IJCS | Volume 31, Nº6, November / December 2018

616 Moura et al. Diabetic rats and aerobic training Int J Cardiovasc Sci. 2018;31(6)610-618 Original Article used herein. However, they found that diabetic rats had increased levels of collagen fibers after DM1 induction, which may indicate that there is some variability in the cardiac collagen fraction response of rats when DM1 is induced by STZ. 30 It may be possible that the period of DM1 may influence this response. Aside from that, the different techniques and types of collagen volume fraction assessment may also influence this response. Bakth et al., 31 induced mild DM1 in canines by administering low doses of alloxan for a period of 1 year. Despite the low doses and mild DM1, this greater period of DM1 led to increased collagen volume fraction in the myocardium of canines. Silva et al., 28 also observed increased collagen in the left ventricle of rats. However, in this case, Silva et al. induced DM1 in rats by administering STZ, 60 mg/kg, intraperitoneally. 28 Another cardiac morphological change that was observed in this study was the left ventricular wet weight. As expected, the DMC group had a decrease in ventricular weight when compared with the CTR group, similar to other studies that have found a decrease in heart weight 30,32 and left ventricular weight. 27 Neither of the exercise training protocols used in the present study was able to change the left ventricular wet weight. On the other hand, cardiomyocyte hypertrophy was not observed in the DMC group, but the exercise training protocols used in the present study were able to increase the cross-sectional diameter of cardiomyocytes. There may have been a physiological hypertrophy since it is believed that ET can influence DNA replication and mitotic activity, which offsets the larger diameter induced by exercise training, compensating for the left ventricular weight. Thus, it may have decreased the number of cells, while increasing the size of those that remained. However, it is important to remember that we evaluated the wet weight of the ventricle, not the dry weight, which may result in other components influencing this remodeling of cardiomyocytes and ventricular mass. The fact that swimming showed a more prominent increase in cardiac cross-sectional diameter may be due to the characteristics of this type of exercise, where the hydrostatic pressure exerted by water on the blood vessels leads to vasoconstriction and therefore, promotes increased venous return and preload. 33 It is known that increasing the preload promotes the addition of new sarcomeres in series, leading to an eccentric hypertrophy. 34 Furthermore, this increase in vasoconstriction also promotes increased post-load and stimulates the growth of cardiomyocytes by adding new sarcomeres in parallel and increasing the thickness of the heart wall to produce a stronger contraction to overcome this increased resistance. 34 In fact, swimming training was effective in increasing the width and cell volume of cardiomyocytes in rats with DM. 4 Another factor that may have contributed to this more notable increase in cardiac cross-sectional diameter after swimming training is the increased adrenergic activity. There are studies in rats showing that swimming training leads to higher norepinephrine and adrenaline levels than exercise training on a treadmill, favoring the activation of the sympathetic nervous system. 35 It is known that catecholamines exert cardiac hypertrophic effects through adrenergic action, especially in pathological conditions. 36-39 Therefore, perhaps a higher cardiac adrenergic action may have contributed to increased cardiomyocyte hypertrophy in DMS, when compared with DMT. It is likely that this cardiac remodeling observed in the trained animals reflected an improvement in cardiac function, which must have influenced the superior performance in terms of exercise tolerance compared with the DMC group. However, it is important to note that our study did not evaluate the number of cardiomyocytes, or the adrenergic activity of each training protocol and its association with cardiac hypertrophy is speculative. Nevertheless, it is undoubtedly an interesting topic for future investigations. Conclusion The aerobic ET protocols applied to animals with DM1, irrespective of the ergometer, offered benefits in some regards: increased glycogen content of the soleus and EDLmuscles and increased cross-sectional diameter of cardiomyocytes. For some variables, such as exercise tolerance and the cross-sectional area of the soleus and EDL muscles, the DMT showed better results than DMS group. On the other hand, the DMS showed increased cardiac cross-sectional diameter when compared with the DMT group. Author contributions Conception and design of the research: Moura EOC, Nogueira E, Gomes R, Medeiros A. Acquisition of data: Moura EOC, Tanaka K, Mattos K, Brum PC. Analysis and interpretation of the data: Moura EOC, Tanaka K.