Karl-Heinz Hiller

Age- and Training-Dependent Development of Arrhythmogenic Right Ventricular Cardiomyopathy in Heterozygous Plakoglobin-Deficient Mice

Background— Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disorder that causes sudden death and right ventricular heart failure in the young. Clinical data suggest that competitive sports may provoke ARVC in susceptible persons. Genetically, loss-of-function mutations in desmosomal proteins (plakophilin, desmoplakin, or plakoglobin) have been associated with ARVC. To test the hypothesis that reduced desmosomal protein expression causes ARVC, we studied the cardiac effects of heterozygous plakoglobin deficiency in mice. Methods and Results— Ten-month-old heterozygous plakoglobin-deficient mice (plakoglobin+/−) had increased right ventricular volume, reduced right ventricular function, and spontaneous ventricular ectopy (all P<0.05). Left ventricular size and function were not altered. Isolated, perfused plakoglobin+/− hearts had spontaneous ventricular tachycardia of right ventricular origin and prolonged right ventricular conduction times compared with wild-type hearts. Endurance training accelerated the development of right ventricular dysfunction and arrhythmias in plakoglobin+/− mice. Histology and electron microscopy did not identify right ventricular abnormalities in affected animals. Conclusions— Heterozygous plakoglobin deficiency provokes ARVC. Manifestation of the phenotype is accelerated by endurance training. This suggests a functional role for plakoglobin and training in the development of ARVC.

Dobutamine-Stress Magnetic Resonance Microimaging in Mice Acute Changes of Cardiac Geometry and Function in Normal and Failing Murine Hearts

Abstract— The aim of this study was to assess the capability of MRI to characterize systolic and diastolic function in normal and chronically failing mouse hearts in vivo at rest and during inotropic stimulation. Applying an ECG-gated FLASH-cine sequence, MRI at 7 T was performed at rest and after administration of 1.5 &mgr;g/g IP dobutamine. There was a significant increase of heart rate, cardiac output, and ejection fraction and significant decrease of end-diastolic and end-systolic left ventricular (LV) volumes (P <0.01 each) in normal mice during inotropic stimulation. In mice with heart failure due to chronic myocardial infarction (MI), MRI at rest revealed gross LV dilatation. There was a significant decrease of LV ejection fraction in infarcted mice (29%) versus sham mice (58%). Mice with MI showed a significantly reduced maximum LV ejection rate (P <0.001) and LV filling rate (P <0.01) and no increase of LV dynamics during dobutamine action, indicating loss of contractile and relaxation reserve. In 4-month-old transgenic mice with cardiospecific overexpression of the &bgr;1-adrenergic receptor, which at this early stage do not show abnormalities of resting cardiac function, LV filling rate failed to increase after dobutamine stress (transgenic, 0.19±0.03 &mgr;L/ms; wild type, 0.36±0.01 &mgr;L/ms;P <0.01). Thus, MRI unmasked diastolic dysfunction during dobutamine stress. Dobutamine-stress MRI allows noninvasive assessment of systolic and diastolic components of heart failure. This study shows that MRI can demonstrate loss of inotropic and lusitropic response in mice with MI and can unmask diastolic dysfunction as an early sign of cardiac dysfunction in a transgenic mouse model of heart failure.

In vivo assessment of absolute perfusion and intracapillary blood volume in the murine myocardium by spin labeling magnetic resonance imaging.

The absolute perfusion and the intracapillary or regional blood volume (RBV) in murine myocardium were assessed in vivo by spin labeling magnetic resonance imaging. Pixel‐based perfusion and RBV maps were calculated at a pixel resolution of 469 × 469 μm and a slice thickness of 2 mm. The T1 imaging module was a segmented inversion recovery snapshot fast low angle shot sequence with velocity compensation in all three gradient directions. The group average myocardial perfusion at baseline was determined to be 701 ± 53 mL (100 g · min)−1 for anesthesia with isoflurane (N = 11) at a mean heart rate (HR) of 455 ± 10 beats per minute (bpm). This value is in good agreement with perfusion values determined by invasive microspheres examinations. For i.v. administration of the anesthetic Propofol, the baseline perfusion decreased to 383 ± 40 mL (100 g · min)−1 (N = 17, P < 0.05 versus. isoflurane) at a mean heart rate of 261 ± 13 bpm (P < 0.05 versus isoflurane). In addition, six mice with myocardial infarction were studied under isoflurane anesthesia (HR 397 ± 7 bpm). The perfusion maps showed a clear decrease of the perfusion in the infarcted area. The perfusion in the remote myocardium decreased significantly to 476 ± 81 mL (100 g · min)−1 (P < 0.05 versus sham). Regarding the regional blood volume, a mean value of 11.8 ± 0.8 vol % was determined for healthy murine myocardium under anesthesia with Propofol (N = 4, HR 233 ± 17 bpm). In total, the presented techniques provide noninvasive in vivo assessment of the perfusion and the regional blood volume in the murine myocardium for the first time and seem to be promising tools for the characterization of mouse models in cardiovascular research. Magn Reson Med 53:584–592, 2005.

Serial cine-magnetic resonance imaging of left ventricular remodeling after myocardial infarction in rats.

The purpose of the present study was the serial investigation of morphological and functional changes after left coronary artery ligation in the intact rat using cine‐magnetic resonance imaging (MRI). MRI studies were performed 4, 8, 12, and 16 weeks after myocardial infarction (MI) with an echocardiogram (ECG)‐triggered cine‐fast low‐angle shot (FLASH)‐sequence in a 7‐Tesla magnet. MI‐size, left ventricular (LV) mass and volumes, cardiac index, ejection fraction (EF), and remote wall and scar thickness of 11 Wistar rats were compared to four sham‐operated rats. Stress MRI with dobutamine (10 μl/kg × minute) was performed at 16 weeks. In MI groups (small MI < 30%, N = 5, large MI > 30%, N = 6), there was significant increase of LV mass (small MI + 47.8% increase, large MI + 74.1%) and wall thickness (large MI 1.21 ± 0.03 to 1.84 ± 0.07 mm). Scar thickness declined from four to 16 weeks (large MI 0.92 ± 0.06 to 0.38 ± 0.02mm, P < 0.05). End‐diastolic volume of both MI groups was significantly elevated but increased further only in animals with large MI from four to 16 weeks (657.1 ± 38.6 to 869.7 ± 60.7 μL, P < 0.05). Compared to sham, EF was significantly depressed in MI (large MI 31.5 ± 2.0%). Wall thickening declined from four to 16 weeks post‐MI (large MI 50.9 ± 9.9 to 28.9 ± 4.4%, P < 0.05). During stress, sham and MI rats increased wall thickening from 66.5 ± 8.2 to 111.2 ± 6.7% and from 30.8 ± 4.3 to 47.5 ± 5.8%, respectively (P < 0.05). Hypertrophy was found in all animals with MI throughout the entire period of observation, whereas dilatation after four weeks was only detected in animals with large MI. These morphologic changes were accompanied by an early decline of EF; myocardial function characterized by wall thickening deteriorated later. J. Magn. Reson. Imaging 2001;14:547–555.

Cardiac magnetic resonance imaging in small animal models of human heart failure

The aim of this study was to test the feasibility of cine magnetic resonance imaging (MRI) for assessment of the infarcted rat and mouse heart and to compare the results with established methods. These models have been proven to predict genesis and prevention of heart failure in patients. The value of cine MRI was tested in studies investigating interventions to change the course of the remodeling process. MRI was performed for determination of left ventricular (LV) volumes and mass, myocardial infarct (MI) size and cardiac output. LV wet weight was determined after MRI. Rats underwent conventional hemodynamic measurements for determination of cardiac output and LV volumes by electromagnetic flowmeter and pressure-volume curves. Infarct size was determined by histology. MRI-acquired MI-size (18.5+/-2%) was smaller than that found by histology (22.8+/-2.5%, p<0.05) with close correlation (r=0.97). There was agreement in LV mass between MRI and wet weight (r=0.97, p<0.05) and in the MRI- and flowmeter measurements of cardiac output (r=0.80, p<0.05). Volume by MRI differed from pressure-volume curves with good correlation (r=0.96, p<0.05). In a serial study of mice after coronary ligation, LV hypertrophy at 8 weeks was detected (Sham 105.1+/-7.9 mg, MI 144.4+/-11.7 mg, p<0.05). Left ventricles were enlarged in infarcted mice (end-diastolic volume, week 8: Sham 63.5+/-4 microl, MI 94.2 microl, p<0.05). In conclusion, cine MRI is a valuable diagnostic tool applicable to the rat and mouse model of MI. Being non-invasive and exact it offers new insights into the remodeling process after MI because serial measurements are possible. The technique was applied to study several interventions and proved its usefulness.

Fast High-Resolution Magnetic Resonance Imaging Demonstrates Fractality of Myocardial Perfusion in Microscopic Dimensions

Abstract— The fractal nature of heterogeneity of myocardial blood flow and its implications for the healthy and diseased heart is not yet understood. The main hindrance for investigation of blood flow heterogeneity and its role in physiology and pathophysiology is that conventional methods for determination of myocardial perfusion have severe limitations concerning temporal and spatial resolution and invasiveness. In isolated rat hearts, we developed a nuclear magnetic resonance technique that does not depend on contrast agents and in which the apparent longitudinal relaxation time is made perfusion sensitive by selective preparation of the imaging slice. This perfusion-sensitive relaxation time is determined within 40 seconds as a map with a high spatial in-plane resolution of 140×140 &mgr;m2 and a thickness of 1.5 mm. Perfusion imaging was validated with the established microsphere technique. Additionally, the congruence between perfusion-sensitive T1 maps and first-pass perfusion imaging was demonstrated. As an application of high-resolution perfusion imaging, fractal analysis of the spatial distribution of perfusion was performed. We were able to demonstrate that the fractality of this distribution exists even in microscopic dimensions. Vasodilation by nitroglycerin modulated the fractal pattern of perfusion, and the decrease of the fractal dimension indicated a shift toward homogeneity. This implies that parameters of the fractal distribution depend on the microvascular tone rather than on anatomic preformations; ie, fractality is a functional characteristic of perfusion.

Serial Magnetic Resonance Imaging of Microvascular Remodeling in the Infarcted Rat Heart

BackgroundAlterations in the coronary circulation are important determinants of myocardial function. Few data are available, however, about microvascular changes in reactive hypertrophy. With MRI, serial determination of myocardial microcirculation after myocardial infarction (MI) is feasible. Methods and ResultsWe quantitatively determined myocardial perfusion and relative intracapillary blood volume using an MRI technique. Infarct size, myocardial mass, and left ventricular volumes were determined with cine MRI. Rats were investigated at 8, 12, and 16 weeks after MI (mean MI size 24.1±2.0%) or sham operation. Vasodilation was induced by adenosine. In the infarcted group, maximum perfusion decreased significantly from 8 to 16 weeks (5.6±0.3 versus 3.5±0.2 mL · g−1 · min−1, P <0.01) compared with sham animals (5.5±0.3 versus 5.0±0.2 mL · g−1 · min−1, P =0.17). Myocardial mass increased significantly (559.1±20.8 mg at 8 weeks versus 690.9±42.7 mg at 16 weeks, P <0.05) compared with sham-operated animals (516.3±41.7 versus 549.2±32.3 mg). Basal relative intracapillary blood volume increased significantly to 15.7±0.5 vol% at 8 weeks after MI and remained elevated (16.8±0.6 vol%) at 16 weeks compared with 12.1±0.3 vol% (P <0.01) in sham-operated rats. ConclusionsOur results indicate that significant microvascular changes occur during cardiac remodeling. Hypoperfusion in the hypertrophied myocardium is related to an increase in vascular capacity, suggesting a compensatory vasodilatory response at the capillary level. These microvascular changes may therefore contribute to the development of heart failure.

Impact of hydroxymethylglutaryl coenzyme a reductase inhibition on left ventricular remodeling after myocardial infarction: An experimental serial cardiac magnetic resonance imaging study☆

OBJECTIVES We sought to assess the influence of long-term hydroxymethylglutaryl coenzyme A reductase inhibition (statin) therapy on left ventricular (LV) remodeling after myocardial infarction (MI) by use of serial cardiac magnetic resonance imaging (CMRI) studies. BACKGROUND Statin therapy has been shown to reduce cardiac hypertrophy in vitro and in vivo, but the influence on LV post-MI remodeling is largely unknown. METHODS The CMRI measurements were taken four and 12 weeks after left coronary artery ligation in a 7.05-tesla Biospec. The MI size, LV mass and volumes, cardiac output (CO), and ejection fraction were determined. Rats were treated for 12 weeks with either placebo (P), cerivastatin (C; 0.6 mg/kg body weight per day) as a dietary supplement, or cerivastatin plus the nitric oxide synthase (NOS) inhibitor N-methyl-L-arginine methyl ester (L-NAME, 76 mg/100 ml) and hydralazine (8 mg/100 ml) in drinking water (CLH) to assess the contribution of endogenous nitric oxide formation. RESULTS Administration of cerivastatin attenuated hypertrophy after MI, and this effect was completely abolished by NOS inhibition (increase of LV mass from 4 to 12 weeks after MI: 235.3 +/- 33.7 mg with P vs. 59.8 +/- 20.5 mg with C vs. 239.5 +/- 16.0 mg with CLH; p < 0.05 vs. P and CLH). Left ventricular dilation was not changed (increase of end-diastolic volume from 4 to 12 weeks after MI: 108.7 +/- 28.8 with P vs. 126.6 +/- 20.5 with C vs. 173.7 +/- 25.1 with CLH; p = NS). The CO was higher in the cerivastatin group (12 weeks: 76.1 +/- 2.9 ml/min with P vs. 95.8 +/- 4.8 ml/min with C; p < 0.05). The effects of cerivastatin were abolished by NOS inhibition in the CLH group (CO at 12 weeks: 69.3 +/- 2.8 ml/min, p < 0.05 vs. C). CONCLUSIONS Left ventricular remodeling was profoundly changed by statin treatment. Hypertrophy was attenuated, and global function was improved. These positive effects were abolished by NOS inhibition.

Assessment of Cardiovascular Apoptosis in the Isolated Rat Heart by Magnetic Resonance Molecular Imaging

Apoptosis, an active process of cell self-destruction, is associated with myocardial ischemia. The redistribution of phosphatidylserine (PS) from the inner to the outer leaflet of the cell membrane is an early event in apoptosis. Annexin V, a protein with high specificity and tight binding to PS, was used to identify and localize apoptosis in the ischemic heart. Fluorescein-labeled annexin V has been used routinely for the assessment of apoptosis in vitro. For the detection of apoptosis in vivo, positron emission tomography and single-photon emission computed tomography have been shown to be suitable tools. In view of the relatively low spatial resolution of nuclear imaging techniques, we developed a high-resolution contrast-enhanced magnetic resonance imaging (MRI) method that allows rapid and noninvasive monitoring of apoptosis in intact organs. Instead of employing superparamagnetic iron oxide particles linked to annexin V, a new T1 contrast agent was used. To this effect, annexin V was linked to gadolinium diethylenetriamine pentaacetate (Gd-DTPA)-coated liposomes. The left coronary artery of perfused isolated rat hearts was ligated for 30 min followed by reperfusion. T1 and T2* images were acquired by using an 11.7-T magnet before and after intracoronary injection of Gd-DTP-labeled annexin V to visualize apoptotic cells. A significant increase in signal intensity was visible in those regions containing cardiomyocytes in the early stage of apoptosis. Because labeling of early apoptotic cell death in intact organs by histological and immunohistochemical methods remains challenging, the use of Gd-DTPA-labeled annexin V in MRI is clearly an improvement in rapid targeting of apoptotic cells in the ischemic and reperfused myocardium.

In Vivo Assessment of Cardiac Remodeling After Myocardial Infarction in Rats by Cine–Magnetic Resonance Imaging

The rat infarct model offers important parallels to the process of remodeling after myocardial infarction (MI) in humans. The aim of this study was to test the feasibility of cine fast low-angle shot (FLASH) magnetic resonance imaging (MRI) for assessment of the infarcted and noninfarcted rat heart and to compare the results with established methods. In group A, MRI was done 8-16 weeks after MI on a 7-T scanner using an electrocardiogram-triggered cine-FLASH sequence. We determined left ventricular (LV) volumes and mass, wall thickness, MI size, cardiac output, and ejection fraction. Afterward, MI size was histologically determined. In group B, after MRI eight controls and eight rats 16 weeks after MI underwent conventional hemodynamic measurements for determination of cardiac output, LV volumes, and ejection fraction by electromagnetic flowmeter and pressure-volume curves. LV wet weight was determined. In group A, MRI-acquired MI size (18.5 +/- 2%) was smaller than histology (22.8 +/- 2.5%, p < 0.05) with close correlation (r = 0.97). In group B, agreement in LV mass was found between MRI and wet weight (controls, 537.6 +/- 19.6 vs. 540.3 +/- 18.4 mg; MI, 865.1 +/- 39.2 vs. 865.1 +/- 41.3 mg; for the difference p = ns, r = 0.97, p < 0.05) and in the MRI and flowmeter measurements (cardiac output, controls 73.1 +/- 2.9 vs. 75.2 +/- 2.6 ml/min; MI 82.4 +/- 5.2 vs. 81.9 +/- 3.7 ml/min; for the difference p = ns, r = 0.80, p < 0.05). End-diastolic volume by MRI differed from pressure-volume curves with good correlation (controls, 343.9 +/- 8.4 vs. 262.7 +/- 12.8 microl; MI, 737.0 +/- 70.5 vs. 671.1 +/- 64.1 microl; p < 0.05 each, r = 0.96, p < 0.05). Cine-FLASH-MRI is a valuable diagnostic tool applicable to the rat model of MI. Being noninvasive and exact, it offers new insights in the remodeling process after MI because serial measurements are possible.