Cine-MRI assessment of cardiac function in mice anesthetized with ketamine/xylazine and isoflurane

Accurate image derived input function in [18F]SynVesT-1 mouse studies using isoflurane and ketamine/xylazine anesthesia

BACKGROUND

Kinetic modeling in positron emission tomography (PET) requires measurement of the tracer plasma activity in the absence of a suitable reference region. To avoid invasive blood sampling, the use of an image derived input function has been proposed. However, an accurate delineation of the blood pool region in the PET image is necessary to obtain unbiased blood activity. Here, to perform brain kinetic modeling in [18F]SynVesT-1 dynamic scans, we make use of non-negative matrix factorization (NMF) to unmix the activity signal from the different tissues that can contribute to the heart region activity, and extract only the left ventricle activity in an unbiased way. This method was implemented in dynamic [18F]SynVesT-1 scans of mice anesthetized with either isoflurane or ketamine-xylazine, two anesthestics that we showed to affect differently radiotracer kinetics. The left ventricle activity (NMF-IDIF) and a manually delineated cardiac activity (IDIF) were compared with arterial blood samples (ABS), and for isoflurane anesthetized mice, arteriovenous (AV) shunt blood data were compared as well. Finally, brain regional 2 tissue compartment modeling was performed using IDIF and NMF-IDIF, and the model fit accuracy (weighted symmetrical mean absolute percentage error, wsMAPE) as well as the total volume of distribution (VT) were compared.

RESULTS

In isoflurane anesthetized mice, the difference between ABS and NMF-IDIF activity (+ 12.8 [Formula: see text] 11%, p = 0.0023) was smaller than with IDIF (+ 16.4 [Formula: see text] 9.8%, p = 0.0008). For ketamine-xylazine anesthetized mice the reduction in difference was larger (NMF-IDIF: 16.9 [Formula: see text] 10%, p = 0.0057, IDIF: 56.3 [Formula: see text] 14%, p < 0.0001). Correlation coefficient between isoflurane AV-shunt time activity curves and NMF-IDIF (0.97 [Formula: see text] 0.01) was higher than with IDIF (0.94 [Formula: see text] 0.03). The brain regional 2TCM wsMAPE was improved using NMF-IDIF compared with IDIF, in isoflurane (NMF-IDIF: 1.24 [Formula: see text] 0.24%, IDIF: 1.56 [Formula: see text] 0.30%) and ketamine-xylazine (NMF-IDIF: 1.40 [Formula: see text] 0.24, IDIF: 2.62 [Formula: see text] 0.27) anesthetized mice. Finally, brain VT was significantly (p < 0.0001) higher using NMF-IDIF compared with IDIF, in isoflurane (3.97 [Formula: see text] 0.13% higher) and ketamine-xylazine (32.7 [Formula: see text] 2.4% higher) anesthetized mice.

CONCLUSIONS

Image derived left ventricle blood activity calculated with NMF improves absolute activity quantification, and reduces the error in the kinetic modeling fit. These improvements are more pronounced in ketamine-xylazine than in isoflurane anesthetized mice.

Relationship of cardiac remodeling and perfusion alteration with hepatic lipid metabolism in a prediabetic high fat high sucrose diet female rat model

BACKGROUND AND AIMS

Cardiovascular disease (CVD) is known to be linked with metabolic associated fatty liver disease and type 2 diabetes, but few studies assessed this relationship in prediabetes, especially among women, who are at greater risk of CVD. We aimed to evaluate cardiac alterations and its relationship with hepatic lipid metabolism in prediabetic female rats submitted to high-fat-high-sucrose diet (HFS).

METHODS AND RESULTS

Wistar female rats were divided into 2 groups fed for 5 months with standard or HFS diet. We analyzed cardiac morphology, function, perfusion and fibrosis by Magnetic Resonance Imaging. Hepatic lipid contents along with inflammation and lipid metabolism gene expression were assessed. Five months of HFS diet induced glucose intolerance (p < 0.05), cardiac remodeling characterized by increased left-ventricular volume, wall thickness and mass (p < 0.05). No significant differences were found in left-ventricular ejection fraction and cardiac fibrosis but increased myocardial perfusion (p < 0.01) and reduced cardiac index (p < 0.05) were shown. HFS diet induced hepatic lipid accumulation with increased total lipid mass (p < 0.001) and triglyceride contents (p < 0.05), but also increased mitochondrial (CPT1a, MCAD; (p < 0.001; p < 0.05) and peroxisomal (ACO, LCAD; (p < 0.05; p < 0.001) β-oxidation gene expression. Myocardial wall thickness and perfusion were correlated with hepatic β-oxidation genes expression. Furthermore, myocardial perfusion was also correlated with hepatic lipid content and glucose intolerance.

CONCLUSION

This study brings new insights on the relationship between cardiac sub-clinical alterations and hepatic metabolism in female prediabetic rats. Further studies are warranted to explore its involvement in the higher CVD risk observed among prediabetic women.

Co-Clinical Imaging Metadata Information (CIMI) for Cancer Research to Promote Open Science, Standardization, and Reproducibility in Preclinical Imaging

Preclinical imaging is a critical component in translational research with significant complexities in workflow and site differences in deployment. Importantly, the National Cancer Institute's (NCI) precision medicine initiative emphasizes the use of translational co-clinical oncology models to address the biological and molecular bases of cancer prevention and treatment. The use of oncology models, such as patient-derived tumor xenografts (PDX) and genetically engineered mouse models (GEMMs), has ushered in an era of co-clinical trials by which preclinical studies can inform clinical trials and protocols, thus bridging the translational divide in cancer research. Similarly, preclinical imaging fills a translational gap as an enabling technology for translational imaging research. Unlike clinical imaging, where equipment manufacturers strive to meet standards in practice at clinical sites, standards are neither fully developed nor implemented in preclinical imaging. This fundamentally limits the collection and reporting of metadata to qualify preclinical imaging studies, thereby hindering open science and impacting the reproducibility of co-clinical imaging research. To begin to address these issues, the NCI co-clinical imaging research program (CIRP) conducted a survey to identify metadata requirements for reproducible quantitative co-clinical imaging. The enclosed consensus-based report summarizes co-clinical imaging metadata information (CIMI) to support quantitative co-clinical imaging research with broad implications for capturing co-clinical data, enabling interoperability and data sharing, as well as potentially leading to updates to the preclinical Digital Imaging and Communications in Medicine (DICOM) standard.

Female hormones prevent sepsis-induced cardiac dysfunction: an experimental randomized study

Although epidemiologic research has demonstrated significant differences in incidence and outcomes of sepsis according to sex, their underlying biological mechanisms are poorly understood. Here, we studied the influence of hormonal status by comparing in vivo cardiac performances measured by MRI in non-ovariectomized and ovariectomized septic female rats. Control and ovariectomized rats were randomly allocated to the following groups: sham, sepsis and sepsis plus landiolol. Sepsis was induced by caecum ligation and punction (CLP). Landiolol, a short-acting selective β1-adrenergic blocker improving the in vivo cardiac performance of septic male rats was perfused continuously after sepsis induction. Cardiac MRI was carried out 18 h after induction of sepsis to assess in vivo cardiac function. Capillary permeability was evaluated by Evans Blue administration and measurement of its tissue extravasation. Variation in myocardial gene and protein expression was also assessed by qPCR and western-blot in the left ventricular tissue. Sepsis reduced indexed stroke volume, cardiac index and indexed end-diastolic volume compared to sham group in ovariectomized females whereas it had no effect in control females. This was associated with an overexpression of JAK2 expression and STAT3 phosphorylation on Ser727 site, and an inhibition of the adrenergic pathways in OVR females. Landiolol increased the indexed stroke volume by reversing the indexed end-diastolic volume reduction after sepsis in ovariectomized females, while it decreased indexed stroke volume and cardiac index in control. This was supported by an overexpression of genes involved in calcium influx in OVR females while an inactivation of the β-adrenergic and a calcium efflux pathway was observed in control females. Sepsis decreased in vivo cardiac performances in ovariectomized females but not in control females, presumably associated with a more pronounced inflammation, inhibition of the adrenergic pathway and calcium efflux defects. Administration of landiolol prevents this cardiac dysfunction in ovariectomized females with a probable activation of calcium influx, while it has deleterious effects in control females in which calcium efflux pathways were down-regulated.

Multiparametric MRI identifies subtle adaptations for demarcation of disease transition in murine aortic valve stenosis

Aortic valve stenosis (AS) is the most frequent valve disease with relevant prognostic impact. Experimental model systems for AS are scarce and comprehensive imaging techniques to simultaneously quantify function and morphology in disease progression are lacking. Therefore, we refined an acute murine AS model to closely mimic human disease characteristics and developed a high-resolution magnetic resonance imaging (MRI) approach for simultaneous in-depth analysis of valvular, myocardial as well as aortic morphology/pathophysiology to identify early changes in tissue texture and critical transition points in the adaptive process to AS. AS was induced by wire injury of the aortic valve. Four weeks after surgery, cine loops, velocity, and relaxometry maps were acquired at 9.4 T to monitor structural/functional alterations in valve, aorta, and left ventricle (LV). In vivo MRI data were subsequently validated by histology and compared to echocardiography. AS mice exhibited impaired valve opening accompanied by significant valve thickening due to fibrotic remodelling. While control mice showed bell-shaped flow profiles, AS resulted not only in higher peak flow velocities, but also in fragmented turbulent flow patterns associated with enhanced circumferential strain and an increase in wall thickness of the aortic root. AS mice presented with a mild hypertrophy but unaffected global LV function. Cardiac MR relaxometry revealed reduced values for both T1 and T2 in AS reflecting subtle myocardial tissue remodelling with early alterations in mitochondrial function in response to the enhanced afterload. Concomitantly, incipient impairments of coronary flow reserve and myocardial tissue integrity get apparent accompanied by early troponin release. With this, we identified a premature transition point with still compensated cardiac function but beginning textural changes. This will allow interventional studies to explore early disease pathophysiology and novel therapeutic targets.

19F MRI Imaging Strategies to Reduce Isoflurane Artifacts in In Vivo Images

Purpose

Isoflurane (ISO) is the most commonly used preclinical inhalation anesthetic. This is a problem in ¹⁹F MRI of fluorine contrast agents, as ISO signals cause artifacts that interfere with unambiguous image interpretation and quantification; the two most attractive properties of heteronuclear MRI. We aimed to avoid these artifacts using MRI strategies that can be applied by any pre-clinical researcher.

Procedures

Three strategies to avoid ISO chemical shift displacement artifacts (CSDA) in ¹⁹F MRI are described and demonstrated with measurements of ¹⁹F-containing agents in phantoms and in vivo (n = 3 for all strategies). The success of these strategies is compared to a standard Rapid Acquisition with Relaxation Enhancement (RARE) sequence, with phantom and in vivo validation. ISO artifacts can successfully be avoided by (1) shifting them outside the region of interest using a narrow signal acquisition bandwidth, (2) suppression of ISO by planning a frequency-selective suppression pulse before signal acquisition or by (3) preventing ISO excitation with a 3D sequence with a narrow excitation bandwidth.

Results

All three strategies result in complete ISO signal avoidance (p < 0.0001 for all methods). Using a narrow acquisition bandwidth can result in loss of signal to noise ratio and distortion of the image, and a frequency-selective suppression pulse can be incomplete when B₁-inhomogeneities are present. Preventing ISO excitation with a narrow excitation pulse in a 3D sequence yields the most robust results (relative SNR 151 ± 28% compared to 2D multislice methods, p = 0.006).

Conclusion

We optimized three easily implementable methods to avoid ISO signal artifacts and validated their performance in phantoms and in vivo. We make recommendation on the parameters that pre-clinical studies should report in their method section to make the used approach insightful.

Co-Clinical Imaging Resource Program (CIRP): Bridging the Translational Divide to Advance Precision Medicine

The National Institutes of Health’s (National Cancer Institute) precision medicine initiative emphasizes the biological and molecular bases for cancer prevention and treatment. Importantly, it addresses the need for consistency in preclinical and clinical research. To overcome the translational gap in cancer treatment and prevention, the cancer research community has been transitioning toward using animal models that more fatefully recapitulate human tumor biology. There is a growing need to develop best practices in translational research, including imaging research, to better inform therapeutic choices and decision-making. Therefore, the National Cancer Institute has recently launched the Co-Clinical Imaging Research Resource Program (CIRP). Its overarching mission is to advance the practice of precision medicine by establishing consensus-based best practices for co-clinical imaging research by developing optimized state-of-the-art translational quantitative imaging methodologies to enable disease detection, risk stratification, and assessment/prediction of response to therapy. In this communication, we discuss our involvement in the CIRP, detailing key considerations including animal model selection, co-clinical study design, need for standardization of co-clinical instruments, and harmonization of preclinical and clinical quantitative imaging pipelines. An underlying emphasis in the program is to develop best practices toward reproducible, repeatable, and precise quantitative imaging biomarkers for use in translational cancer imaging and therapy. We will conclude with our thoughts on informatics needs to enable collaborative and open science research to advance precision medicine.

Nitrous oxide improves cardiovascular, respiratory, and thermal stability during prolonged isoflurane anesthesia in juvenile guinea pigs

Anesthesia is frequently used to facilitate physiological monitoring during interventional animal studies. However, its use may induce cardiovascular (central and peripheral), respiratory, and thermoregulatory depression, confounding results in anesthetized animals. Despite the wide utility of guinea pigs as a translational platform, anesthetic protocols remain unstandardized for extended physiological studies in this species. Therefore, optimizing an anesthetic protocol that balances stable anesthesia with intact cardiorespiratory and metabolic function is crucial. To achieve this, 12 age and sex-matched juvenile Dunkin Hartley guinea pigs underwent extended anesthesia (≤150 min) with either (a) isoflurane (ISO: 1.5%), or (b) isoflurane + N2 O (ISO+ N2 O: 0.8% +70%), in this randomized cross-over designed study. Cardiovascular (HR, SBP, peripheral microvascular blood flow), respiratory (respiratory rate, SpO2 ), and thermal (Tre and Tsk ) measures were recorded continuously throughout anesthesia. Blood gas measures pre- and post- anesthesia were performed. Incorporation of 70% N2 O allowed for significant reductions in isoflurane (to 0.8%) while maintaining an effective anesthetic depth for prolonged noninvasive physiological examination in guinea pigs. ISO+N2 O maintained heart rate, peripheral blood flow, respiratory rate, and thermoregulatory function at levels closest to those of conscious animals, especially in females; however, it did not fully rescue anesthesia-induced hypotension. These results suggest that for studies requiring prolonged physiological examination (≤150 min) in guinea pigs, 0.8% isoflurane with a 70% N2 O adjuvant provides adequate anesthesia, while minimizing associated cardiorespiratory depression. The preservation of cardiorespiratory status is most marked throughout the first hour of anesthesia.

Preparation and Monitoring of Small Animals in Renal MRI

Renal diseases remain devastating illnesses with unacceptably high rates of mortality and morbidity worldwide. Animal models are essential tools to better understand the pathomechanism of kidney-related illnesses and to develop new, successful therapeutic strategies. Magnetic resonance imaging (MRI) has been actively explored in the last decades for assessing renal function, perfusion, tissue oxygenation as well as the degree of fibrosis and inflammation. This chapter aims to provide an overview of the preparation and monitoring of small animals before, during, and after surgical interventions or MR imaging. Standardization of experimental settings such as body temperature or hydration of animals and minimizing pain and distress are essential for diminishing nonexperimental variables as well as for conducting ethical research.This publication is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers.

Serial examination of cardiac function and perfusion in growing rats using SPECT/CT for small animals

Few studies have evaluated myocardial perfusion and ventricular function in normal, growing rats. We, therefore, evaluated serial changes in cardiac perfusion and function during the growth of normal rats using single photon emission computed tomography (SPECT) with technetium (^99mTc)-sestamibi. Gated SPECT was serially performed in six normal rats. The left ventricular end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV) and ejection fraction (EF) were calculated with Quantitative Gated SPECT software. The perfusion distribution was calculated as the percentage uptake of each of the 17 segments using Quantitative Perfusion SPECT software. As expected, the body weight (BW) of the rats increased with growth, but their heart rates (HR) did not change over time. EF decreased very slowly over time and showed a negative correlation with BW. EDV, ESV and SV showed strong positive correlations with BW. There were no significant differences in the percentage segmental uptake in 13 of the 17 segments during growth, except for three basal and one apical segments. Therefore, a single normal database could be applied for the evaluation of perfusion abnormalities in rats of at least 8 to 28 weeks old.

Sex-Mediated Response to the Beta-Blocker Landiolol in Sepsis: An Experimental, Randomized Study

OBJECTIVES

To investigate any gender effect of the beta-1 adrenergic blocker, landiolol, on cardiac performance and energy metabolism in septic rats, and to explore the expression of genes and proteins involved in this process.

DESIGN

Randomized animal study.

SETTING

University research laboratory.

SUBJECTS

Male and female Wistar rats.

INTERVENTIONS

One hour after cecal ligation and puncture, male and female rats were randomly allocated to the following groups: sham male, cecal ligation and puncture male, cecal ligation and puncture + landiolol male, sham female, cecal ligation and puncture female, and cecal ligation and puncture + landiolol female. Cardiac MRI was carried out 18 hours after cecal ligation and puncture to assess in vivo cardiac function. Ex vivo cardiac function measurement and P magnetic resonance spectroscopy were subsequently performed using an isovolumic isolated heart preparation. Finally, we assessed cardiac gene and protein expression.

MEASUREMENTS AND MAIN RESULTS

In males, landiolol increased indexed stroke volume by reversing the indexed end-diastolic volume reduction without affecting left ventricle ejection fraction. In females, landiolol did not increase indexed stroke volume and indexed end-diastolic volume but decreased left ventricle ejection fraction. Landiolol had no effect on ex vivo cardiac function and on high-energy phosphate compounds. The effect of landiolol on the gene expression of natriuretic peptide receptor 3 and on protein expression of phosphorylated-AKT:AKT ratio and endothelial nitric oxide synthase was different in males and females.

CONCLUSIONS

Landiolol improved the in vivo cardiac performance of septic male rats while deleterious effects were reported in females. Expression of natriuretic peptide receptor 3, phosphorylated-AKT:AKT, and endothelial nitric oxide synthase are signaling pathways to investigate to better understand the sex differences in sepsis.

Self-Gated Respiratory Motion Rejection for Optoacoustic Tomography

Respiratory motion in living organisms is known to result in image blurring and loss of resolution, chiefly due to the lengthy acquisition times of the corresponding image acquisition methods. Optoacoustic tomography can effectively eliminate in vivo motion artifacts due to its inherent capacity for collecting image data from the entire imaged region following a single nanoseconds-duration laser pulse. However, multi-frame image analysis is often essential in applications relying on spectroscopic data acquisition or for scanning-based systems. Thereby, efficient methods to correct for image distortions due to motion are imperative. Herein, we demonstrate that efficient motion rejection in optoacoustic tomography can readily be accomplished by frame clustering during image acquisition, thus averting excessive data acquisition and post-processing. The algorithm’s efficiency for two- and three-dimensional imaging was validated with experimental whole-body mouse data acquired by spiral volumetric optoacoustic tomography (SVOT) and full-ring cross-sectional imaging scanners.

The mode of anesthesia influences outcome in mouse models of arterial thrombosis

BACKGROUND

Arterial thrombosis models are important for preclinical evaluation of antithrombotics but how anesthetic protocol can influence experimental results is not studied.

OBJECTIVES

We studied how three most commonly used rodent anesthetics affect the induction of thrombosis and thrombus resolution with antiplatelet agent integrilin (Eptifibatide).

METHODS

The Folts, electrolytic, and FeCl3 models of carotid artery thrombosis were evaluated. The extent of blood flow reduction required to elicit cyclic flow reductions (CFR) was examined in the Folts model. The occlusion time and stability following electrolytic or FeCl3 injury was assessed. The efficacy of Eptifibatide was studied in each cohort and clot composition following FeCl3 application was assessed histologically.

RESULTS

Isoflurane and ketamine-xylazine (ket-x) elicited higher basal blood flow velocities. For reliable CFR in the Folts model, a higher degree of blood flow reduction was required under ket-x and isoflurane. For the FeCl3 and electrolytic models, injury severity had to be increased in mice under ket-x anesthesia to achieve rapid occlusion. FeCl3-injured artery sections from ket-x and isoflurane-treated mice showed vessel dilatation and clots that were more fibrin/red-cell rich compared to pentobarbitone. Integrilin led to cycle abolishment for all three Folts-injury cohorts but for the electrolytic model a 2.5-fold higher dose was required to restore blood flow under pentobarbitone. Integrilin after FeCl3 arterial injury was partially ineffective in isoflurane-treated mice.

CONCLUSIONS

Anesthesia impacts rodent carotid artery occlusion experiments and alters integrilin efficacy. It is important to consider anesthetic protocols in animal experiments involving pharmacological agents for treatment of atherothrombosis.

4D cardiac magnetic resonance imaging, 4D and 2D transthoracic echocardiography: a comparison of in-vivo assessment of ventricular function in rats

Preclinical cardiovascular research is the foundation of our understanding and broad knowledge of heart function and cardiovascular disease. Reliable cardiac imaging modalities are the basis for applicable results. Four-dimensional cardiac magnetic resonance (4D CMR) has been set as the gold standard for in-vivo assessment of ventricular function in rodents. However, technical improvements in echocardiography now allow us to image the whole heart, which makes four-dimensional echocardiography (4DE) a possible alternative to 4D CMR. To date, no study has systematically assessed 4DE in comparison with 4D CMR in rats. In total we studied 26 juvenile Sprague-Dawley rats (Crl: CD (SD) IGS). Twenty rats underwent echocardiographic imaging (2D and 4D) and 4D CMR. Five of those rats underwent a ligation of the superior and inferior vena cava to reduce the cardiac inflow as a disease model. Six additional rats were used to assess reproducibility of echocardiography and underwent three echocardiographic examinations. 4D CMR was performed on a 7T scanner; 2D and 4D echocardiography was conducted using a 40 MHz transducer. Correlation between 4D CMR, 4DE and 2DE for left-ventricular ejection fraction (LVEF) was assessed. An excellent correlation was observed between 4DE and 4D CMR ( r = 0.95, p < 0.001). Correlation of 2DE and 4D CMR was weak ( r = 0.57, p < 0.01). 4DE provides results that are equally precise as 4D CMR and highly reproducible with less technical effort than 4D CMR.

Fluoride induced tissue hypercalcemia, IL-17 mediated inflammation and apoptosis lead to cardiomyopathy: Ultrastructural and biochemical findings

An increased prevalence of cardiac complications has been observed in residents of fluorosis endemic areas chronically exposed to fluoride. Fluoride induces soft tissue injury due to oxidative stress, lipid peroxidation (LPO) and mitochondriopathy. It was hypothesized that chronic fluoride exposure induces apoptosis in cardiomyocytes due to inflammation, lysis of extra cellular matrix and altered calcium metabolism. This study was planned to evaluate the effects of chronic fluoride exposure and the mechanism of action in the cardiac muscle. Fifteen week old male Wistar rats were administered a human equivalent dose of fluoride (50 and 100 ppm ad-libitum, HED = 5 & 10 ppm in human) for 75-days. After 75-days of fluoride exposure, the animals were euthanized and fluoride, oxidative stress (SOD, GPX, Catalase activities) and LPO were measured. Histopathological and ultrastructural pathological examinations were conducted on the cardiac tissues using light, atomic force and electron microscopies. The cardiac tissues were also assessed for apoptosis (TUNEL/Caspase assays), and tissue calcium levels (Alizarin-assay and SEM-EDX). Tissue inflammation and expression of IL-17, MMP-9, Caspase-3 and Bcl-2 were evaluated. In the fluoride exposed groups, a significant (≤0.05) increase in levels of oxidative stress, LPO and apoptosis were observed. The IL-17, MMP-9 and Caspase-3 were significantly (≤0.05) higher in the cardiac muscle after chronic fluoride exposure. The fluoride seems to have induced inflammation in the cardiac tissues, as well as an increase in tissue calcium (≤0.05). There was significant damage to cardiac muscle fibres including, thinning, distortion and neo-vasculogenesis following chronic fluoride exposure. Mitochondriopathy, lysis of ground substance, oedema, and hyper-vacuolation was seen in fluoride treated groups. Remarkable levels of distortion and bending in Z band were observed under the AFM. Many of these observed changes mimic those occurring in cardiomegaly, cardiac hypertrophy and cardiomyopathies.

Type-2 diabetic aldehyde dehydrogenase 2 mutant mice (ALDH 2*2) exhibiting heart failure with preserved ejection fraction phenotype can be determined by exercise stress echocardiography

E487K point mutation of aldehyde dehydrogenase (ALDH) 2 (ALDH2*2) in East Asians intrinsically lowers ALDH2 activity. ALDH2*2 is associated with diabetic cardiomyopathy. Diabetic patients exhibit heart failure of preserved ejection fraction (HFpEF) i.e. while the systolic heart function is preserved in them, they may exhibit diastolic dysfunction, implying a jeopardized myocardial health. Currently, it is challenging to detect cardiac functional deterioration in diabetic mice. Stress echocardiography (echo) in the clinical set-up is a procedure used to measure cardiac reserve and impaired cardiac function in coronary artery diseases. Therefore, we hypothesized that high-fat diet fed type-2 diabetic ALDH2*2 mutant mice exhibit HFpEF which can be measured by cardiac echo stress test methodology. We induced type-2 diabetes in 12-week-old male C57BL/6 and ALDH2*2 mice through a high-fat diet. At the end of 4 months of DM induction, we measured the cardiac function in diabetic and control mice of C57BL/6 and ALDH2*2 genotypes by conscious echo. Subsequently, we imposed exercise stress by allowing the mice to run on the treadmill until exhaustion. Post-stress, we measured their cardiac function again. Only after treadmill running, but not at rest, we found a significant decrease in % fractional shortening and % ejection fraction in ALDH2*2 mice with diabetes compared to C57BL/6 diabetic mice as well as non-diabetic (control) ALDH2*2 mice. The diabetic ALDH2*2 mice also exhibited poor maximal running speed and distance. Our data suggest that high-fat fed diabetic ALDH2*2 mice exhibit HFpEF and treadmill exercise stress echo test is able to determine this HFpEF in the diabetic ALDH2*2 mice.

Test-retest reproducibility of cardiac magnetic resonance imaging in healthy mice at 7-Tesla: effect of anesthetic procedures

Cardiac magnetic resonance (CMR) has emerged as a powerful tool for in vivo assessments of cardiac parameters in experimental animal models of cardiovascular diseases, but its reproducibility in this setting remains poorly explored. To address this issue, we investigated the test-retest reproducibility of preclinical cardiac magnetic resonance imaging (CMR) at 7 Tesla in healthy C57BL/6 mice, including an analysis of the impact of different anesthetic procedures (isoflurane or pentobarbital). We also analyzed the intra-study reproducibility and the intra- and inter-observer post-processing reproducibility of CMR images. Test-retest reproducibility was high for left ventricular parameters, especially with the isoflurane anesthetic procedure, whereas right ventricular parameters and deformation measurements were less reproducible, mainly due to physiological variability. Post-processing reproducibility of CMR images was high both within and between observers. These results highlight that anesthetic procedures might influence CMR test-retest reproducibility, an important ethical consideration for longitudinal studies in rodent models of cardiomyopathy to limit the number of animals used.

Acute exhaustive aerobic exercise training impair cardiomyocyte function and calcium handling in Sprague-Dawley rats

Introduction

Recent data from long-distance endurance participants suggest that cardiac function is impaired after completion. Existing data further indicate that right ventricular function is more affected than left ventricular function. The cellular mechanisms underpinning cardiac deterioration are limited and therefore the aim of this study was to examine cardiomyocyte and molecular responses of the right and left ventricle to an acute bout of exhaustive endurance exercise.

Materials and methods

Male Sprague-Dawley rats were assigned to sedentary controls or acute exhaustive endurance exercise consisting of a 120 minutes long forced treadmill run. The contractile function and Ca²⁺ handling properties in isolated cardiomyocytes, protein expression levels of sarcoplasmic reticulum Ca²⁺-ATPase and phospholamban including two of its phosphorylated states (serine 16 and threonine 17), and the mitochondrial respiration in permeabilized cardiac muscle fibers were analyzed.

Results

The exercise group showed a significant reduction in cardiomyocyte fractional shortening (right ventricle 1 Hz and 3 Hz p<0.001; left ventricle 1 Hz p<0.05), intracellular Ca²⁺ amplitude (right ventricle 1 and 3 Hz p<0.001; left ventricle 1 Hz p<0.01 and 3 Hz p<0.05) and rate of diastolic Ca²⁺ decay (right ventricle 1 Hz p<0.001 and 3 Hz p<0.01; left ventricle 1 and 3 Hz p<0.01). Cardiomyocyte relaxation during diastole was only significantly prolonged at 3 Hz in the right ventricle (p<0.05) compared to sedentary controls. We found an increase in phosphorylation of phospholamban at serine 16 and threonine 17 in the left (p<0.05), but not the right, ventricle from exhaustively exercised animals. The protein expression levels of sarcoplasmic reticulum Ca²⁺-ATPase and phospholamban was not changed. Furthermore, we found a reduction in maximal oxidative phosphorylation and electron transport system capacities of mitochondrial respiration in the right (p<0.01 and p<0.05, respectively), but not the left ventricle from rats subjected to acute exhaustive treadmill exercise.

Conclusion

Acute exhaustive treadmill exercise is associated with impairment of cardiomyocyte Ca²⁺ handling and mitochondrial respiration that causes depression in both contraction and diastolic relaxation of cardiomyocytes.

Temporal accumulation and localization of isoflurane in the C57BL/6 mouse and assessment of its potential contamination in 19 F MRI with perfluoro-crown-ether-labeled cardiac progenitor cells at 9.4 Tesla

Purpose

To assess the uptake, accumulation, temporal stability, and spatial localization of isoflurane (ISO) in the C57BL/6 mouse, and to identify its potential interference with the detection of labeled cardiac progenitor cells using ¹⁹F MRI/MR spectroscopy (MRS).

Materials and Methods

Objectives are demonstrated using (a) in vitro ISO tests, (b) in vivo temporal accumulation/spatial localization C57BL/6 studies (n = 3), and (c) through injections of perfluoro‐crown‐ether (PFCE) labeled cardiac progenitor cells into femoral muscle areas of the murine hindlimb post‐mortem (n = 1) using ¹H/¹⁹F MRI/MRS at 9.4 Tesla. Data were acquired using double‐gated spoiled gradient echo images and pulse‐acquire spectra. For the in vivo study, the temporal stability of ISO resonances was quantified using coefficient of variability (CV) (5 min) estimates.

Results

Two ISO resonances were observed in vivo that correspond to the ‐CF₃ and ‐OCHF₂ moieties. CV values ranged between 3.2 and 6.4% (‐CF₃) and 6.4 and 11.2% (‐OCHF₂). Reductions of the ISO dose (2.0 to 1.7%) at 80 min postinduction had insignificant effects on ISO signals (P = 0.23; P = 0.71). PFCE‐labeled cells exhibited a resonance at −16.25 ppm in vitro that did not overlap with the ISO resonances, a finding that is confirmed with MRS post‐mortem using injected, labeled cells. Based on ¹⁹F MRI, similar in vivo/post‐mortem ISO compartmentalization was also confirmed in peripheral and thoracic skeletal muscles.

Conclusion

Significant ISO accumulation was observed by ¹⁹F MRS in vivo with temporally stable signals over 90 min postinduction. ISO effects on PFCE labels are anticipated to be minimal but may be more prominent for perfluoropolyether or perfluorooctyl bromide labels.

Level of Evidence: 1

Technical Efficacy: Stage 1

J. MAGN. RESON. IMAGING 2017;45:1659–1667

Molecular and Integrative Physiological Effects of Isoflurane Anesthesia: The Paradigm of Cardiovascular Studies in Rodents using Magnetic Resonance Imaging

To-this-date, the exact molecular, cellular, and integrative physiological mechanisms of anesthesia remain largely unknown. Published evidence indicates that anesthetic effects are multifocal and occur in a time-dependent and coordinated manner, mediated via central, local, and peripheral pathways. Their effects can be modulated by a range of variables, and their elicited end-effect on the integrative physiological response is highly variable. This review summarizes the major cellular and molecular sites of anesthetic action with a focus on the paradigm of isoflurane (ISO) - the most commonly used anesthetic nowadays - and its use in prolonged in vivo rodent studies using imaging modalities, such as magnetic resonance imaging (MRI). It also presents established evidence for normal ranges of global and regional physiological cardiac function under ISO, proposes optimal, practical methodologies relevant to the use of anesthetic protocols for MRI and outlines the beneficial effects of nitrous oxide supplementation.

Myocardial arterial spin labeling

Arterial spin labeling (ASL) is a cardiovascular magnetic resonance (CMR) technique for mapping regional myocardial blood flow. It does not require any contrast agents, is compatible with stress testing, and can be performed repeatedly or even continuously. ASL-CMR has been performed with great success in small-animals, but sensitivity to date has been poor in large animals and humans and remains an active area of research. This review paper summarizes the development of ASL-CMR techniques, current state-of-the-art imaging methods, the latest findings from pre-clinical and clinical studies, and future directions. We also explain how successful developments in brain ASL and small-animal ASL-CMR have helped to inform developments in large animal and human ASL-CMR.

Cardiovascular Imaging in Mice

The mouse is the mammalian model of choice for investigating cardiovascular biology, given our ability to manipulate it by genetic, pharmacologic, mechanical, and environmental means. Imaging is an important approach to phenotyping both function and structure of cardiac and vascular components. This review details commonly used imaging approaches, with a focus on echocardiography and magnetic resonance imaging and brief overviews of other imaging modalities. We also briefly outline emerging imaging approaches but caution that reliability and validity data may be lacking.

Time course of cardiometabolic alterations in a high fat high sucrose diet mice model and improvement after GLP-1 analog treatment using multimodal cardiovascular magnetic resonance

BACKGROUND

Cardiovascular complications of obesity and diabetes are major health problems. Assessing their development, their link with ectopic fat deposition and their flexibility with therapeutic intervention is essential. The aim of this study was to longitudinally investigate cardiac alterations and ectopic fat accumulation associated with diet-induced obesity using multimodal cardiovascular magnetic resonance (CMR) in mice. The second objective was to monitor cardiac response to exendin-4 (GLP-1 receptor agonist).

METHODS

Male C57BL6R mice subjected to a high fat (35 %) high sucrose (34 %) (HFHSD) or a standard diet (SD) during 4 months were explored every month with multimodal CMR to determine hepatic and myocardial triglyceride content (HTGC, MTGC) using proton MR spectroscopy, cardiac function with cine cardiac MR (CMR) and myocardial perfusion with arterial spin labeling CMR. Furthermore, mice treated with exendin-4 (30 μg/kg SC BID) after 4 months of diet were explored before and 14 days post-treatment with multimodal CMR.

RESULTS

HFHSD mice became significantly heavier (+33 %) and displayed glucose homeostasis impairment (1-month) as compared to SD mice, and developed early increase in HTGC (1 month, +59 %) and MTGC (2-month, +63 %). After 3 months, HFHSD mice developed cardiac dysfunction with significantly higher diastolic septum wall thickness (sWtnD) (1.28 ± 0.03 mm vs. 1.12 ± 0.03 mm) and lower cardiac index (0.45 ± 0.06 mL/min/g vs. 0.68 ± 0.07 mL/min/g, p = 0.02) compared to SD mice. A significantly lower cardiac perfusion was also observed (4 months:7.5 ± 0.8 mL/g/min vs. 10.0 ± 0.7 mL/g/min, p = 0.03). Cardiac function at 4 months was negatively correlated to both HTGC and MTGC (p < 0.05). 14-day treatment with Exendin-4 (Ex-4) dramatically reversed all these alterations in comparison with placebo-treated HFHSD. Ex-4 diminished myocardial triglyceride content (-57.8 ± 4.1 %), improved cardiac index (+38.9 ± 10.9 %) and restored myocardial perfusion (+52.8 ± 16.4 %) under isoflurane anesthesia. Interestingly, increased wall thickness and hepatic steatosis reductions were independent of weight loss and glycemia decrease in multivariate analysis (p < 0.05).

CONCLUSION

CMR longitudinal follow-up of cardiac consequences of obesity and diabetes showed early accumulation of ectopic fat in mice before the occurrence of microvascular and contractile dysfunction. This study also supports a cardioprotective effect of glucagon-like peptide-1 receptor agonist.

Variability of Mouse Left Ventricular Function Assessment by 11.7 Tesla MRI

We studied intraobserver (n = 24), interobserver (n = 24) and interexperiment (n = 12) reproducibility of left ventricular (LV) mass and volume measurements in mice using an 11.7 T MRI system. The LV systolic function was assessed with a short-axis FLASH-cine sequence in 29 mice, including animals having undergone transverse aortic constriction. Bland-Altman and regression analysis were used to compare the different data sets. Reproducibility was excellent for the LV mass and end-diastolic volume (coefficient of variability (CoV) between 5.4 and 11.8 %), good for end-systolic volume (CoV 15.2-19.4 %) and moderate for stroke volume and ejection fraction (CoV 14.7-20.9 %). We found an excellent correlation between LV mass determined by MRI and ex vivo morphometric data (r = 0.92). In conclusion, LV systolic function can be assessed on an 11.7 T MRI scanner with high reproducibility for most parameters, as needed in longitudinal studies. However, data should be interpreted taking into account the moderate reproducibility of small volumes.

Effect of isoproterenol on myocardial perfusion, function, energy metabolism and nitric oxide pathway in the rat heart - a longitudinal MR study

The chronic administration of the β-adrenoreceptor agonist isoproterenol (IsoP) is used in animals to study the mechanisms of cardiac hypertrophy and failure associated with a sustained increase in circulating catecholamines. Time-dependent changes in myocardial blood flow (MBF), morphological and functional parameters were assessed in rats in vivo using multimodal cardiac MRI. Energy metabolism, oxidative stress and the nitric oxide (NO) pathway were evaluated in isolated perfused rat hearts following 7 days of treatment. Male Wistar rats were infused for 7 days with IsoP or vehicle using osmotic pumps. Cine-MRI and arterial spin labeling were used to determine left ventricular morphology, function and MBF at days 1, 2 and 7 after pump implantation. Isolated hearts were then perfused, and high-energy phosphate compounds and intracellular pH were followed using ³¹P MRS with simultaneous measurement of contractile function. Total creatine and malondialdehyde (MDA) contents were measured by high-performance liquid chromatography. The NO pathway was evaluated by NO synthase isoform expression and total nitrate concentration (NO(x)). In IsoP-treated rats, left ventricular mass was increased at day 1 and maintained. Wall thickness was increased with a peak at day 2 and a tendency to return to baseline values at day 7. MBF was markedly increased at day 1 and returned to normal values between days 1 and 2. The rate-pressure product and phosphocreatine/adenosine triphosphate ratio in perfused hearts were reduced. MDA, endothelial NO synthase expression and NO(x) were increased. Sustained high cardiac function and normal MBF after 24 h of IsoP infusion indicate imbalance between functional demand and blood flow, leading to morphological changes. After 1 week, cardiac hypertrophy and decreased function were associated with impaired phosphocreatine, increased oxidative stress and up-regulation of the NO pathway. These results provide supplemental information on the evolution of the different contributing factors leading to morphological and functional changes in this model of cardiac hypertrophy and failure.

An optical fiber-based gating device for prospective mouse cardiac MRI

Prospective synchronization of MRI acquisitions on living organisms involves the monitoring of respiratory and heart motions. The electrocardiogram (ECG) signal is conventionally used to measure the cardiac cycle. However, in some circumstances, obtaining an uncorrupted ECG signal recorded on small animals with radio frequency (RF) pulses and gradient switching is challenging. To monitor respiratory motion, an air cushion associated with a pressure sensor is commonly used but the system suffers from bulkiness. For many applications, the physiological gating information can also be derived from an MR navigated signal. However, a compact device that can simultaneously provide respiratory and cardiac information, for both prospective gating and physiological monitoring, is desirable. This is particularly valid since small volume coils or dedicated cardiac RF coil arrays placed directly against the chest wall are required to maximize measurement sensitivity. An optic-based device designed to synchronize MRI acquisitions on small animal's respiratory and heart motion was developed using a transmit-receive pair of optical fibers. The suitability of the developed device was assessed on mice ( n = 10) and was based on two sets of experiments with dual cardiac and respiratory synchronization. Images acquired with prospective triggering using the optical-based signal, ECG, and the pressure sensor during the same experiment were compared between themselves in the first set. The second set compared prospective technique using optical-based device and ECG to a retrospective technique. The optical signal that was correlated to both respiratory and heart motion was totally unaffected by radiofrequency pulses or currents induced by the magnetic field gradients used for imaging. Mice heart MR images depict low-visible motion artifacts with all sensors or techniques used. No significant SNR differences were found between each series of image. Full fiber-optic-based signal derived from heart and respiratory motion was suitable for prospective triggering of heart MR imaging. The fiber optic device performed similarly to the ECG and air pressure sensors, while providing an advantage for imaging with dedicated cardiac array coils by reducing bulk. It can be an attractive alternative for small animal MRI in difficult environments such as limited space and strong gradient switching.

Influence of anesthetic agent, depth of anesthesia and body temperature on cardiovascular functional parameters in the rat

Sedating animals is sometimes necessary in experimental research. This paper presents and discusses the influence of four of the most common anesthetic agents on cardiovascular parameters in rats. We also studied the influence of body temperature. Ten-week-old Sprague-Dawley rats were anesthetized with either isoflurane, pentobarbital, ketamine/xylazine or tiletamine/zolazepam (n = 12 in each group). A pressure-sensing catheter was placed in the right carotid artery for the continuous measurement of arterial pressure, and echocardiography was performed. Indices of cardiac function were significantly higher in the tiletamine/zolazepam rats compared with the other groups. Heart rate was highest but stroke volume lowest with pentobarbital. Left ventricular diastolic dimension was lower in the pentobarbital and tiletamine/zolazepam rats compared with the isoflurane or ketamine/xylazine rats. Intraventricular diastolic pressure was similar in all groups whereas intraventricular systolic pressure, as well as both systolic and diastolic aortic pressures, was significantly higher in the tiletamine/zolazepam rats compared with the other groups. No hemodynamic indices differed significantly among the isoflurane, pentobarbital and ketamine/xylazine rats. Lowering body temperature significantly reduced heart rate and cardiac output but had no apparent effect on hemodynamic parameters. In conclusion, although cardiac functional parameters differed between the different anesthetic agents in ways that could be of relevance to the researcher, they may all have a role in experimental cardiology. Importantly, tiletamine/zolazepam anesthesia resulted in significantly higher indices of cardiac function and elevated blood pressures compared with the other anesthetic agents, a finding that should be kept in mind when interpreting data obtained in rats sedated on this regimen.

Combination of ketamine and xylazine exacerbates cardiac dysfunction in severely scalded rats during the shock stage

Cardiac inhibition due to burn injury and anesthetics have been documented previously. However, little is known about their combined effects on cardiac function. The aim of the present study was to observe the effects of a ketamine/xylazine (K/X) combination on the cardiac function of rats with severe scalds and compare them with those of avertin. Adult rats were randomly distributed into four groups: the KXB group (scalds anesthetized with K/X, n=10), the KXC group (sham scalds anesthetized with K/X, n=10), the AVB group (scalds anesthetized with avertin, n=10) and the AVC group (sham scalds anesthetized with avertin, n=10). Ketamine and xylazine were administered at 25 and 6 mg/kg, respectively, and avertin at 200 mg/kg before full-thickness scalds or sham scalds of 30% total body surface area (TBSA) were produced. Echocardiographic parameters were assessed following injury. The heart rate (HR) in the KXB group was fatally low during the study period. Fractional shortening (FS%) and ejection fraction (EF) in the KXB group were extremely low initially and remained low. The left ventricular end-diastolic volume (LVEDV) and left ventricular end-systolic volume (LVESV) were reduced in the burned rats. Serum levels of cardiac troponin I (cTnI) were significantly higher in the KXB group than in the AVB group (1.66±0.28 vs. 1.16±0.34 ng/ml, P<0.01). The highest lung wet/dry weight ratio was observed in the KXB group. However, no evident heart tissue pathological changes were observed in these groups. The apoptotic index of myocardial cells and caspase 3 expression level were highest in the KXB group (P<0.01). In conclusion, K/X exacerbated cardiac inhibition in severely scalded rats during the shock stage by a mechanism which may involve mitochondrial apoptosis.

Passive targeting of lipid-based nanoparticles to mouse cardiac ischemia-reperfusion injury

Reperfusion therapy is commonly applied after a myocardial infarction. Reperfusion, however, causes secondary damage. An emerging approach for treatment of ischemia-reperfusion (IR) injury involves the delivery of therapeutic nanoparticles to the myocardium to promote cell survival and constructively influence scar formation and myocardial remodeling. The aim of this study was to provide detailed understanding of the in vivo accumulation and distribution kinetics of lipid-based nanoparticles (micelles and liposomes) in a mouse model of acute and chronic IR injury. Both micelles and liposomes contained paramagnetic and fluorescent lipids and could therefore be visualized with magnetic resonance imaging (MRI) and confocal laser scanning microscopy (CLSM). In acute IR injury both types of nanoparticles accumulated massively and specifically in the infarcted myocardium as revealed by MRI and CLSM. Micelles displayed faster accumulation kinetics, probably owing to their smaller size. Liposomes occasionally co-localized with vessels and inflammatory cells. In chronic IR injury only minor accumulation of micelles was observed with MRI. Nevertheless, CLSM revealed specific accumulation of both micelles and liposomes in the infarct area 3 h after administration. Owing to their specific accumulation in the infarcted myocardium, lipid-based micelles and liposomes are promising vehicles for (visualization of) drug delivery in myocardial infarction.

Cardiac magnetic resonance, transthoracic and transoesophageal echocardiography: a comparison of in vivo assessment of ventricular function in rats

In vivo assessment of ventricular function in rodents has largely been restricted to transthoracic echocardiography (TTE). However 1.5 T cardiac magnetic resonance (CMR) and transoesophageal echocardiography (TOE) have emerged as possible alternatives. Yet, to date, no study has systematically assessed these three imaging modalities in determining ejection fraction (EF) in rats. Twenty rats underwent imaging four weeks after surgically-induced myocardial infarction. CMR was performed on a 1.5 T scanner, TTE was conducted using a 9.2 MHz transducer and TOE was performed with a 10 MHz intracardiac echo catheter. Correlation between the three techniques for EF determination and analysis reproducibility was assessed. Moderate-strong correlation was observed between the three modalities; the greatest between CMR and TOE (intraclass correlation coefficient (ICC) = 0.89), followed by TOE and TTE (ICC = 0.70) and CMR and TTE (ICC = 0.63). Intra- and inter-observer variations were excellent with CMR (ICC = 0.99 and 0.98 respectively), very good with TTE (0.90 and 0.89) and TOE (0.87 and 0.84). Each modality is a viable option for evaluating ventricular function in rats, however the high image quality and excellent reproducibility of CMR offers distinct advantages even at 1.5 T with conventional coils and software.

Anaesthesia and physiological monitoring during in vivo imaging of laboratory rodents: considerations on experimental outcomes and animal welfare

The implementation of imaging technologies has dramatically increased the efficiency of preclinical studies, enabling a powerful, non-invasive and clinically translatable way for monitoring disease progression in real time and testing new therapies. The ability to image live animals is one of the most important advantages of these technologies. However, this also represents an important challenge as, in contrast to human studies, imaging of animals generally requires anaesthesia to restrain the animals and their gross motion. Anaesthetic agents have a profound effect on the physiology of the animal and may thereby confound the image data acquired. It is therefore necessary to select the appropriate anaesthetic regime and to implement suitable systems for monitoring anaesthetised animals during image acquisition. In addition, repeated anaesthesia required for longitudinal studies, the exposure of ionising radiations and the use of contrast agents and/or imaging biomarkers may also have consequences on the physiology of the animal and its response to anaesthesia, which need to be considered while monitoring the animals during imaging studies. We will review the anaesthesia protocols and monitoring systems commonly used during imaging of laboratory rodents. A variety of imaging modalities are used for imaging rodents, including magnetic resonance imaging, computed tomography, positron emission tomography, single photon emission computed tomography, high frequency ultrasound and optical imaging techniques such as bioluminescence and fluorescence imaging. While all these modalities are implemented for non-invasive in vivo imaging, there are certain differences in terms of animal handling and preparation, how the monitoring systems are implemented and, importantly, how the imaging procedures themselves can affect mammalian physiology. The most important and critical adverse effects of anaesthetic agents are depression of respiration, cardiovascular system disruption and thermoregulation. When anaesthetising rodents, one must carefully consider if these adverse effects occur at the therapeutic dose required for anaesthesia, if they are likely to affect the image acquisitions and, importantly, if they compromise the well-being of the animals. We will review how these challenges can be successfully addressed through an appropriate understanding of anaesthetic protocols and the implementation of adequate physiological monitoring systems.

Distribution of lipid-based nanoparticles to infarcted myocardium with potential application for MRI-monitored drug delivery

Adverse cardiac remodeling after myocardial infarction ultimately causes heart failure. To stimulate reparative processes in the infarct, efficient delivery and retention of therapeutic agents is desired. This might be achieved by encapsulation of drugs in nanoparticles. The goal of this study was to characterize the distribution pattern of differently sized long-circulating lipid-based nanoparticles, namely micelles (~15 nm) and liposomes (~100 nm), in a mouse model of myocardial infarction (MI). MI was induced in mice (n=38) by permanent occlusion of the left coronary artery. Nanoparticle accumulation following intravenous administration was examined one day and one week after surgery, representing the acute and chronic phase of MI, respectively. In vivo magnetic resonance imaging of paramagnetic lipids in the micelles and liposomes was employed to monitor the trafficking of nanoparticles to the infarcted myocardium. Ex vivo high-resolution fluorescence microscopy of fluorescent lipids was used to determine the exact location of the nanoparticles in the myocardium. In both acute and chronic MI, micelles permeated the entire infarct area, which renders them very suited for the local delivery of cardioprotective or anti-remodeling drugs. Liposomes displayed slower and more restricted extravasation from the vasculature and are therefore an attractive vehicle for the delivery of pro-angiogenic drugs. Importantly, the ability to non-invasively visualize both micelles and liposomes with MRI creates a versatile approach for the development of effective cardioprotective therapeutic interventions.

Inflammatory immune responses in a reproducible mouse brain death model

BACKGROUND

Brain death impairs donor organ quality and accelerates immune responses after transplantation. Detailed aspects of immune activation following brain death remain unclear. We have established a mouse model and investigated the immediate consequences of brain death and anesthesia on immune responses.

METHODS

C57JBl/6 mice (n=6/group) were anesthetized with isoflurane (ISF) or ketamine/xylazine (KX); subsequently, animals underwent brain death induction and were followed for 3h under continuous ventilation. Blood pressure was monitored continuously and animals were resuscitated with normal saline to achieve normotension. Immune activation in brain dead animals was analyzed by IFNγ-ELispot, MLR, and flow-cytometry. Sham-operated and naïve animals served as controls.

RESULTS

Blood pressure remained stable in both BD/KX and BD/ISF animals during the 3h observation time. Brain death was linked to systemic immune activation: IFNγ-expression of splenocytes and lymphocyte proliferation rates was significantly elevated subsequent to brain death (p<0.02, <0.01); T-cell activation markers CD28 and CD69 had increased in brain dead animals (p<0.03, <0.02). Isoflurane treatment in sham controls throughout the observation period (3.5h) revealed anesthesia associated IFNγ-expression and lymphocyte activation which were not observed when animals were treated with ketamine/xylazine (p<0.04, <0.009).

CONCLUSIONS

This study reports on a reproducible and hemodynamically stable brain death mouse model. Hemodynamic stability was not impacted through either isoflurane or ketamine/xylazine induction. Of clinical relevance, prolonged anesthesia with isoflurane had been linked to pro-inflammatory cytokine activation. Brain death caused systemic immune activation in organ donors.

Inhibition of firefly luciferase by general anesthetics: effect on in vitro and in vivo bioluminescence imaging

Bioluminescence imaging is routinely performed in anesthetized mice. Often isoflurane anesthesia is used because of its ease of use and fast induction/recovery. However, general anesthetics have been described as important inhibitors of the luciferase enzyme reaction.

Aim

To investigate frequently used mouse anesthetics for their direct effect on the luciferase reaction, both in vitro and in vivo.

Materials and Methods

isoflurane, sevoflurane, desflurane, ketamine, xylazine, medetomidine, pentobarbital and avertin were tested in vitro on luciferase-expressing intact cells, and for non-volatile anesthetics on intact cells and cell lysates. In vivo, isoflurane was compared to unanesthetized animals and different anesthetics. Differences in maximal photon emission and time-to-peak photon emission were analyzed.

Results

All volatile anesthetics showed a clear inhibitory effect on the luciferase activity of 50% at physiological concentrations. Avertin had a stronger inhibitory effect of 80%. For ketamine and xylazine, increased photon emission was observed in intact cells, but this was not present in cell lysate assays, and was most likely due to cell toxicity and increased cell membrane permeability. In vivo, the highest signal intensities were measured in unanesthetized mice and pentobarbital anesthetized mice, followed by avertin. Isoflurane and ketamine/medetomidine anesthetized mice showed the lowest photon emission (40% of unanesthetized), with significantly longer time-to-peak than unanesthetized, pentobarbital or avertin-anesthetized mice. We conclude that, although strong inhibitory effects of anesthetics are present in vitro, their effect on in vivo BLI quantification is mainly due to their hemodynamic effects on mice and only to a lesser extent due to the direct inhibitory effect.

Cardiovascular magnetic resonance imaging in small animals

Noninvasive imaging studies involving small animals are becoming increasingly important in preclinical pharmacological, genetic, and biomedical cardiovascular research. Especially small animal magnetic resonance imaging (MRI) using high field and clinical MRI systems has gained significant importance in recent years. Compared to other imaging modalities, like computer tomography, MRI can provide an excellent soft tissue contrast, which enables the characterization of different kinds of tissues without the use of contrast agents. In addition, imaging can be performed with high spatial and temporal resolution. Small animal MRI cannot only provide anatomical information about the beating murine heart; it can also provide functional and molecular information, which makes it a unique imaging modality. Compared to clinical MRI examinations in humans, small animal MRI is associated with additional challenges. These included a smaller size of all cardiovascular structures and a up to ten times higher heart rate. Dedicated small animal monitoring devices make a reliable cardiac triggering and respiratory gating feasible. MRI in combination with molecular probes enables the noninvasive imaging of biological processes at a molecular level. Different kinds of iron oxide or gadolinium-based contrast agents can be used for this purpose. Compared to other molecular imaging modalities, like single photon emission computed tomography (SPECT) and positron emission tomography (PET), MRI can also provide imaging with high spatial resolution, which is of high importance for the assessment of the cardiovascular system. The sensitivity for detection of MRI contrast agents is however lower compared to sensitivity of radiation associated techniques like PET and SPECT. This chapter is divided into the following sections: (1) "Introduction," (2) "Principals of Magnetic Resonance Imaging," (3) "MRI Systems for Preclinical Imaging and Experimental Setup," and (4) "Cardiovascular Magnetic Resonance Imaging."

Flushing induction chambers used for rodent anesthesia to reduce waste anesthetic gas

Anesthetic induction chambers used for medical research are a substantial source of waste anesthetic gas (WAG). Ideally, any generated waste gas should be actively vented away from personnel operating the chamber by either a ventilated hood or snorkel. Unfortunately, the ideal environment for anesthetizing rodents is not always available. In an effort to create a safer environment, the authors designed a system to reduce WAG. This system is portable, can be adapted to different precision vaporizing anesthetic systems and fits in a variety of physical locations. The system flushes anesthetic gas out of an induction chamber before operators open the chamber. To ensure that the system was adequately flushing the anesthetic gas, the authors measured WAG concentration in the environment above the induction chamber and directly behind the vent of an activated charcoal filter. They also compared the efficiency of the filters in vertical and horizontal positions. Finally, they measured the recovery time for mice and rats after flushing the anesthetic gas from an induction chamber. The results show that flushing the induction chamber was an inexpensive and effective method for reducing WAG accumulation in the air surrounding the chamber.

Paeoniflorin improves survival in LPS-challenged mice through the suppression of TNF-α and IL-1β release and augmentation of IL-10 production

Lipopolysaccharide (LPS) plays an important role in Gram-negative bacteria-induced sepsis and multiple organ dysfunction syndrome, which are still the leading cause of high mortality in intensive care units. Although paeoniflorin (Pae) has reportedly exhibited anti-inflammatory effect and protection against immunological liver injury in mice, it is not known whether Pae improve survival in endotoxemic mice. The purpose of this study was to determine the effect of Pae on the mortality, multiple organ dysfunction and cytokine production in lipopolysaccharide (LPS)-treated mice. We found that pretreatment with Pae decreased mortality, reduced lung and kidney injury, decreased serum creatinine level and improve systolic function of heart in mice challenged with LPS. Further experiments showed that Pae inhibited LPS-stimulated tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) release and promoted LPS-induced interleukin-10 (IL-10) production. Our results indicate that Pae protects mice against lethal LPS challenge, at least in part, through inhibiting TNF-α and IL-1β production and accelerating IL-10 expression.

Cardiac phenotype induced by a dysfunctional α 1C transgene: a general problem for the transgenic approach

Based on stable integration of recombinant DNA into a host genome, transgenic technology has become an important genetic engineering methodology. An organism whose genetic characteristics have been altered by the insertion of foreign DNA is supposed to exhibit a new phenotype associated with the function of the transgene. However, successful insertion may not be sufficient to achieve specific modification of function. In this study we describe a strain of transgenic mouse, G7-882, generated by incorporation into the mouse genome of human CaV 1.2 α(1C) cDNA deprived of 3'-UTR to exclude transcription. We found that, in response to chronic infusion of isoproterenol, G7-882 develops dilated cardiomyopathy, a misleading "transgenic artifact" compatible with the expected function of the incorporated "correct" transgene. Specifically, using magnetic resonance imaging (MRI), we found that chronic β-adrenergic stimulation of G7-882 mice caused left ventricular hypertrophy and aggravated development of dilated cardiomyopathy, although no significant changes in the kinetics, density and voltage dependence of the calcium current were observed in G7-882 cardiomyocytes as compared to cells from wild type mice. This result illustrates the possibility that even when a functional transgene is expressed, an observed change in phenotype may be due to the artifact of "incidental incorporation" leading to misleading conclusions. To exclude this possibility and thus provide a robust tool for exploring biological function, the new transgenic phenotype must be replicated in several independently generated transgenic strains.

Cardiac imaging using clinical 1.5 t MRI scanners in a murine ischemia/reperfusion model

To perform cardiac imaging in mice without having to invest in expensive dedicated equipment, we adapted a clinical 1.5 Tesla (T) magnetic resonance imaging (MRI) scanner for use in a murine ischemia/reperfusion model. Phase-sensitive inversion recovery (PSIR) sequence facilitated the determination of infarct sizes in vivo by late gadolinium enhancement. Results were compared to histological infarct areas in mice after ischemia/reperfusion procedure with a good correlation (r = 0.807, P < .001). In addition, fractional area change (FAC) was assessed with single slice cine MRI and was matched to infarct size (r = −0.837) and fractional shortening (FS) measured with echocardiography (r = 0.860); both P < .001. Here, we demonstrate the use of clinical 1.5 MRI scanners as a feasible method for basic phenotyping in mice. These widely available scanners are capable of investigating in vivo infarct dimensions as well as assessment of cardiac functional parameters in mice with reasonable throughput.

Effects of heart rate and anesthetic timing on high-resolution echocardiographic assessment under isoflurane anesthesia in mice

OBJECTIVE

Anesthesia provides sedation and immobility, facilitating echocardiography in mice, but it influences cardiovascular function and therefore outcomes of measurement. This study aimed to determine the effect of the optimal heart rate (HR) and anesthetic timing on echocardiographic reproducibility under isoflurane anesthesia.

METHODS

Male C57BL/6J mice underwent high-resolution echocardiography with relative fixed HRs and anesthetic timing. The same experiment was repeated once again after 1 week.

RESULTS

Echocardiography was highly reproducible in repeated measurements under low-HR (350-400 beats per minute [bpm]) and high-HR (475-525 bpm) conditions except some M-mode parameters under low-HR conditions. With similar anesthetic timing, mice with a high HR had decreased preload indices and increased ejection phase and Doppler indices. Inversely, when the HR was similar, the echocardiographic results of mice under short anesthetic timing showed little difference from the ones under long anesthetic timing.

CONCLUSIONS

This study shows that echocardiographic assessment is greatly reproducible under a high HR. The HR is more important than anesthetic timing for echocardiographic evaluation in mice.

Physiologically based pharmacokinetics of molecular imaging nanoparticles for mRNA detection determined in tumor-bearing mice

Disease detection and management might benefit from external imaging of disease gene mRNAs. Previously we designed molecular imaging nanoparticles (MINs) based on peptide nucleic acids complementary to cancer gene mRNAs. The MINs included contrast agents and analogs of insulin-like growth factor 1 (IGF-1). Analysis of MIN tumor uptake data showed stronger binding in tumors than in surrounding tissues. We hypothesized that MINs with an IGF-1 analog stay in circulation by binding to IGF-binding proteins. To test that hypothesis, we fit the tissue distribution results of several MINs in xenograft-bearing mice to a physiological pharmacokinetics model. Fitting experimental tissue distribution data to model-predicted mass transfer of MINs from blood into organs and tumors converged only when the parameter for MINs bound to circulating IGF-binding proteins was set to 10%-20% of the injected MIN dose. This result suggests that previous mouse imaging trials used more MINs than necessary. This prediction can be tested by a ramp of decreasing doses.

Cardiovascular MRI in small animals

Imaging studies of cardiovascular disease in small rodents have become a prerequisite in preclinical cardiovascular research. Transgenic and gene-knockout models of cardiovascular diseases enables the investigation of the influence of single genes or groups of genes on disease pathogenesis. In addition, experimental and genetically altered models provide valuable in vivo platforms to investigate the efficacy of novel drugs and contrast agents. Owing to the excellent soft tissue contrast, high spatial and temporal resolution, as well as the tomographic nature of MRI, anatomy and function can be assessed with unique accuracy and reproducibility. Furthermore, using novel targeted MRI contrast agents, molecular changes associated with cardiovascular disease can be investigated in the same imaging session. This review focuses on recent advances in hardware, imaging sequences and probe design.

Effects of deep sedation or general anesthesia on cardiac function in mice undergoing cardiovascular magnetic resonance

BACKGROUND

Genetically engineered mouse models of human cardiovascular disease provide an opportunity to understand critical pathophysiological mechanisms. Cardiovascular magnetic resonance (CMR) provides precise reproducible assessment of cardiac structure and function, but, in contrast to echocardiography, requires that the animal be immobilized during image acquisition. General anesthetic regimens yield satisfactory images, but have the potential to significantly perturb cardiac function. The purpose of this study was to assess the effects of general anesthesia and a new deep sedation regimen, respectively, on cardiac function in mice as determined by CMR, and to compare them to results obtained in mildly sedated conscious mice by echocardiography.

RESULTS

In 6 mildly sedated normal conscious mice assessed by echo, heart rate was 615 +/- 25 min-1 (mean +/- SE) and left ventricular ejection fraction (LVEF) was 0.94 +/- 0.01. In the CMR studies of normal mice, heart rate was slightly lower during deep sedation with morphine/midazolam (583 +/- 30 min-1), but the difference was not statistically significant. General anesthesia with 1% inhaled isoflurane significantly depressed heart rate (468 +/- 7 min-1, p < 0.05 vs. conscious sedation). In 6 additional mice with ischemic LV failure, trends in heart rate were similar, but not statistically significant. In normal mice, deep sedation depressed LVEF (0.79 +/- 0.04, p < 0.05 compared to light sedation), but to a significantly lesser extent than general anesthesia (0.60 +/- 0.04, p < 0.05 vs. deep sedation). In mice with ischemic LV failure, ejection fraction measurements were comparable when performed during light sedation, deep sedation, and general anesthesia, respectively. Contrast-to-noise ratios were similar during deep sedation and during general anesthesia, indicating comparable image quality. Left ventricular mass measurements made by CMR during deep sedation were nearly identical to those made during general anesthesia (r2 = 0.99, mean absolute difference < 4%), indicating equivalent quantitative accuracy obtained with the two methods. The imaging procedures were well-tolerated in all mice.

CONCLUSION

In mice with normal cardiac function, CMR during deep sedation causes significantly less depression of heart rate and ejection fraction than imaging during general anesthesia with isoflurane. In mice with heart failure, the sedation/anesthesia regimen had no clear impact on cardiac function. Deep sedation and general anesthesia produced CMR with comparable image quality and quantitative accuracy.

Assessment of right and left ventricular function in healthy mice by blood-pool pinhole gated SPECT

The feasibility of blood-pool pinhole ECG gated SPECT was investigated in healthy mice to assess right and left ventricular function analysis. Anaesthetized (isoflurane 1-1.5%) adult CD1 mice (n=11) were analyzed after intravenous administration of 0.2 ml of 550 MBq of (99m)Tc human albumin. For blood-pool gated SPECT imaging, 48 ventral step and shoot projections with eight time bins per RR over 180 degrees with 64 x 64 word images were acquired with a small animal gamma camera equipped with a pinhole collimator of 12 cm in focal length and 1.5 mm in diameter. For appropriate segmentation of right and left ventricular volumes, a 4D Fourier analysis was performed after reconstruction and reorientation of blood-pool images with a voxel size of 0.55 x 0.55 x 0.55 mm(3). Average right and left ejection fractions were respectively 52+/-4.7% and 65+/-5.2%. Right end diastolic and end systolic volumes were significantly higher compared with the corresponding left ventricular volumes (P<0.0001 each). A linear correlation between right and left stroke volumes (r=0.9, P<0.0001) was obtained and right and left cardiac outputs were not significantly different 14.2+/-1.9 and 14.1+/-2 ml/min, respectively.

Flow-compensated self-gating

OBJECTIVE

Self-gating (SG) is a method to record cardiac movement during MR imaging. It uses information from an additional short, non-spatially encoded data acquisition. This usually lengthens TE and increases the sensitivity to flow artifacts. A new flow compensation scheme optimized for self-gating sequences is introduced that has very little or no time penalty over self-gating sequences without flow compensation.

MATERIALS AND METHODS

Three variants of a self-gated 2D spoiled gradient echo or fast low angle shot (FLASH) sequence were implemented: without (noFC), with a conventional, serial (cFC), and with a new, time-efficient flow compensation (sFC). In experiments on volunteers and small animals, the sequence variants were compared with regard to the SG signal and the flow artifacts in the images.

RESULTS

Both cFC and sFC reduce flow artifacts in cardiac images. The SG signal of the sFC is more sensitive to physiological motion, so that a cardiac trigger can be extracted more precisely as in cFC. In a typical setting for small animal imaging, sFC technique reduces the echo/repetition time over cFC by about 23%/14%.

CONCLUSION

The time-efficient sFC technique provides flow-compensated images with cardiac triggering in both volunteers and small animals.

Dystrophin-deficient cardiomyopathy in mouse: expression of Nox4 and Lox are associated with fibrosis and altered functional parameters in the heart

Duchenne muscular dystrophy (DMD; dystrophin-deficiency) causes dilated cardiomyopathy in the second decade of life in affected males. We studied the dystrophin-deficient mouse heart (mdx) using high-frequency echocardiography, histomorphometry, and gene expression profiling. Heart dysfunction was prominent at 9-10months of age and showed significantly increased LV internal diameter (end systole) and decreased posterior wall thickness. This cardiomyopathy was associated with a 30% decrease in shortening fraction. Histologically, there was a 10-fold increase in connective tissue volume (fibrosis). mRNA profiling with RT-PCR validation showed activation of key pro-fibrotic genes, including Nox4 and Lox. The Nox gene family expression differed in mdx heart and skeletal muscle, where Nox2 was specifically induced in skeletal muscle while Nox4 was specifically induced in heart. This is the first report of an altered profibrotic gene expression profile in cardiac tissue of dystrophic mice showing echocardiographic evidence of cardiomyopathy.