Effect of Physical Training on Nitric Oxide Levels in Patients with Arterial Hypertension: An Integrative Review

The regular practice of physical exercise as a non-pharmacological treatment of arterial hypertension (AH) has been encouraged due to causing a series of physiological responses in the cardiovascular system, such as the production of vasoactive substances, including nitric oxide (NO). NO is a relaxation factor released by the endothelium, and the decrease in its bioavailability is related to coronary and arterial diseases, such as AH. This study aimed to perform an integrative literature review to elucidate the effect of physical training on NO levels in patients with AH and to establish a relationship between these levels and blood pressure (BP) control. A literature review was was performed by searching PubMed / MEDLINE, Lilacs, Scielo, Cinahl and Embase databases. The search string used was ("arterial hypertension" OR hypertension) AND (exercise OR "physical exercise" OR "aerobic exercise" OR "exercise training" or "physical activity") AND ("nitric oxide"). We included fully available controlled and uncontrolled clinical trials published in English and Portuguese languages in the last 10 years. The review consisted of 16 articles, of which 13 reported an increase in NO production after the physical training intervention, and three studies found no change. In addition, 15 studies observed a reduction in BP after the intervention. In conclusion, regular practice of physical exercises, advocating moderate intensity, can improve NO bioavailability in pre-hypertensive and hypertensive individuals, which seems to be one of the mechanisms responsible for BP reduction.


Introduction
Arterial hypertension (AH) is characterized as a multifactorial clinical condition and considered one of the main risk factors for cardiovascular morbidity and mortality. In addition to a sustained elevation in blood pressure (BP), AH is also associated with metabolic disorders and functional and structural changes in target organs, which can be aggravated by the presence of other risk factors and is responsible for several other complications. 1 The practice of physical exercise as a nonpharmacological therapeutic approach to AH has been increasingly encouraged by health professionals, as it causes many physiological responses in body systems, especially in the cardiovascular system. 2 Physical training, when performed regularly, causes important autonomic and hemodynamic adaptations, as well as humoral changes related to the production of vasoactive substances, such as nitric oxide (NO). 3 These changes are responsible for the reduction or even normalization of the BP levels in patients with mild to moderate hypertension, using or not using medications. 1,4,5 NO, a relaxation factor released by the endothelium, 6 is a gaseous mediator responsible for a variety of physiological phenomena, 7 and a decrease in its enzyme nitric oxide synthase (NOS), or BP levels of prehypertensive or hypertensive individuals. Studies on acute physical training only and those that included individuals with pulmonary hypertension were excluded.
Two independent researchers participated in the four steps of the review: literature search; duplicate analysis; reading of titles and abstracts; and full reading of each article. In the 3rd and 4th steps, each researcher classified the articles in a binary way, with zero (0) for articles that did not meet the inclusion criteria or had any of the exclusion criteria and one (1) for articles that fulfilled the inclusion criteria. Articles that scored one (1) from both researchers carried on to the next step, and those articles that were already at the fourth step were included immediately. Articles that scored zero from both researchers were immediately excluded. Articles that were assigned zero from one researcher and one (1) from the other researcher were evaluated by a third reviewer to ultimately determine if the article would be included (or moved to the next step) or not. Figure 1 presents the flow diagram of identification and selection of the articles included in this review, according to the PRISMA flow diagram. 15 A total of 16 articles were included, with the main results described in Table 1.

Intervention
Aerobic exercise was the most common intervention, identified in 11 of the 16 articles included, 16,18,19,21,[25][26][27][28][29][30][31] three of which addressed high-intensity aerobic training. 16,18,19 In addition, two studies performed resistance exercise, 20,28 one of which combined aerobic and resistance exercises. 28 Four studies addressed training with different techniques bioavailability is related to coronary and arterial diseases, among others. In systemic AH, the increase in oxidative stress and endothelial dysfunction promotes a reduction in the bioavailability of NO and its action on the vascular wall, affecting vascular relaxation. 8,9 In this sense, moderate physical exercise can be an effective non-pharmacological medicated means to increase NO bioavailability and, hence, mediate positive adjustments in the tissues. The main functions of NO in the cardiovascular system include regulation of vascular tone by the vasodilating action on smooth muscle cells; inhibition of platelet activity; leukocyte aggregation; and proliferation of smooth muscle cells in the vascular endothelium, 10 which altogether contribute to BP control and prevention or control of cardiovascular diseases. 3,11,12 Therefore, for an effective clinical application of physical training in the management of hypertensive individuals, it is necessary to know the effect of different physical exercises on NO and BP levels. In this regard, defining the study population and clarifying issues related to exercise -type (aerobic or anaerobic), intensity and training duration 3,12-14 is crucial to guide the therapeutic approach by health professionals. Therefore, this study aimed to conduct a literature review to elucidate the effect of physical training on NO levels in patients with AH and to establish a relationship between NO levels and BP control in this population.

Methods
An integrative review was performed by searching PubMed / MEDLINE, Lilacs, Scielo, Cinahl and Embase databases, using terms indexed in the DeCS -Health Sciences Descriptors -which was developed from the Medical Subject Headings of the US National Library of Medicine, to allow the use of common terminology in Portuguese, English and Spanish. The search string used in all databases was ("arterial hypertension" OR hypertension) AND (exercise OR "physical exercise" OR "aerobic exercise" OR "exercise training" or "physical activity") AND "nitric oxide".
The search was conducted between October 2019 and April 2020, covering studies published in the last 10 years, i.e., from October 2009 until the present moment. Fully available controlled and uncontrolled clinical trials published in English and Portuguese languages were included in the review. We selected articles that evaluated the effect of physical training on blood / urinary concentrations of NO or its metabolites, activity of the such as vibrating platform, 22 yoga, 23 Tai Chi practice 24 and mat Pilates. 17 In the studies that used aerobic physical training, different parameters and percentages were used to measure training intensity, ranging from 60% to 100% of maximum heart rate (HR max ), 23,25,27,30 30% to 100% of maximum oxygen consumption (VO 2max ), 18,24,27,29 50% to 90% of HR max reserve 19,28,31 and between 11 and 13 points on Borg's rating scale of perceived exertion. 21 Duration of exercise training varied between six 18

Outcomes
Regarding the effect of physical exercise on NO, 13 out of the 16 studies analyzed reported an increase in NO or NOS production after the intervention. [16][17][18][19][20][21][22][23][24][25][26]29,31 In these cases, exercise modality, time and intensity was not homogeneous throughout the studies. The other three studies found no change. 27,28,30 With respect to BP, only one article carried out with African American individuals did not report a reduction in this variable after the intervention, most likely because the parameters of normotensive and hypertensive individuals were analyzed together. Despite this, there   -MICT: 5 min warm-up at 40% of the reserve heart rate (RHR) followed by 35 min of continuous jogging at 60% of the RHR.
-HIIT: 5 min warm-up at 40% of the reserve heart rate (RHR) and 5 min warm-up at 60% of continuous jogging at 60% of the RHR, followed by five 3 min breaks at 80% of the RHR with a 3 min active recovery at 40% of the RHR between each break.

Significant increase in plasma
NOx levels for HIITG only. Significant reduction in SBP and DBP for both groups.
HIIT had greater effect than MICT in reducing resting HR, in dilatation mediated by flow, and in the epicardial fat thickness.
Significant increase in NOS activity and l-arginine transport in platelets and levels of intra-platelet cGMP and significant reduction in SBP and DBP in EG. was an increase in plasma NO levels and an improvement in vascular structure and function after training. 25 On the other hand, three studies showed a decrease in BP, but unrelated to changes in NO or NOS production. 27,28,30 In these studies, the hypotensive effect was associated with an improvement in the balance between vasodilator and vasoconstrictor factors, with changes in prostanoids levels, 27,28 increased hydrogen sulfide-producing enzyme (cystathionine gamma-lyase) and reduced thromboxane, 28 or with decreased levels of total cholesterol and LDL. 30

Discussion
In the present review it was verified that physical training was able to increase NO production and reduce BP in hypertensive and prehypertensive individuals. Most studies used an exercise intensity ranging from 60% to 100% of HR max , 50% to 100% of VO 2max , 30% to 90% of HR max reserve, and between 11 and 13 points on the scale of perceived exertion (Borg). Based on analysis of the relationship between these parameters, we can verify that exercises of intensities of 60-79% of HR max , 50-74% of VO 2max or reserve of HR max and Borg of 12-13 are considered of moderate intensity. 32,33 Based on the literature, approximately 75% of hypertensive individuals when submitted to physical training, mainly of moderate intensity, have reduced BP levels. 34 The practice of physical exercise may be responsible for promoting several adaptations, such as attenuation of vascular and cardiac sympathetic activity, decrease in serum levels of vasoconstrictor factors and increase in endothelial dilating factors, resulting in a reduction of peripheral vascular resistance. 35,36 The time, frequency and duration of training are also important factors to be considered. Despite the great discrepancy between the training protocols of the selected studies, ranging from 20 to 60 min per session, three to four days per week, and from six to 24 weeks, this did not affect the results on NO concentrations. In this context, the Brazilian Society of Cardiology (Sociedade Brasileira de Cardiologia) recommends that individuals diagnosed with AH initiate regular exercise programs, three to five times a week, in sessions of at least 30 min, with ideal duration between 40 and 50 minutes. 1 Furthermore, aerobic exercises are preferred, of light to moderate intensity, between 60% and 80% of HR max , or between 50% and 70% of VO 2max , and complemented by resistance exercises. 1,4,37 Dynamic or isotonic resistance training should be performed with caution, since there are still few randomized and controlled studies with this type of exercise in AH, and its isolated effect on resting BP is not yet well established. 38,39 In this case, it is recommended an overload of up to 50-60% of one-repetition maximum (1RM) from two to three times a week, one to three series, 8 to 15 repetitions up to moderate fatigue, and passive breaks of 90 to 120 seconds. 1 In this sense, the study by Tomeleri et al.,20 evaluated the effect of resisted exercise -series of 10 to 15 repetitions according to 1RM, twice a week -in pre-and hypertensive women. Although they did not specify the length of breaks and the percentage of RM, the parameters used in this study were consistent with the recommendations of the Hypertension Guideline 1 and indicated an increase in plasma NO levels with resistance training.
Three articles included in the present review showed improvement in NO levels and consequent decrease in BP due to increased vascular mechanical stress imposed by high-intensity interval training (HIIT) in hypertensive patients. 16,18,19 HIIT consists of alternating short periods of high-intensity aerobic exercise (85-100% VO 2max ) with active periods of moderate to low intensity exercise. Hence, blood flow varies between high and low intensities, representing a greater challenge to the heart, improving cardiorespiratory fitness. 19 The authors justify that in this type of training, the increase in shear stress induces an increase in the apelin pathway, which is positively correlated to the increase in NO production, generating a vasodilatation with a consequent reduction in BP. 19 Nevertheless, this type of training is still best suited to healthy adult individuals, as described by the Update of the Cardiovascular Prevention Guideline of the Brazilian Society of Cardiology. 40 The shear stress caused by the increased unidirectional blood flow during physical exercise is the main mechanism of improvement of endothelial function. 33,41 This mechanical stress produced by the friction between red blood cells and endothelial cells activates endothelial NOS, increasing the production of NO. NO diffuses into the underlying vascular smooth muscle and activates the enzyme guanylate cyclase. This, in turn, induces the cGMP production that activates the metabolic pathways of cGMP-dependent protein kinase G (PKG), causing vascular relaxation. 42 Thus, shear stress is considered a powerful stimulus for the release of vasodilator factors produced by the vascular endothelium. 41 In addition to its potent vasodilating action, NO can induce other important vascular, renal and cardiac effects, including inhibition of platelet aggregation, modulation of glomerular filtration rate, and an effect on vascular and cardiac remodeling. 43 On the other hand, the endogenous reduction of NO synthesis is related to several Int J Cardiovasc Sci. 2022; 35(2), 253-264 pathophysiological disorders or associated conditions, such as reduction of endothelium-dependent vasodilation in patients with hypertension, hypercholesterolemia, diabetes or arteriosclerosis. 44 Studies have shown that the responses in BP control are related to humoral mechanisms, especially with involvement of NO. In fact, in the studies by Firoenza et al., 18 Pan et al., 24 Nyberg et al., 27 and Hansen et al., 28 it was observed that hypertensive individuals had lower levels of muscle eNOS and plasma NO compared to normotensive individuals. In addition, Pan et al. 24 and Tomeleri et al. 20 demonstrated a negative correlation between NO and BP values. Also, there is evidence that one cause of AH is the presence of products analogous to endothelial L-arginine, which hampers its action on eNOS, resulting in a substantial decrease in NO production. 45 Furthermore, the increase in BP is not only caused by elimination of the vasodilating action of NO, but also by elimination of its influence in central regions of the autonomic cardiovascular control, especially of the sympathetic nervous system. 45 Therefore, characteristics of physical exercise, i.e., its intensity, duration, frequency, and the muscle groups involved (larger or smaller muscle groups), can be determinant in the greater production of NO and in the control of BP in hypertensive patients. 46,47 The increase in NO bioavailability promotes relaxation of smooth muscle cells in the blood vessel wall, leading to an increase in its diameter and a decrease of vascular resistance and systemic BP. 6 Besides, the decrease in sympathetic activity induced by physical exercise also suggests that the increase in NO production promotes a buffering action to the lowfrequency oscillations in BP, acting in opposition to the vascular sympathetic modulation. [48][49][50] In 2018, Pagan et al. 51 published an editorial addressing the role of exercise in endothelial function, with emphasis on NO, and discussed the studies with animal models that obtained improvement of this function associated with increased levels of NO, 52,53 also in hypertensive animals. 54 The authors emphasized the need to establish better training intensity, type, and duration for this objective. In the present integrative review, a diversity of training parameters in humans was found, as also pointed out by Pagan et al. 51 Therefore, among the limitations of this review, we can point out the lack of information and standardization of tests and training protocols, which made it difficult to interpret the effectiveness of exercise intervention on NO bioavailability. Therefore, we concluded that the regular practice of physical exercises in pre-hypertensive and hypertensive individuals can increase the bioavailability of NO and, consequently, cause a hypotensive effect. Thus, we can establish a relationship between NO levels and BP control in hypertensive individuals, that is, the greater the NO production, the lower the BP values. However, it is important to note that the higher bioavailability of NO depends on the type -different by controlled, and of moderate intensity -of physical exercise and the muscle mass involved.

Potential Conflict of Interest
No potential conflict of interest relevant to this article was reported.

Sources of Funding
This study was partially funded by Universidade de Ribeirão Preto.

Study Association
This article is part of the thesis of lato sensu specialization submitted by Tábata P. Facioli, from Universidade de Ribeirão Preto.

Ethics approval and consent to participate
This article does not contain any studies with human participants or animals performed by any of the authors.