Abstract 255 Antiarrhythmic Effects of Ionizing Radiation on Neonatal Rat Cardiomyocytes

Background 

Ventricular tachycardia in patients with heart failure (HF) contributes to significant morbidity and mortality. Despite progress in HF therapy and ablation strategies, therapy options for some patients are still limited. Stereotactic Body Radiotherapy (SBRT) is a promising new treatment option for refractory ventricular tachycardia. Antiarrhythmic response becomes apparent within hours to days after SBRT. The molecular mechanisms underlying the clinically demonstrated effect are still poorly understood and cannot be explained solely by the induction of fibrosis, which usually takes several months. We hypothesized that exposing ventricular myocytes to single-fraction radiation has direct cellular electrophysiological impact. This may be attributed to changes in the expression and function of associated proteins and subsequent changes in action potential duration and propagation.

Methods 

Ventricular neonatal rat cardiomyocytes were isolated, exposed to X- rays with a single-dose of 20Gy and analyzed at time intervals ranging from 24h to 96h after radiation. Expression alterations of ion channels, proteins of intracellular calcium handling and structural proteins affecting cellular electrophysiology were assessed via qRT-PCR and Western blotting. Functional data was so far obtained by video analyses of beating cardiomyocytes. The occurrence of apoptotic cardiomyocytes was quantified by TUNEL assays.

Results

Significant changes in transcript expression of Gja1 (Cx43) and ion channels were observed. Increased levels of Kcnd3 (Kv4.3), Cacna1c (Cav1.2), Kcnh2 (Kv11.1) and decreased levels of Cacna1d (Cav1.3) and Kcnj8 (Kir6.1) were shown. Regarding calcium handling, significant upregulations of Ryr2 (RYR2) and Slc8a1 (NCX) transcript levels were detected. Western blotting data confirmed significant findings. Most distinct changes were observed 96h after radiation. No significant changes in radiation-induced DNA damage and apoptotic rate could be determined.

Conclusion 

In summary, these results indicate a connection between single- fraction irradiation and short- term cellular changes in electrophysiological cardiac function. Alterations in protein expression of ion channels after irradiation suggest action potential shortening. Although it seems that ion channel remodeling is a possible explanation for early antiarrhythmic effects, further studies will focus on the functional relevance and elucidate molecular pathways underlying distinct ion channel remodeling.

Clin Res Cardiol (2021). 10.1007/s00392-021-01933-9
Antiarrhythmic Effects of Ionizing Radiation on Neonatal Rat Cardiomyocytes
C. Mages¹, H. Gampp², A.-K. Rahm¹, K. Seidensaal³, B. Rhein³, J. Debus³, N. Frey¹, D. Thomas¹, P. Lugenbiel¹
¹Klinik für Innere Med. III, Kardiologie, Angiologie u. Pneumologie, Universitätsklinikum Heidelberg, Heidelberg; ²Innere Medizin III, Molekulare und Translationale Kardiale Elektrophysiologie, Universitätsklinikum Heidelberg, Heidelberg; ³Klinik für Radioonkologie und Strahlentherapie, Universitätsklinik Heidelberg, Heidelberg;
Background  Ventricular tachycardia in patients with heart failure (HF) contributes to significant morbidity and mortality. Despite progress in HF therapy and ablation strategies, therapy options for some patients are still limited. Stereotactic Body Radiotherapy (SBRT) is a promising new treatment option for refractory ventricular tachycardia. Antiarrhythmic response becomes apparent within hours to days after SBRT. The molecular mechanisms underlying the clinically demonstrated effect are still poorly understood and cannot be explained solely by the induction of fibrosis, which usually takes several months. We hypothesized that exposing ventricular myocytes to single-fraction radiation has direct cellular electrophysiological impact. This may be attributed to changes in the expression and function of associated proteins and subsequent changes in action potential duration and propagation. Methods  Ventricular neonatal rat cardiomyocytes were isolated, exposed to X- rays with a single-dose of 20Gy and analyzed at time intervals ranging from 24h to 96h after radiation. Expression alterations of ion channels, proteins of intracellular calcium handling and structural proteins affecting cellular electrophysiology were assessed via qRT-PCR and Western blotting. Functional data was so far obtained by video analyses of beating cardiomyocytes. The occurrence of apoptotic cardiomyocytes was quantified by TUNEL assays. Results Significant changes in transcript expression of Gja1 (Cx43) and ion channels were observed. Increased levels of Kcnd3 (Kv4.3), Cacna1c (Cav1.2), Kcnh2 (Kv11.1) and decreased levels of Cacna1d (Cav1.3) and Kcnj8 (Kir6.1) were shown. Regarding calcium handling, significant upregulations of Ryr2 (RYR2) and Slc8a1 (NCX) transcript levels were detected. Western blotting data confirmed significant findings. Most distinct changes were observed 96h after radiation. No significant changes in radiation-induced DNA damage and apoptotic rate could be determined. Conclusion  In summary, these results indicate a connection between single- fraction irradiation and short- term cellular changes in electrophysiological cardiac function. Alterations in protein expression of ion channels after irradiation suggest action potential shortening. Although it seems that ion channel remodeling is a possible explanation for early antiarrhythmic effects, further studies will focus on the functional relevance and elucidate molecular pathways underlying distinct ion channel remodeling.
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