What Doesn't Kill You Makes You Stronger—Even Heart Radiation?

Abstract

Radiation therapy (RT) is a commonly used treatment for thoracic cancers. However, the risk of coronary events increase by 4‐16% per Gy of heart dose. Our prior studies show that localized heart RT leads to radiation‐induced heart disease (RIHD) and increased mortality in Dahl Salt Sensitive (SS) rats starting at 3mo post‐RT. We have also shown that whole thoracic RT increases susceptibility to ischemia‐reperfusion (IR) injury in Wistar rats. Taken together, we hypothesized that 10weeks after targeted cardiac RT (prior to RIHD) SS rat hearts would have worsened cardiac function as measured by echocardiography and be more susceptible to IR injury. Echocardiograms were performed 10weeks after 24Gy dose of CT‐guided localized cardiac RT and in age‐matched controls (C). Next, hearts were isolated and perfused ex vivo using the Langendorf method ‐ without ischemia reperfusion (time controls; TC; n=3 in C and RT) or exposed to 25min of global ischemia followed by 60min reperfusion (IR; n=6 in C and RT). During TC and IR, cardiac function, mitochondrial redox state, and vascular reactivity were assessed; the hearts were then sectioned and stained to assess infarct size. Echocardiography of the RT group when compared to C showed an increase in interventricular septal thickness (1.32±0.1 vs 0.78±0.07 cm, p<0.0001; mean±SD) and left ventricular posterior wall thickness (1.03±0.08 vs 0.75±0.06 cm, p<0.005). Accordingly, a decrease in end diastolic (1.79±0.46 vs 2.38±0.35 ml/kg, p<0.05) and end systolic (0.10±0.04 vs 0.23±0.08 ml/kg, p<0.05) volume was also seen, with no significant differences in ejection fraction (94.5±2.1 vs 90.3±3.9 %) or stroke volume (1.69±0.44 vs 2.15±0.27 ml; p>0.05). Interestingly, in the ex vivo perfused hearts, TC RT had a 2 times higher rate‐pressure product – a measure of cardiac work – compared to C (p<0.05), and following IR, the RT group recovered to 2.5x higher levels to C (p<0.0001). Following IR, the RT group compared to C also showed a greater recovery of coronary flow rate (1.6x, p<0.05) and rates of contractility (2.25x) and relaxation (lusitropy; 2.05x) (p<0.0001). No difference in mitochondrial redox state, infarct size, and endothelium‐dependent and independent vascular reactivity were seen between the groups. In conclusion, our results show for the first time that 10weeks post‐targeted cardiac RT, ex vivo hearts show better recovery and cardiac function compared to C after IR through mechanisms other than differences in redox state and vascular reactivity.