The present study sought to evaluate the impact of regular exercise and 2 weeks of detraining on blood pressure, cardiac hypertrophy and cardiac function in an AngII-induced hypertensive rat model. Also, we investigated the impact of exercise and detraining on pro- and anti-inflammatory cytokines and oxidative stress within the PVN of these hypertensive rats. Three novel and important findings emerge from this study. First, two weeks of detraining did not abolish the exercise-induced attenuation in MAP in hypertensive rats, whereas, detraining failed to completely preserve the exercise-mediated improvement in cardiac hypertrophy and diastolic function in these rats. Second, two weeks of detraining did not have any detrimental effects on exercise-induced improvement in PICs; whereas, it abolished the exercise-induced improvement in IL-10 in the PVN of hypertensive rats. Third, 2 weeks of detraining in exercising hypertensive rats abolished the exercise-induced attenuation in oxidative stress within the PVN, as indicated by increased levels of iNOS as well as reduction in Cu/ZnSOD after detraining. Collectively, these results led us to conclude that 2 weeks of detraining is not long enough to completely abolish the exercise-induced beneficial effects; however, further cessation of exercise may lead to complete reversal of the beneficial effects.
It is now well established that an overactivation of the RAS within the brain plays a key role in the pathogenesis of hypertension. AngII, which is a major effector molecule of the RAS, induces vasoconstriction, aldosterone secretion, increased sympathetic activity and sodium retention, ultimately leading to increased BP 
. Over time, sustained elevation of AngII leads to cardiac hypertrophy and remodeling, further deteriorating the hypertensive condition 
. Previous findings from our laboratory and others have shown that blockade of vasoconstrictor components of the RAS 
or overexpression of vasoprotective components of the RAS 
within the cardiovascular regulatory centers of the brain (such as PVN) attenuates BP, reduces cardiac hypertrophy and improves cardiac function in animal models of hypertension. These reports emphasize the importance of blocking RAS specifically within the PVN in mitigating the hypertensive response and associated cardiac damage. Therefore, the results of the present study are important from clinical perspective as they demonstrate that regular exercise not only reduces BP and improves cardiac function but also reduces inflammatory cytokines and oxidative stress within the PVN of hypertensive animals. Additionally, our current finding that the transient cessation of exercise could reverse exercise-induced beneficial effects in hypertension further emphasizes the importance of regular exercise in attenuating hypertension.
At the end of the study, we observed significant reduction in MAP in trained hypertensive rats compared with their sedentary counterparts and saw no comparable changes in trained normotensive controls. As depicted in , the continuous recording of MAP in conscious rats by implanted telemetry device showed that AngII infusion resulted in significant increase in MAP in sedentary rats beginning from day 8 of infusion and this increase in MAP reached to plateau at day 23 of infusion. Regular exercise resulted in significant reduction in MAP beginning from day 16 of training and remained significantly lower until the end of the study. These results are in accordance with previous findings from our laboratory and others 
. It is noteworthy that the exercise training protocols presently used did not completely normalize the increased MAP in hypertensive rats. However, intensity, frequency, duration, and type of exercise have previously shown to affect the magnitude of the BP reduction in hypertensive animals and humans 
. Therefore, future studies are still warranted to determine which is the best exercise training intensity or frequency to completely normalize BP. Nevertheless, the results of the present study suggest that regular exercise delays the progression of hypertension. This finding is significant from a clinical perspective, because evidence suggest that a reduction of BP by only 5 mmHg can significantly reduces the risk of stroke, heart failure, and mortality from cardiovascular diseases 
. Interestingly, 2 weeks of detraining preceded by 4 weeks of exercise in AngII-induced hypertensive rats was found to be insufficient to abolish exercise-induced attenuation in MAP as indicated by no significant difference in MAP between AngII+Ex and AngII+Det rats. In accordance with these findings, previous reports have demonstrated that 10 weeks of exercise attenuated BP in spontaneously hypertensive rats (SHRs) and 1 or 2 weeks of detraining did not affect attenuated BP in these rats 
. It is noteworthy that previous studies from our lab and others have used tail-cuff method for BP measurements and most of those studies reported BP as measured only before and/or after the study. Whereas, to best of our knowledge, this is the first study that has employed telemetry recording of MAP in conscious sedentary and exercising animals without causing any undue stress on animals. This methodological improvement in the present study not only allowed us to obtain the most accurate measurements but also allowed us to monitor day-to-day changes in BP in relation to exercise and detraining. Nonetheless, the data suggests that although two weeks of detraining may not be long enough to revert MAP back to sedentary values, continuing detraining may lead to complete reversal.
Our echocardiographic data showed that regular moderate-intensity exercise resulted in reduced cardiac hypertrophy and improved diastolic function in hypertensive rats Interestingly, 2 weeks of detraining failed to completely preserve this exercise-induced improvements in cardiac hypertrophy and function as suggested by significant increase in LVPWTd and a not significant but considerable increase in IVSTd and Tei index in AngII+Det when compared to AngII+Ex rats. These results extended the observations of Bocalini et al
, who demonstrated that 2 weeks of detraining was sufficient to reverse LVPWT in healthy female rats 
. However, our study examined in detail cardiac function using M-mode and Doppler echocardiography performed in the same animal at baseline and at the end of the study, thus providing greater insight into the effects of detraining on cardiac function and morphology.
In the present study, the detraining could not fully preserve the cardioprotective effects of exercise; however, it is noteworthy that the 2 weeks of detraining was not sufficient to completely reverse the benefits either. Therefore, it is plausible to suggest that cessation of exercise for more than 2 weeks may lead to complete reversal of the cardioprotection offered by regular exercise. In support of this, it has previously been reported that resting cardiac output is reduced in trained SHRs, and that it returns to sedentary values only after 5 weeks of detraining 
. Additionally, 5 weeks of detraining in these SHRs led to reversal of resting HR and peripheral vascular resistance to pre-training levels 
. Furthermore, Mostarda et al
has also demonstrated that 3 weeks of detraining did not cause reversal of hemodynamic benefits in diabetic animals. Taken together, the current findings along with previous studies clearly suggest that shorter periods of detraining may prove to be insufficient in abolishing the beneficial effects of exercise in hypertension. Continued absence of exercise can certainly have detrimental effects and hence emphasis should be given to regular active life-style to maintain the benefits.
Besides cardiac hypertrophy and diastolic dysfunction, hypertension is characterized by chronic inflammation which is reflected by a two- to threefold increase in circulating levels of several PICs 
. In addition, the past few years of research have implicated brain cytokines, particularly in the PVN of the brain, in the pathogenesis of hypertension as well. It is apparent from these studies that PICs such as TNF-α and IL-1β act as neuromodulators and play a pivotal role in sympathetic regulation of BP 
. For instance, an increased levels of PICs such as TNF-α and IL-1β have been found in the PVN of hypertensive rats 
. Moreover, infusion of IL-1β intracerebroventricularly 
or microinjection into the PVN 
increases sympathetic activity and resting arterial BP in conscious animals. Additionally, anti-inflammatory cytokines (AIC) such as IL-10 have a significant impact on arterial pressure 
. IL-10 is known to exert inhibitory effects on PICs in the peripheral immune system and it also has a similar role in the CNS 
. Overexpression of IL-10 in the brain (particularly within the PVN) ameliorates hypertension and associated organ damage in hypertensive rats 
. We have recently reported that chronic regular exercise of 16 weeks duration decreases PICs and upregulates IL-10 levels in the brain of SHRs 
. In the present study, we found that regular exercise induces similar improvements in PIC and AIC in the PVN of AngII-induced hypertensive rats. Interestingly, 2 weeks of detraining did not abolish the exercise-mediated improvement in TNF-α and IL-1β levels in the PVN. In contrast, detraining reversed the IL-10 levels back to near sedentary values in hypertensive rats. Given that it is not only the PICs but the balance between PIC and AIC that determines the outcome of the disease, there is a possibility that the reduction of IL-10 levels by detraining may ultimately lead to upregulation of PICs, if continued longer than 2 weeks. Nevertheless, our data suggest that the anti-inflammatory defense system of the body is vulnerable and sensitive to detraining. These data also emphasize the importance of regular physical activity in improving the anti-inflammatory status in hypertension.
Research over past several decades has established that PICs contribute to the AngII-induced increase in BP via induction of oxidative stress 
. AngII is a potent activator of NADPH oxidase (NOX), a primary source of reactive oxygen species (ROS), particularly the superoxide anion (O2−
. NOX-derived ROS acts as potent intra- and intercellular second messengers in signaling pathways causing hypertension 
. Of the various isoforms of NOX, the role of NOX2 (gp91phox
) in AngII-induced hypertension is well established 
. Activity and expression of gp91phox
within the cardiovascular regulatory centers of the brain has been shown to be increased in various rat models of hypertension 
. Recent reports also showed that the AngII-induced increase in BP and cardiac damage is attenuated by treatment with NOX inhibitors or Tempol, an O2−
. Given the role of AngII-induced oxidative stress within the brain in hypertension, it is interesting to investigate whether training and detraining has the ability to influence ROS generation within the brain of hypertensive rats. Our data illustrates that regular exercise dramatically downregulated gp91phox
and iNOS levels and significantly improved Cu/ZnSOD levels in hypertensive rats, suggesting attenuated oxidative stress. Although not in the brain, similar increases in SOD expression have previously been shown in heart and thoracic aorta of exercising rats 
. Interestingly, 2 weeks of detraining abolished the effects of exercise on iNOS; whereas, gp91phox
levels remained unchanged in detrained animals when compared with trained hypertensive rats. These changes were associated with complete reversal of exercise-induced improvement in Cu/ZnSOD in detrained animals. Taken together, these results indicate that 2 weeks of detraining abolishes the exercise-induced reduction in oxidative stress within the PVN of hypertensive rats.
Previous studies have investigated the effects of detraining on heart and skeletal muscle of hypertensive and normal rats in relation to insulin sensitivity 
. For instance, 48 hours 
to 1 week 
of detraining was found to reduce GLUT4 gene expression in the skeletal muscle of normotensive rats. In another study, cessation of training for 1 week resulted in reduced levels of GLUT4 in the heart and white fat tissue in both normotensive and hypertensive rats 
. However, to the best of our knowledge, the present study is the first to demonstrate the effects of detraining on inflammatory cytokines and oxidative stress, in particular within the brain of AngII-induced hypertensive animals. Also, the effects of detraining on cardiac morphology and function in hypertension have rarely been studied before.
In summary, this study demonstrated that 2 weeks of detraining could partially revert the exercise-induced improvements in cardiac hypertrophy, cardiac function, anti-inflammatory cytokine (IL-10) and oxidative stress in the PVN of hypertensive rats, although, positive effects in MAP and PICs remained unchanged. These results indicate that although 2 weeks of detraining is not long enough to completely abolish the beneficial effects of regular exercise, continuing cessation of exercise may lead to detrimental effects.
Given that exercise is recommended as a current guideline for the treatment of hypertension and non-compliance with the recommended treatment is a universal phenomenon, it is imperative to understand the cardiac and molecular changes associated with detraining. A few previous studies have examined the effects of detraining on heart and skeletal muscle of hypertensive and normal rats in relation to insulin sensitivity 
.The results of the current study provides a greater insight in to how detraining can influence the mean arterial blood pressure, cardiac function, inflammatory cytokines, and redox status within the brain of hypertensive rats. Investigating the effects of exercise and detraining on other components of the AngII-induced signaling pathway such as downstream transcription factors and sympathetic activity could certainly be important perspectives of this study ().
A schematic depicting the proposed pathways of effects of exercise training and detraining on AngII-induced hypertensive response.
We have previously demonstrated that the beneficial effects of exercise in hypertension are mediated by reduced PICs, improved cellular redox homeostasis, and downregulation of NFκB activity (). However, one can raise the possibility for role of RAS in exercise-induced beneficial effects as well. For instance, recent reports from our laboratory as well as others have showed that chronic exercise decreases circulating AngII and modulates vasoconstrictor and vasodilatory components of the RAS within the brain of spontaneously hypertensive rats 
and heart failure rabbits 
. Although we have not measured AngII levels within the PVN, the beneficial effects of exercise in AngII-induced hypertensive rats presently observed cannot be completely explained by a decrease in brain AngII because AngII was continuously infused in these rats through subcutaneously implanted minipumps. Nonetheless, it is becoming clear from all these studies that both AngII-dependent and –independent mechanisms of beneficial effects of exercise are taking place. However, further studies are still warranted to achieve deeper understanding of molecular mechanism involved in exercise-induced effects and how detraining modulates them. The understanding of the underlying molecular mechanisms and the time taken for each signaling pathway to lose adaptation induced by regular exercise will lead us to improve the current guidelines for the treatment of hypertension on the basis of scientific evidence.