Molecular imaging of myocardial remodeling after infarction.

Myocardial fibrosis after myocardial infarction plays a major role in cardiac remodeling, development of heart failure, and arrhythmias. Both replacement and interstitial fibrosis are determined by the extent of myofibroblastic proliferation and hence the extent of collagen deposition. It is logical to propose that a molecular imaging strategy that is able to determine the rate of myofibroblastic proliferation should be able to foretell the magnitude of myocardial remodeling and the likelihood of development of heart failure. Of various plausible targets on the proliferating myofibroblasts, receptors for neurohumoral agonists and overexpression of integrin moieties may offer best options for molecular imaging. In this chapter we describe the assessment of angiotensin II receptor and αvß3 integrin upregulation in a mouse model after myocardial infarction using real time in vivo fluorescence imaging and nuclear imaging.

Annexin A5 uptake in ischemic myocardium: demonstration of reversible phosphatidylserine externalization and feasibility of radionuclide imaging.

UNLABELLED: Ischemic insult to the myocardium is associated with cardiomyocyte apoptosis. Because apoptotic cell death is characterized by phosphatidylserine externalization on cell membrane and annexin-A5 (AA5) avidly binds to phosphatidylserine, we hypothesized that radiolabeled AA5 should be able to identify the regions of myocardial ischemia. METHODS: Models of brief myocardial ischemia by the occlusion of the coronary artery for 10 min (I-10) and reperfusion for 180 min (R-180) for the detection of phosphatidylserine exteriorization using (99m)Tc-labeled AA5 and gamma-imaging were produced in rabbits. (99m)Tc-AA5 uptake after brief ischemia was compared with an I-40/R-180 infarct model. Histologic characterization of both myocardial necrosis and apoptosis was performed in ischemia and infarct models. Phosphatidylserine exteriorization was also studied in a mouse model, and the dynamics and kinetics of phosphatidylserine exposure were assessed using unlabeled recombinant AA5 and AA5 labeled with biotin, Oregon Green, or Alexa 568. Appropriate controls were established. RESULTS: Phosphatidylserine exposure after ischemia in the rabbit heart could be detected by radionuclide imaging with (99m)Tc-AA5. Pathologic characterization of the explanted rabbit hearts did not show apoptosis or necrosis. Homogenization and ultracentrifugation of the ischemic myocardial tissue from rabbit hearts recovered two thirds of the radiolabeled AA5 from the cytoplasmic compartment. Murine experiments demonstrated that the cardiomyocytes expressed phosphatidylserine on their cell surface after an ischemic insult of 5 min. Phosphatidylserine exposure occurred continuously for at least 6 h after solitary ischemic insult. AA5 targeted the exposed phosphatidylserine on cardiomyocytes; AA5 was internalized into cytoplasmic vesicles within 10-30 min. Twenty-four hours after ischemia, cardiomyocytes with internalized AA5 had restored phosphatidylserine asymmetry of the sarcolemma, and no detectable phosphatidylserine remained on the cell surface. The preadministration of a pan-caspase inhibitor, zVAD-fmk, prevented phosphatidylserine exposure after ischemia. CONCLUSIONS: After a single episode of ischemia, cardiomyocytes express phosphatidylserine, which is amenable to targeting by AA5, for at least 6 h. Phosphatidylserine exposure is transient and internalized in cytoplasmic vesicles after AA5 binding, indicating the reversibility of the apoptotic process.

Macrophage depletion in hypertensive rats accelerates development of cardiomyopathy.

Inflammation contributes to the process of ventricular remodeling after acute myocardial injury. To investigate the role of macrophages in the chronic process of cardiac remodeling, they were selectively depleted by intravenous administration of liposomal clodronate in heart failure-prone hypertensive Ren-2 rats from the age of 7 until 13 weeks. Plain liposomes were used for comparison. Liposomal clodronate treatment reduced the number of blood monocytes and decreased the number of macrophages in the myocardium. Compared to plain liposomes, liposomal clodronate treatment rapidly worsened left ventricular ejection function in hypertensive rats. Liposomal clodronate- treated Ren-2 rat hearts showed areas of myocyte loss with abundant inflammatory cell infiltration, predominantly comprising CD4 positive T lymphocytes. The current study showed that lack of macrophages was associated with earlier development of myocardial dysfunction in hypertensive rats. Modulation of macrophage function may be of value in the evolution of cardiomyopathy.

Noninvasive imaging of angiotensin receptors after myocardial infarction.

OBJECTIVES: 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). BACKGROUND: 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. METHODS: 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. RESULTS: 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). CONCLUSIONS: 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.