Publications by authors named "David Hauton"

37 Publications

L-Carnitine Stimulates In Vivo Carbohydrate Metabolism in the Type 1 Diabetic Heart as Demonstrated by Hyperpolarized MRI.

Metabolites 2021 Mar 23;11(3). Epub 2021 Mar 23.

Cardiac Metabolism Research Group, Department of Physiology Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK.

The diabetic heart is energetically and metabolically abnormal, with increased fatty acid oxidation and decreased glucose oxidation. One factor contributing to the metabolic dysfunction in diabetes may be abnormal handling of acetyl and acyl groups by the mitochondria. L-carnitine is responsible for their transfer across the mitochondrial membrane, therefore, supplementation with L-carnitine may provide a route to improve the metabolic state of the diabetic heart. The primary aim of this study was to use hyperpolarized magnetic resonance imaging (MRI) to investigate the effects of L-carnitine supplementation on the in vivo metabolism of [1-C]pyruvate in diabetes. Male Wistar rats were injected with either vehicle or streptozotocin (55 mg/kg) to induce type-1 diabetes. Three weeks of daily i.p. treatment with either saline or L-carnitine (3 g/kg/day) was subsequently undertaken. In vivo cardiac function and metabolism were assessed with CINE and hyperpolarized MRI, respectively. L-carnitine supplementation prevented the progression of hyperglycemia, which was observed in untreated streptozotocin injected animals and led to reductions in plasma triglyceride and ß-hydroxybutyrate concentrations. Hyperpolarized MRI revealed that L-carnitine treatment elevated pyruvate dehydrogenase flux by 3-fold in the diabetic animals, potentially through increased buffering of excess acetyl-CoA units in the mitochondria. Improved functional recovery following ischemia was also observed in the L-carnitine treated diabetic animals.
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http://dx.doi.org/10.3390/metabo11030191DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8004902PMC
March 2021

E2F1: Cause and Consequence of DNA Replication Stress.

Front Mol Biosci 2020 16;7:599332. Epub 2021 Feb 16.

Department of Oncology, Medical Research Council Oxford Institute for Radiation Oncology, University of Oxford, Oxford, United Kingdom.

In mammalian cells, cell cycle entry occurs in response to the correct stimuli and is promoted by the transcriptional activity of E2F family members. E2F proteins regulate the transcription of S phase cyclins and genes required for DNA replication, DNA repair, and apoptosis. The activity of E2F1, the archetypal and most heavily studied E2F family member, is tightly controlled by the DNA damage checkpoints to modulate cell cycle progression and initiate programmed cell death, when required. Altered tumor suppressor and oncogenic signaling pathways often result in direct or indirect interference with E2F1 regulation to ensure higher rates of cell proliferation independently of external cues. Despite a clear link between dysregulated E2F1 activity and cancer progression, literature on the contribution of E2F1 to DNA replication stress phenotypes is somewhat scarce. This review discusses how dysfunctional tumor suppressor and oncogenic signaling pathways promote the disruption of E2F1 transcription and hence of its transcriptional targets, and how such events have the potential to drive DNA replication stress. In addition to the involvement of E2F1 upstream of DNA replication stress, this manuscript also considers the role of E2F1 as a downstream effector of the response to this type of cellular stress. Lastly, the review introduces some reflections on how E2F1 activity is integrated with checkpoint control through post-translational regulation, and proposes an exploitable tumor weakness based on this axis.
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http://dx.doi.org/10.3389/fmolb.2020.599332DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7921158PMC
February 2021

Hyperpolarized magnetic resonance shows that the anti-ischemic drug meldonium leads to increased flux through pyruvate dehydrogenase in vivo resulting in improved post-ischemic function in the diabetic heart.

NMR Biomed 2021 04 17;34(4):e4471. Epub 2021 Jan 17.

Cardiac Metabolism Research Group (CMRG), Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, UK.

The diabetic heart has a decreased ability to metabolize glucose. The anti-ischemic drug meldonium may provide a route to counteract this by reducing l-carnitine levels, resulting in improved cardiac glucose utilization. Therefore, the aim of this study was to use the novel technique of hyperpolarized magnetic resonance to investigate the in vivo effects of treatment with meldonium on cardiac metabolism and function in control and diabetic rats. Thirty-six male Wistar rats were injected either with vehicle, or with streptozotocin (55 mg/kg) to induce a model of type 1 diabetes. Daily treatment with either saline or meldonium (100 mg/kg/day) was undertaken for three weeks. in vivo cardiac function and metabolism were assessed with CINE MRI and hyperpolarized magnetic resonance respectively. Isolated perfused hearts were challenged with low-flow ischemia/reperfusion to assess the impact of meldonium on post-ischemic recovery. Meldonium had no significant effect on blood glucose concentrations or on baseline cardiac function. However, hyperpolarized magnetic resonance revealed that meldonium treatment elevated pyruvate dehydrogenase flux by 3.1-fold and 1.2-fold in diabetic and control animals, respectively, suggesting an increase in cardiac glucose oxidation. Hyperpolarized magnetic resonance further demonstrated that meldonium reduced the normalized acetylcarnitine signal by 2.1-fold in both diabetic and control animals. The increase in pyruvate dehydrogenase flux in vivo was accompanied by an improvement in post-ischemic function ex vivo, as meldonium elevated the rate pressure product by 1.3-fold and 1.5-fold in the control and diabetic animals, respectively. In conclusion, meldonium improves in vivo pyruvate dehydrogenase flux in the diabetic heart, contributing to improved cardiac recovery after ischemia.
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http://dx.doi.org/10.1002/nbm.4471DOI Listing
April 2021

Dimethyl-2-oxoglutarate improves redox balance and mitochondrial function in muscle pericytes of individuals with diabetes mellitus.

Diabetologia 2020 10 30;63(10):2205-2217. Epub 2020 Jul 30.

Bristol Medical School, Translational Health Sciences, University of Bristol, Upper Maudlin Street, Bristol, BS2 8HW, UK.

Aims/hypothesis: Treatment of vascular complications of diabetes remains inadequate. We reported that muscle pericytes (MPs) from limb muscles of vascular patients with diabetes mellitus display elevated levels of oxidative stress causing a dysfunctional phenotype. Here, we investigated whether treatment with dimethyl-2-oxoglutarate (DM-2OG), a tricarboxylic acid cycle metabolite with antioxidant properties, can restore a healthy metabolic and functional phenotype.

Methods: MPs were isolated from limb muscles of diabetes patients with vascular disease (D-MPs) and from non-diabetic control participants (ND-MPs). Metabolic status was assessed in untreated and DM-2OG-treated (1 mmol/l) cells using an extracellular flux analyser and anion-exchange chromatography-mass spectrometry (IC-MS/MS). Redox status was measured using commercial kits and IC-MS/MS, with antioxidant and metabolic enzyme expression assessed by quantitative RT-PCR and western blotting. Myogenic differentiation and proliferation and pericyte-endothelial interaction were assessed as functional readouts.

Results: D-MPs showed mitochondrial dysfunction, suppressed glycolytic activity and reduced reactive oxygen species-buffering capacity, but no suppression of antioxidant systems when compared with ND-MP controls. DM-2OG supplementation improved redox balance and mitochondrial function, without affecting glycolysis or antioxidant systems. Nonetheless, this was not enough for treated D-MPs to regain the level of proliferation and myogenic differentiation of ND-MPs. Interestingly, DM-2OG exerted a positive effect on pericyte-endothelial cell interaction in the co-culture angiogenesis assay, independent of the diabetic status.

Conclusions/interpretation: These novel findings support the concept of using DM-2OG supplementation to improve pericyte redox balance and mitochondrial function, while concurrently allowing for enhanced pericyte-endothelial crosstalk. Such effects may help to prevent or slow down vasculopathy in skeletal muscles of people with diabetes. Graphical abstract.
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http://dx.doi.org/10.1007/s00125-020-05230-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7476972PMC
October 2020

Anion-exchange chromatography mass spectrometry provides extensive coverage of primary metabolic pathways revealing altered metabolism in IDH1 mutant cells.

Commun Biol 2020 05 20;3(1):247. Epub 2020 May 20.

Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK.

Altered central carbon metabolism is a hallmark of many diseases including diabetes, obesity, heart disease and cancer. Identifying metabolic changes will open opportunities for better understanding aetiological processes and identifying new diagnostic, prognostic, and therapeutic targets. Comprehensive and robust analysis of primary metabolic pathways in cells, tissues and bio-fluids, remains technically challenging. We report on the development and validation of a highly reproducible and robust untargeted method using anion-exchange tandem mass spectrometry (IC-MS) that enables analysis of 431 metabolites, providing detailed coverage of central carbon metabolism. We apply the method in an untargeted, discovery-driven workflow to investigate the metabolic effects of isocitrate dehydrogenase 1 (IDH1) mutations in glioblastoma cells. IC-MS provides comprehensive coverage of central metabolic pathways revealing significant elevation of 2-hydroxyglutarate and depletion of 2-oxoglutarate. Further analysis of the data reveals depletion in additional metabolites including previously unrecognised changes in lysine and tryptophan metabolism.
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http://dx.doi.org/10.1038/s42003-020-0957-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7239943PMC
May 2020

Energy Metabolism in the Failing Right Ventricle: Limitations of Oxygen Delivery and the Creatine Kinase System.

Int J Mol Sci 2019 Apr 12;20(8). Epub 2019 Apr 12.

Multidisciplinary Cardiovascular Research Centre, University of Leeds, Leeds LS2 9JT, UK.

Pulmonary arterial hypertension (PAH) results in hypertrophic remodeling of the right ventricle (RV) to overcome increased pulmonary pressure. This increases the O consumption of the myocardium, and without a concomitant increase in energy generation, a mismatch with demand may occur. Eventually, RV function can no longer be sustained, and RV failure occurs. Beta-adrenergic blockers (BB) are thought to improve survival in left heart failure, in part by reducing energy expenditure and hypertrophy, however they are not currently a therapy for PAH. The monocrotaline (MCT) rat model of PAH was used to investigate the consequence of RV failure on myocardial oxygenation and mitochondrial function. A second group of MCT rats was treated daily with the beta-1 blocker metoprolol (MCT + BB). Histology confirmed reduced capillary density and increased capillary supply area without indications of capillary rarefaction in MCT rats. A computer model of O flux was applied to the experimentally recorded capillary locations and predicted a reduction in mean tissue P in MCT rats. The fraction of hypoxic tissue (defined as P < 0.5 mmHg) was reduced following beta-1 blocker (BB) treatment. The functionality of the creatine kinase (CK) energy shuttle was measured in permeabilized RV myocytes by sequential ADP titrations in the presence and absence of creatine. Creatine significantly decreased the K in cells from saline-injected control (CON) rats, but not MCT rats. The difference in K with or without creatine was not different in MCT + BB cells compared to CON or MCT cells. Improved myocardial energetics could contribute to improved survival of PAH with chronic BB treatment.
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http://dx.doi.org/10.3390/ijms20081805DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6514649PMC
April 2019

Modulation of the orthostatic blood pressure response by acute nitrate consumption is dependent upon ethnic origin.

Clin Exp Pharmacol Physiol 2018 11 12;45(11):1106-1117. Epub 2018 Aug 12.

Department of Physiology, Anatomy & Genetics, University of Oxford, Oxford, UK.

Orthostatic stress triggers a response to maintain cerebral perfusion and prevent syncope. Given the hypotensive effects of inorganic nitrate this response to orthostasis may be altered by acute supplementation with inorganic nitrate and modified by ethnic origin. Caucasian and SE Asian (n = 30 for both), were recruited and subjected to an 'active stand test' and brachial artery blood pressure (BP), digit blood flow and ECG were recorded. Following inorganic nitrate supplementation, (10 mg/kg body mass) the tests were repeated. For both Caucasian and SE Asians transition to standing increased diastolic pressure (DP) and heart rate (HR) (P < 0.001 for both) and by calculation increased rate-pressure product (P < 0.001) and decreased pulse pressure (P < 0.01 for both) indicative of decreased ventricular filling. Nitrate supplementation decreased both DP (P < 0.001) and HR (P < 0.001). Assessment of HR variability suggested sympathetic nerve activity, was higher throughout in Caucasians (P < 0.05) coupled with higher parasympathetic tone (P < 0.01). Nitrate had no effect on cardiac autonomic nerve activity, as estimated using HR variability, for supine or standing subjects. The tachycardia and hypertension associated with orthostatic stress were preserved in both Caucasian and SE Asian subjects, however, we highlight possible differences in autonomic nervous system activity between Caucasians and SE Asians. SE Asians are resistant to the hypotensive effects of inorganic nitrate supplementation suggesting the absence of a crucial mechanism for activation of the nitrate-nitrite-nitric oxide system.
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http://dx.doi.org/10.1111/1440-1681.13010DOI Listing
November 2018

Caffeine, gravity, and baroreceptor function: the integration of diet and cardiovascular control.

Adv Physiol Educ 2018 Sep;42(3):454-461

Institute of Clinical Sciences, College of Medical and Dental Sciences, University of Birmingham , Birmingham , United Kingdom.

We describe a simple, cost-effective experiment to demonstrate cardiovascular integration of heart rate and blood pressure to accommodate the environmental and dietary factors of gravity and caffeine. Specific learning objectives associated with this include understanding the effects of posture on blood pressure and heart rate, coupled with the role of caffeine in modifying this response. Inclusion of ECG measurements, coupled with heart rate variability analysis, added a demonstration of the contribution made by the autonomic nervous system under these conditions. We clearly demonstrate that the cardiac work, estimated as rate-pressure product, necessary to undertake the transition from supine to standing, is fixed for a given group of subjects. However, the individual contribution of heart rate and systolic pressure to the cardiac workload is subject to the external factors of gravity and caffeine. Such an activity also demonstrates additional benefits, including unstructured teaching opportunities to augment classroom learning associated with integrative physiology and also the discussion of ethical issues with regard to human experimentation.
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http://dx.doi.org/10.1152/advan.00003.2017DOI Listing
September 2018

Oral nitrate and citrulline decrease blood pressure and increase vascular conductance in young adults: a potential therapy for heart failure.

Eur J Appl Physiol 2016 Sep 22;116(9):1651-61. Epub 2016 Jun 22.

School of Food Science and Nutrition, Faculty of Maths and Physical Sciences, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK.

Purpose: Both inorganic nitrate and citrulline are known to alter the arginine-nitric oxide-nitrate system to increase the bioavailability of nitric oxide with potential benefits in the treatment of heart failure. However, their effects on cardiac electrical activity, vascular compliance and peripheral conductance are less well understood. This study examined the effect of nitrate and citrulline on cardiac electrical activity and blood flow.

Methods: Young adult subjects (n = 12) were recruited to investigate the effects of acute oral nitrate consumption (8 mg/kg) and chronic citrulline consumption (3 g/day) on cardiac electrical activity measured by ECG recording and blood pressure. Blood flow and vascular compliance were measured by IR-plethysmography at the thumb and the hallux.

Results: Nitrate (p < 0.05) and citrulline (p < 0.01) consumption both decreased diastolic blood pressure but had no effect on either pulse pressure or rate-pressure product (NS for both). Citrulline also decreased systolic pressure (p < 0.01). Nitrate and citrulline both decreased vascular compliance (p < 0.05 for both) prior to isometric grip exercise, but this was increased for nitrate following exercise (NS). Citrulline decreased R-R interval 9 % (p < 0.05) at rest and increased heart rate (p < 0.05) in addition to significantly decreasing pulse transit duration (6 %; p < 0.05). QRS duration was also decreased by 5 % for citrulline (p < 0.05) with the reduction in R-R interval.

Conclusion: Both nitrate and citrulline supplementation decreased vascular tone at rest but citrulline also altered sympathovagal balance to increase sympathetic tone. We suggest that both oral nitrate and citrulline may be suitable adjuvants for patients with heart failure to improve peripheral tissue oxygenation.
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http://dx.doi.org/10.1007/s00421-016-3418-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4983290PMC
September 2016

The role of triacylglycerol in cardiac energy provision.

Biochim Biophys Acta 2016 10 12;1861(10):1481-91. Epub 2016 Mar 12.

School of Food Science and Nutrition, Faculty of Mathematics & Physics, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK. Electronic address:

Triacylglycerols (TAGs) constitute the main energy storage resource in mammals, by virtue of their high energy density. This in turn is a function of their highly reduced state and hydrophobicity. Limited water solubility, however, imposes specific requirements for delivery and uptake mechanisms on TAG-utilising tissues, including the heart, as well as intracellular disposition. TAGs constitute potentially the major energy supply for working myocardium, both through blood-borne provision and as intracellular TAG within lipid droplets, but also provide the heart with fatty acids (FAs) which the myocardium cannot itself synthesise but are required for glycerolipid derivatives with (non-energetic) functions, including membrane phospholipids and lipid signalling molecules. Furthermore they serve to buffer potentially toxic amphipathic fatty acid derivatives. Intracellular handling and disposition of TAGs and their FA and glycerolipid derivatives similarly requires dedicated mechanisms in view of their hydrophobic character. Dysregulation of utilisation can result in inadequate energy provision, accumulation of TAG and/or esterified species, and these may be responsible for significant cardiac dysfunction in a variety of disease states. This review will focus on the role of TAG in myocardial energy provision, by providing FAs from exogenous and endogenous TAG sources for mitochondrial oxidation and ATP production, and how this can change in disease and impact on cardiac function. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.
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http://dx.doi.org/10.1016/j.bbalip.2016.03.010DOI Listing
October 2016

Maternal hypoxia decreases capillary supply and increases metabolic inefficiency leading to divergence in myocardial oxygen supply and demand.

PLoS One 2015 1;10(6):e0127424. Epub 2015 Jun 1.

School of Biomedical Science, University of Leeds, Leeds, United Kingdom.

Maternal hypoxia is associated with a decrease in left ventricular capillary density while cardiac performance is preserved, implying a mismatch between metabolism and diffusive exchange. We hypothesised this requires a switch in substrate metabolism to maximise efficiency of ATP production from limited oxygen availability. Rat pups from pregnant females exposed to hypoxia (FIO2=0.12) at days 10-20 of pregnancy were grown to adulthood and working hearts perfused ex vivo. 14C-labelled glucose and 3H-palmitate were provided as substrates and metabolism quantified from recovery of 14CO2 and 3H2O, respectively. Hearts of male offspring subjected to Maternal Hypoxia showed a 20% decrease in cardiac output (P<0.05), despite recording a 2-fold increase in glucose oxidation (P<0.01) and 2.5-fold increase (P<0.01) in palmitate oxidation. Addition of insulin to Maternal Hypoxic hearts, further increased glucose oxidation (P<0.01) and suppressed palmitate oxidation (P<0.05), suggesting preservation in insulin signalling in the heart. In vitro enzyme activity measurements showed that Maternal Hypoxia increased both total and the active component of cardiac pyruvate dehydrogenase (both P<0.01), although pyruvate dehydrogenase sensitivity to insulin was lost (NS), while citrate synthase activity declined by 30% (P<0.001) and acetyl-CoA carboxylase activity was unchanged by Maternal Hypoxia, indicating realignment of the metabolic machinery to optimise oxygen utilisation. Capillary density was quantified and oxygen diffusion characteristics examined, with calculated capillary domain area increased by 30% (P<0.001). Calculated metabolic efficiency decreased 4-fold (P<0.01) for Maternal Hypoxia hearts. Paradoxically, the decline in citrate synthase activity and increased metabolism suggest that the scope of individual mitochondria had declined, rendering the myocardium potentially more sensitive to metabolic stress. However, decreasing citrate synthase may be essential to preserve local PO2, minimising regions of hypoxia and hence maximising the area of myocardium able to preserve cardiac output following maternal hypoxia.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0127424PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4452690PMC
March 2016

Changes to both cardiac metabolism and performance accompany acute reductions in functional capillary supply.

Biochim Biophys Acta 2015 Apr 18;1850(4):681-90. Epub 2014 Dec 18.

School of Biomedical Sciences, University of Leeds, Clarendon Way, Leeds LS2 9JT, United Kingdom.

Background: The relative importance of arteriole supply or ability to switch between substrates to preserve cardiac performance is currently unclear, but may be critically important in conditions such as diabetes.

Methods: Metabolism of substrates was measured before and after infusion of polystyrene microspheres in the perfused working heart to mimic random capillary loss due to microvascular disease. The effect of acute loss of functional capillary supply on palmitate and glucose metabolism together with function was quantified, and theoretical tissue oxygen distribution calculated from histological samples and ventricular VO(2) estimated.

Results: Microsphere infusion led to a dose-dependent decrease in rate-pressure product (RPP) and oxygen consumption (P<0.001). Microsphere infusion also increased work/unit oxygen consumption of hearts ('efficiency') by 25% (P<0.01). When corrected for cardiac work palmitate oxidation remained tightly coupled to very low workloads (RPP<2500 mmHg/min), illustrating a high degree of metabolic control. Arteriole occlusion by microspheres decreased the density of patent capillaries (P<0.001) and correspondingly increased the average capillary supply area by 40% (P<0.01). Calculated rates of oxygen consumption declined from 16.6±7.2 ml/100 ml/min to 12.4±9 ml/100 ml/min following arteriole occlusion, coupled with increases in size of regions of myocardial hypoxia (Control=22.0% vs. Microspheres=42.2%).

Conclusions: Cardiac mechanical performance is very sensitive to arteriolar blockade, but metabolite switching from fatty acid to glucose utilisation may also support cardiac function in regions of declining PO(2).

General Significance: Preserving functional capillary supply may be critical for maintenance of cardiac function when metabolic flexibility is lost, as in diabetes.
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http://dx.doi.org/10.1016/j.bbagen.2014.12.014DOI Listing
April 2015

Glycogen metabolism protects against metabolic insult to preserve carotid body function during glucose deprivation.

J Physiol 2014 Oct 25;592(20):4493-506. Epub 2014 Jul 25.

School of Clinical and Experimental Medicine, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

The view that the carotid body (CB) type I cells are direct physiological sensors of hypoglycaemia is challenged by the finding that the basal sensory neuronal outflow from the whole organ is unchanged in response to low glucose. The reason for this difference in viewpoint and how the whole CB maintains its metabolic integrity when exposed to low glucose is unknown. Here we show that, in the intact superfused rat CB, basal sensory neuronal activity was sustained during glucose deprivation for 29.1 ± 1.2 min, before irreversible failure following a brief period of excitation. Graded increases in the basal discharge induced by reducing the superfusate PO2 led to proportional decreases in the time to the pre-failure excitation during glucose deprivation which was dependent on a complete run-down in glycolysis and a fall in cellular energy status. A similar ability to withstand prolonged glucose deprivation was observed in isolated type I cells. Electron micrographs and immunofluorescence staining of rat CB sections revealed the presence of glycogen granules and the glycogen conversion enzymes glycogen synthase I and glycogen phosphorylase BB, dispersed throughout the type I cell cytoplasm. Furthermore, pharmacological attenuation of glycogenolysis and functional depletion of glycogen both significantly reduced the time to glycolytic run-down by ∼33 and 65%, respectively. These findings suggest that type I cell glycogen metabolism allows for the continuation of glycolysis and the maintenance of CB sensory neuronal output in periods of restricted glucose delivery and this may act as a key protective mechanism for the organ during hypoglycaemia. The ability, or otherwise, to preserve energetic status may thus account for variation in the reported capacity of the CB to sense physiological glucose concentrations and may even underlie its function during pathological states associated with augmented CB discharge.
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http://dx.doi.org/10.1113/jphysiol.2014.276105DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4287740PMC
October 2014

How ubiquitous is endothelial NOS?

Comp Biochem Physiol A Mol Integr Physiol 2013 Sep 30;166(1):207-14. Epub 2013 May 30.

Centre for Cardiovascular Sciences, University of Birmingham, UK.

The ability to regulate vascular tone is an essential cardiovascular control mechanism, with nitric oxide (NO) assumed to be a ubiquitous smooth muscle relaxant. However, the literature contains reports of vasoconstrictor, vasodilator and no response to nitroergic stimulation in non-mammalian vertebrates. We examined functional (branchial artery myography), structural (immunohistochemistry of skeletal muscle), proteomic (Western analysis) and genomic (RT-PCR, sequence orthologues, syntenic analysis) evidence for endothelial NO synthase (NOS3) in model and non-model fish species. A variety of nitrodilators failed to elicit any changes in vascular tone, although a dilatation to exogenous cyclic GMP was noted. NOS3 antibody staining does not localise to endothelial markers in cryosections, and gives rise to non-specific staining of Western blots. Abundant NOS2 mRNA was found in all species but NOS3 was not found in any fish, while putative orthologues are not flanked by similar genes to NOS3 in humans. We conclude that NOS3 does not exist in fish, and that previous reports of its presence may reflect use of antibodies raised against mammalian epitopes.
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http://dx.doi.org/10.1016/j.cbpa.2013.05.027DOI Listing
September 2013

Is cold acclimation of benefit to hibernating rodents?

J Exp Biol 2013 Jun 21;216(Pt 11):2140-9. Epub 2013 Feb 21.

School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.

The thermal challenge associated with cold acclimation (CA) and hibernation requires effective cardio-respiratory function over a large range of temperatures. We examined the impact of acute cooling in a cold-naïve hibernator to quantify the presumed improvement in cardio-respiratory dysfunction triggered by CA, and estimate the role of the autonomic nervous system in optimising cardiac and respiratory function. Golden hamsters (Mesocricetus auratus) were held at a 12 h:12 h light:dark photoperiod and room temperature (21°C euthermic control) or exposed to simulated onset of winter in an environmental chamber, by progression to 1 h:23 h light:dark and 4°C over 4 weeks. In vivo acute cooling (core temperature Tb=25°C) in euthermic controls led to a hypotension and bradycardia, but preserved cardiac output. CA induced a hypertension at normothermia (Tb=37°C) but on cooling led to decreases in diastolic pressure below euthermic controls and a decrease in cardiac output, despite an increase in left ventricular conductance. Power spectral analysis of heart rate variability suggested a decline in vagal tone on cooling euthermic hamsters (Tb=25°C). Following CA, vagal tone was increased at Tb=37°C, but declined more quickly on cooling (Tb=25°C) to preserve vagal tone at levels similar to euthermic controls at Tb=37°C. For the isolated heart, CA led to concentric hypertrophy with decreased end-diastolic volume, but with no change in intrinsic heart rate at either 37 or 25°C. Mechanical impairment was noted at 37°C following CA, with peak developed pressure decreased by 50% and peak rate-pressure product decreased by 65%; this difference was preserved at 25°C. For euthermic hearts, coronary flow showed thermal sensitivity, decreasing by 65% on cooling (T=25°C). By contrast, CA hearts had low coronary flow compared with euthermic controls, but with a loss of thermal sensitivity. Together, these observations suggest that CA induced a functional impairment in the myocardium that limits performance of the cardiovascular system at euthermia, despite increased autonomic input to preserve cardiac function. On acute cooling this autonomic control was lost and cardiac performance declined further than for cold-naïve hamsters, suggesting that CA may compromise elements of cardiovascular function to facilitate preservation of those more critical for subsequent rewarming.
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http://dx.doi.org/10.1242/jeb.079160DOI Listing
June 2013

The impact of acute and chronic catecholamines on respiratory responses to hypoxic stress in the rat.

Pflugers Arch 2013 Feb 6;465(2):209-19. Epub 2013 Jan 6.

School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham, UK.

Chronic catecholamine production is associated with desensitisation and down-regulation of adrenergic receptors and occurs in conditions, such as heart failure and myocardial infarction. The effects of further acute adrenergic stimulation, which may occur during exercise, and their subsequent effects on chemosensitivity and ventilation are unclear. Chronic isoprenaline (ISO) increased ventilation by 50 % (P < 0.05) yet the sensitivity to graded hypoxia was preserved. Acute noradrenaline (NA) in control animals led to a doubling of ventilation in hyperoxia (P < 0.001), and this difference was preserved in graded hypoxia (P < 0.001). Yet, combination of NA + ISO did not increase ventilation beyond ISO at baseline or in hypoxia. ISO, NA, and NA + ISO all induced a metabolic acidosis (P < 0.05) with enhanced ventilation in partial compensation. Carotid sinus nerve (CSN) section led to a partial loss of catecholamine-induced augmentation in ventilation (P < 0.05), yet direct recording from CSN in vitro suggests catecholamine is inhibitory for CSN discharge. These observations suggest that chronic catecholamine exposure may result in decreased exercise performance as a direct consequence of the hyperpnea to compensate for an increased metabolic rate coupled with acidosis and leading to increased central chemosensitivity. A limited contribution from peripheral chemoreceptors was noted but was not a consequence of catecholamine stimulation of the carotid body.
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http://dx.doi.org/10.1007/s00424-012-1210-zDOI Listing
February 2013

The interaction between low glucose and hypoxia in the in vitro, rat carotid body.

Adv Exp Med Biol 2012 ;758:123-7

University of Birmingham, Birmingham, West Midlands, UK.

A role for the carotid body (CB) in systemic glycaemic control is yet to be fully characterised. Observations made on fasted, anaesthetised cats, rats and dogs in vivo showed that intra-arterial injection of sodium cyanide into the carotid sinus region immediately increased carotid sinus nerve (CSN) discharge frequency and elicited a subsequent significant increase in the systemic arterial glucose concentration, within 2-8 min of drug administration (Alvarez-Buylla and Alvarez-Buylla 1988). These responses were abolished in animals in which both CSNs had been surgically sectioned, demonstrating that the increased arterial glucose concentration detected following CB stimulation was dependent on CSN input into the NTS. Although not directly tested by these authors, it was proposed that low plasma glucose directly stimulated the CB, as the increase in CSN discharge frequency elicited with NaCN was attenuated by direct injection of a hyperglycaemic solution into the common carotid artery (Alvarez-Buylla and Alvarez-Buylla 1988; Alvarez-Buylla et al. 1997). Additionally, in dogs with bilateral CB resection (CBR), the rate of exogenous glucose infusion required to maintain a fixed hypoglycaemic level was significantly higher, whilst the endogenous hepatic glucose production was significantly lower, compared to control (CSN intact) animals (Koyama et al. 2000). These results further suggested a dependence on CB stimulation for the maintenance of a physiologically normal plasma glucose concentration, but again no direct measure of CB response to hypoglycaemia had been made.
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http://dx.doi.org/10.1007/978-94-007-4584-1_17DOI Listing
April 2013

The effects of asymmetric ventricular filling on left-right ventricular interaction in the normal rat heart.

Pflugers Arch 2012 Nov 22;464(5):523-34. Epub 2012 Sep 22.

School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.

Heart failure is characterised by ventricular dysfunction and with the potential for changes to ventricular volumes constraining the mechanical performance of the heart. The contribution of this interaction from geometric changes rather than fibrosis or metabolic changes is unclear. Using the constant pressure Langendorff-perfused rat heart, the volume interaction between left ventricle (LV) and right ventricle (RV) was investigated. RV diastolic stiffness (P < 0.001) and developed pressure (P < 0.001) were significantly lower than LV. When the RV was fixed at the end-diastolic volume (EDV) or EDV + 50 %, both LV systolic and diastolic performance were unaffected with increasing LV balloon volume. However, at fixed LV volume, RV systolic performance was significantly decreased when LV volume increased to EDV + 50 % when RV volume was increased incrementally between 50 and 300 μl (P < 0.001). Systolic interaction in RV was noted as declining RV peak systolic load with increasing LV systolic pressure (P < 0.05) and diastolic interaction was noted for RV when LV volume was increased from EDV to EDV + 50 % (P < 0.05). RV diastolic wall stress was increased with increasing LV balloon volume (P < 0.05), but LV wall stress was unaltered at fixed RV balloon volume. Taken together, increasing LV volume above EDV decreased systolic performance and triggered ventricular constraint in the RV but the RV itself had no effect on the performance of the LV. These results are consistent with overload of the LV impairing pulmonary perfusion by direct ventricular interaction with potential alteration to ventilation-perfusion characteristics within the lung.
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http://dx.doi.org/10.1007/s00424-012-1154-3DOI Listing
November 2012

Hypoxia in early pregnancy induces cardiac dysfunction in adult offspring of Rattus norvegicus, a non-hypoxia-adapted species.

Authors:
David Hauton

Comp Biochem Physiol A Mol Integr Physiol 2012 Nov 7;163(3-4):278-85. Epub 2012 Aug 7.

School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.

Environmental stresses such as hypoxia can alter the development of the fetus that are manifested later in life, but the impact of early maternal hypoxia (MH) on cardiac performance, coronary flow and catecholamine responsiveness in adult offspring is less clear. The effects of exposure to chronic hypoxia (FIO(2)=0.12) in early intrauterine development (days E1-10) on cardiac performance of the adult offspring were estimated using the Langendorff-perfused rat heart. Cardiac dysfunction is presented as increased end-diastolic volume, with decreased ventricular stiffness in both male and female adult offspring (P<0.01 for both). While developed pressures were preserved in female MH rats, males demonstrated a decrease in systolic function, estimated as peak developed pressure (P<0.01). Challenge with dobutamine (300 nM), an adrenergic positive inotrope, increased cardiac work for control rats (P<0.01 for male and female rats) but not in MH-male rats. Coronary flow was reduced (P<0.01) and SERCA2 protein expression increased (2-fold, P<0.05) in female offspring, while eNOS protein levels were increased (2.5-fold, P<0.05) in females. This suggests gender-specific differences in compensatory responses to early MH, with female rats increasing calcium turnover to improve contractility and increasing coronary flow through increased expression of eNOS protein, partially restoring coronary perfusion while male rats show little compensation.
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http://dx.doi.org/10.1016/j.cbpa.2012.07.020DOI Listing
November 2012

Cardiac lipoprotein lipase activity in the hypertrophied heart may be regulated by fatty acid flux.

Biochim Biophys Acta 2012 Apr 29;1821(4):627-36. Epub 2011 Dec 29.

School of Clinical and Experimental Medicine, College of Medicinal and Dental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom.

Cardiac hypertrophy is characterised by an imbalance between lipid uptake and fatty acid β-oxidation leading to an accumulation of lipids, particularly triacylglycerol (TAG). It is unclear whether uptake mechanisms such as lipoprotein lipase (LPL) can be attenuated to diminish this uptake. Rats were cold acclimated to induce cardiac hypertrophy and increase cardiac LPL. Lipid uptake and metabolism were altered by feeding a 'Western-style' high fat diet (WSD) or feeding oxfenicine (2g/L) in the drinking water. Diastolic stiffness (increased volume change/unit pressure change) was induced in hypertrophied hearts for rats fed WSD (P<0.05) or WSD+oxfenicine (P<0.01), although absolute performance of cardiac muscle, estimated from stress-strain calculations was unchanged. Cold acclimation increased cardiac endothelial LPL (P<0.05) but this was diminished following oxfenicine. Following WSD LPL was further decreased below WSD-fed control hearts (P<0.05) with no further decrease by oxfenicine supplementation. A negative correlation was noted between plasma TAG and endothelial LPL (correlation coefficient=-0.654; P<0.001) but not cardiac TAG concentration. Transcript levels of angiopoietin-like protein-4 (ANGPTL4) were increased 6-fold by WSD (P<0.05) and increased 15-fold following WSD+oxfenicine (P<0.001). For CA-hearts fed WSD or WSD+oxfenicine ANGPTL4 mRNA levels were preserved at chow-fed levels. VLDLR protein levels were increased 10-fold (P<0.01) by CA. ANGPTL4 protein levels were increased 2-fold (P<0.05) by WSD, but restored following oxfenicine. For CA-hearts WSD increased ANGPTL4 protein levels 3-fold (P<0.01) with WSD+oxfenicine increasing ANGPTL4 protein 4-fold (P<0.01). These data suggest that endothelial LPL levels in the heart are altered to maintain FA flux and may exploit ANGPTL4.
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http://dx.doi.org/10.1016/j.bbalip.2011.12.004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3793859PMC
April 2012

Regulation of lipogenesis by glucocorticoids and insulin in human adipose tissue.

PLoS One 2011 14;6(10):e26223. Epub 2011 Oct 14.

Centre for Endocrinology, Diabetes and Metabolism, Institute of Biomedical Research, School of Clinical and Experimental Medicine, University of Birmingham, Birmingham, United Kingdom.

Patients with glucocorticoid (GC) excess, Cushing's syndrome, develop a classic phenotype characterized by central obesity and insulin resistance. GCs are known to increase the release of fatty acids from adipose, by stimulating lipolysis, however, the impact of GCs on the processes that regulate lipid accumulation has not been explored. Intracellular levels of active GC are dependent upon the activity of 11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) and we have hypothesized that 11β-HSD1 activity can regulate lipid homeostasis in human adipose tissue (Chub-S7 cell line and primary cultures of human subcutaneous (sc) and omental (om) adipocytes. Across adipocyte differentiation, lipogenesis increased whilst β-oxidation decreased. GC treatment decreased lipogenesis but did not alter rates of β-oxidation in Chub-S7 cells, whilst insulin increased lipogenesis in all adipocyte cell models. Low dose Dexamethasone pre-treatment (5 nM) of Chub-S7 cells augmented the ability of insulin to stimulate lipogenesis and there was no evidence of adipose tissue insulin resistance in primary sc cells. Both cortisol and cortisone decreased lipogenesis; selective 11β-HSD1 inhibition completely abolished cortisone-mediated repression of lipogenesis. GCs have potent actions upon lipid homeostasis and these effects are dependent upon interactions with insulin. These in vitro data suggest that manipulation of GC availability through selective 11β-HSD1 inhibition modifies lipid homeostasis in human adipocytes.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0026223PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3194822PMC
February 2012

Endonuclease G is a novel determinant of cardiac hypertrophy and mitochondrial function.

Nature 2011 Oct 5;478(7367):114-8. Epub 2011 Oct 5.

Medical Research Council Clinical Sciences Centre, Faculty of Medicine, Imperial College London, Hammersmith Hospital, Du Cane Road, London W12 ONN, UK.

Left ventricular mass (LVM) is a highly heritable trait and an independent risk factor for all-cause mortality. So far, genome-wide association studies have not identified the genetic factors that underlie LVM variation, and the regulatory mechanisms for blood-pressure-independent cardiac hypertrophy remain poorly understood. Unbiased systems genetics approaches in the rat now provide a powerful complementary tool to genome-wide association studies, and we applied integrative genomics to dissect a highly replicated, blood-pressure-independent LVM locus on rat chromosome 3p. Here we identified endonuclease G (Endog), which previously was implicated in apoptosis but not hypertrophy, as the gene at the locus, and we found a loss-of-function mutation in Endog that is associated with increased LVM and impaired cardiac function. Inhibition of Endog in cultured cardiomyocytes resulted in an increase in cell size and hypertrophic biomarkers in the absence of pro-hypertrophic stimulation. Genome-wide network analysis unexpectedly implicated ENDOG in fundamental mitochondrial processes that are unrelated to apoptosis. We showed direct regulation of ENDOG by ERR-α and PGC1α (which are master regulators of mitochondrial and cardiac function), interaction of ENDOG with the mitochondrial genome and ENDOG-mediated regulation of mitochondrial mass. At baseline, the Endog-deleted mouse heart had depleted mitochondria, mitochondrial dysfunction and elevated levels of reactive oxygen species, which were associated with enlarged and steatotic cardiomyocytes. Our study has further established the link between mitochondrial dysfunction, reactive oxygen species and heart disease and has uncovered a role for Endog in maladaptive cardiac hypertrophy.
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http://dx.doi.org/10.1038/nature10490DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3189541PMC
October 2011

Cold-impaired cardiac performance in rats is only partially overcome by cold acclimation.

J Exp Biol 2011 Sep;214(Pt 18):3021-31

Centre for Cardiovascular Sciences, School of Clinical and Experimental Medicine, University of Birmingham, Birmingham B15 2TT, UK.

The consequences of acute hypothermia include impaired cardiovascular performance, ultimately leading to circulatory collapse. We examined the extent to which this results from intrinsic limitations to cardiac performance or physiological dysregulation/autonomic imbalance, and whether chronic cold exposure could ameliorate the impaired function. Wistar rats were held at a 12 h:12 h light:dark (L:D) photoperiod and room temperature (21°C; euthermic controls), or exposed to a simulated onset of winter in an environmental chamber by progressive acclimation to 1 h:23 h L:D and 4°C over 4 weeks. In vivo, acute cold exposure (core temperature, T(b)=25°C) resulted in hypotension (approximately -20%) due to low cardiac output (approximately -30%) accompanying a bradycardia (approximately -50%). Cold acclimation (CA) induced only partial compensation for this challenge, including increased coronary flow at T(b)=37°C (but not at T(b)=25°C), maintenance of ventricular capillarity and altered sympathovagal balance (increased low:high frequency in power spectral analysis, PSA), suggesting physiological responses alone were insufficient to maintain cardiovascular performance. However, PSA showed maintenance of cardiorespiratory coupling on acute cold exposure in both groups. Ex vivo cardiac performance revealed no change in intrinsic heart rate, but a mechanical impairment of cardiac function at low temperatures following CA. While CA involved an increased capacity for β-oxidation, there was a paradoxical reduction in developed pressure as a result of adrenergic down-regulation. These data suggest that integrated plasticity is the key to cardiovascular accommodation of chronic exposure to a cold environment, but with the potential for improvement by intervention, for example with agents such as non-catecholamine inotropes.
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http://dx.doi.org/10.1242/jeb.053587DOI Listing
September 2011

Physiological angiogenesis is a graded, not threshold, response.

J Physiol 2011 Jan 8;589(Pt 1):195-206. Epub 2010 Nov 8.

Angiogenesis Research Group, Centre for Cardiovascular Sciences, University of Birmingham Medical School, Birmingham B15 2TT, UK.

Angiogenesis may be induced in skeletal muscle by metabolic or mechanical factors, but whether an in vivo stimulus threshold applies for physiological angiogenesis is unknown. We compared three models of muscle overload inducing varying degrees of stretch on angiogenesis. Rat extensor digitorum longus (EDL) was overloaded by (a) extirpation of the synergist tibialis anterior (TA), (b) sectioning the distal tendon of the TA, or (c) release of the TA tendon by sectioning the retaining ligament. EDL samples were taken after 4, 7, 14 and 28 days to quantify capillary supply (alkaline phosphatase staining), and co-labelling for cell proliferation (using PCNA). The gradation of overload was confirmed by Western analysis of SERCA and CPT expression (1.6- to 7.2-fold and 8.3- to 33.9-fold changes, respectively), and the force characteristics of EDL. There was a significant increase in the number of new myonuclei only in the extirpated group after 7 days, while there was a graded increase in capillary-linked PCNA density (PCNAcap) among groups compared to controls. However, extirpation caused significant increase in PCNAcap after 7 days, whereas tenotomy showed a more modest and delayed increase at 14 days, and ligament transection induced no significant change. Muscle capillary supply followed a similar trend to that of PCNA, whereas the pro-angiogenic VEGF and Flk-1 protein levels were both up-regulated to a similar extent in all three experimental models 7–14 days after surgery. These results are consistent with the hypothesis that overload-induced angiogenesis is primarily a mechanical response, and that it is graded according to stimulus intensity.
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http://dx.doi.org/10.1113/jphysiol.2010.194951DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3039269PMC
January 2011

Does long-term metformin treatment increase cardiac lipoprotein lipase?

Authors:
David Hauton

Metabolism 2011 Jan 12;60(1):32-42. Epub 2010 Feb 12.

School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, B152TT Birmingham, United Kingdom.

Acute activation of adenosine monophosphate-activated protein kinase (AMPK) or jumps in cardiac work increased cardiac endothelial lipoprotein lipase (LPL), yet it is unclear whether chronic AMPK activation maintains this elevated LPL. To activate AMPK chronically, metformin at low (300 mg/kg/d) and high dose (600 mg/kg/d) was administered in drinking water for 14 days. Control, metformin-treated, and 5-amino-imidazole-4-carboxamide riboside (AICAR)-treated (0.5 mmol/L) ex vivo hearts were perfused to investigate uptake of triacylglycerol and cardiac LPL activity. For perfused rat hearts, increased uptake of labeled Intralipid and β-oxidation of Intralipid-fatty acid were noted for both AICAR (P < .05) and high-dose metformin (P < .01). Intralipid incorporation into tissue lipids was decreased by AICAR (P < .05) and increased after high-dose metformin (P < .05), the increase manifest as enhanced triacylglycerol deposition (P < .05). Low-dose metformin did not alter lipid uptake or tissue deposition. Both high-dose metformin and AICAR decreased cardiac acetyl-coenzyme A carboxylase activity (P < .01). Heparin-releasable LPL was increased after treatment with AICAR (P < .05) and high-dose metformin (P < .01). Low-dose metformin did not alter cardiac LPL. High-dose metformin doubled immunoreactive AMPK and phospho-AMPK protein (P < .001) and increased phosphorylation of p38-mitogen-activated protein kinase (P < .05). After heparin pretreatment, the rate of recruitment of LPL to the cardiac endothelium was increased by AICAR (P < .05) but not by high-dose metformin. These data suggest that AMPK activation increased cardiac endothelial LPL, yet acute and chronic activation of AMPK may yield increased LPL through differing mechanisms.
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http://dx.doi.org/10.1016/j.metabol.2009.12.015DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3004047PMC
January 2011

Both substrate availability and utilisation contribute to the defence of core temperature in response to acute cold.

Comp Biochem Physiol A Mol Integr Physiol 2009 Dec 25;154(4):514-22. Epub 2009 Aug 25.

School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham, United Kingdom.

Acute cooling significantly increases energy demand in non-hibernators for the defence of core temperature but the contribution of the liver to thermogenesis is poorly understood. A two-tracer method to estimate lipid metabolism in cold-naïve control (CON) and cold-acclimated (CA) rats was employed to quantify hepatic rates of fat metabolism. Both fenofibrate, to increase liver mass and fat oxidation and dichloroacetate (DCA) to inhibit fat oxidation were used to alter lipid metabolism in CON animals. Following acute cooling, CA led to a doubling of the time to reach a core temperature 25 degrees C (P<0.001), whereas DCA treatment decreased time of cooling (P<0.01). DCA-treatment increased the gradient of Arrhenius-transformed rate-pressure product (P<0.01). CA increased both palmitate uptake (P<0.001) and beta-oxidation (P<0.01) whilst DCA treatment decreased uptake (P<0.01) and beta-oxidation (P<0.05). Tissue-specific estimates of metabolism revealed that CA led to a 12-fold increase in beta-oxidation for brown adipose tissue (P<0.001) whilst fenofibrate halved beta-oxidation in the liver (P<0.01) despite doubling the liver mass (P<0.001) and DCA decreased hepatic beta-oxidation to 15% of control levels. Taken together, these results suggest that the liver has minimal contribution to thermogenesis in the rat, with brown adipose tissue significantly increasing both fat uptake and oxidation in response to CA.
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http://dx.doi.org/10.1016/j.cbpa.2009.08.008DOI Listing
December 2009

Prenatal hypoxia induces increased cardiac contractility on a background of decreased capillary density.

BMC Cardiovasc Disord 2009 Jan 6;9. Epub 2009 Jan 6.

Department of Physiology, School of Clinical and Experimental Medicine, University of Birmingham, Birmingham, UK.

Background: Chronic hypoxia in utero (CHU) is one of the most common insults to fetal development and may be associated with poor cardiac recovery from ischaemia-reperfusion injury, yet the effects on normal cardiac mechanical performance are poorly understood.

Methods: Pregnant female wistar rats were exposed to hypoxia (12% oxygen, balance nitrogen) for days 10-20 of pregnancy. Pups were born into normal room air and weaned normally. At 10 weeks of age, hearts were excised under anaesthesia and underwent retrograde 'Langendorff' perfusion. Mechanical performance was measured at constant filling pressure (100 cm H2O) with intraventricular balloon. Left ventricular free wall was dissected away and capillary density estimated following alkaline phosphatase staining. Expression of SERCA2a and Nitric Oxide Synthases (NOS) proteins were estimated by immunoblotting.

Results: CHU significantly increased body mass (P < 0.001) compared with age-matched control rats but was without effect on relative cardiac mass. For incremental increases in left ventricular balloon volume, diastolic pressure was preserved. However, systolic pressure was significantly greater following CHU for balloon volume = 50 microl (P < 0.01) and up to 200 microl (P < 0.05). For higher balloon volumes systolic pressure was not significantly different from control. Developed pressures were correspondingly increased relative to controls for balloon volumes up to 250 microl (P < 0.05). Left ventricular free wall capillary density was significantly decreased in both epicardium (18%; P < 0.05) and endocardium (11%; P < 0.05) despite preserved coronary flow. Western blot analysis revealed no change to the expression of SERCA2a or nNOS but immuno-detectable eNOS protein was significantly decreased (P < 0.001) in cardiac tissue following chronic hypoxia in utero.

Conclusion: These data offer potential mechanisms for poor recovery following ischaemia, including decreased coronary flow reserve and impaired angiogenesis with subsequent detrimental effects of post-natal cardiac performance.
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http://dx.doi.org/10.1186/1471-2261-9-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2627821PMC
January 2009

Lack of hexose-6-phosphate dehydrogenase impairs lipid mobilization from mouse adipose tissue.

Endocrinology 2008 May 24;149(5):2584-91. Epub 2008 Jan 24.

Division of medical Sciences (Medicine), University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.

In adipose tissue, glucocorticoids regulate lipogenesis and lipolysis. Hexose-6-phosphate dehydrogenase (H6PDH) is an enzyme located in the endoplasmic reticulum that provides a cofactor for the enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1), regulating the set point of its activity and allowing for tissue-specific activation of glucocorticoids. The aim of this study was to examine the adipose tissue biology of the H6PDH null (H6PDH/KO) mouse. Real-time PCR analysis confirmed similar mRNA levels of 11beta-HSD1 and glucocorticoid receptor-alpha in wild-type (WT) and H6PDH/KO mice in liver and gonadal fat depots. Microsomal 11beta-HSD1 protein levels shown by Western blot analysis corresponded well with mRNA expression in gonadal fat of WT and H6PDH/KO mice. Despite this, the enzyme directionality in these tissues changed from predominately oxoreductase in WT to exclusively dehydrogenase activity in the H6PDH/KO mice. In the fed state, H6PDH/KO mice had reduced adipose tissue mass, but histological examination revealed no difference in average adipocyte size between genotypes. mRNA expression levels of the key lipogenic enzymes, acetyl CoA carboxylase, adiponutrin, and stearoyl-coenzyme A desaturase-2, were decreased in H6PDH/KO mice, indicative of impaired lipogenesis. In addition, lipolysis rates were also impaired in the H6PDH/KO as determined by lack of mobilization of fat and no change in serum free fatty acid concentrations upon fasting. In conclusion, in the absence of H6PDH, the set point of 11beta-HSD1 enzyme activity is switched from predominantly oxoreductase to dehydrogenase activity in adipose tissue; as a consequence, this leads to impairment of fat storage and mobilization.
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http://dx.doi.org/10.1210/en.2007-1705DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2329282PMC
May 2008

Hypoglycemia with enhanced hepatic glycogen synthesis in recombinant mice lacking hexose-6-phosphate dehydrogenase.

Endocrinology 2007 Dec 6;148(12):6100-6. Epub 2007 Sep 6.

Division of Medical Sciences, University of Birmingham, Birmingham, United Kingdom.

Hexose-6-phosphate dehydrogenase (H6PDH) knockout (KO) mice have reduced generation of nicotinamide adenine dinucleotide phosphate (reduced) within the endoplasmic reticulum. As a consequence, 11beta-hydroxysteroid dehydrogenase type 1 enzyme activity switches from a reductase to a dehydrogenase leading to glucocorticoid inactivation. 11beta-Hydroxysteroid dehydrogenase type 1 has emerged as an important factor in regulating hepatic glucose output; therefore, we examined aspects of glucose homeostasis in KO mice. Compared with wild-type mice, KO mice reduced weight gain, displayed peripheral fasting hypoglycemia, improved glucose tolerance, and elevated plasma corticosterone concentrations. Plasma insulin levels in fed and fasted KO mice are normal; however, insulin and plasma glucose levels are reduced 4 h after fasted animals are refed, indicating improved insulin sensitivity. There is preserved induction and activity of the glucocorticoid-responsive gluconeogenic enzymes phosphoenolpyruvate carboxykinase and glucose-6-phosphatase in fasted KO mice. Glycogen storage is elevated in fed KO liver, with fed glycogenesis rates increased in KO mice. There is normal flux of lactate through gluconeogenesis recovered as plasma glucose, coupled with increased glycogen derived from lactate. These data suggest partial retention of glucocorticoid sensitivity at the level of the liver. We therefore postulate that increased glycogen synthesis may reflect increased flux of glucose-6-phosphate (H6PDH substrate) through to glycogen in the absence of H6PDH mediated metabolism.
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http://dx.doi.org/10.1210/en.2007-0963DOI Listing
December 2007

The role of the liver in lipid metabolism during cold acclimation in non-hibernator rodents.

Comp Biochem Physiol B Biochem Mol Biol 2006 Jul 6;144(3):372-81. Epub 2006 Apr 6.

Department of Physiology, Division of Medical Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.

Cold exposure increases the demand for energy substrates. Cold acclimation of rats led to a 3-fold increase in fatty acid (FA) beta-oxidation (P<0.01) for ex vivo livers perfused at 37 degrees C. This increase was preserved following perfusion at 25 degrees C (P<0.001). In vitro measurement of absolute rates of hepatic beta-oxidation revealed no significant difference following cold acclimation, implying changes in fatty acid flux through beta-oxidation rather than increased oxidation capacity. Total FA uptake was increased one-third following perfusion at 25 degrees C (P<0.001) and cold acclimation (P<0.05) and cold acclimation led to diversion of tissue FA from storage to beta-oxidation (P<0.01). In separate experiments, in vivo hepatic lipogenesis rates for saponifiable lipids doubled (P<0.01) and cholesterol synthesis increased one-third (P<0.001). Taken together these data suggest the oxidation and synthesis of lipids occur simultaneously in hepatic tissue possibly to increase prevailing tissue FA concentrations and to generate heat through increased metabolic flux rates.
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http://dx.doi.org/10.1016/j.cbpb.2006.03.013DOI Listing
July 2006
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