Brown Adipose Tissue Blood Flow Research Articles

The effects of the β1-adrenergic receptor antagonist bisoprolol administration on mirabegron-stimulated human brown adipose tissue thermogenesis.

Pharmacological stimulation of human brown adipose tissue (BAT) has been hindered by ineffective activation or undesirable off-target effects. Oral administration of the maximal allowable dose of mirabegron (200 mg), a β3-adrenergic receptor (β3-AR) agonist, has been effective in stimulating BAT thermogenesis and whole-body energy expenditure. However, this has been accompanied by undesirable cardiovascular effects. Therefore, we hypothesized that combining mirabegron with a β1-AR antagonist could suppress these unwanted effects and increase the stimulation of the β3-AR and β2-AR in BAT. We performed a randomized crossover trial (NCT04823442) in 8 lean men. Mirabegron (200 mg) was administered orally with or without the β1-AR antagonist bisoprolol (10 mg). Dynamic [11C]-acetate and 2-deoxy-2-[18F]fluoro-d-glucose PET/CT scans were performed sequentially after oral administration of mirabegron ± bisoprolol. Compared to room temperature, mirabegron alone increased BAT oxidative metabolism (0.84 ± 0.46 vs. 1.79 ± 0.91 min-1, p = 0.0433), but not when combined with bisoprolol. The metabolic rate of glucose in BAT, measured using [18F]FDG PET, was significantly higher with mirabegron than mirabegron with bisoprolol (24 ± 10 vs. 16 ± 8 nmol/g/min, p = 0.0284). Bisoprolol inhibited the mirabegron-induced increase in systolic blood pressure and heart rate. The administration of bisoprolol decreases the adverse cardiovascular effects of mirabegron. However, the provided dose also blunted the mirabegron-stimulated increase in BAT lipolysis, thermogenesis, and glucose uptake. The attenuation in BAT blood flow induced by the large dose of bisoprolol may have limited BAT thermogenesis.

Xenon-enhanced computed tomography assessment of brown adipose tissue distribution and perfusion in lean, obese, and diabetic primates.

ObjectiveThis study aimed to validate xenon‐enhanced computed tomography (XECT) for the detection of brown adipose tissue (BAT) and to use XECT to assess differences in BAT distribution and perfusion between lean, obese, and diabetic nonhuman primates (NHPs).MethodsWhole‐body XECT imaging was performed in anesthetized rhesus and vervet monkeys during adrenergic stimulation of BAT thermogenesis. In XECT images, BAT was identified as fat tissue that, during xenon inhalation, underwent significant radiodensity enhancement compared with subcutaneous fat. To measure BAT blood flow, BAT radiodensity enhancement was measured over time on the six computed tomography scans acquired during xenon inhalation. Postmortem immunohistochemical staining was used to confirm imaging findings.ResultsXECT was able to correctly identify all BAT depots that were confirmed at necropsy, enabling construction of the first comprehensive anatomical map of BAT in NHPs. A significant decrease in BAT perfusion was found in diabetic animals compared with obese animals and healthy animals, as well as absence of axillary BAT and significant reduction of supraclavicular BAT in diabetic animals compared with obese and lean animals.ConclusionsThe use of XECT in NHP models of obesity and diabetes allows the analysis of the impact of metabolic status on BAT mass and perfusion.

Brown Adipose Tissue Energy Metabolism in Humans.

The demonstration of metabolically active brown adipose tissue (BAT) in humans primarily using positron emission tomography coupled to computed tomography (PET/CT) with the glucose tracer 18-fluorodeoxyglucose (18FDG) has renewed the interest of the scientific and medical community in the possible role of BAT as a target for the prevention and treatment of obesity and type 2 diabetes (T2D). Here, we offer a comprehensive review of BAT energy metabolism in humans. Considerable advances in methods to measure BAT energy metabolism, including nonesterified fatty acids (NEFA), chylomicron-triglycerides (TG), oxygen, Krebs cycle rate, and intracellular TG have led to very good quantification of energy substrate metabolism per volume of active BAT in vivo. These studies have also shown that intracellular TG are likely the primary energy source of BAT upon activation by cold. Current estimates of BAT's contribution to energy expenditure range at the lower end of what would be potentially clinically relevant if chronically sustained. Yet, 18FDG PET/CT remains the gold-standard defining method to quantify total BAT volume of activity, used to calculate BAT's total energy expenditure. Unfortunately, BAT glucose metabolism better reflects BAT's insulin sensitivity and blood flow. It is now clear that most glucose taken up by BAT does not fuel mitochondrial oxidative metabolism and that BAT glucose uptake can therefore be disconnected from thermogenesis. Furthermore, BAT thermogenesis is efficiently recruited upon repeated cold exposure, doubling to tripling its total oxidative capacity, with reciprocal reduction of muscle thermogenesis. Recent data suggest that total BAT volume may be much larger than the typically observed 50–150 ml with 18FDG PET/CT. Therefore, the current estimates of total BAT thermogenesis, largely relying on total BAT volume using 18FDG PET/CT, may underestimate the true contribution of BAT to total energy expenditure. Quantification of the contribution of BAT to energy expenditure begs for the development of more integrated whole body in vivo methods.

Inhibition of Intracellular Triglyceride Lipolysis Suppresses Cold-Induced Brown Adipose Tissue Metabolism and Increases Shivering in Humans

Indirect evidence from human studies suggests that brown adipose tissue (BAT) thermogenesis is fueled predominantly by fatty acids hydrolyzed from intracellular triglycerides (TGs). However, no direct experimental evidence to support this assumption currently exists in humans. The aim of this study was to determine the role of intracellular TG in BAT thermogenesis, in cold-exposed men. Using positron emission tomography with 11C-acetate and 18F-fluorodeoxyglucose, we showed that oral nicotinic acid (NiAc) administration, an inhibitor of intracellular TG lipolysis, suppressed the cold-induced increase in BAT oxidative metabolism and glucose uptake, despite no difference in BAT blood flow. There was a commensurate increase in shivering intensity and shift toward a greater reliance on glycolytic muscle fibers without modifying total heat production. Together, these findings show that intracellular TG lipolysis is critical for BAT thermogenesis and provides experimental evidence for a reciprocal role of BAT thermogenesis and shivering in cold-induced thermogenesis in humans.

Relationship of brown adipose tissue perfusion and function: a study through β2-adrenoreceptor stimulation.

Brown adipose tissue (BAT) activation increases glucose and lipid consumption; as such, it is been considered as a potential therapy to decrease obesity. BAT is highly vascularized and its activation is associated with a necessary increase in blood flow. However, whether increasing BAT blood flow per se increases BAT activity is unknown. To examine this hypothesis, we investigated whether an isolated increase in BAT blood flow obtained by β2-adrenoreceptor (β2-AR) stimulation with salbutamol increased BAT activity. BAT blood flow was estimated in vivo in mice using contrast-enhanced ultrasound. The absence of direct effect of salbutamol on the function of isolated brown adipocytes was assessed by measuring oxygen consumption. The effect of salbutamol on BAT activity was investigated by measuring BAT glucose uptake in vivo. BAT blood flow increased by 2.3 ± 0.6-fold during β2-AR stimulation using salbutamol infusion in mice (P= 0.003). β2-AR gene expression was detectable in BAT but was extremely low in isolated brown adipocytes. Oxygen consumption of isolated brown adipocytes did not change with salbutamol exposure, confirming the absence of a direct effect of β2-AR agonist on brown adipocytes. Finally, β2-AR stimulation by salbutamol increased BAT glucose uptake in vivo (991 ± 358 vs. 135 ± 49 ng glucose/mg tissue/45 min in salbutamol vs. saline injected mice, respectively,P= 0.046). In conclusion, an increase in BAT blood flow without direct stimulation of the brown adipocytes is associated with increased BAT metabolic activity. Increasing BAT blood flow might represent a new therapeutic target in obesity.

Adiposity-Related Cancer and Functional Imaging of Brown Adipose Tissue

Brown adipose tissue (BAT) is involved in energy dissipation and cytokine production and is potentially beneficial for the human body. The aim of the paper is to review the literature on adiposity-related cancer and functional imaging of BAT. We performed a review on adiposity-related cancer and functional imaging of BAT. We extensively researched papers for information on BAT molecular biology, as well as functional imaging modalities. Adipose tissue is linked to the development of many cancers. Multiple drugs including fenofibrate, spironolactone, and other substances, as well as experimental agents like β-3 receptor agonists, caffeine, green tea extract, medium chain triglycerides (MCTs), and adenosine are known to stimulate and activate BAT. However, cold and nonshivering thermogenesis are the main activators of BAT. BAT has been detected on both magnetic resonance imaging (MRI) and 18F-fluorodexoxyglucose positron emission tomography (18F-FDG-PET)-based imaging in multiple studies. Different methods of cold stimulation and static and dynamic protocols have been used to detect and image BAT. Factors like sex, fasting or fed state, surface skin temperature, and/or body mass index (BMI) may influence PET-based BAT detection. BAT has also been detected using MRI, (99m)Technetium (Tc)-sestamibi, and 123I-metaiodobenzylguanidine single-photon emission computed tomography/computed tomography (MIBG SPECT/CT). Stimulation of BAT offers promise in the management of obesity-related conditions. Tracers like [(15)O]-H2O, [(11)C] acetate, and 18F-fluoro-6-thia-heptadecanoic acid (18F-FTHA) that measure BAT blood flow, oxygen utilization, and nonessential fatty acid (NEFA) uptake, respectively, have been studied in humans. Future studies should focus on BAT tissue generation by altering the genetic pathways of adiposity-linked genes.

Adrenergically stimulated blood flow in brown adipose tissue is not dependent on thermogenesis.

Brown adipose tissue (BAT) thermogenesis relies on blood flow to be supplied with nutrients and oxygen and for the distribution of the generated heat to the rest of the body. Therefore, it is fundamental to understand the mechanisms by which blood flow is regulated and its relation to thermogenesis. Here, we present high-resolution laser-Doppler imaging (HR-LDR) as a novel method for noninvasive in vivo measurement of BAT blood flow in mice. Using HR-LDR, we found that norepinephrine stimulation increases BAT blood flow in a dose-dependent manner and that this response is profoundly modulated by environmental temperature acclimation. Surprisingly, we found that mice lacking uncoupling protein 1 (UCP1) have fully preserved BAT blood flow response to norepinephrine despite failing to perform thermogenesis. BAT blood flow was not directly correlated to systemic glycemia, but glucose injections could transiently increase tissue perfusion. Inguinal white adipose tissue, also known as a brite/beige adipose tissue, was also sensitive to cold acclimation and similarly increased blood flow in response to norepinephrine. In conclusion, using a novel noninvasive method to detect BAT perfusion, we demonstrate that adrenergically stimulated BAT blood flow is qualitatively and quantitatively fully independent of thermogenesis, and therefore, it is not a reliable parameter for the estimation of BAT activation and heat generation.

Brown adipose tissue--not as hot as we thought.

The article by Blondin et al. (2015) in the current issue of The Journal of Physiology brings together and expands upon three adipose/muscle-related concepts that emphasize the importance of understanding physiology before jumping to conclusions when new findings are reported. First, the excitement following the reports of brown adipose tissue (BAT) in 2009 (Cypess et al. 2009; van Marken Lichtenbelt et al. 2009; Virtanen et al. 2009) has spawned a number of additional, enthusiastic reports of human BAT activity (Orava et al. 2011). Some authors suggest that BAT accounts for up to 20% of the cold-stimulated increase in energy expenditure in humans (Cypess et al. 2009). In the rush to characterize BAT as the next frontier in obesity treatment investigators have largely ignored the obligate relationship between energy expenditure and oxygen delivery, which in turn is dependent upon blood flow and tissue mass. Adipose tissue blood flow is notoriously low relative to metabolically active tissues (Summers et al. 1996; Karpe et al. 2002), and it turns out that BAT is not an exception. Even during cold exposure BAT blood flow averages only 13 ml (100 g tissue)−1 min−1 (Muzik et al. 2012, 2013). Under conditions of normal oxygen saturation and 16 g dl−1 of haemoglobin, 100 ml of arterial blood contains only 21.3 ml of O2. With an average total body BAT volume of 57 ml, the amount of oxygen delivered to total body BAT would be only ∼1.6 ml min−1. Furthermore, BAT oxygen extraction is reported to be ∼56% (Muzik et al. 2012). Thus, under steady-state, cold-stimulated conditions, average whole-body BAT oxygen consumption should be ∼1 ml min−1, very similar to that measured using oxygen isotopes and PET approaches (Muzik et al. 2012). This is underwhelming considering the cold-induced increase in oxygen uptake of ∼270 ml min−1 that was observed by Blondin et al. (2015). The BAT oxygen uptake data (Muzik et al. 2012, 2013) are consistent with the limited total body BAT glucose uptake relative to the increase in total glucose disposal in response to cold (Blondin et al. 2015). Even individuals at the high end of BAT blood flow (25 ml (100 g tissue)−1 min−1; Orava et al. 2011) and with 100 g of total body BAT, actual BAT thermogenesis would account for only ∼1% of the increase in energy expenditure above basal induced by cold exposure. The second important observation is the contribution of subclinical muscle shivering to cold-induced increases in muscle glucose uptake and presumably energy expenditure. Blondin et al.’s careful and extensive use of surface electromyography showed that muscle contraction (shivering) is the major component of the increase in whole-body glucose uptake. This suggests that investigations directed at the role of muscle, rather than BAT, will be more promising. Finally, the observation by Blondin et al. (2015) that the cold-induced increase in non-esterified fatty acid release from adipose tissue lipolysis relates to the activation of BAT is important. It is consistent with animal data indicating that the sympathetic innervation of BAT and white adipose tissue (WAT) is similar (reviewed in Vaughan et al. 2014). This implies that variations in sympathetic activation or the sensitivity to such activation is similar in human BAT and WAT. If the former, then variations in observed BAT activation during cold exposure could reflect more of a central nervous system sympathetic response to cold rather than a peripheral, e.g. BAT, difference. In summary, the findings of Musik (Muzik et al. 2012, 2013) and Blondin et al. (2015) suggest to me that BAT is most likely a vestigial and largely irrelevant tissue when it comes to adult human energy expenditure. They also provide some renewed excitement about the role of muscle in resting energy expenditure. None declared.

Editor’s Note

HomeCirculation: Cardiovascular ImagingVol. 7, No. 1Editor’s Note Free AccessEditorialPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessEditorialPDF/EPUBEditor’s Note Marcelo F. Di Carli, MD Ravin Davidoff, MBBCh Marcelo F. Di CarliMarcelo F. Di Carli Editor Search for more papers by this author Ravin DavidoffRavin Davidoff Senior Associate Editor Search for more papers by this author Originally published1 Jan 2014https://doi.org/10.1161/CIRCIMAGING.113.001572Circulation: Cardiovascular Imaging. 2014;7:2Circulation: Cardiovascular Imaging is proud to announce the establishment of a Best Paper Award in Cardiovascular Imaging to recognize the contribution of our authors to the journal’s excellence. The award will be granted to 2 recipients each year, in the areas of basic and clinical science. All original articles published in Circulation: Cardiovascular Imaging in a given calendar year will be eligible for the award, which will be selected by the editors based on the originality of the work as well as scientific and clinical impact.The first 2 awards have been presented to the authors of 2 articles published in 2012.The Best Paper in Cardiovascular Imaging Basic Science has been awarded to David M. Baron, MD, and Maeva Clerte, MD, for their article “In Vivo Noninvasive Characterization of Brown Adipose Tissue Blood Flow by Contrast Ultrasound in Mice.” Circulation: Cardiovascular Imaging. 2012;5:652–659. This innovative research demonstrates and validates the use of contrast ultrasound to characterize the blood flow to brown adipose tissue in mice. The ability to measure the effect of different stimuli on blood flow has important therapeutic potential in humans.The Best Paper in Cardiovascular Imaging Clinical Science has been awarded to Jérôme Adda, MD, for his article “Low-Flow, Low-Gradient Severe Aortic Stenosis Despite Normal Ejection Fraction Is Associated With Severe Left Ventricular Dysfunction as Assessed by Speckle-Tracking Echocardiography: A Multicenter Study.” Circulation: Cardiovascular Imaging. 2012;5:27–35. This research provides new insights into our understanding of this clinically challenging problem and is an important foundation for on-going work in this area.We hope our readership will join the editors in extending our warmest congratulations to these outstanding researchers and their work. Previous Back to top Next FiguresReferencesRelatedDetails January 2014Vol 7, Issue 1 Advertisement Article InformationMetrics © 2014 American Heart Association, Inc.https://doi.org/10.1161/CIRCIMAGING.113.001572 Originally publishedJanuary 1, 2014 PDF download Advertisement

Quantification of Human and Rodent Brown Adipose Tissue Function Using99mTc-Methoxyisobutylisonitrile SPECT/CT and18F-FDG PET/CT

For brown adipose tissue (BAT) to be effective at consuming calories, its blood flow must increase enough to provide sufficient fuel to sustain energy expenditure and also transfer the heat created to avoid thermal injury. Here we used a combination of human and rodent models to assess changes in BAT blood flow and glucose utilization. (99m)Tc-methoxyisobutylisonitrile (MIBI) SPECT (n = 7) and SPECT/CT (n = 74) scans done in adult humans for parathyroid imaging were reviewed for uptake in regions consistent with human BAT. Site-directed biopsies of subcutaneous and deep neck fat were obtained for electron microscopy and gene expression profiling. In mice, tissue perfusion was measured with (99m)Tc-MIBI (n = 16) and glucose uptake with (18)F-FDG (n = 16). Animals were kept fasting overnight, anesthetized with pentobarbital, and given intraperitoneally either the β3-adrenergic receptor agonist CL-316,243, 1 mg/kg (n = 8), or saline (n = 8) followed by radiotracer injection 5 min later. After 120 min, the mice were imaged using SPECT/CT or PET/CT. Vital signs were recorded over 30 min during the imaging. BAT, white adipose tissue (WAT), muscle, liver, and heart were resected, and tissue uptake of both (99m)Tc-MIBI and (18)F-FDG was quantified by percentage injected dose per gram of tissue and normalized to total body weight. In 5.4% of patients (4/74), (99m)Tc-MIBI SPECT/CT showed increased retention in cervical and supraclavicular fat that displayed multilocular lipid droplets, dense capillary investment, and a high concentration of ovoid mitochondria. Expression levels of the tissue-specific uncoupling protein-1 were 180 times higher in BAT than in subcutaneous WAT (P < 0.001). In mice, BAT tissue perfusion increased by 61% (P < 0.01), with no significant changes in blood flow to WAT, muscle, heart, or liver. CL-316,243 increased glucose uptake in BAT even more, by 440% (P < 0.01). Pharmacologic activation of BAT requires increased blood flow to deliver glucose and oxygen for thermogenesis. However, the glucose consumption far exceeds the vascular response. These findings demonstrate that activated BAT increases glucose uptake beyond what might occur by increased blood flow alone and suggest that activated BAT likely uses glucose for nonthermogenic purposes.

Brown Adipose Tissue Blood Flow and Mass in Obesity: A Contrast Ultrasound Study in Mice

When activated by the sympathetic nervous system, brown adipose tissue (BAT) increases energy expenditure to produce heat. Augmenting BAT mass or increasing BAT activation could potentially be used to decrease obesity. Noninvasive methods to detect and monitor BAT mass are needed. Contrast ultrasound can estimate BAT blood flow and is able to measure the perfused volume of an organ and thus its mass. The objective of this study was to evaluate whether contrast ultrasound could characterize BAT mass in two mouse models of obesity: wild-type mice fed a high-fat diet and mutant db/db mice. Contrast ultrasound of BAT (Definity 2 μL/min; 14-MHz linear probe) was performed before and after stimulation of BAT with norepinephrine (NE). BAT replenishment curves were obtained, and blood flow was estimated by the product of the curve's plateau and slope. Additionally, consecutive two-dimensional images of perfused BAT were acquired at 1-mm intervals after stimulation with NE and used to assess BAT volume and mass. BAT blood flow increased after NE infusion in all mice studied. Blood flow response to NE was similar in wild-type mice fed either a low-fat diet or a high-fat diet. BAT blood flow was lower in db/db mice than in wild-type mice (P= .02). Contrast ultrasound-derived BAT mass was correlated with BAT mass obtained at necropsy (R(2)= 0.83, P < .001). BAT mass was higher in mice fed a high-fat diet than in those fed a low-fat diet. Contrast ultrasound can be used to estimate BAT mass in mice when BAT vascularization is not significantly impaired. This noninvasive technique may potentially allow the serial evaluation of therapies designed to augment BAT mass.

Blunted metabolic responses to cold and insulin stimulation in brown adipose tissue of obese humans

Inactive brown adipose tissue (BAT) may predispose to weight gain. This study was designed to measure metabolism in the BAT of obese humans, and to compare it to that in lean subjects. The impact of weight loss on BAT and the association of detectable BAT with various metabolic characteristics were also assessed. Using positron emission tomography (PET), cold- and insulin-stimulated glucose uptake and blood flow in the BAT of obese and lean humans were quantified. Further, cold-induced glucose uptake was measured in obese subjects before and after a 5-month conventional weight loss. Mean responses in BAT glucose uptake rate to both cold and insulin stimulation were twice as large in lean as in obese subjects. Blood flow in BAT was also lower in obese subjects under cold conditions. The increase in cold-induced BAT glucose uptake rate after weight loss was not statistically significant. Subjects with cold-activated detectable BAT were leaner and had higher whole-body insulin sensitivity than BAT-negative subjects, irrespective of age and gender. The effects of cold and insulin on BAT activity are severely blunted in obesity, and the presence of detectable BAT may contribute to a metabolically healthy status.

In Vivo Noninvasive Characterization of Brown Adipose Tissue Blood Flow by Contrast Ultrasound in Mice

Interventions to increase brown adipose tissue (BAT) volume and activation are being extensively investigated as therapies to decrease the body weight in obese subjects. Noninvasive methods to monitor these therapies in animal models and humans are rare. We investigated whether contrast ultrasound (CU) performed in mice could detect BAT and measure its activation by monitoring BAT blood flow. After validation, CU was used to study the role of uncoupling protein 1 and nitric oxide synthases in the acute regulation of BAT blood flow. Blood flow of interscapular BAT was assessed in mice (n=64) with CU by measuring the signal intensity of continuously infused contrast microbubbles. Blood flow of BAT estimated by CU was 0.5±0.1 (mean±SEM) dB/s at baseline and increased 15-fold during BAT stimulation by norepinephrine (1 µg·kg(-1)·min(-1)). Assessment of BAT blood flow using CU was correlated to that performed with fluorescent microspheres (R(2)=0.86, P<0.001). To evaluate whether intact BAT activation is required to increase BAT blood flow, CU was performed in uncoupling protein 1-deficient mice with impaired BAT activation. Norepinephrine infusion induced a smaller increase in BAT blood flow in uncoupling protein 1-deficient mice than in wild-type mice. Finally, we investigated whether nitric oxide synthases played a role in acute norepinephrine-induced changes of BAT blood flow. Genetic and pharmacologic inhibition of nitric oxide synthase 3 attenuated the norepinephrine-induced increase in BAT blood flow. These results indicate that CU can detect BAT in mice and estimate BAT blood flow in mice with functional differences in BAT.

Assessment of Oxidative Metabolism in Brown Fat Using PET Imaging

Objective: Although it has been believed that brown adipose tissue (BAT) depots disappear shortly after the perinatal period in humans, positron emission tomography (PET) imaging using the glucose analog 18F-deoxy-d-glucose (FDG) has shown unequivocally the existence of functional BAT in humans, suggesting that most humans have some functional BAT. The objective of this study was to determine, using dynamic oxygen-15 (15O) PET imaging, to what extent BAT thermogenesis is activated in adults during cold stress and to establish the relationship between BAT oxidative metabolism and FDG tracer uptake. Methods: Fourteen adult normal subjects (9F/5M, 30 ± 7 years) underwent triple oxygen scans (H215O, C15O, 15O2) as well as indirect calorimetric measurements at both rest and following exposure to mild cold (16°C). Subjects were divided into two groups (BAT+ and BAT−) based on the presence or absence of FDG tracer uptake (SUV > 2) in cervical–supraclavicular BAT. Blood flow and oxygen extraction fraction (OEF) was calculated from dynamic PET scans at the location of BAT, muscle, and white adipose tissue (WAT). The metabolic rate of oxygen (MRO2) in BAT was determined and used to calculate the contribution of activated BAT to daily energy expenditure (DEE). Results: The median mass of activated BAT in the BAT+ group (5F, age 31 ± 8) was 52.4 g (range 14–68 g) and was 1.7 g (range 0–6.3 g) in the BAT − group (5M/4F, age 29 ± 6). Corresponding SUV values were significantly higher in the BAT+ as compared to the BAT− group (7.4 ± 3.7 vs. 1.9 ± 0.9; p = 0.03). Blood flow values in BAT were significantly higher in the BAT+ group as compared to the BAT− group (13.1 ± 4.4 vs. 5.7 ± 1.1 ml/100 g/min, p = 0.03), but were similar in WAT (4.1 ± 1.6 vs. 4.2 ± 1.8 ml/100 g/min) and muscle (3.7 ± 0.8 vs. 3.3 ± 1.2 ml/100 g/min). Moreover, OEF in BAT was similar in the two groups (0.56 ± 0.18 in BAT+ vs. 0.46 ± 0.19 in BAT−, p = 0.39). Calculated MRO2 values in BAT increased from 0.95 ± 0.74 to 1.62 ± 0.82 ml/100 g/min in the BAT+ group and were significantly higher than those determined in the BAT− group (0.43 ± 0.27 vs. 0.56 ± 0.24, p = 0.67). The DEE associated with BAT oxidative metabolism was highly variable in the BAT+ group, with an average of 5.5 ± 6.4 kcal/day (range 0.57–15.3 kcal/day). Conclusion: BAT thermogenesis in humans accounts for less than 20 kcal/day during moderate cold stress, even in subjects with relatively large BAT depots. Furthermore, due to the large differences in blood flow and glucose metabolic rates in BAT between humans and rodents, the application of rodent data to humans is problematic and needs careful evaluation.

Abstract 16795: Detection of Brown Adipose Tissue Activation and Mass Using Contrast Ultrasound

Obesity is an increasingly frequent condition and a risk factor for cardiovascular diseases. Brown adipose tissue (BAT), located mostly in the paravertebral and supraclavicular space in humans and between the scapulae in rodents, consumes lipids and glucose to produce heat when it is activated by norepinephrine (NE) or cold. Increasing metabolic activity and/or mass of BAT could open new therapeutic avenues to treat obesity. Currently,18F-fluorodeoxyglucose Positron Emission Tomography is the only imaging technique detecting BAT and its activation in humans. Since increased BAT blood flow is associated with BAT activation, we hypothesized that BAT contrast ultrasound (CU) could estimate the degree of BAT activation and BAT mass, both in mice and humans. To validate the ability of CU to estimate BAT perfusion, the BAT and kidney of C57BL6 wild type mice were imaged with a 14 MHz linear transducer during continuous infusion of DefinityTM microbubble contrast. The time intensity curves of the contrast were quantified and blood flow was estimated by the product of the curve plateau A and slope ß. The BAT/kidney blood flow ratio obtained with CU was compared to that measured with fluorescent microspheres. In another experiment, to validate the ability of CU to estimate BAT mass, the maximal area of perfusion of BAT was compared to the BAT weight, in mice. Finally, the BAT vascular response to activation was assessed in 6 mice before and after infusing 1µg/kg/min NE and in 7 young and lean human volunteers before and after cold exposure (16°C for 2 hours).The BAT/kidney blood flow ratio obtained by CU in mice correlated with microsphere measurements (R2=0.85, p&lt;0.05). The maximal area of BAT measured by CU correlated with BAT weight (R2=0.58, p&lt;0.002). Mice demonstrated a 10-fold increase in BAT perfusion after infusion of NE (Aß 0.6±0.1 vs. 5.5±0.9 dB/s, p&lt;0.05) while kidney perfusion decreased. Similarly, BAT perfusion increased in humans after cold exposure (Aß 0.1±0.0 vs. 1.0±0.4 dB/s,p&lt;0.05). In conclusion, contrast ultrasound imaging can be used to estimate BAT activation and its mass. This noninvasive technique free of ionizing radiation offers a method for serial evaluation and monitoring of therapies designed to activate BAT or augment its mass.

Reversal of high pancreatic islet and white adipose tissue blood flow in type 2 diabetic GK rats by administration of the β3-adrenoceptor inhibitor SR-59230A

Previous studies have shown that the Goto-Kakizaki (GK) rat, a nonobese type 2 diabetes model, has an increased white adipose tissue (WAT) and islet blood flow when compared with control rats. The aim of the study was to examine if these increased blood flow values in GK rats could be affected by the beta(3)-adrenoceptor antagonist SR-59230A. We measured organ blood flow with a microsphere technique 10 min after administration of SR-59230A (1 mg/kg body wt), or the corresponding volume of 0.9% NaCl solution (1 ml/kg body wt) in rats anaesthetized with thiobutabarbital. The GK rat had an increased blood flow in all intra-abdominal adipose tissue depots except for the sternal fat pad compared with Wistar-Furth (WF) rats. However, no differences were seen in the blood perfusion of subcutaneous white or brown adipose tissue. The blood flow was also increased in both the pancreas and in the islets in the GK rat compared with WF rats. SR-59230A treatment affected neither WAT nor pancreatic blood flow in WF rats. In GK rats, on the other hand, SR-59230A decreased both WAT and islet blood flow values to values similar to those seen in control WF rats. The whole pancreatic blood flow was not affected by SR-59230A administration in GK rats. Interestingly, the brown adipose tissue blood flow in GK rats increased after SR-59230A administration. These results suggest that beta(3)-adrenoceptors are involved in regulation of blood flow both in islet and in adipose tissue.

Comparison of Uptake of Multiple Clinical Radiotracers into Brown Adipose Tissue Under Cold-Stimulated and Nonstimulated Conditions

Our objective was to determine whether multiple clinically useful radiotracers accumulate in brown adipose tissue (BAT) and to assess their uptake in rats kept at room temperature or exposed to a cold environment. The following radiotracers were injected intravenously into groups of 6 female Wistar rats: (201)Tl-chloride (TlCl), (123)I-metaiodobenzylguanidine (MIBG), (99m)Tc-sestamibi (MIBI), (18)F- or (3)H-FDG, (3)H-l-methionine, and (3)H-thymidine. BAT-stimulated animals were maintained at 4 degrees C for 4 h before tracer injection, whereas control animals were kept at approximately 22.5 degrees C. The animals were sacrificed at 20-60 min after tracer injection, and BAT, major organs, and blood were extracted, weighed, and measured for radioactivity. The localization of uncoupling protein-1, glucose transporter-1, and norepinephrine transporter was evaluated with immunohistochemical staining in both groups. We determined the percentage injected dose (%ID) per gram of each radiotracer in interscapular BAT, normalized to blood %ID/g. In control animals, this uptake ratio (+/-SD) was 8.44 +/- 3.39 for (201)TlCl, 9.77 +/- 6.06 for (123)I-MIBG, 37.30 +/- 14.42 for (99m)Tc-MIBI, 5.47 +/- 4.44 for (18)F- or (3)H-FDG, 1.93 +/- 0.87 for (3)H-l-methionine, and 1.22 +/- 0.74 for (3)H-thymidine. Compared with uptake at room temperature, uptake after exposure to cold increased 26.4-fold (P < 0.01) for (18)F- or (3)H-FDG and increased significantly (P < 0.05) for (201)Tl (2.04-fold), (123)I-MIBG (3.25-fold), and (3)H-l-methionine (3.11-fold). Immunohistochemical staining revealed increased glucose transporter-1 and norepinephrine transporter expression in BAT cell membranes and blood vessels after exposure to cold, whereas uncoupling protein-1 was expressed in the cytoplasm under both control and cold-stimulated conditions. BAT uptake of (18)F- or (3)H-FDG, (123)I-MIBG, and (3)H-l-methionine was significantly increased over the control state by exposure to cold. Increased uptake of (201)TlCl relative to blood in cold-stimulated BAT suggests that blood flow in BAT is increased by exposure to cold. The greater increased uptake with (18)F- or (3)H-FDG, (123)I-MIBG, and (3)H-l-methionine, and the immunohistostaining findings, suggest that other factors in addition to blood flow (e.g., increased metabolism, increased transport, or metabolic trapping of the tracers) are involved in cold-stimulated BAT activation. Knowledge that high uptake in BAT may possibly be observed on clinical scans using several radiotracers, especially after patients are exposed to the cold, may lead to more accurate interpretation of clinical studies.

Enhanced gene expression of endothelial nitric oxide synthase in brown adipose tissue during cold exposure.

It has been shown that norepinephrine (NE) can mediate vasodilatation by stimulating the production of nitric oxide (NO) in brown adipose tissue (BAT), resulting in an increase in BAT blood flow. We speculated that constitutive NO synthase (NOS) is involved in this NO production. However, it is not known whether constitutive NOS is expressed in BAT. To answer this question, we assessed the expression of two types of constitutive NOS, endothelial (eNOS) and neuronal NOS (nNOS), in BAT of rats. eNOS was abundantly expressed in both BAT and isolated brown adipocytes, whereas nNOS was not. Cold exposure, which is known to stimulate NE release from sympathetic nerve terminals in BAT, led to a significant increase in eNOS mRNA in this tissue. In contrast, very low levels of inducible NOS (iNOS) mRNA were expressed, and cold stimulation failed to increase iNOS mRNA levels in BAT. These results suggest that eNOS is the primary isoform that is responsible for NO production in BAT and that its expression may be under sympathetic control.

Blood flow to the brown adipose tissue of conscious young rabbits during hypoxia in cold and warm conditions.

Brown adipose tissue (BAT) non-shivering thermogenesis is stimulated by cold temperature and depressed by hypoxia. We investigated the extent to which changes in metabolic rate during cold and hypoxia, singly or combined, were accompanied by changes in BAT perfusion. One-month-old rabbits were instrumented for measurements of regional blood flow by the coloured microsphere technique. One group of rabbits was tested in warm (24 degrees C, n=17), and the other in cold (13 degrees C, n=9) conditions, first in normoxia (inspired oxygen concentration FIO2 about 21%, arterial oxygen saturation SaO2 approximately 88%) followed by hypoxia (FIO2 approximately 10%, SaO2 approximately 54%). In warm conditions, oxygen consumption (VO2, measured by an open-flow method) averaged 22 ml.kg-1.min-1 (STPD), and BAT blood flow 98 ml.100g-1.min-1. In hypoxia, VO2 dropped on average to 87%, whereas BAT flow dropped to 43% of the normoxic values. In the cold during normoxia, VO2 averaged 31 ml.kg-1.min-1 (STPD), and BAT blood flow was 155 ml.100g-1.min-1. In cold and hypoxia VO2 dropped to 19 ml.kg-1.min-1 (STPD) (i.e. 60% of the normoxic value), whereas BAT blood flow was not altered significantly (148 ml.100g-1.min-1). Hence, BAT blood flow decreased in hypoxia in absence of cold stimuli, whereas it remained high when hypoxia occurred during cold, despite the major drop in VO2. We conclude that cold is more important than hypoxia in determining BAT perfusion, and that changes in BAT blood flow are not a mechanism for the hypoxic control of V.O2.

The role of β- and α-adrenoreceptors on blood flow and temperature of brown adipose tissue and involvement of nitric oxide in their effects

1. o 2. 1. Isoproterenol increased interscapular brown adipose tissue (BAT) blood flow and temperature. 3. 2. Phenylephrine increased BAT temperature, but did not affect blood flow. The effect was completely suppressed by N ω -nitro- l-arginine methyl ester ( l-NAME), an inhibitor of endogenous NO synthesis. 4. 3. Isoproterenol plus phenylephrine increased BAT blood flow and temperature earlier than isoproterenol alone. 5. 4. CL316,243 increased BAT blood flow and temperature. These effects were also inhibited by l-NAME. 6. 5. These data suggest that BAT blood flow as well as thermogenesis is regulated mainly by β-adrenoreceptors and partly by α 1-adrenoreceptors, and NO may be a common mediator for their regulations.