Early Changes in Rat Heart After High‐Dose Irradiation: Implications for Antiarrhythmic Effects of Cardiac Radioablation

Figures

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  Figure 1. Ventricular myocardium irradiated with 30 Gy at 4 weeks. Irradiated left ventricular free wall subendocardium of rats was stained with hematoxylin and eosin (H&E), Masson trichrome (MT), and immunohistochemistry (Desmin, TUNEL [terminal deoxynucleotidal transferase–mediated biotin–deoxyuridine triphosphate nick‐end labeling] assay, and C4d immunostaining). Myocyte necrosis and apoptosis were not observed in rat hearts irradiated by 20 to 50 Gy. Intercellular space (interstitium) was widened but not fibrotic. Cellular rupture and destruction of myocytes were not observed. (×400 Magnification.)

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  Figure 2. Electron microscope examination of the left ventricular myocardium of irradiated rat heart (30 Gy single fraction). Compared with normal intercalated discs (A), irradiated intercalated discs exhibited irregular widening focal protrusion (B) or segmental separation (C) peaking at 1 week. These abnormalities were observed in 30 Gy‐irradiated rats harvested at day 1 and were rarely observed after week 2. From week 2, intercalated discs exhibited irregularly widened longitudinal cell connections with adjacent gap junction vesicles (D). Damaged intercalated discs were narrowed after 3 weeks. Remodeling was uneven and twisted (E). (×50 000 Magnification.)

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  Figure 3. Interstitial vacuolization. A, Temporal changes of the left ventricular subendocardial vacuolization. (×400 Magnification.) B, Boxplot of the percentage of vacuolization at 3 weeks according to radiation dose. The extent of vacuolization in 5 high‐power field was measured at left ventricular subendocardium using an Aperio Imagescope (Leica). All groups were statistically different from each group, except the 20 and 25 Gy groups and the 30, 40, 50 Gy groups. *** indicates P<0.001. C, The vacuolized area on hematoxylin and eosin (H&E) staining showed positive staining for CD68. (×400 Magnification.) Statistical difference was determined by 1‐way ANOVA with Tukey post hoc analysis.

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  Figure 4. Interstitial and cytoplasmic edema after irradiation. A, Rat left ventricular midmyocardium on electron microscopy (×5000 magnification). B, Rat left ventricular midmyocardium on electron microscopy (×40 000 magnification), left ventricle irradiated with 30 Gy (3 weeks). Cellular membranes were dilated because of cytoplasmic swelling (red arrows). Interstitial edema widened intercellular spaces (red asterisks and white arrow). Subsarcolemmal and interfibrillar mitochondria (marked as “Mi”) were damaged and variable in size. Myofibrils (MFs) with Z and M lines were intact. No remarkable abnormalities in myocyte nuclei (marked as “Nu”) were noted.

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  Figure 5. Main histopathologic findings. Irradiated rat heart myocardium (right) and normal rat heart myocardium (left) were compared. No cellular necrosis or apoptosis after irradiation was noted. Diffuse vacuolization was correlated with interstitial and subsarcolemmal edema. Intercalated discs between myocytes were widened with diverse patterns.

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  Figure 6. ECG changes. Average values of 10 measurements of randomly selected beats during a 1‐minute recording were determined. The sample size was 15 per group at each time point. A, Resting heart rate was increased after irradiation compared with baseline measurements. B, The PR interval, representing the conduction velocity from atrium to ventricle, was slightly delayed and peaked at 3 weeks. C, The corrected QT interval, representing the intraventricular conduction property, was prolonged relative to baseline. D, The ST segment was depressed after irradiation but elevated after 2 weeks in most cases. The statistical difference between groups was calculated using 1‐way ANOVA with Dunnett post hoc analysis.