Baroreflex is essential for the short-term control of blood pressure (BP) and modulation of sympathetic activity (Malpas et al., 1997; Grassi et al., 1998; Julien, 2008). Convincing evidence from research with both animals and humans have reported a relationship between decreased arterial baroreflex sensitivity, sympathetic overactivity and arterial hypertension (Gao et al., 2002; Nagai et al., 2003; Salgado et al., 2007).

Our laboratory and other research groups have demonstrated that augmented oxidative stress, characterized by downregulation of the antioxidant capacity and/or increased pro-oxidant factors, depresses baroreflex sensitivity, promotes endothelial dysfunction, augments sympathetic activity and may contribute to the development and maintenance of arterial hypertension (Guimaraes et al., 2012; de Queiroz et al., 2015; Cavalcanti et al., 2016). The role of oxidative stress in baroreflex dysfunction is much more complex than a simple reflex dysfunction and needs to be further elucidated.

The two-kidney one-clip (2K1C) model of renovascular hypertensive is characterized by augmented angiotensin II, oxidative stress, inflammation, depressed baroreflex, sympathetic hyperactivity and endothelial dysfunction (Wang et al., 2005). The depressed arterial baroreflex sensitivity has also been reported in patients with renovascular hypertension (Gao et al., 2002).

For these reasons, it is reasonable to suggest that therapies or interventions with the potential to counteract the adverse effects of oxidative stress may be an important strategy for improving sympathetic baroreflex sensitivity and lower BP under hypertensive states (Guimaraes et al., 2012). In fact, recent studies from our research group have shown that an antioxidant therapy improves baroreflex sensitivity and BP control in hypertensive rats (Botelho-Ono et al., 2011; Guimaraes et al., 2012; Monteiro et al., 2012; Queiroz et al., 2012; Mendes-Junior et al., 2013). A recent meta-analysis suggested that baroreflex activation therapy could exert beneficial effects on BP in resistant hypertension. However, further experimental and clinical data are needed to confirm the application of baroreflex activation therapy under hypertensive conditions (Wallbach and Koziolek, 2017). Thus, we sought to identify antioxidant compounds that could be used as a potential anti-hypertensive strategy, mainly by modulating arterial baroreceptors.

Carboxymethyl-glucan (CM-G) is a water-soluble derivative of Saccharomyces spp. cell wall β(1-3)(1-6) glucan that appears to be translocated from the gastrointestinal (GI) tract into the systemic circulation by epithelial cells in the GI tract and their absorption kinetics differs among the different types of β-glucan (Vetvicka et al., 2015). Well-known by its antioxidant properties, CM-G reduces malondialdehyde levels in healthy men and it has an important role in the protection of biological membranes, probably by its main accepted mechanism that involves the capacity of CM-G to scavenge reactive oxygen species (Babincova et al., 2002; Araujo et al., 2015). Despite the free radical scavenging activity of CM-G, whether an oral treatment with CM-G could improve baroreflex sensitivity and ameliorate arterial hypertension in renovascular hypertensive rats remains unknown. Therefore, we assessed the ability of CM-G treatment to reduce arterial hypertension and restore baroreflex sensitivity in 2K1C hypertensive rats.

We have demonstrated that oral CM-G treatment for 2 weeks reduced arterial hypertension and restored depressed baroreflex sensitivity in conscious renovascular hypertensive rats. These benefits seem to be related to a reduction in sympathetic tone.

The pre-clinical and clinical use of CM-G has been proposed as part of a combination therapy for a variety of diseases mainly because of its strong capability to scavenge reactive oxygen species (ROS) (Magnani et al., 2011; Araujo et al., 2015). For example, it has been reported that oral administration of CM-G helps to protect DNA against damage in prostate cancer patients (Magnani et al., 2011) and reduced the malondialdehyde serum levels in healthy individuals (Araujo et al., 2015). In addition, it has been reported that the daily oral treatment with CM-G (20 mg/kg) for eight days reduced interleukin 8 (IL-8), improved vascular response to nitric oxide and exhibited anti-aggregation activity. This suggests that CM-G could have a beneficial effect on the vascular system (Bezerra et al., 2017).

Considering the antioxidant capacity of the CM-G, we may suggest that our treatment with CM-G might have reduced BP in the 2K1C rats, secondary to the capability of scavenging reactive oxygen species. In agreement with other studies that used acute antioxidant treatment for reducing BP in hypertensive rats (Costa et al., 2009; Guimaraes et al., 2012; Mengal et al., 2016), our study showed that CM-G treatment may be a relevant strategy to ameliorate BP in renovascular hypertension. However, we point out that CM-G treatment reduced blood pressure after 2 weeks of treatment, while most studies using antioxidant intervention with similar dose found reduction in BP only after 3 or 4 weeks of treatment (Alves et al., 2015; de Queiroz et al., 2015; Mengal et al., 2016). A recent study examined the antioxidant effects of green tea demonstrated that 1-week administration of the tea reduced blood pressure and sympathoexcitation in hypertensive rats (Garcia et al., 2017). Similarly, daily treatment with quercetin (25 mg/kg, for 1 week) reduced blood pressure in spontaneously hypertensive rats (Monteiro et al., 2012).

One of the key mechanisms in controlling blood pressure in health and disease is the baroreflex (Grassi et al., 1998). Baroreceptors located in the carotid sinuses and aortic arch detect changes in blood pressure and trigger reflex autonomic adjustments that buffer alterations in blood pressure (Malpas et al., 1997; Kanbar et al., 2007; Salgado et al., 2007). In pathological conditions such as hypertension, there is impairment in the autonomic control of blood pressure resulting in changes in baroreflex sensitivity (Gao et al., 2002; Nagai et al., 2003; Martinka et al., 2005).

The 2K1C model increased levels in ANG-II circulation, which might act in specific areas of the brainstem, augmenting oxidative stress and promoting autonomic dysfunction and reduction of baroreflex sensitivity (Heitzer et al., 1999). Our results showed that CM-G treatment improved the baroreflex sensitivity in animals with renovascular hypertension. Considering the well-known efficacy of antioxidant therapies for improving of baroreflex sensitivity in ANG-II-dependent hypertension models (Botelho-Ono et al., 2011; Queiroz et al., 2012; Mendes-Junior et al., 2013; Alves et al., 2015), it is probable that the improvement of baroreflex sensitivity was a result of the antioxidant therapy promoted by the oral CM-G treatment. In fact, previous studies have demonstrated that the antioxidant therapy had no effect on baroreflex function in normotensive animals (Li et al., 1996; Pickering et al., 2008), but improved the baroreflex in hypertensive rats (Guimaraes et al., 2012; Monteiro et al., 2012; Mendes-Junior et al., 2013). This suggests that antioxidant administration in the absence of oxidative stress has no influence on baroreflex sensitivity.

Additionally, CM-G treatment plays an important role in reducing sympathetic tone in rats with renovascular hypertension. Reactive oxygen species (ROS) in the brainstem have a potential role in the modulation of sympathetic activity and BP in hypertensive rats, suggesting that oxidative stress can contribute to higher sympathetic activity and hypertension (Malpas, 2010; Chan and Chan, 2014). Given this, further study is needed to see whether the reduced sympathetic overactivity was a consequence of the re-establishment of baroreflex sensitivity or if CM-G treatment could act as antioxidant therapy directly on the neuronal network involved in sympathetic control (Huber and Schreihofer, 2010). One possible limitation of our study is that the sympathetic modulation of blood pressure was evaluated one hour after the vasoactive drugs protocol. One could argue that using vasoactive drugs to evaluate baroreflex could trigger vasopressin release. However, it is important to note that vasopressin release is far more sensitive to changes in plasma osmolarity than to changes in blood volume. Therefore, repeated doses of the vasoactive drugs would be needed to trigger effective changes in vasopressin release, which was not the case. The use of a single bolus injection of vasoactive drugs in very small volumes/concentrations for the baroreflex sensitivity test allows a considerable margin of safety regarding the intervention of other mechanisms such as osmolality changes and release of vasopressin.

In order to further assess the idea that CM-G treatment reduces arterial hypertension, we performed the spectral analysis of systolic blood pressure. Our findings indicated that CM-G treatment significantly reduces the LF component of the spectral analysis. The LF component is a well-accepted index of sympathetic modulation (Inoue et al., 1991). Therefore, although we were not able to perform direct recordings of the sympathetic activity, which would be the gold standard for sympathetic evaluation, both the hexamethonium and the spectral analysis data suggest that CM-G treatment reduces sympathetic activity in renovascular hypertensive rats.

Further studies investigating the underlying mechanisms involved in CM-G treatment and reduction of arterial hypertension, improvement of the baroreflex sensitivity and the involvement of the sympathetic tone will be needed. Our experience leads to the following suggestions. First, it is important knowing if CM-G alters antioxidant enzymes, such as superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase activities, in the vascular or brainstem level. Second, we need to understand if CM-G can improve other control mechanisms of arterial blood pressure, such as the peripheral chemoreflex or renin-angiotensin-aldosterone system.

In summary, we reported that acute oral CM-G treatment reduced arterial hypertension and restored baroreflex sensitivity via reduction of the sympathetic tone in renovascular hypertensive rats. In fact, the precise site of action where CM-G therapy produces its beneficial effects in order to ameliorate hypertension remains unknown. However, we have identified one more antioxidant compound with baroreceptor modulation properties that could be used in the future as an additional therapy for renovascular hypertension.

AC-G and VB designed the experiments. AC-G and DG performed and analyzed the experiments. AC-G, DG, JB, and VB wrote the manuscript. AC-G, DG, JB, BK, RC-G, JC, MM, and VB reviewed the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.