Dolichoarteriopathies of the extracranial internal carotid artery: The Plaque At RISK study.

BACKGROUND AND PURPOSE: Dolichoarteriopathies of the extracranial part of the internal carotid artery (ICA) are associated with cerebrovascular events, yet information on their prevalence and risk factors remains limited. The aim of the present study therefore was to study the prevalence and risk factors of dolichoarteriopathies in a sample of patients with cerebrovascular symptoms from the Plaque At RISK (PARISK) study. METHODS: In a random sample of 100 patients from the PARISK study, multidetector computed tomography angiography (MDCTA) was performed as part of clinical workup. On MDCTA, we evaluated the extracranial trajectory of the ICA by measuring the length (in millimeters), the tortuosity index (TI; defined as the ICA length divided by the shortest possible distance from bifurcation to skull base), and dolichoarteriopathy type (tortuosity, coiling or kinking). Next, we investigated the association between cardiovascular risk factors and these measurements using linear and logistic regression analyses. RESULTS: The mean (standard deviation) length of the ICA was 93 (± 14) mm, with a median (interquartile range) TI of 1.2 (1.1-1.3). The overall prevalence of dolichoarteriopathies was 69%, with tortuosity being the most common (72%), followed by coiling (20%), and kinking (8%). We found that age and obesity were associated with a higher TI: difference per 10-year increase in age: 0.05 (95% confidence interval [CI] 0.02-0.08) and 0.16 (95% CI 0.07-0.25) for obesity. Obesity and hypercholesterolemia were associated with a more severe type of dolichoarteriopathy (odds ratio [OR] 2.07 [95% CI 1.04-4.12] and OR 2.17 [95% CI 1.02-4.63], respectively). CONCLUSION: Dolichoarteriopathies in the extracranial ICA are common in patients with cerebrovascular symptoms, and age, obesity and hypercholesterolemia may play an important role in the pathophysiology of these abnormalities.

Plaque components in symptomatic moderately stenosed carotid arteries related to cerebral infarcts: the plaque at RISK study.

BACKGROUND AND PURPOSE: Carotid plaque composition is a major determinant of cerebrovascular events. In the present analysis, we evaluated the relationship between intraplaque hemorrhage (IPH) and a thin/ruptured fibrous cap (TRFC) in moderately stenosed carotid arteries and cerebral infarcts on MRI in the ipsilateral hemisphere. METHODS: A total of 101 patients with a symptomatic 30% to 69% carotid artery stenosis underwent MRI of the carotid arteries and the brain, within a median time of 45 days from onset of symptoms. The presence of ipsilateral infarcts in patients with and without IPH and TRFC was evaluated. RESULTS: IPH was seen in 40 of 101 plaques. TRFC was seen in 49 of 86 plaques (postcontrast series were not obtained in 15 patients). In total, 51 infarcts in the flow territory of the symptomatic carotid artery were found in 47 patients. Twenty nine of these infarcts, found in 24 patients, were cortical infarcts. No significant relationship was found between IPH or TRFC and the presence of ipsilateral infarcts. CONCLUSIONS: MRI detected IPH and TRFC are not related to the presence of old and recent cortical and subcortical infarcts ipsilateral to a symptomatic carotid artery stenosis of 30% to 69%. CLINICAL TRIAL REGISTRATION URL: http://www.clinicaltrials.gov. Unique identifier: NCT01208025.

Intraplaque hemorrhage, fibrous cap status, and microembolic signals in symptomatic patients with mild to moderate carotid artery stenosis: the Plaque at RISK study.

BACKGROUND AND PURPOSE: In patients with mild to moderate symptomatic carotid artery stenosis, intraplaque hemorrhage (IPH) and a thin/ruptured fibrous cap (FC) as evaluated with MRI, and the presence of microembolic signals (MESs) as detected with transcranial Doppler, are associated with an increased risk of a (recurrent) stroke. The objective of the present study is to determine whether the prevalence of MES differs in patients with and without IPH and thin/ruptured FC, and patients with only a thin/ruptured FC without IPH. METHODS: In this multicenter, diagnostic cohort study, patients with recent transient ischemic attack or minor stroke in the carotid territory and an ipsilateral mild to moderate carotid artery plaque were included. IPH and FC status were dichotomously scored. Analysis of transcranial Doppler data was done blinded for the MRI results. Differences between groups were analyzed with Fisher exact test. RESULTS: A total of 113 patients were included. Transcranial Doppler measurements were feasible in 105 patients (average recording time, 219 minutes). A total of 26 MESs were detected in 8 of 105 patients. In 44 of 105 plaques IPH was present. In 92 of 105 plaques FC status was assessable, 36 of these had a thin/ruptured FC. No significant difference in the prevalence of MES between patients with and without IPH (P=0.46) or with thick versus thin/ruptured FC (P=0.48) was found. CONCLUSIONS: In patients with a symptomatic mild to moderate carotid artery stenosis, IPH and FC status are not associated with MES. This suggests that MRI and transcranial Doppler provide different information on plaque vulnerability. CLINICAL TRIAL REGISTRATION URL: http://www.clinicaltrials.gov. Unique identifier: NCT01709045.

Combined 18F-FDG PET-CT and DCE-MRI to assess inflammation and microvascularization in atherosclerotic plaques.

BACKGROUND AND PURPOSE: Hallmarks of vulnerable atherosclerotic plaques are inflammation that can be assessed with 18fluorine-fluorodeoxyglucose positron emission tomography/computed tomography, and increased neovascularization that can be evaluated by dynamic contrast-enhanced-MRI. It remains unclear whether these parameters are correlated or represent independent imaging parameters. This study determines whether there is a correlation between inflammation and neovascularization in atherosclerotic carotid plaques. METHODS: A total of 58 patients with transient ischemic attack or minor stroke in the carotid territory and ipsilateral carotid artery stenosis of 30% to 69% were included. All patients underwent positron emission tomography/computed tomography and dynamic contrast-enhanced-MRI of the carotid plaque. 18Fluorine-fluorodeoxyglucose standard uptake values with target/background ratio were determined. Neovascularization was quantified by the mean (leakage) volume transfer constant Ktrans. Spearman rank correlation coefficients between target/background ratio and Ktrans were calculated. RESULTS: Images suitable for further analysis were obtained in 49 patients. A weak but significant positive correlation between target/background ratio and mean Ktrans (Spearman ρ=0.30 [P=0.035]) and 75th percentile Ktrans (Spearman ρ=0.29 [P=0.041]) was found. CONCLUSIONS: There is a weak but significant positive correlation between inflammation on positron emission tomography/computed tomography and neovascularization as assessed with dynamic contrast-enhanced-MRI. Future studies should investigate which imaging modality has the highest predictive value for recurrent stroke, as these are not interchangeable. CLINICAL TRIAL REGISTRATION URL: http://www.clinicaltrials.gov. Unique identifier: NCT00451529.

Pearls and Pitfalls of Carotid Artery Imaging: Ultrasound, Computed Tomography Angiography, and MR Imaging.

Stroke represents a major cause of morbidity and mortality worldwide with carotid atherosclerosis responsible for a large proportion of ischemic strokes. Given the high burden of the disease , early diagnosis and optimal secondary prevention are essential elements in clinical practice. For a long time, the degree of stenosis had been considered the parameter to judge the severity of carotid atherosclerosis. Over the last 30 years, literature has shifted attention from stenosis to structural characteristics of atherosclerotic lesion, eventually leading to the "vulnerable plaque" model. These "vulnerable plaques" frequently demonstrate high-risk imaging features that can be assessed by various non-invasive imaging modalities.

Higher Leukocyte Count Is Associated with Lower Presence of Carotid Lipid-Rich Necrotic Core: A Sub-Study in the Plaque at RISK (PARISK) Study.

BACKGROUND: Increasing evidence suggests that inflammation inside the vessel wall has a prominent role in atherosclerosis. In carotid atherosclerosis in particular, vulnerable plaque characteristics are strongly linked to an increased stroke risk. An association between leukocytes and plaque characteristics has not been investigated before and could help with gaining knowledge on the role of inflammation in plaque vulnerability, which could contribute to a new target for intervention. In this study, we investigated the association of the leukocyte count with carotid vulnerable plaque characteristics. METHODS: All patients from the Plaque At RISK (PARISK) study whom had complete data on their leukocyte count and CTA- and MRI-based plaque characteristics were included. Univariable logistic regression was used to detect associations of the leukocyte count with the separate plaque characteristics (intra-plaque haemorrhage (IPH), lipid-rich-necrotic core (LRNC), thin or ruptured fibrous cap (TRFC), plaque ulceration and plaque calcifications). Subsequently, other known risk factors for stroke were included as covariates in a multivariable logistic regression model. RESULTS: 161 patients were eligible for inclusion in this study. Forty-six (28.6%) of these patients were female with a mean age of 70 [IQR 64-74]. An association was found between a higher leukocyte count and lower prevalence of LRNC (OR 0.818 (95% CI 0.687-0.975)) while adjusting for covariates. No associations were found between the leucocyte count and the presence of IPH, TRFC, plaque ulceration or calcifications. CONCLUSIONS: The leukocyte count is inversely associated with the presence of LRNC in the atherosclerotic carotid plaque in patients with a recently symptomatic carotid stenosis. The exact role of leukocytes and inflammation in plaque vulnerability deserves further attention.

Magnetic resonance imaging contrast-enhancement with superparamagnetic iron oxide nanoparticles amplifies macrophage foam cell apoptosis in human and murine atherosclerosis.

AIMS: (Ultra) Small superparamagnetic iron oxide nanoparticles, (U)SPIO, are widely used as magnetic resonance imaging contrast media and assumed to be safe for clinical applications in cardiovascular disease. As safety tests largely relied on normolipidaemic models, not fully representative of the clinical setting, we investigated the impact of (U)SPIOs on disease-relevant endpoints in hyperlipidaemic models of atherosclerosis. METHODS AND RESULTS: RAW264.7 foam cells, exposed in vitro to ferumoxide (dextran-coated SPIO), ferumoxtran (dextran-coated USPIO), or ferumoxytol [carboxymethyl (CM) dextran-coated USPIO] (all 1 mg Fe/mL) showed increased apoptosis and reactive oxygen species accumulation for ferumoxide and ferumoxtran, whereas ferumoxytol was tolerated well. Pro-apoptotic (TUNEL+) and pro-oxidant activity of ferumoxide (0.3 mg Fe/kg) and ferumoxtran (1 mg Fe/kg) were confirmed in plaque, spleen, and liver of hyperlipidaemic ApoE-/- (n = 9/group) and LDLR-/- (n = 9-16/group) mice that had received single IV injections compared with saline-treated controls. Again, ferumoxytol treatment (1 mg Fe/kg) failed to induce apoptosis or oxidative stress in these tissues. Concomitant antioxidant treatment (EUK-8/EUK-134) largely prevented these effects in vitro (-68%, P < 0.05) and in plaques from LDLR-/- mice (-60%, P < 0.001, n = 8/group). Repeated ferumoxtran injections of LDLR-/- mice with pre-existing atherosclerosis enhanced plaque inflammation and apoptosis but did not alter plaque size. Strikingly, carotid artery plaques of endarterectomy patients who received ferumoxtran (2.6 mg Fe/kg) before surgery (n = 9) also showed five-fold increased apoptosis (18.2 vs. 3.7%, respectively; P = 0.004) compared with controls who did not receive ferumoxtran. Mechanistically, neither coating nor particle size seemed accountable for the observed cytotoxicity of ferumoxide and ferumoxtran. CONCLUSIONS: Ferumoxide and ferumoxtran, but not ferumoxytol, induced apoptosis of lipid-laden macrophages in human and murine atherosclerosis, potentially impacting disease progression in patients with advanced atherosclerosis.

Geometrical models for cardiac MRI in rodents: comparison of quantification of left ventricular volumes and function by various geometrical models with a full-volume MRI data set in rodents.

MRI has been proven to be an accurate method for noninvasive assessment of cardiac function. One of the current limitations of cardiac MRI is that it is time consuming. Therefore, various geometrical models are used, which can reduce scan and postprocessing time. It is unclear how appropriate their use is in rodents. Left ventricular (LV) volumes and ejection fraction (EF) were quantified based on 7.0 Tesla cine-MRI in 12 wild-type (WT) mice, 12 adipose triglyceride lipase knockout (ATGL(-/-)) mice (model of impaired cardiac function), and 11 rats in which we induced cardiac ischemia. The LV volumes and function were either assessed with parallel short-axis slices covering the full volume of the left ventricle (FV, gold standard) or with various geometrical models [modified Simpson rule (SR), biplane ellipsoid (BP), hemisphere cylinder (HC), single-plane ellipsoid (SP), and modified Teichholz Formula (TF)]. Reproducibility of the different models was tested and results were correlated with the gold standard (FV). All models and the FV data set provided reproducible results for the LV volumes and EF, with interclass correlation coefficients ≥0.87. All models significantly over- or underestimated EF, except for SR. Good correlation was found for all volumes and EF for the SR model compared with the FV data set (R(2) ranged between 0.59-0.95 for all parameters). The HC model and BP model also predicted EF well (R(2) ≥ 0.85), although proved to be less useful for quantitative analysis. The SP and TF models correlated poorly with the FV data set (R(2) ≥ 0.45 for EF and R(2) ≥ 0.29 for EF, respectively). For the reduction in acquisition and postprocessing time, only the SR model proved to be a valuable method for calculating LV volumes, stroke volume, and EF.

Evaluation of a Dedicated Radiofrequency Carotid PET/MRI Coil.

Carotid radiofrequency coils inside a PET/MRI system can result in PET quantification errors. We compared the performance of a dedicated PET/MRI carotid coil against a coil for MRI-only use. An 18F-fluorodeoxyglucose (18F-FDG) phantom was scanned without and with an MRI-only coil and with the PET/MRI coil. The decay-corrected normalized activity was compared for the different coil configurations. Eighteen patients were scanned with the three coil configurations. The maximal standardized uptake values (SUVmax) and signal-to-noise ratios (SNR) were calculated. Repeated measures ANOVA was performed to assess the differences in SUVmax and SNR between the coil configurations. In the phantom study, the PET/MRI coil demonstrated a slight decrease (<5%), while the MRI-only coil showed a substantial decrease (up to 10%) in normalized activity at the position of coil elements compared to no dedicated coil configuration. In the patient study, the SUVmax values for both no surface coil (3.59 ± 0.15) and PET/MRI coil (3.54 ± 0.15) were significantly higher (p = 0.03 and p = 0.04, respectively) as compared to the MRI-only coil (3.28 ± 0.16). No significant difference was observed between PET/MRI and no surface coil (p = 1.0). The SNR values for both PET/MRI (7.31 ± 0.44) and MRI-only (7.62 ± 0.42) configurations demonstrated significantly higher (p < 0.001) SNR values as compared to the no surface coil (3.78 ± 0.22), while no significant difference was observed in SNR between the PET/MRI and MRI-only coil (p = 1.0). This study demonstrated that the PET/MRI coil can be used for PET imaging without requiring attenuation correction while acquiring high-resolution MR images.

Intrahepatic lipid content is independently associated with soluble E-selectin levels: The Maastricht study.

BACKGROUND: Evidence is accumulating that liver sinusoidal endothelial cells are involved in the pathogenesis of non-alcoholic fatty liver disease. Previous studies have suggested that the endothelial biomarker soluble E-selectin (sE-selectin) is to an important extent liver-derived. AIMS: To study the relationship of intrahepatic lipid (IHL) content with sE-selectin at the population level. METHODS: This study was conducted in participants of The Maastricht Study (n = 1,634), a population-based cohort study enriched with patients with type 2 diabetes. We assessed the cross-sectional association between IHL content, quantified by MRI, and sE-selectin via multivariable regression with adjustment for age, sex, type 2 diabetes, educational level, BMI, Dutch Healthy Diet index, physical activity, and the Matsuda index. RESULTS: Standardized IHL content was associated with (log) sE-selectin (age-, sex- and type 2 diabetes-adjusted regression coefficient [B]: 0.048 [95%CI:0.038;0.058], p<0.001), even after full adjustment (B: 0.030 [0.019;0.042], p<0.001). Such an association was not observed for soluble vascular cell adhesion molecule 1 (sVCAM-1) levels. CONCLUSION: IHL content is an independent determinant of sE-selectin at the population level. These findings support further studies to unravel the involvement of liver sinusoidal endothelial cells in the different stages of non-alcoholic fatty liver disease and the specific role of E-selectin herein.

Development and validation of novel imaging technologies to assist translational studies in atherosclerosis.

In the past decade, significant progress has been made to visualize atherosclerotic disease. Until recently, imaging technologies mainly focused on lumen and vessel wall visualization. Current advances and knowledge on the molecular mechanisms of initiation and progression of atherosclerosis has emphasized the need for imaging technologies and probes that can image function and biology rather than anatomy. This field of molecular imaging is now in rapid development with new imaging agents that aim at visualizing processes involved in atherosclerosis such as inflammation, macrophage activation, protease activity, angiogenesis, apoptosis, lipid accumulation and thrombus formation.:

Mitochondrial Function and Diabetes: Consequences for Skeletal and Cardiac Muscle Metabolism.

SIGNIFICANCE: An early hallmark in the development of type 2 diabetes is the resistance to the effect of insulin in skeletal muscle and in the heart. Since mitochondrial function was found to be diminished in patients with type 2 diabetes, it was suggested that this defect might be involved in the etiology of insulin resistance. Although several hypotheses were suggested, yet unclear is the mechanistic link between these two phenomena. RECENT ADVANCES: Herein, we review the evidence for disturbances in mitochondrial function in skeletal muscle and the heart in the diabetic state. Also the mechanisms involved in improving mitochondrial function are considered and, whenever possible, human data is cited. CRITICAL ISSUES: Reported evidence shows that interventions that improve skeletal muscle mitochondrial function also improve insulin sensitivity in humans. In the heart, available data from animal studies suggests that enhancement of mitochondrial function can reverse aging-induced changes in heart function, and can be protective against cardiomyopathy and heart failure. FUTURE DIRECTIONS: Mitochondria and their functions can be targeted with the aim of improving skeletal muscle insulin sensitivity and cardiac function. However, human clinical intervention studies are needed to fully substantiate the potential of mitochondria as a target to prevent cardiometabolic disease.

Carotid plaques in transient ischemic attack and stroke patients: one-year follow-up study by magnetic resonance imaging.

OBJECTIVE: To investigate the natural course of carotid plaque progression in transient ischemic attack/stroke patients by using serial multisequence magnetic resonance imaging (MRI). MATERIALS AND METHODS: Forty transient ischemic attack/stroke patients with ipsilateral <70% carotid stenosis underwent MRI of the plaque ipsilateral to the symptomatic side at baseline and after 1 year. The MRI protocol consisted of T1-weighted turbo field-echo, time-of-flight, T2-weighted turbo spin-echo (TSE), and pre- and postgadopentetate dimeglumine-enhanced T1-weighted TSE images. For each plaque, carotid lumen volume, wall volume, total vessel volume (=carotid lumen volume + wall volume), the presence of a lipid-rich necrotic core (LRNC), fibrous cap (FC) status, and the presence of intraplaque hemorrhage (IPH) were assessed at both time points. RESULTS: Over a 1-year period, mean carotid lumen volume decreased with 4.8% ± 2.0% (±standard error) (P = 0.013). Mean wall volume increased with 11.2% ± 2.2% (P < 0.001). Total vessel volume did not significantly change (P = 0.147). At baseline, there were 18 plaques with a LRNC, which also had a LRNC at 1-year follow-up. No plaque without a LRNC at baseline developed a LRNC during the follow-up period. All plaques with a LRNC had a thin and/or ruptured FC at both time points. Twelve patients had IPH both at baseline and at follow-up. In one patient, IPH disappeared, whereas in another patient, new IPH appeared at follow-up. The presence of IPH and a LRNC with a thin and/or ruptured FC were not significantly associated with plaque progression (P > 0.05). CONCLUSIONS: In symptomatic patients with an ipsilateral carotid plaque causing <70% stenosis, we found evidence for inward plaque remodeling over a 1-year period. Overall, the presence/absence of IPH, a LRNC, and FC status did not change over 1 year.

Molecular MRI of early thrombus formation using a bimodal alpha2-antiplasmin-based contrast agent.

OBJECTIVES: We aimed to investigate whether early thrombus formation can be visualized with in vivo magnetic resonance imaging (MRI) by the use of a novel bimodal alpha(2)-antiplasmin-based contrast agent (CA). BACKGROUND: Thrombus formation plays a central role in several vascular diseases. During the early phases of thrombus formation, activated factor XIII (FXIIIa) covalently cross-links alpha(2)-antiplasmin to fibrin, indicating the potential of alpha(2)-antiplasmin-based CAs in the detection of early thrombus formation. METHODS: A bimodal CA was synthesized by coupling gadolinium-diethylene triamine pentaacetic acid and rhodamine to an alpha(2)-antiplasmin-based peptide. For the control CA, a glutamine residue essential for cross-linking was replaced by alanine. In vitro-generated thrombi were exposed to both CAs and imaged by MRI and 2-photon laser-scanning microscopy. Immunohistochemistry was performed on human pulmonary thromboemboli sections to determine the presence of alpha(2)-antiplasmin and FXIII in different thrombus remodeling phases. In vivo feasibility of the CA in detecting early thrombus formation specifically was investigated with MRI. RESULTS: In vitro-generated thrombi exposed to the alpha(2)-antiplasmin-based CA showed hyperintense magnetic resonance signal intensities at the thrombus edge. No hyperintense signal was observed when we used the alpha(2)-antiplasmin-based CA in the presence of FXIII inhibitor dansylcadaverine nor when we used the control CA. Two-photon laser-scanning microscopy demonstrated that the alpha(2)-antiplasmin-based CA bound to fibrin. Immunohistochemistry demonstrated substantial alpha(2)-antiplasmin staining in fresh compared with lytic and organized thrombi. The administration of CA in vivo within seconds after inducing thrombus formation increased contrast-to-noise ratios (CNRs 2.28 +/- 0.39, n=6) at the site of thrombus formation compared with the control CA (CNRs -0.14 +/- 0.55, p = 0.003, n = 6) and alpha(2)-antiplasmin-based CA administration 24 to 48 h after thrombus formation (CNRs 0.11 +/- 0.23, p = 0.006, n = 6). CONCLUSIONS: A bimodal CA was developed, characterized, and validated. Our results showed that this bimodal CA enabled noninvasive in vivo magnetic resonance visualization of early thrombus formation.

Lower intrinsic ADP-stimulated mitochondrial respiration underlies in vivo mitochondrial dysfunction in muscle of male type 2 diabetic patients.

OBJECTIVE: A lower in vivo mitochondrial function has been reported in both type 2 diabetic patients and first-degree relatives of type 2 diabetic patients. The nature of this reduction is unknown. Here, we tested the hypothesis that a lower intrinsic mitochondrial respiratory capacity may underlie lower in vivo mitochondrial function observed in diabetic patients. RESEARCH DESIGN AND METHODS: Ten overweight diabetic patients, 12 first-degree relatives, and 16 control subjects, all men, matched for age and BMI, participated in this study. Insulin sensitivity was measured with a hyperinsulinemic-euglycemic clamp. Ex vivo intrinsic mitochondrial respiratory capacity was determined in permeabilized skinned muscle fibers using high-resolution respirometry and normalized for mitochondrial content. In vivo mitochondrial function was determined by measuring phosphocreatine recovery half-time after exercise using (31)P-magnetic resonance spectroscopy. RESULTS: Insulin-stimulated glucose disposal was lower in diabetic patients compared with control subjects (11.2 +/- 2.8 vs. 28.9 +/- 3.7 micromol x kg(-1) fat-free mass x min(-1), respectively; P = 0.003), with intermediate values for first-degree relatives (22.1 +/- 3.4 micromol x kg(-1) fat-free mass x min(-1)). In vivo mitochondrial function was 25% lower in diabetic patients (P = 0.034) and 23% lower in first-degree relatives, but the latter did not reach statistical significance (P = 0.08). Interestingly, ADP-stimulated basal respiration was 35% lower in diabetic patients (P = 0.031), and fluoro-carbonyl cyanide phenylhydrazone-driven maximal mitochondrial respiratory capacity was 31% lower in diabetic patients (P = 0.05) compared with control subjects with intermediate values for first-degree relatives. CONCLUSIONS: A reduced basal ADP-stimulated and maximal mitochondrial respiratory capacity underlies the reduction in in vivo mitochondrial function, independent of mitochondrial content. A reduced capacity at both the level of the electron transport chain and phosphorylation system underlies this impaired mitochondrial capacity.

Intramyocellular lipid content in human skeletal muscle.

Fat can be stored not only in adipose tissue but also in other tissues such as skeletal muscle. Fat droplets accumulated in skeletal muscle [intramyocellular lipids (IMCLs)] can be quantified by different methods, all with advantages and drawbacks. Here, we briefly review IMCL quantification methods that use biopsy specimens (biochemical quantification, electron microscopy, and histochemistry) and non-invasive alternatives (magnetic resonance spectroscopy, magnetic resonance imaging, and computed tomography). Regarding the physiological role, it has been suggested that IMCL serves as an intracellular source of energy during exercise. Indeed, IMCL content decreases during prolonged submaximal exercise, and analogously to glycogen, IMCL content is increased in the trained state. In addition, IMCL content is highest in oxidative, type 1 muscle fibers. Together, this, indeed, suggests that the IMCL content is increased in the trained state to optimally match fat oxidative capacity and that it serves as readily available fuel. However, elevation of plasma fatty acid levels or dietary fat content also increases IMCL content, suggesting that skeletal muscle also stores fat simply if the availability of fatty acids is high. Under these conditions, the uptake into skeletal muscle may have negative consequences on insulin sensitivity. Besides the evaluation of the various methods to quantify IMCLs, this perspective describes IMCLs as valuable energy stores during prolonged exercise, which, however, in the absence of regular physical activity and with overconsumption of fat, can have detrimental effects on muscular insulin sensitivity.

Intramyocellular lipid content and molecular adaptations in response to a 1-week high-fat diet.

OBJECTIVE: To investigate molecular adaptations that accompany the elevation of intramyocellular lipid (IMCL) content on a high-fat (HF) diet for 1 week. RESEARCH METHODS AND PROCEDURES: Ten subjects consumed a normal-fat (NF) diet for 1 week, followed by an HF diet for another week. After both dietary periods, we determined the IMCL content by proton magnetic resonance spectroscopy in the vastus lateralis muscle and quantified changes in gene expression, protein content, and activity in biopsy samples. We investigated genes involved in carbohydrate and fatty acid handling [lipoprotein lipase, acetyl-coenzyme A carboxylase (ACC) 2, hormone-sensitive lipase, hexokinase II, and glucose transporter 4] and measured protein levels of CD36 and phosphorylated and unphosphorylated ACC2 and the activity of adenosine monophosphate-activated kinase. RESULTS: IMCL content was increased by 54% after the HF period. Lipoprotein lipase mRNA concentration was increased by 33%, whereas ACC2 mRNA concentration tended to be increased after the HF diet. Hexokinase II, glucose transporter 4, and hormone-sensitive lipase mRNA were unchanged after the HF diet. ACC2 and CD36 protein levels, phosphorylation status of ACC2, and adenosine monophosphate-activated kinase activity did not change in response to the HF diet. DISCUSSION: We found that IMCL content in skeletal muscle increased after 1 week of HF feeding, accompanied by molecular adaptations that favor fat storage in muscle rather than oxidation.