br Materials and methods br Results br

Materials and methods
Results
Discussion Although some conditioning therapies have provided satisfactory clinical outcomes, mammalian species have little or no ability to replace injured cardiac tissue [34]. Therefore, therapeutic strategies based on the prevention, rather than treatment after the heart has already been damaged, emerge as promising strategies. In this context, the main findings of this study are as follows: (1) RE enhances cardiac eNOS activity converting NO signal into RIPC effect; (2) RE improves post-ischemic cardiac contractility recovery, decreases life-threatening arrhythmias, and attenuates acute MI size by limiting cardiac eNOS uncoupling. Previously, we have shown that high intensity RE (70%) acutely led to the greatest activation of eNOS/NO pathway in mesenteric 2768 synthesis [21]; however, this intensity might be correlated with excessive exercise overload and adverse cardiovascular events. Moreover, a large number of studies, including guidelines by the American College of Sports Medicine (ACSM), recommend light-to-moderate exercise intensity for patients with heart diseases due to its efficacy and safety [35]. Nevertheless, high-intensity interval training (HIIT) has challenged the clinical practice regarding the most suitable exercise intensity as a complementary therapy for patients with HF, since HIIT was superior to moderate continuous training at improving health-related quality of life and cardiovascular outcomes in HF patients with reduced ejection fraction (HFrEF) [36]. However, a recently launched clinical trial (SMARTEX Heart Failure Study) could not confirm the notion that HIIT was superior to moderate continuous exercise at reducing left ventricular remodeling in stable HFrEF patients [37]. As it is still a matter of debate whether the magnitude of cardiac benefits is dependent of exercise intensity, we followed the current recommendation by ACSM to investigate whether RE performed at a light intensity confers a RIPC effect. Acute cardioprotective effects of exercise have been attributed to a number of preconditioning molecules, G-coupled protein receptors, and ion channels [15,30]. Here, we showed that a single bout of RE acutely increases plasma nitrite concentration and cardiac eNOS activity. Interestingly, our results revealed that nitrite levels rapidly returned to the basal condition after 1 h of RE. However, although it is well known that nitrite is a stable product of NO metabolism [38], the cardioprotection might be present even several hours later of RE, as previously shown [[16], [17], [18]], likely due to S-nitrosylation on a variety of proteins [2,39,40]. Previous studies have demonstrated that NO-based pharmacological therapies that augment levels of nitrite and S-nitrosothiols provide protection against myocardial IR injury [[41], [42], [43]]. Based on the results of those studies and the results from this study, RE may induce RIPC through a NOS-dependent mechanism. Accordingly, a previous study reported that augmented circulating nitrite levels derived from eNOS stimulation contributes to the cardioprotection mediated by remote ischemic preconditioning [44]. Indeed, aerobic and resistance exercise have been associated with beneficial cardiovascular actions [45,46]. Moreover, there is a vast knowledge regarding the mechanisms involved in the cardioprotective phenotype evoked by aerobic exercise [2,14,33], while RE remained largely elusive until now. In addition, it is well known that long-term exercise training produces a continuous activation of preconditioning signaling pathways [15], however little is known regarding the short-term exercise mechanisms. Among the multiple mechanisms recruited by different conditioning strategies to protect against IR injury, the 3 major signaling pathways are eNOS–protein kinase G, reperfusion injury salvage kinase (RISK), and survivor activating factor enhancement (SAFE) [30]. However, some recent reviews highlight existing controversy regarding results and limitations of those findings [40,47,48]. Although these decisive mechanisms converge on cardiomyocyte survival or death, they are individually complex and inter-related. We show here that RE induces robust cardioprotection in rat hearts subjected to IR, as validated by enhanced post-ischemic cardiac contractility recovery and preventing the rise in coronary perfusion pressure. We also demonstrated a significant dependence on the eNOS/NO signaling pathway, since the inhibition of NOS fully blunted the RIPC evoked by RE. These findings are consistent with previous studies showing that transgenic mice lacking eNOS expression [13] or pharmacological inhibition [2] abolish the exercise-induced cardioprotective effects. Moreover, the protective nature of NO in the heart may be attributed to the activation of downstream effectors involved in contractility and rhythmicity [10,31].