French researchers concluded in their paper published in 2008 that there was no histological and morphological change and no arrhythmia in rats' hearts after chronic ingestion of low levels of cesium-137, though they did observe cardiovascular system impairments.
The researchers were trying to determine whether chronic ingestion of cesium-137 caused cardiovascular system impairment observed in children and liquidators after the Chernobyl nuclear power plant accident.
The researchers' conclusion runs counter to the assertion by several researchers like Dr. Yury Bandazhevsky, former director of the Medical Institute in Gomel, Belarus, that cesium accumulates in hearts, causes heart arrhythmia and results in histological and morphological change. In fact, after Dr. Bandazhevsky made lecture tours in Japan earlier this year, it is widely believed as truism, at least among lay people (non-researcher).
From Cardiovascular Toxicology March 2008, Volume 8, Issue 1, pp 33-40, via Springer Link (emphasis is mine):
Chronic Contamination of Rats with 137Cesium Radionuclide: Impact on the Cardiovascular System (Yann Guéguen,Philippe Lestaevel,Line Grandcolas,Cédric Baudelin,Stéphane Grison, Jean-René Jourdain,Patrick Gourmelon,Maâmar Souidi)
Cardiovascular system impairment has been observed in children and in liquidators exposed to the Chernobyl nuclear power plant accident. No experimental studies of animals have analyzed whether these disorders might be attributed to chronic ingestion of low levels of cesium 137 (137Cs). Biochemical, physiological, and molecular markers of the cardiovascular system were analyzed in rats exposed through drinking water to 137Cs at a dose of 500 Bq kg−1 (6500 Bq l−1). Plasma concentrations of CK and CK-MB were higher (+52%, P < 0.05) in contaminated rats. No histological alteration of the heart was observed, but gene expression was modified in the atria. Specifically, levels of ACE (angiotensin converting enzyme) and BNP (brain natriuretic peptide) gene expression increased significantly (P < 0.05). ECG analysis did not disclose any arrhythmia except ST- and RT-segment shortening (−9% and −11%, respectively, P < 0.05) in rats exposed to 137Cs. Mean blood pressure decreased (−10%, P < 0.05), and its circadian rhythm disappeared. Overall, chronic contamination by an extreme environmental dose of 137Cs for 3 months did not result in cardiac morphological changes, but the cardiovascular system impairments we observed could develop into more significant changes in sensitive animals or after longer contamination.
(H/T Dr. Masahiro Kami)