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Noninvasive Imaging of Angiotensin ReceptorsAfter Myocardial Infarction

Johan W. H. Verjans, MD,*� Dagfinn Lovhaug, PHD,§ Navneet Narula, MD,†Artiom D. Petrov, PHD,* Bård Indrevoll, MS,§ Emma Bjurgert, PHD,§Tatiana B. Krasieva, PHD,‡ Lizette B. Petersen, MS,§ Grete M. Kindberg, PHD,§Magne Solbakken, PHD,§ Alan Cuthbertson, PHD,§ Mani A. Vannan, MD, FACC,*Chris P. M. Reutelingsperger, PHD,� Bruce J. Tromberg, PHD,‡Leonard Hofstra, MD, PHD,� Jagat Narula, MD, PHD, FACC*

Irvine, California; Oslo, Norway; and Maastricht, the Netherlands

O B J E C T I V E S The purpose of this study was to evaluate the feasibility of noninvasive imaging of

angiotensin II (AT) receptor upregulation in a mouse model of post-myocardial infarction (MI) heart failure (HF).

B A C K G R O U N D Circulating AT levels do not reflect the status of upregulation of renin-angiotensin

axis in the myocardium, which plays a central role in ventricular remodeling and evolution of HF after

MI. Appropriately labeled AT or AT receptor blocking agents should be able to specifically target AT

receptors by molecular imaging techniques.

M E T H O D S AT receptor imaging was performed in 29 mice at various time points after permanent

coronary artery ligation or in controls using a fluoresceinated angiotensin peptide analog (APA) and

radiolabeled losartan. The APA was used in 19 animals for intravital fluorescence microscopy on a

beating mouse heart. Tc-99m losartan was used for in vivo radionuclide imaging and quantitative

assessment of AT receptor expression in 10 mice. After imaging, hearts were harvested for pathological

characterization using confocal and 2-photon microscopy.

R E S U L T S No or little APA uptake was observed in control animals or within infarct regions on days 0 and

1. Distinct uptake occurred in the infarct area at 1 to 12 weeks after MI; the uptake was at maximum at 3

weeks and reduced markedly at 12 weeks after MI. Ultrasonographic examination demonstrated left

ventricular remodeling, and pathologic characterization revealed localization of the APA tracer with

collagen-producing myofibroblasts. Tc-99m losartan uptake in the infarct region (0.524 � 0.212% injected

dose/g) increased 2.4-fold as compared to uptake in the control animals (0.215 � 0.129%; p � 0.05).

C O N C L U S I O N S The present study demonstrates the feasibility of in vivo molecular imaging of AT

receptors in the remodeling myocardium. Noninvasive imaging studies aimed at AT receptor expression

could play a role in identification of subjects likely to develop heart failure. In addition, such a strategy

could allow for optimization of anti-angiotensin therapy in patients after MI. (J Am Coll Cardiol Img

2008;1:354–62) © 2008 by the American College of Cardiology Foundation

From the *Department of Cardiology, †Department of Pathology, and the ‡Beckman Laser Institute, University of Californiaat Irvine, Irvine, California; §GE Healthcare, Oslo, Norway; and the �Cardiovascular Research Institute, Maastricht University,Maastricht, the Netherlands. Supported in part by GE Healthcare (Oslo, Norway) that provided the fluorescent and radiolabeledprobes for imaging; the Laser Microbeam and Medical Program (LAMMP) and NIH Biomedical Technology Resource grant#P41-RR01192, both of which supported the optical imaging and analysis; and by DiPalma-Brodsky funds that provided researchfellowship support to Dr. Verjans. Drs. Lovhaug, B. Indrevoll, Dr. Bjurgert, L. Peterson, Dr. Kindberg, Dr. Solbakken, and Dr.Cuthbertson are employees of GE Healthcare, Oslo, Norway. H. William Strauss, MD, acted as Guest Editor for this paper.

Manuscript received September 4, 2007; revised manuscript received November 14, 2007, accepted November 20, 2007.

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Verjans et al.

Imaging Angiotensin Receptors After MI

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t is well established that myocardial, more sothan the systemic neurohumoral upregulation,contributes to ventricular remodeling and de-velopment of heart failure (HF) after myocar-ial infarction (MI) (1). In particular, myocardialngiotensin II (AT) type-1 receptor expression isssociated with interstitial fibrosis and ventricularysfunction (2). Transgenic mice with deficientT type-1 receptor expression exhibit minimal

See page 363

eometric and structural remodeling following MI,nd show lesser upregulation of fetal genes, trans-orming growth factor beta-1, and collagen depo-ition in comparison with the wild-type animals (3).linical use of angiotensin-converting enzyme

ACE) inhibitors and angiotensin receptor blockersn MI retards ventricular remodeling, delays devel-pment of HF, reduces rehospitalization for wors-ning of HF, and improves survival (4–8).

Because myocardial neurohumoral upregulations associated with ventricular remodeling, we hy-othesized that a novel diagnostic strategy targetedt detecting the extent of myocardial AT receptorxpression would allow identification of patientst risk of developing HF. In addition, such atrategy should allow optimization of pharmaco-ogic therapy in HF patients. Because subcellularrocesses are amenable to noninvasive molecularmaging techniques, we labeled an angiotensineptide analog (APA) with a fluorescent tracer.he APA targeted both angiotensin type-1 and

2 receptors with high affinity (Ki � 3 pmol/l),nd optical imaging was performed to determinehe myocardial upregulation of AT receptors after

I in an experimental mouse model. To morepecifically target AT type-1 receptors, we alsoadiolabeled losartan, a commonly used AType-1 receptor blocker, and performed imagingith micro–single-photon emission computed

omograpy and micro-computed tomography.

Table 1. Optical and Nuclear Imaging Groups and Timeline of Im

Tracer ProbeControlMice

0days

1day

GFP APA 3 3 399mTc Losartan 6

APA � angiotensin peptide analog; GFP � green fluorescent protein; MI � myocar

E T H O D S

he present study was performed in 29 mice (Table). Of these, 19 mice were studied with fluorescentPA; 16 mice were imaged up to 12 weeks after MI

nd compared with 3 noninfarcted control animals.or radionuclide imaging, technetium-99m (99mTc)

osartan was used in 4 mice 3 weeks after MI and 6oninfarcted control animals (Table 1). Echocardi-graphy was performed for the assessment of leftentricular (LV) dimensions and function in allice prior to optical imaging. Optical imaging was

erformed in open chest preparation. Noninvasiveadionuclide imaging was performed on mice byicro-single-photon emission computed tomogra-

hy (SPECT)/micro-computed tomography (CT)oregistration. All animals were sacrificed afterptical or nuclear imaging. The myocardial spec-mens from animals receiving the fluo-escent tracer were characterized histo-ogically by localizing the injected probeith confocal and 2-photon microscopy

n frozen samples as well as by hematox-lin and eosin staining of paraffin-mbedded tissue. Myocardial specimensbtained from animals after nuclear im-ging were used to determine quantita-ive uptake of radiolabeled losartan inhe myocardium. All experiments werepproved by the Institutional Animalare and Use Committee at the Univer-

ity of California, Irvine and were con-ucted in accordance with U.S. guide-

ines on the humane care and use ofesearch animals.ynthesis of fluorescein APA for optical imaging. Theeptidyl resin H-Arg(Pmc)-Val-Tyr(tBu)-Ile-is(Trt)-Pro-Ile-R was synthesized on an Appliediosystems 433A peptide synthesizer (Appliediosystems, Foster City, California) starting withmoc-Ile-Wang resin. The N-terminus was bro-oacetylated using bromoacetic anhydride. The

romoacetylated peptidyl resin was then treated

ing

Post-MI Mice

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ith N-Boc-ethylenediamine. The simultaneousemoval of protecting groups and cleavage of theeptide from the resin was carried out in triflu-roacetic acid. Subsequently, trifluoroacetic acidas removed in vacuo, diethyl ether added to

he residue, and the precipitated peptide washedith diethyl ether and dried. The crude pep-

ide, fluorescein N-hydroxysuccinimide ester,nd N-methylmorpholine were dissolved inimethylformamide-affording fluorescein APAFig. 1A). The reaction product was purified byreparative reversed-phase high performance liq-id chromatography. The pure fluorescein APAas analyzed by analytical high performance

iquid chromatography and electrospray masspectrometry.ynthesis of chelate-coupled losartan-Leu-Diglycoloyl-EG(4)-Tetraamine and radiolabeling. The optimalharmacokinetic profile of an imaging agent differsadically from therapeutic drugs in that rapid excre-ion from blood is advantageous. Because peptidesre particularly better suited for targeting in vivoue to a superior biodistribution profile, we intro-uced peptide characteristics into the losartan phar-acophore. To facilitate a convenient and rapid

ynthesis, we adopted a solid phase approach whereosartan was immobilized on a trityl resin throughts tetrazole group. Further replacement of theydroxyl group by an amino group enabled attach-ent of amino acids, linkers, and tetracycline che-

ates by standard peptide synthesis methods. Thistrategy did also increase the affinity of losartan forT type-1 receptors. Structure activity data showed

Figure 1. Structure of Tracers Used for AT Receptor Targeting

Fluorescent angiotensin peptide analog (APA) (A), technetium Tc-99m l

hat the introduction of the amino acid leucine andshort linker comprising tetraethylene glycol and

iglycolic acid between the losartan moiety and theetra-amine chelate yielded a losartan analog (Fig.B) with superior affinity (Ki � 60 pmol/l) to ATeceptors compared with losartan (Ki � 10 nmol/l)nd an acceptable biodistribution profile withainly urinary excretion.ouse model of HF. Male Swiss Webster mice

weight 20 to 30 g, Charles River, Wilmington,assachusetts) were anesthetized with pentobarbi-

al (75 mg/kg by intraperitoneal injection) andntubated with a 21-G tube for mechanical venti-ation. The electrocardiogram was monitored con-inuously, and body temperature was maintainedhroughout the procedure at 36.5°C. Ventilationas maintained at a 250-�l tidal volume and a

espiratory rate of 200 breaths/min (MiniVentSE-HA, Harvard Apparatus, Holliston, Massa-

husetts). The heart was exposed through theourth left intercostal thoracotomy, and the leftoronary artery was ligated at its second branchingoint with a 6-0 Prolene suture. After watching thenimal carefully for 30 min, the chest was closedsing a 6-0 Monocryl suture. The animals wereubjected to optical or radionuclide imaging atarious time points in follow-up. Before imaging,nesthetized animals were examined by transtho-acic echocardiography using a 14-MHz linearrobe (Acuson 15L8, Siemens, Malvern, Pennsyl-ania). B-mode images of LV parasternal long axisnd short axis were obtained and digitally analyzeds described previously (9).

osartan (B). AT � angiotensin II.

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ptical imaging using fluorescent APA. Frozen driedliquot (100 �g) of the compound was dissolved in00 �l of distilled water. After cannulation of thenternal jugular vein, 50 �g of fluorescent APA wasnjected. Real-time optical imaging was performedrom baseline to 30 min as previously described (10)sing a fluorescence stereomicroscope (LeicaZ16FA, Leica Microsystems, Heerbrugg, Swit-

erland) equipped with a charge-coupled deviceamera (Orca 285, Hamamatsu, Japan). A 470/0-nm bandpass excitation filter was used, andmitted fluorescence was collected through a band-ass filter of 525/50 nm. Images were analyzedsing SimplePCI software (Compix, Sewickley,ennsylvania). In vivo optical imaging was per-

ormed on the day of MI, 30 min after coronaryigation (n � 3); day 1 (n � 3); and weeks 1 (n � 3),

(n � 3), 6 (n � 3), and 12 (n � 1) after MI inpen-chested mice. Fluorescence APA imaging waslso performed in 3 unmanipulated control mice.adionuclide imaging using radiolabeled losartan.adiolabeled losartan was used in 4 mice, 3 weeks

fter MI and 6 unmanipulated age-matched controlice, using a dual-head micro-SPECT gamma

amera and a micro-CT (X-SPECT, Gammaedica, Northridge, California). The SPECT im-

ges were acquired in a 64 � 64 matrix, 32 steps at0 s (0 h) or 120 s (at 4 h) per step at 140-KeVhotopeak of 99mTc-losartan with a 15% windowsing a low-energy, high-resolution parallel colli-ator. The CT images were acquired withoutoving the animals. The micro-CT uses an X-ray

ube operating at 50 kVp and 0.6 mA, with imagesaptured for 2.5 s per view for 256 views in 360°otation. Micro-CT images were transferred to a56 � 256 matrix and micro-CT tomographictudies were fused, facilitating scintigraphic andnatomic information in all tomographic scans in 3patial axes. Animals were killed by an overdose ofentobarbital (120 mg/kg) after the imaging exper-ments. Hearts were carefully extracted and planarmages of ex vivo heart were acquired for 15 min in

128 � 128 matrix using a low-energy, high-esolution, pinhole collimator. Thereafter, eacheart was cut into 3 bread-loaf slices (infarct,eri-infarct, and remote areas). The quantitativeadiolabeled losartan uptake was determined with aamma scintillation counter (1480 Wizard 3-inch;

allac Co., Turku, Finland).athologic characterization of the myocardial tissue.earts harvested after the optical imaging experi-ents were used for histopathologic characteriza-

ion taking advantage of the intravenously admin- h

stered fluorescent AT receptor probe. For thisurpose, half of the myocardial tissue specimensere snap-frozen. Frozen sections (7 �m) of theeart were cut and examined by confocal micros-opy (Axioplan 2, Zeiss, Göttingen, Germany)irectly without further preparation. Subsequently,he same myocardial tissue specimens were stainedith �-smooth muscle actin (SMA) antibody

Sigma, St. Louis, Missouri) for localization ofyofibroblasts. The antibody for SMA was conju-

ated to a rhodamine-linked secondary antibody.ndividual localization of APA and SMA wereegistered and merged subsequently to investigateolocalization. To further characterize cellular APAptake, the infarcted myocardial region was alsonalyzed by 2-photon microscopy. This imagingetup consisted of a mode-locked Ti:Sapphire laser170-fs pulse width, 76 MHz repetition rate; Mira00F, Coherent Radiation, Palo Alto, California)umped by a 5-W Verdi laser (Coherent Radia-ion). The 800-nm wavelength excitation beamas deflected into the back port of an invertedxiovert 100 microscope (Zeiss) and scanned across

he sample by a personal computer– controlledalvanometer-driven x-y scanner (Series 603X,ambridge Technology, Watertown, Massachu-

etts). The beam was then reflected by a short-pass75-nm dichroic beam splitter (Chroma Technol-gy, Brattleboro, Vermont) and focused onto theample with a 63�, water-immersion microscopebjective (C-Apochromat, numerical aperture �.2, Zeiss). The average excitation power enteringhe microscope was approximately P � 60 mWcorresponding to 5 mW at the sample site). Theuorescence and second harmonic generation sig-als from the sample were discriminated by a75-nm short-pass dichroic mirror, transmittedhrough a short-pass 600-nm filter (CVI Laser,ivermore, California), and further split by second-ry long-pass 510-nm dichroic mirror onto 2 pho-omultiplier tubes with narrow (10-nm bandwidth)andpass 400-nm filter (Chroma Technology) forecond harmonic generation collection and 535-nm50-nm bandwidth) band-center filter (Chromaechnology) for green fluorescence collection. Im-

ges (256 � 256 pixels) were taken at variousepths (z), covering an area of 35 � 35 �m2 for the3� microscope objective. Each acquisition wasntegrated for 10 s. The remaining half of the

yocardial tissue specimens were fixed with 4%hosphate-buffered paraformaldehyde; fixed myo-ardial tissue specimens were analyzed routinely by
ematoxylin and eosin staining.

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tatistical analysis. All quantitative results are pre-ented as the mean � SD. To determine statisticalignificance of differences, Mann-Whitney test wassed and a p value smaller than 0.05 was consideredignificant.

E S U L T S

eal-time optical imaging of myocardial angiotensineceptors in HF. Optical imaging was performedith fluoresceinated APA. Although all infarctsere non-reperfused, the tracer entry in the in-

arcted myocardium occurred within 3 min andaximized in 20 min (Figs. 2A to 2D). In addition

o myocardial localization, coronary arteries andrterioles showed intense uptake of the ligand; the

Optical Imaging of AT Receptors

icroscopy images show a 3-week old MI in the murine heartdel from baseline (A), and 3 min (B), and at 20 min (C) aftertion of green fluorescent angiotensin peptide analog (APA), dem-gradual increase of the fluorescent APA in infarcted region. Fluo-take was maximized at 20 min after intravenous APA injection.o image (D), demonstrates APA uptake in the border zone, ast infarct zone has collapsed and is therefore not visible. Panel Euptake in smooth muscle cell layer of a small coronary artery

rrows) as well as minimal uptake in thinner veins (diagonal

d

ptake was minimal in the venous tree (Fig. 2E).o uptake of fluorescent APA probe was observed

n the infarct area immediately after MI and 1 dayfter MI. However, distinct APA uptake was ob-erved in the infarct regions at 1 week following

I, which increased substantially by 3 weeks (Fig.A). Minimal uptake of fluorescent probe wasbserved in the remote areas. Subsequently, theptake in infarct region decreased at 6 weeks afterI and was predominantly seen in the infarct

order zone (Fig. 2D). Uptake resolved by the 12theek. Echocardiographic examination demon-

trated left ventricular dilation over time indicatingrogressive cardiac remodeling (Fig. 3B). Unma-ipulated control animals receiving fluorescentPA did not show tracer uptake (Fig. 3A).athologic characterization of receptor targeting. Di-ect localization of systemically administered fluo-escent APA probe allowed characterization ofinding sites of AT receptors in the myocardialissue specimens. The APA uptake (green, Figs. 4And 4B) was almost exclusively localized in cellsther than cardiomyocytes. The fluorescently la-eled cells in the replacement fibrosis in infarctednd peri-infarcted regions (green, Fig. 4D) stainedositively for �-SMA (red, Fig. 4E). These datauggest that binding of the APA occurs, at least inlarge part, to myofibroblasts (yellow, Figs. 4D toF). Although no uptake was seen in the cardio-yocytes, endothelial cells, or interstitial cells, APA

ptake was distinctly observed in the vascular me-ial layers in arterioles. Two-photon microscopyevealed internalization of the green APA probeithin the myofibroblasts and demonstrated intra-

ellular and nuclear localization of the APA (Fig.C). Using second harmonic generation imaging,ollagen fibers (blue) were seen emanating from thePA-positive cells, which further suggests theyofibroblastic origin of these cells (Fig. 4C).uclear imaging of angiotensin receptors. For nonin-asive imaging of angiotensin receptors, we usedadiolabeled losartan, in the 3-week post-MI ani-als for micro-SPECT/micro-CT imaging; a

-week post-MI model was chosen for radionuclidemaging because of maximal upregulation of ATeceptors observed in the optical imaging experi-ents. Radiolabeled losartan uptake was comparedith unmanipulated control animals (Fig. 5). The

ombination of micro-SPECT and -CT imagesllowed for precise localization of myocardial up-ake, which was observed in the anterolateral car-iac region. Ex vivo imaging of the explanted hearts

Figure 2.

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erritory (Fig. 5C). Quantitative radiotracer uptaken infarcted myocardium, represented as percentagef injected radiotracer uptake per gram of myocar-ial tissue, was significantly higher than in controlearts (0.52 � 0.21% vs. 0.21 � 0.13%, p � 0.05).he mean infarct-to-remote region uptake ratioas 2.4:1. The maximum non-target organ radia-

ion burden was observed in liver and kidney.

I S C U S S I O N

he pivotal role of the renin-angiotensin systemn ventricular remodeling following MI has beenstablished in numerous experimental and clinicaltudies. More than circulating renin-angiotensinevels, it is the myocardial upregulation of ACE,T and its receptors, which determine the like-

ihood of ventricular remodeling (3,11). As such,he use of angiotensin receptor blockers (6,12)nd ACE inhibitors (13–15) has resulted inmproved survival in patients with manifest andubclinical HF. It has been suggested that max-mization of anti-angiotensin therapy, including

Figure 3. Evolution of APA Uptake After MI

Intravital microscopy images demonstrating uptake of fluorescent Aand 12 weeks after MI (A). The uptake increases from 1 to 3 weeksgrams at 1, 3, and 12 weeks, indicating left ventricular dilation and

ncrease in ACE-inhibitor dose or addition of A

ngiotensin receptor blockers over ACE-inhibitorherapy further reduces morbidity (6,16) andortality (4) in HF. It is therefore conceivable

hat accurate assessment of myocardial AT recep-or expression should guide optimization of anti-ngiotensin therapy. It should also allow periodicvaluation of the efficacy of anti-angiotensinntervention. Current diagnostic imaging of HFemains generally focused on geometric andtructural cardiac alterations (17,18), and it cane hypothesized that the ability to visualizenterstitial processes at the molecular level, whichrecede the geometric and functional deteriora-ion of the left ventricle, should help predict theikelihood and rate of remodeling and develop-

ent of HF (19).The present study demonstrates the feasibility of

n vivo imaging of myocardial angiotensin receptorsn an experimental HF model. Currently, as atrategy for prevention of advanced HF, significantmphasis is being placed on the recognition ofatients at risk for developing HF, including thoseith no apparent structural LV abnormality (stage

from 6 groups: no infarction, and 0 day, 1 day, 1 week, 3 weeksdecreases thereafter. Panel B shows the respective echocardio-

gressive heart failure in these mice with MI.

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ural LV abnormality (stage B) (20). As the currentlinical practice allows diagnosis of HF only afterhe LV has undergone substantial remodeling, de-elopment of a technology that actually predictsccurrence of cardiac remodeling is of critical im-ortance. Our immunohistochemical analyses showhat binding of the tracer uptake was almost exclu-ively localized to myofibroblasts; we did not ob-erve APA uptake in cardiomyocytes. Upregulationf AT receptors on myofibroblasts (Fig. 4F) allows

Figure 4. Pathologic Characterization APA Localization

The remote zone (A) demonstrates no APA uptake in interstitium (*strate fluorescent APA uptake in the infarct zone (B), with an enlarg(D, green) also stained positively for smooth muscle actin (SMA) (E(ATR) on myofibroblasts. (C) Ex vivo 2-photon microscopy demonstwith collagen surrounding the myofibroblast using second harmoni

Figure 5. Noninvasive Imaging of AT Receptors With Radiolabe

The micro–SPECT and micro-CT images are shown in a control mouheart can be seen (A) in the in vivo and ex vivo images. There is onthe 3-week post-MI animal, significant radiolabeled losartan uptakethe in vivo image is confirmed in the ex vivo image. The histogram
region (0.524 � 0.212% ID/g) as compared to control noninfarcted anim
or growth factor (such as AT)–induced myofibro-last proliferation and collagen production, which iselieved to contribute to healing and the remodel-ng process following MI (2,21,22). Although an-iotensin receptor upregulation in the kidneys haseen observed earlier by a carbon-11 labeled AType-1 antagonist (23), the hypothesis of the role ofargeting neurohumoral upregulation for ventricularemodeling has been supported by imaging ofenin-angiotensin axis with the help of radiolabeled

cardiomyocytes (arrows). The cells of nonmyocytic origin demon-image of cellular uptake (inset). These cells in the infarct region), showing colocalization (F, yellow), indicating AT receptorsfluorescent APA intracellularly within the myofibroblast (green),

neration imaging (blue). Abbreviations as in Figure 1.

Losartan

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enzoyl lisinopril (21,24). Dilsizian et al. (24)ncubated myocardial specimens explanted fromatients undergoing cardiac transplantation fornd-stage ischemic cardiomyopathy with F-18uoro-benzoyl lisinopril. There was specific bind-

ng of radiotracer to ACE; mean binding was 6.6 �.2, compared with 3.4 � 2.5 luminescence/mm2 inegments pre-incubated with cold lisinopril (p �.0001). Further, mean radiotracer binding was 6.3

4.5 in infarcted, 7.6 � 4.7 in peri-infarcted, and.0 � 1.0 luminescence/mm2 in remote non-nfarcted (p � 0.02) segments. The distribution of

ast cell chymase was nonuniform in these samplesnd disparate from ACE. This and the studyresented here demonstrate that ACE (and proba-ly AT) upregulation induces the proliferation ofyofibroblasts (which have increased angiotensin

eceptor density) and contributes to collagen depo-ition. Both studies have also demonstrated thathese key steps in the tissue renin-angiotensinascade involve receptor upregulation of only about- to 3-fold. It therefore remains to be seen whethermaging targeted to AT receptors and/or ACE will

Group. Effects of enalapril on mortal- OPTIMAAL rand

O N C L U S I O N S

he present study explains the development of aiagnostic probe seeking AT receptor upregula-ion within the myocardium and shows the fea-ibility of noninvasive radionuclide imaging. Theuorescent angiotensin receptor probe confirmseceptor localization on myofibroblasts, whichay contribute to the process of interstitial fibro-

is. It is proposed that monitoring of angiotensineceptor expression after infarction may predicthe evolution of HF. The present study offers aroof of concept, with modest uptake of radiola-eled losartan and it will be necessary to engineerrobes with better targeting characteristics forealization of potential clinical application.

eprint requests and correspondence: Dr. Jagat Narula ORr. Leo Hofstra, University of California, Irvine, School

f Medicine, 101 The City Drive, Bldg. 53, Mail Route1, Orange, California 92868-4080. E-mail: narula@

esult in clinically robust imaging strategy. uci.edu OR [email protected].

1

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