. Insulin does not affect the intracellular distribution or the kinetics of hexokinase.
Effect of Ca2+ and cAMP on capacitance-measured hormone secretion in human GH-secreting adenoma cells
Authors: Takei Tsukasa , Yasufuku-Takano Junko , Takano Koji , Fujita Toshiro , Yamashita Naohide
Published: December 2017

Membrane capacitance ( C m) was measured as an index of exocytosis in human growth hormone-secreting adenoma cells using the perforated whole cell, patch-clamp technique; the effects of membrane depolarization, growth hormone-releasing hormone, and 8-bromoadenosine 3′,5′-cyclic monophosphate (8-BrcAMP) were examined. C m was increased by membrane depolarization to potentials beyond the threshold necessary to open voltage-gated Ca2+channels. These voltage-dependent changes in C m varied as a function of both depolarization amplitude and duration and were blocked in the presence of the Ca2+channel antagonist nitrendipine (10−6 M). When membrane potential was clamped at the holding potential (−78 mV), voltage-gated Ca2+ channels were closed, and neither application of growth hormone-releasing hormone nor 8-BrcAMP affected C m. However, when these agents were applied to depolarized cells, where the voltage-gated Ca2+ channels were open, the increases in C mwere augmented. From these data, it was concluded that elevation of intracellular cAMP, per se, did not stimulate exocytosis. Rather, Ca2+ influx through voltage-gated channels was a prerequisite for cAMP-induced exocytosis.
Reduced epinephrine clearance and glycemic sensitivity to epinephrine in older individuals
Authors: Marker James C. , Clutter William E. , Cryer Philip E.
Published: December 2017

To test the hypothesis that glycemic sensitivity to epinephrine is reduced in older individuals and to assess the impact of a sedentary lifestyle on responses to the hormone, we performed 30-min sequential intravenous infusions of epinephrine (0, 41, 82, 164, 246, and 328 pmol ⋅ kg−1⋅ min−1) in young ( n = 10) and older ( n = 23) healthy subjects. We performed these again after 12 mo of physical training, which raised peak O2consumption from 24.4 ± 1.0 to 30.4 ± 1.4 ml ⋅ kg−1⋅ min−1( P < 0.01) in most of the older subjects ( n = 21). During epinephrine infusions, plasma epinephrine concentrations were higher ( P = 0.0001) in older than in young subjects (e.g., final values of 7,280 ± 500 vs. 4,560 ± 380 pmol/l, respectively), indicating that the clearance of epinephrine from the circulation was reduced in the older individuals. Plasma epinephrine concentration-response curves disclosed reduced glycemic sensitivity to the hormone in the older subjects ( P = 0.0001), a finding plausibly attributed to increased sympathetic neural activity, as evidenced here by higher plasma norepinephrine concentrations ( P = 0.0001) in the older subjects and consequent desensitization of cellular responsiveness to catecholamines. Training did not correct reduced epinephrine clearance, reduced glycemic sensitivity to epinephrine, or raised norepinephrine levels. We conclude that aging is associated with reduced clearance of epinephrine from the circulation and reduced glycemic sensitivity to epinephrine, the latter plausibly attributed to an age-associated increase in sympathetic neural norepinephrine release. These age-associated changes are not the result of a sedentary lifestyle.
Synergistic actions of insulin and troglitazone on contractility in endothelium-denuded rat aortic rings
Authors: Goud Chetan , Pitt Bertram , Webb R. Clinton , Richey Joyce M.
Published: December 2017

Insulin attenuates vascular contraction via inhibition of voltage-operated Ca2+ channels and by enhancement of endothelium-dependent vasodilation. Thus it has been suggested that hypertension-associated insulin resistance results from an insensitivity to the hormone’s effects on vascular reactivity. This hypothesis has been strengthened by reports that thiazolidinediones, a class of insulin-sensitizing agents, lower blood pressure and improve insulin responsiveness in hypertensive, insulin-resistant animal models. We tested the hypothesis that troglitazone enhances the vasodilating effect of insulin via inhibition of voltage-operated Ca2+ channels in vascular smooth muscle cells. Rat thoracic aortic rings (no endothelium) were suspended in tissue baths for isometric force measurement. Rings were incubated with 0.1 DMSO vehicle (control), troglitazone (10−5 M), insulin (10−7 U/l), or both troglitazone and insulin (1 h) and then contracted with phenylephrine (PE), KCl, or BAY K 8644. Troglitazone increased the EC50 values for PE and KCl. Contractions to BAY K 8644 in troglitazone-treated rings were virtually abolished. Insulin alone had no effect on contraction. However, when insulin was combined with troglitazone, the EC50 values for PE and KCl were further increased. Additionally, the maximum contractions to both PE (14 ± 4% of control) and KCl (12 ± 2% of control) were reduced. Measurement of Ca2+concentration ([Ca2+]) with fura 2-AM in dispersed vascular smooth muscle cells indicated that neither insulin nor troglitazone alone altered PE-induced increases in intracellular [Ca2+]. However, troglitazone and insulin together caused a significant reduction in PE-induced increases in intracellular [Ca2+] (expressed as percentage of preincubation stimulation to PE: 47 ± 10%, treated; 102 ± 13%, vehicle). These results demonstrate that troglitazone inhibits Ca2+ influx and that it acts synergistically with insulin to attenuate further vascular contraction via inhibition of voltage-operated Ca2+ channels.
Measurement of interstitial insulin in human muscle
Authors: Sjöstrand Mikaela , Holmäng Agneta , Lönnroth Peter
Published: December 2017

Previous measurements in lymph and adipose tissue have indicated that interstitial insulin concentrations are ∼40% lower than in plasma. Measurements of insulin in human muscle interstitial fluid have not been performed yet. We developed a new external reference technique for calibration of microdialysis catheters in situ. This technique allows correct assessments of interstitial peptide concentrations and was employed to estimate the insulin concentration in medial quadriceps femoris muscle in 11 individuals (age: 37 ± 3 yr; body mass index: 25.2 ± 1.2 kg/m2) during a two-step euglycemic hyperinsulinemic clamp. At steady-state insulin and glucose infusion, plasma glucose was 5.9 ± 0.2 mmol/l, plasma insulin was 155 ± 17 mU/l, and interstitial muscle insulin was 67 ± 19 mU/l ( n = 9; P < 0.01). At a higher insulin infusion rate, the steady-state plasma insulin concentration was 379 ± 58 mU/l, and interstitial insulin concentration was 180 ± 40 mU/l ( P < 0.01). The data show for the first time that high physiological and supraphysiological plasma insulin levels give 30–50% lower interstitial concentrations of insulin in the muscle. The importance of capillary delivery as a rate-limiting step for the insulin effect is suggested.
3,5,3′-Triiodothyronine actively stimulates UCP in brown fat under minimal sympathetic activity
Authors: Branco Marcelo , Ribeiro Miriam , Negrão Nubio , Bianco Antonio C.
Published: December 2017

To investigate the role of type II 5′-deiodinase (5′D-II) in the expression of mitochondrial uncoupling protein (UCP) in brown adipose tissue (BAT), we injected intact male rats with reverse (r) 3,5,3′-triiodothyronine (T3; 100 μg ⋅ 100 g body wt−1⋅ day−1), an inhibitor of 5′D-II, for 2–5 days. UCP decreased by ∼20% in rats kept at 28°C and failed to increase during cold exposure (4°C). Next, thyroxine treatment (1–10 μg ⋅ 100 g body wt−1⋅ day−1) increased nuclear T3in rats kept at 28 or 4°C. In these rats, nuclear T3correlated positively with UCP. In addition, T3(1–50 μg ⋅ 100 g body wt−1⋅ day−1) given to intact rats (5–15 days; 28°C) induced an approximately twofold increase in UCP. In these T3-treated animals, the interscapular BAT thermal response to norepinephrine infusion also correlated positively with T3dose and UCP content. Treatment with propranolol or reserpine failed to block the T3induction of UCP (∼1.8- and ∼2.3-fold). The results emphasize the importance of local 5′D-II and reveal an independent role of T3in the expression of UCP.
The endocrine secretion of mammalian digestive enzymes by exocrine glands
Authors: Isenman Lois , Liebow Charles , Rothman Stephen
Published: December 2017

The exocrine pancreas and certain salivary glands of mammals secrete a variety of enzymes into the gastrointestinal tract, where they digest food. The same glands also release these enzymes into the bloodstream. This latter process has commonly been assumed to occur solely as the result of a pathological condition or as an inadvertent by-product of exocrine secretion due to the leakage of trace quantities of the enzymes into blood. However, a variety of evidence suggests that the endocrine secretion of digestive enzymes is a normal occurrence that can be of substantial magnitude in healthy individuals, is responsive to various physiological stimuli, and is distinct from exocrine secretion. Recent research has focused attention on this process as a promising means for the delivery of engineered proteins into the systemic circulation for pharmaceutical purposes. In this review, we survey research in this area and consider the evidence for the existence of an endocrine secretion of digestive enzymes, the cause of enzyme release into the bloodstream, its source within the tissue, and, finally, the physiological purposes that this secretion process might serve.
Glycerol and nonesterified fatty acid metabolism in human muscle and adipose tissue in vivo
Authors: Coppack Simon W. , Persson Mai , Judd Robert L. , Miles John M.
Published: December 2017

To determine the relationship between glycerol and nonesterified fatty acid (NEFA) release from adipose tissue, and to test whether forearm muscle and abdominal adipose tissue are capable of extracting these two lipolytic products from the circulation, 13 male subjects were studied after an overnight fast during combined infusion of radiolabeled palmitate and glycerol. Blood samples were taken from a radial artery, a deep forearm vein, and a superficial abdominal vein before and during a 2-h infusion of glucose at ∼7 mg ⋅ kg−1⋅ min−1. The ratio of the appearance rates of total NEFA to glycerol was ∼3/1 during the baseline period but decreased to 1.3/1 during glucose infusion. There was significant extraction of both glycerol and NEFA by forearm muscle. In contrast, there was no apparent uptake of glycerol by adipose tissue. Adipose tissue NEFA uptake was undetectable during the baseline period but became significant during glucose infusion. These data indicate that there is very little to no in situ reesterification of NEFA in adipose tissue after an overnight fast. During glucose infusion, there was apparently a relative increase in the fraction of glycerol derived from the action of lipoprotein lipase and an increase in reesterification in situ.
Does estradiol mediate leptin’s effects on adiposity and body weight?
Authors: Pelleymounter Mary Ann , Baker Mary Beth , McCaleb Michael
Published: December 2017

The role of estradiol in mediating leptin’s effects on body weight was assessed in ovariectomized (OVX) mice before and after the onset of obesity. Ovariectomy did not alter leptin levels before the onset of obesity, and estradiol adminstration (0.05–17 μg/day for 14 days) did not significantly alter leptin levels if they were corrected for the estradiol-induced reduction in body fat. The converse was also true, in that leptin administration (0.4–140 μg/day) did not alter estradiol levels in intact mice. Furthermore, neither estradiol reduction (via ovariectomy) nor addition (via exogenous administration) significantly altered leptin’s ability to reduce fat mass. Leptin was equally effective in reducing body weight in lean or obese OVX mice and intact controls. Finally, estradiol did not change the magnitude of leptin’s effect on fat mass reduction when it was given in combination with leptin to lean intact or OVX mice. Estradiol may have indirectly affected leptin efficacy, because leptin did not produce as large a change in fat mass at lower doses in lean OVX mice as it did in intact counterparts. Taken together, these data suggested that 1) estradiol does not directly regulate leptin secretion or its effects on fat mass and 2) leptin does not directly regulate estradiol secretion or its effects on fat mass. Leptin and estradiol, however, may interact in an indirect fashion to affect fat utilization.