Dynamics of development of exercise-induced hypertrophy: continuous and high-intensity interval training in small animal models

Abstract Background Although the phenomen of athlete's heart has been intensively investigated, only few research group conducted complex, longitudinal studies to understand the physiology and importance of underlying mechanisms. The relation of morhological and functional alterations still needs to be elucidated. Experimental investigations allow us to examine certain phenomena in a unified manner and reveal the underlying mechanisms. Purpose In this complex study, we would characterize the dynamics of development of exercise-induced hypertrophy in small animal models of continuous (swim exercise) and high-intensity interval training (HIIT). Methods Young adult male rats have been subjected to swim-training (Sw group, n=10, 200 min/day, continuous type training) or treadmill training (Tr group, n=10, alternation of 90%/60% of maximal speed, high-intensity interval training) and were compared to control animals (Co group, n=10). Echocardiography and left-ventricular pressure-volume analysis were performed at 2, 5 and 12 weeks after the start of training program to reveal the morhological and functional alterations in detail, respectively. Results In the early phase (2 weeks) both type of exercise training was associated with improved functional parameters (increased stroke volume, stroke work and load-independent contractility indices) without myocardial hypertrophy (post-mortem heart weight Co: 1.05±0.07 g, Sw: 1.10±0.13 g, Tr: 1.07±0.07 g). At week 5, a developing hypertrophy was observed along with the already observed improved systolic function. After completion of exercise program, we experienced similar degree of hypertrophy (approximately 15%) in swim- and treadmill-trained animals post-mortem heart weight Co: 1.12±0.07 g, Sw: 1.26±0.10 g, Tr: 1.26±0.09 g) with comparable functional improvement (as shown by load-indepenndent contractility indices and mechanoenergetic parameters). While systolic function and contractility showed early suparnormal values, the improvement of diastolic function (active relaxation) could be observed only in the later phase. Conclusions Our results suggest that exercise-induced functional improvement is (at least partly) independent from myocardial hypertrophy. The systolic mechanical enhancement develops in the early phase, while growth of myocardium and diastolic improvement follows it later. Additionally HIIT and continuous training might be associated with similar degree of hypertrophy and functional improvement.