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Flexibility in myocardial substrate metabolism for energy production is fundamental to cardiac health. This loss in plasticity or flexibility leads to an over dependence on the metabolism of an individual category of substrates with the predominance in fatty acid metabolism characteristic of diabetic heart disease and the accelerated glucose use associated with pressure-overload left ventricular hypertrophy being prime examples. Important unresolved questions include the extent to which these metabolic patterns are adaptive and have the propensity to become maladaptive, what are the key determinants of these metabolic perturbations, do they alter prognosis, and do they represent robust targets for novel therapeutics. Accelerating our understanding of the role of alterations in myocardial substrate metabolism cardiac disease is the development of transgenic models targeting key aspects of myocardial substrate use. However, the relevance of the metabolic phenotype of these models to the corresponding human condition is frequently unclear. In addition, applied genomics have identified numerous gene variants intimately involved in the regulation of myocardial substrate use. Yet, identifying the clinically significant genetic variants remains elusive. For all these reasons, there is a strong demand for accurate non-invasive imaging approaches of myocardial substrate metabolism that can facilitate the crosstalk between the bench and the bedside leading to improved patient management paradigms. Currently the most successful example is the detection of ischemic but viable myocardium with PET and 18F-fluorodexoglucose (FDG) for the management of patients with ischemic cardiomyopathy. In this review potential future applications of metabolic imaging, particularly radionuclide approaches, for assessment of cardiovascular disease are discussed.
The heart is an omnivore capable of switching between one substrate to another for energy production. This flexibility in substrate use occurs in response to numerous stimuli including substrate availability, the hormonal environment, the level of tissue perfusion, and the level of workload by the heart (Figure 1).1, 2 The control of substrate switching can either represent an acute or chronic adaptation in response to either short or prolonged alterations in the physiological environment. Examples of acute or short term adaptations would include inhibitory effects of fatty acid oxidation on glucose oxidation as well as the converse, in addition of the oxidation of fatty acids by glucose oxidation as well as the increasing oxidation of glycogen, lactate and glucose in response to increasing workload. Regulating these rapid changes are a host enzymes such as pyruvate dehydrogenase complex and enzyme carnitine palmitoyl transferase 1 which is regulated by the concentration of malonyl-CoA.3–7
In contrast, chronic metabolic adaptations reflect alterations in the metabolic machinery of the heart. These changes occur primarily at the transcriptional level through the coordinated up-regulation of enzymes and proteins in key metabolic pathways. A prominent example in this case is the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARα), which is a key regulator of myocardial fatty acid uptake, oxidation and storage.8 For example, in diabetes mellitus, PPARα activity is increased leading to an up-regulation in genes controlling fatty acid uptake and oxidation.9 In contrast, pressure-overload hypertrophy PPARα activity is reduced leading to a down-regulation of genes controlling fat metabolism and in turn leading to an up-regulation of glucose use.10 These chronic adaptations can induce numerous detrimental effects that extend beyond alterations in energy production and may include increases in oxygen free radical production, impaired energetics, increases in apoptosis, and the induction of left ventricular dysfunction. Discussed in the subsequent sections is how metabolic imaging has helped characterize this loss of metabolic flexibility due to these chronic adaptations in various disease processes.
There are currently three methods to image myocardial metabolism noninvasively, magnetic resonance spectroscopy (MRS), single photon emission computed tomography (SPECT) and positron emission tomography (PET). A summary of each technique is listed below:
This technology offers numerous advantages for the measurement of myocardial metabolism. They include the ability to measure multiple metabolic pathways simultaneously, relative ease in performing serial measurements, and the lack of ionizing radiation. When combined with MRI, near simultaneous measurements of myocardial perfusion and mechanical function are possible. MRS allows direct measurement of biochemical information about in vivo processes. This biochemical data can be acquired repeatedly with minimal interference to tissue function. A number of biologically important nuclei can be measured including phosphorous (31P), hydrogen (1H), carbon (13C), sodium (23Na), nitrogen (15N) and fluorine (19F). The basic principle of MRS relies on the fact that the chemical environment of a nuclei induces local magnetic fields that shifts its resonance frequency. The different frequency shift for different metabolites results in an NMR signal consisting of one or more discrete resonance frequencies. The Fourier transform of the acquired signal produces a spectrum with peaks at distinct frequencies. The MRS spectrum displays the signal intensity as a function of frequency measured in parts per million (ppm) relative to the frequency of a reference compound. The signal intensity at a given frequency is proportional to the amount of the respective metabolite and can be used to determine the absolute concentration of the metabolite using appropriate calibrating reference signal.11, 12
MRS is limited by inherent low signal-to-noise, concomitant limited spatial resolution, intravoxel signal contamination and long acquisition times. Compared with nuclear imaging methods, MRS has a much lower sensitivity (detecting millimolar as opposed to nanomolar concentrations). Thus, although recent studies suggest imaging of cardiac metabolism using C-13 labeled agents is possible in intact animals, studies in humans are still not possible.13 Of note, cardiac applications for MRS become more limited as one moves from rodent to man as opposed to nuclear methods where the reverse occurs. This appears to be a function of both the higher field strength in the small bore systems and the use of radiofrequency coils that are in closer proximity to the entire heart used in small animal imaging. These advantages overcome the need for markedly improved spatial resolution. Indeed, as opposed to rodent hearts where the measurements of the left ventricular myocardium is obtained, measurements in human myocardium are typically limited to the anterior myocardium. Currently, only 31P and 1H have been widely used for in vivo clinical cardiac examinations focusing on myocardial energetics (31P) and lipid accumulation (1H).11, 13, 14
An advantage of SPECT is the inherent high sensitivity of the radionuclide method to measure metabolic processes. The technology is widely available in both the clinical and research setting. With ECG-gating, measurements of myocardial function can be obtained simultaneously. Because of the long-physical half-life of SPECT radionuclides radiopharmecutical delivery to multiple sites is possible, facilitating the performance of multi-center studies that incorporate measurements of myocardial substrate metabolism. Theoretically, assessing more than one metabolic process simultaneously is possible if the heart is imaged after the administration of radiopharmaceuticals labeled with radionuclides with different primary photon energies. Finally, small animal SPECT and SPECT/CT systems are rapidly advancing, facilitating the performance of myocardial metabolic studies in rodent models of cardiac disease. The major disadvantage of SPECT is the inability to quantify cellular metabolic processes primarily because of the technical limitations of SPECT (relatively poor temporal and spatial resolution and inaccurate correction for photon attenuation).
Metabolic processes that can be measure by SPECT include:
The two major advantages of PET are its intrinsic quantitative capability and the use of radiopharmaceuticals labeled with the positron-emitting radionuclides. The PET detection scheme permits accurate quantification of activity in the field of view. The positron-emitting radionuclides of the biologically ubiquitous elements oxygen (15O), carbon (11C), and nitrogen (13N), as well as fluorine (18F) substituting for hydrogen, can be incorporated into a wide variety of substrates or substrate analogues that participate in diverse biochemical pathways without altering the biochemical properties of the substrate of interest (Figure 2). By combining the knowledge of the metabolic pathways of interest with kinetic models that faithfully describe the fate of the tracer in tissue, an accurate interpretation of the tracer kinetics as they relate to the metabolic process of interest can be achieved. The major disadvantages of PET are its complexity in both radiotracer design and image quantification schemes and expense. Metabolic processes that are typically measured with PET are:
Both gender and aging impact the myocardial metabolic phenotype. Results of studies in animal models show that there are sex differences in myocardial substrate metabolism, with female rats exhibiting less myocardial glucose and more fatty acid metabolism.52, 53 Recently, using PET with 11C-glucose and 11C-plamitate, these observations were confirmed in young healthy volunteers.54 Women exhibited lower levels of glucose metabolism compared with men (Figure 6). Although, no differences in myocardial fatty acid metabolism were noted, women also exhibited higher MVO2 compared with men as measured by PET with 11C-acetate. These gender differences in substrate metabolism become more pronounced as one transition to more pathologic conditions. For example in addition to the changes in glucose metabolism and MVO2, obese women exhibited higher fatty acid uptake and oxidation compared with obese men.55 Of note, in both these studies, the differences in myocardial metabolism could not be explained by differences in myocardial blood flow, insulin sensitivity, hemodynamics, myocardial work, or the plasma substrate environment.
In various experimental models aging, the contribution of fatty acid oxidation to overall myocardial substrate metabolism declines with age.56, 57 It appears the cause for the decrease in fatty acid oxidation is multifactorial including changes in mitochondrial lipid content, lipid composition and protein interactions as well as oxygen free radical injury, a decline in carnitine palmitoyltransferase-1 activity, and a age-related decline in myocardial PPARα activity.58–60 Using the PET approaches described above, it has been shown that a similar metabolic shift occurs in healthy older humans.61 Moreover, older individuals are not able to increase glucose utilization in response to β-adrenergic stimulation with dobutamine to the same extent as younger individuals. This impaired metabolic response may represent a stress-related energy deprivation state in the aging heart or potentially indicate that the heart is more susceptible to injury during periods of ischemia.62 Recently it has been shown that this impairment in metabolic reserve can be ameliorated by endurance exercise training in older subjects.63 In keeping with the discussion above, it appears the myocardial metabolic response to dobutmaine following endurance exercise training is gender specific with men demonstrating an increase in myocardial metabolism whereas women exhibited increase in both glucose and fatty acid metabolism. Although, requiring further study, these gender and age differences in metabolism may provide a partial explanation for the gender- and age-related outcome differences for various cardiovascular diseases where altered myocardial metabolism plays a role.
Under conditions of mild to moderate myocardial ischemia, fatty acid oxidation ceases and anaerobic metabolism supervenes. Glucose becomes the primary substrate for both increased anaerobic glycolysis and for continued, albeit, diminished oxidative metabolism.64 This metabolic switch is prerequisite for continued a energy production and cell survival. When the ischemic insult is reversed, oxygen availability increases and oxidative metabolism resumes. However it appears that, these abnormalities myocardial substrate metabolism may persist well after the resolution of ischemia, so called “ischemic memory”. Demonstration of either accelerated myocardial glucose metabolism or reduced fatty acid metabolism using FDG and BMIPP, respectively, has been used to document this phenomena. For example, over twenty years ago it was shown that PET myocardial FDG uptake was increased in patients with unstable angina during pain-free episodes.65 Moreover, in patients with stable angina increased FDG uptake was demonstrated following exercise-induced ischemia-in the absence of either perfusion deficits or ECG abnormalities66. Similar observations have been made with SPECT using BMIPP. Results of numerous studies have demonstrated in patients with acute chest pain that abnormalities in myocardial BMIPP uptake may persist 24–36 hrs following the resolution of symptoms (Figure 7).67, 68 Moreover, this “metabolic fingerprint” is superior to perfusion imaging for either identifying coronary artery disease as the cause of the chest pain or assigning prognosis.67 The persistence of the metabolic defect increases the flexibility of radiotracer administration as it allows for delivery of a unit dose after the patient has already been evaluated. This is in contrast to the use of perfusion radiotracers which frequently must be available on-site due to the narrow time window from the resolution of symptoms and normalization of the flow deficit. Based on these observations, BMIPP is currently undergoing Phase 3 evaluation for acute chest pain imaging. Metabolic imaging with either FDG or BMIPP has also been for direct ischemia detection during stress testing. The thought process being that abnormalities in vasodilator reserve with perfusion tracers will underestimate ischemia if oxygen and supply remain balanced. Results of initial studies where FDG were injected during appears to support this contention with a greater detection rate for moderately severe coronary artery stenoses compared with perfusion imaging.15 Despite the promising results with these radiotracers, numerous questions still remain as the optimal imaging protocols, the impact of alterations in the plasma substrate environment on diagnostic accuracy, whether added diagnostic and prognostic information is provided over perfusion imaging, and whether this information alters clinical management.
There is well-established linkage between abnormalities in myocardial substrate metabolism and left ventricular hypertrophy. In animal models of pressure-overload left ventricular hypertrophy there is a reduction in the expression of of β-oxidation enzymes, leading to a fall in myocardial fatty acid oxidation and an increase in glucose use.69, 70 Moreover, interventions in animals that involve inhibition of mitochondrial fatty acid β-oxidation result in cardiac hypertrophy.70 In humans, variants in genes regulating key aspects of myocardial fatty acid metabolism ranging from PPARa to various key β-oxidative enzymes are associated with left ventricular hypertrophy.71, 72
This metabolic shift has been confirmed in-vivo in an animal model of hypertrophy.73 PET with FDG demonstrated myocardial glucose uptake tracked directly with increasing hypertrophy. Similar results have been found in man. PET with 11C-palmitate in humans has shown the reduction in myocardial fatty acid oxidation is an independent predictor of left ventricular mass in hypertension.74 Combining measurements of left ventricular myocardial external work (either by echocardiography or MRI) with measurements of MVO2 performed by PET with 11C-acetate or 15O-oxygen, it is possible to estimate cardiac efficiency23, 75. Using this approach in patients with hypertension-induced left ventricular hypertrophy has shown that the decline in myocardial fatty acid metabolism is associated with a decline in efficiency, a condition that may increase the potential for the development of heart failure. PET has also been used to phenotype patients with hypertrophic cardiomyopathy attributable to a known specific variant in the α-tropomyosin gene.76 It was observed that increased myocardial perfusion, fatty acid metabolism, and efficiency characterize patients with mild hypertrophy whereas these metabolic alterations decrease as hypertrophy becomes more advanced. The results may represent differential penetrance of the gene variant or the effect of modifier gene(s), potentially helping in their identification. Although, requiring further study in larger patient populations, this study suggests that metabolic imaging may identify relevant gene variants without waiting for more end-stage manifestations such as left ventricular remodeling and dysfunction to occur.
In addition to left ventricular hypertrophy, alterations in myocardial substrate metabolism have been implicated in the pathogenesis of contractile dysfunction and heart failure. Animal models of heart failure have shown that in the progression from cardiac hypertrophy to ventricular dysfunction, the expression of genes encoding for enzymes regulating β-oxidation is coordinately decreased, resulting in a shift in myocardial substrate metabolism to primarily glucose use, similar to that seen in the fetal heart.69, 77, 78 These metabolic changes are paralleled by re-expression of fetal isoforms of a variety of contractile and calcium regulatory proteins program. The reactivation of the metabolic fetal gene program may have numerous detrimental consequences on myocardial contractile function ranging from energy deprivation to the inability to process fatty acids leading to accumulation of nonoxidized toxic fatty acid derivatives, resulting in lipotoxicity. It should be noted that alterations in myocardial substrate use are now becoming attractive targets for novel treatments for heart failure with prime examples being the partial fatty acid oxidation antagonists and the insulin sensitizer glucagon-like peptide-1.79
The down-regulation in myocardial fatty acid metabolism leading to an over-dependence on glucose use in heart failure has been well documented using both PET and SPECT techniques. For example, PET using 11C-palmitate and 11C-glucose demonstrated that, myocardial fatty acid uptake and oxidation are lower in patients with nonischemic dilated cardiomyopathy when compared with aged matched controls. In contrast myocardial glucose utilization was higher in the cardiomyopathic patients.42 The metabolic findings cannot be explained by differences in plasma substrates or insulin, blood flow or MVO2. Similarly, SPECT with BMIPP demonstrated reduced myocardial uptake and increased clearance radiotracer in patients with dilated cardiomyopathy compared with controls.80 Moreover, the magnitude of the perturbations correlated with other measurements of heart failure severity such as left ventricular size and plasma b-natruietic peptide levels. PET has also been used provide mechanistic insights into the myocardial metabolic perturbations associated with heart failure. For example, abrupt lowering of fatty acid delivery with acipimox results in reduced fatty acid uptake, MVO2, and cardiac work and no change cardiac efficiency in normal volunteers.81 In contrast, patients with nonischemic dilated cardiomyopathy exhibited a decrease in myocardial fatty acid uptake and cardiac work, no change in MVO2 and a decline in efficiency. Although limited by a small sample size, these results appear to reinforce to the central role of loss flexibility in myocardial substrate metabolism in the pathogenesis of heart failure with even minor changes in substrate delivery having detrimental consequences on cardiac energy transduction.
Metabolic imaging can also been used to study the mechanisms responsible for the effectiveness of treatment in cardiomyopathy. For example, the efficacy of β-blocker therapy in treatment of heart failure patients is well established. Treatment with the selective β-blocker metoprolol results in a reduction in oxidative metabolism and an improvement in cardiac efficiency as measured by PET in patients with left ventricular dysfunction.82 With similar PET techniques, myocardial efficiency has been shown to be improved in patients with heart failure who undergo either exercise training or cardiac resynchronization therapy, implicating improved myocardial energetics as a potential mechanism.83, 84 Moreover, treatment with cardiac resynchronization therapy resulted in homogenization of initially heterogeneously distributed glucose metabolism.85 There is significant interest in the partial fatty acid oxidation inhibitors for the treatment of heart failure. Theoretically, decreasing myocardial fatty acid oxidation should increase the oxidation of glucose leading to a more favorable energetic state and improved left ventricular function. In a recent study, the administration of trimetizidine to patients with dilated cardiomyopathy resulted in a significant improvement in left ventricular ejection fraction.86 However, the improvement in left ventricular function appeared to reflect the complex interplay between a mild decrease in myocardial fatty acid oxidation, improved whole-body insulin resistance and synergestic effects with β-blockade. Metabolic imaging can also be used to predict the response to specific therapies in heart failure patients. For example, in patients with dilated cardiomyopathy, the percent of glucose uptake, as measured by PET with FDG can be used as a predictor for the effectiveness of β-blocker therapy.87 Moreover, in patients with ischemic cardiomyopathy the extent of viable myocardium as measured with PET and FDG correlated with the hemodynamic response after cardiac resynchronization therapy suggesting a role for PET in discriminating responders from non-responders.88
Cardiovascular disease is the leading cause of morbidity and mortality in patients with diabetes mellitus.89 The mechanisms by which diabetes mellitus confers this increased cardiovascular risk are multifactorial and complex with possibilities including an increased prevalence of hyperlipidemia and hypertension, impaired fibrinolysis, abnormal myocardial endothelial function and reduced sympathetic neuronal function. There is a burgeoning body of evidence to suggest that abnormalities in myocardial substrate metabolism contribute to the cardiovascular abnormalities observed in diabetic patients.90, 91 As mentioned previously the metabolic phenotype in diabetes is an overdependence on fatty acid metabolism and a decrease in glucose use. Multiple mechanisms contribute to this phenotype. These include increased plasma delivery of fatty acids due peripheral insulin resistance, decreased insulin signaling, and activation of key transcriptional pathways such as the PPARα/PGC-1 signaling network.92–94 Both insulin-mediated glucose transport and glucose transporter expression decline in diabetes mellitus. However, rates of myocardial glucose uptake are frequently normal due to the presence of hyperglycemia. Further metabolism of extracted glucose declines. The increase in myocardial fatty acid utilization results in increased citrate levels which inhibit phosphofructokinase. Glucose oxidation is inhibited at the level of pyruvate dehydrogenase complex due to increased mitochondrial acetyl-CoA levels and the phosphorylation of pyruvate dehydrogenase kinase 4 by PPARα activation. Consequently, the maintenance of myocardial glucose uptake but a decrease in downstream metabolism results in an accumulation of glucose metabolites. Potential detrimental effects associated with this shift in metabolism include: impaired mechanical function due to the inability to increase glucose metabolism in response to increase myocardial work, depletion of tricarboxylic acid cycle intermediates due to reduced anapleurosis, electrical instability and apoptosis, a greater sensitivity to myocardial ischemia and myocardial lipid accumulation or lipotoxicity leading increased apoptosis.
Small animal imaging has helped clarify the mechanisms responsible for the metabolic alterations that occur in diabetes mellitus. Potential mechanisms underlying diabetic cardiomyopathy have been studied in transgenic mice. For example, mice with cardiac-restricted overexpression of PPARα demonstrate a metabolic phenotype that is similar to diabetic hearts.95 Small animal PET studies with 11C-palmitate and FDG in these mice demonstrate an increase in fatty acid uptake and oxidation and an abnormal suppression of glucose uptake. In contrast, in mice with cardiac-restricted overexpression of PPARβ/σ small animal PET measurements demonstrated an increase glucose uptake and reduced fatty acid uptake and oxidation.96 Taken is sum these observations demonstrate that PPARα and PPARβ/σ drive different metabolic regulatory programs in the heart and that imaging can help characterize genetic manipulations in mouse heart. However, from an imaging perspective, these studies also demonstrate the challenges in imaging the mouse heart due to its small size as only semi-quantitative measurements of tracer uptake were performed. However, quantitative measures of myocardial substrate metabolism are now possible with small animal PET in rat heart. For example, rates of myocardial glucose uptake correlate directly and closely and GLUT 4 gene expression in the Zucker-Diabetic-Fat (ZDF) rat, a model of type-2 diabetes mellitus.97 Moreover, rates of myocardial glucose uptake and fatty acid uptake and oxidation measured with PET in the same disease model demonstrated the importance of increased fatty acid delivery to defining the metabolic phenotype in diabetes (Figure 8).98
Numerous imaging studies have been performed in humans to assess the impact of diabetes mellitus on myocardial glucose metabolism. These studies have been primarily limited to PET using FDG.99–101 In general, rates myocardial glucose uptake are reduced in patients either type 1 or type 2 diabetes mellitus compared with non-diabetics except in under conditions of marked hyperglycemia or supraphysiological levels in plasma insulin (such as occurs with a hyperinsulinemic-euglycemic clamp. Increased myocardial fatty acid uptake measured by arterial-coronary sinus balance studies has been reported in humans with type-1 diabetes mellitus without coronary artery disease.102 Although, the impact of plasma levels of free fatty acids on the level of myocardial fatty acid uptake was not determined, a negative correlation between myocardial glucose uptake and plasma fatty acid levels was observed. Recently studies using PET and 11C-plamitate and 11C-glucose in patients with type-1 diabetes mellitus have helped clarify the myocardial metabolic phenotype in this disease. For example, diabetic patients exhibited higher levels of fatty acid uptake and oxidation compared with non-diabetics primarily due to increased plasma fatty acid levels. In contrast, glucose uptake was reduced in these patients primarily due to decreased glucose transport mechanisms.43 Moreover, the metabolic fate of extracted glucose is impaired in diabetes with reduced rates of glycolysis and glucose oxidation which become more pronounced with increases in cardiac work induced by dobutamine (Figure 9).103 However, the diabetic myocardium is responsive to changes in plasma insulin and fatty acid levels but at a cost. Higher insulin levels are needed to achieve the same level of glucose uptake and glucose oxidation compared with non-diabetics consistent with myocardial insulin resistance (both). Similarly, in response to higher fatty acid plasma levels, myocardial fatty acid uptake is increased however at a cost of a greater esterification rate.103, 104
The increase in plasma fatty acids are an attractive target to reduce the over dependence of the myocardium on fatty acid metabolism and perhaps improve energetics and function of the left ventricle. For example, the use of the insulin sensitizing agent troglitazone in ZDF rats results in reduced plasma fatty acid levels, decreased myocardial lipid accumulation, reduced apoptosis and improved left ventricular function.105 In PET with FDG studies in patients with type-2 diabetes mellitus, before and 26 weeks after treatment with rosiglitazone, demonstrated nearly a 40% increase in insulin-stimulated myocardial glucose uptake, implying reduced fatty acid uptake, which was attributed in large part to suppression in plasma fatty acid levels.106 Of note, similar metabolic changes were not seen with the biguanide, metformin whose mechanism of action is designed to reduce hepatic glucose production. The metabolic response could not be predicted by changes in the plasma glucose or HgBA1C levels. Thus, metabolic imaging can be used to follow the effects of therapies designed to alter myocardial substrate metabolism in patients with diseases such as diabetes mellitus where more readily available clinical parameters are not predictive of a therapeutic response.
It is now apparent that a significant increase in body mass index (BMI) induces marked increases in myocardial fatty acid metabolism. For example, in either dietary-induced or transgenic models of obesity myocardial fatty acid uptake and oxidation are significantly increased.105, 107 This increase at least initially, reflects the increase in fatty acid delivery to the heart due to increased lipolysis from both visceral/abdominal and subcutaneous fat stores secondary to insulin resistance. Similar to diabetes mellitus, the increased delivery of fatty acids likely initiates a cascade of events that lead to increased fatty acid metabolism. Ultimately, fatty acid uptake may exceed oxidation leading to extracted fatty acids entering non-oxidative pathways most likely initially forming triglycerides. As mentioned above, the accumulation of the neutral fats or triglycerides may ultimately become detrimental.105
Imaging of obese young women with PET and 11C-acetate and 11C-palmitate has demonstrated that an increase in BMI is associated with a shift in myocardial substrate metabolism toward greater fatty acid use. Moreover, this dependence on myocardial fatty acid metabolism increased with worsening insulin resistance.44 Of note, little change in myocardial glucose metabolism was observed. Paralleling the preferential use of fatty acids was an increase in MVO2 and a decrease in energy transduction. These findings suggest that metabolic changes in obesity may play a role in the pathogenesis of cardiac dysfunction. Of note, the myocardial metabolic response to obesity appears to gender dependent.55 For example, using similar PET techniques it has been recently demonstrated that in contrast to obese women, obese men had a greater impairment in myocardial glucose metabolism per level plasma insulin, suggesting greater myocardial insulin resistance. In addition, obesity had less effect on myocardial fatty acid metabolism in men. In contrast, MVO2 was higher in the obese women compared with obese men. Thus it appears there appears to be a complex interplay between gender and obesity in influencing myocardial substrate metabolism.
Atherosclerosis is a dynamic immune inflammatory process. It is characterized by cycles of intense activity and progression followed by intervals of stabilization. A common result of this process is coronary artery luminal stenosis that compromises myocardial blood flow and induces ischemia during stress. However, the most devastating event is acute plaque rupture with thrombosis leading to myocardial infarction and frequently, sudden cardiac death. Moreover, for many patients acute plaque rupture is the initial clinical event. Despite the plethora of currently available imaging tools to detect and characterize the extent and severity coronary atherosclerosis, none of them identify patients with active disease and who are at risk of plaque rupture. To this end, FDG is being evaluated for the detection of “biologically active” atherosclerosis based on the premise that the tracer accumulates in activated macrophages which are a key component atherosclerotic plaque. Several groups have established that inflamed arterial vessels have increased uptake of FDG as measured by PET. The increased uptake has been noted in animal models of atherosclerosis, and demonstrated in verified in humans with atherosclerosis of the carotid artery and aorta.108–111 Moreover, a significant correlation between FDG uptake and macrophage staining and CD68 staining has recently been established. Of interest a decrease in carotid artery FDG uptake is correlative with an increase in plasma high-density lipoprotein levels following statin therapy (Figure 10).110 However many questions remain such as the site of localization of radiotracer (e.g., plaque or smooth muscle), the suitability of the method for evaluating the coronary arteries, and whether the information provides more refined risk stratification compared with other more widely applicable methods or if it alters therapy.
The continued growth of metabolic imaging will require advances in several areas. First will be the continued improvement in instrumentation design, both at the human level as well as at the level of imaging of small animals. For example, accurate tracer quantification may be possible with new SPECT/CT systems where accurate attenuation correction can be performed. Advances in PET detector design and post-detector electronics will result in improved counting statistics which should improve the ability to perform more complex compartmental modeling permitting more complete characterization of the metabolism of a given substrate. Second, there is a key need for the development of new radiopharmaceuticals that permit characterization of key metabolic pathways such as uptake, storage or oxidation that are linked to disease manifestations. Moreover, new radiopharmaceuticals are needed to provide insights into the pleiotropic aspects of metabolism such as the relationships between substrate metabolism and cell growth, cell survival and energy transduction. Third, the availability of radiopharmaceuticals radiolabeled with either F-18 or I-123 will be needed for the performance of appropriately powered clinical trials designed to answer key questions about metabolic imaging regarding diagnostic accuracy, risk stratification and monitoring of therapy in specific patient populations. Taken together, these advances will facilitate the translation of metabolic imaging to the clinic.
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