Long-term endurance training-induced cardiac adaptation in new rabbit and dog animal models of the human athlete's heart

Sudden cardiac death in athletes is rare and most often unexpectable. For a better understanding of cardiac remodeling, this study presents the effects of chronic vigorous exercise on cardiac structure and electrophysiology in new rabbit and dog athlete's heart models. Rabbits and dogs were randomized into sedentary ('Sed'), exercised (subjected to 16 weeks chronic treadmill exercise ('Ex') groups, and a testosterone-treated ('Dop') group in dogs. Echocardiography and electrocardiogram were performed. Proarrhythmic sensitivity and autonomic responses were tested in conscious dogs. 'Ex' animals exhibited left ventricular enlargement with bradycardia (mean RR in 'Ex' vs. 'Sed' rabbits: 335 ± 15 vs. 288 ±19 ms, p ≤ 0.05, and in 'Dop' vs. 'Ex' vs. 'Sed' dogs: 718 ± 6 vs. 638 ± 38 vs. 599 ± 49 ms) accompanied by an increase of heart rate variability in both species (e.g. SD RR in 'Ex' vs. 'Sed' rabbits: 3.4 ± 0.9 vs. 1.4 ± 0.1 ms, p ≤ 0.05, and in 'Dop' vs. 'Ex' vs. 'Sed' dogs: 156 ± 59 vs. 163 ± 44 vs. 111 ± 49 ms) indicating an increased vagal tone. A lower response to parasympatholytic agent atropine and more pronounced QTc interval lengthening after dofetilide challenge were found in 'Ex' and 'Dop' dogs compared to the 'Sed' group. No morphological and functional changes were found after chronic steroid treatment in dogs. The structural-functional findings share more similarities with human athlete's heart. Slight repolarization sensitivity in the exercised dogs may indicate an increased risk of arrhythmias in athletes under different circumstances. These animal models might be useful for the further investigations of the cardiovascular effects of competitive training.