Hypertension is a serious medical condition that decreases the quality of life and longevity due to increased in the risks of coronary artery disease, heart failure, stroke, chronic kidney disease, stroke, and peripheral vascular disease (Jardim, 2018). Hypertension can also lead to cognitive decline and dementia, which can start in middle age and progress in an elderly population (de Menezes et al., 2021). Approximately 1.28 billion people worldwide have hypertension, and the majority (two-thirds) live in low- and middle-income countries (WHO, 2023). Essential hypertension has been associated with hyperactivity of the sympathetic nervous system, a nonspecific response to norepinephrine, or abnormalities in cellular cationic metabolism (Boer et al., 2005, Walther et al., 2022).

Previous studies have shown that androgenic hormones in men play an important role in regulating blood pressure, and both low and high testosterone levels were associated with greater hypertension risk. There is a positive correlation between the increase in endogenous testosterone during puberty and the prevalence of hypertension (Geevar et al., 2022, Gerdts et al., 2022, Nsanya et al., 2019, Shen et al., 2017). In contrast, the reduction in plasma concentration due to aging was associated with an increased prevalence of hypertension in older men (Fogari et al., 2005, Qu et al., 2021, Yang et al., 2019). In addition, an increase in blood pressure has also been observed in men using exogenous anabolic-androgenic steroids for hypogonadism (Chasland et al., 2021, Tangredi and Buxton, 2001) or as performance-enhancing drugs, in athletes (Gheshlaghi et al., 2015, Urhausen et al., 2004). Moreover, the influence of testosterone on blood pressure is well documented in animal models, like the spontaneously hypertensive rat (SHR), which was developed as a genetic model of arterial hypertension and is similar in many respects to primary or essential arterial hypertension in men (Okamoto & Aoki, 1963). These animals are born normotensive with a mean blood pressure of 112 mmHg and spontaneously develop an increase in blood pressure from the 8th week after birth, reaching values close to 200 mmHg at 12 months old. In SHR, higher plasma concentrations of testosterone appear to contribute to the exacerbation of blood pressure through reduced natriuresis (Khraibi and Knox, 1989, Reckelhoff et al., 1999) and sympathetic hyperactivity via norepinephrine synthesis by tyrosine hydroxylase (Kumai et al., 1994, Sharma et al., 2002). The reduction of plasma testosterone through orchiectomy is effective in attenuating arterial hypertension in adults (Reckelhoff et al., 1998) and young SHR (Loh et al., 2017).

The reduction in plasma testosterone concentration induced by orchiectomy in normotensive rats caused a significant reduction of ducts and acini in the parotid glands (Dos Santos et al., 2021, Jezek et al., 1996), while in the submandibular glands, there was an increase in acini and reduction of ducts (Dos Santos et al., 2021), in particular the granular convoluted tubule (Dos Santos et al., 2021, Rins de David et al., 1991). In Wistar rats, the castration increased salivary flow rate, calcium, phosphate, and chloride, and decreased total protein and amylase, while not changing the salivary buffer capacity, pH, sodium, and potassium compared to SHAM (Cypriano et al., 2021, Dos Santos et al., 2021). Castration-induced salivary gland dysfunction has been associated with redox state disturbs characterized by increased lipid peroxidation, protein carbonyl, and total antioxidant capacity in parotid and submandibular glands (Dos Santos et al., 2021). These same redox state markers were also higher in the saliva of orchiectomized rats (Cypriano et al., 2021).

Until now, human clinical studies do not provide strog evidences to directly associate hypertension per se with impaired salivary gland function, since most of the induviduals in those studies were also treated with antihypertensive (Dodds et al., 2000, Mohiti et al., 2020, Nimma et al., 2016, Sankar et al., 2002). Therefore, the SHR can be extremely relevant model to analyze the effects of the hypertensive phenotype on salivary flow rate and composition during different stages of growth and maturation (Elias et al., 2006, Picco et al., 2012, Picco et al., 2014). Although 4-week-old SHR demonstrated similar values of salivary amylase activity, the salivary flow rate and total protein concentration were lower when compared to normotensive age-matched Wistar rats (Elias et al., 2006). At 12 weeks old, although salivary flow remained lower in SHR compared to Wistar rats, total protein concentration and amylase activity were similar between strains, while salivary calcium concentration was higher in SHR (Picco et al., 2014). SHR also showed higher pH values and salivary buffering capacity than the 12-week-old Wistar strain (Picco et al., 2012). The reduction in salivary flow in SHR is associated with a dramatic reduction in the expression of the protein aquaporin-5 in the submandibular glands, an important protein involved in the transepithelial transport of water from the interstitium to the acinar lumen (Zhang et al., 2017), and also increased expression of sodium-glucose cotransporter 1 in the luminal membrane of salivary gland duct cells, which was related to the sympathetic activity (Sabino-Silva et al., 2013), while several factors might contribute to the changes in salivary flow in SHR, it is still unknown whether plasma testosterone levels could exert any influence on salivary gland function in hypertensive rats.

Saliva plays an important role in oral homeostasis, creating a favorable environment for cleaning mucosa and teeth, as they present salivary proteins and enzymes with a moisturizing function, facilitating swallowing. Saliva also regulates pH, protects teeth from cavities, and is responsible for the first digestion of starch. Furthermore, saliva has an effective antioxidant capacity, being the first line of gastrointestinal defense against reactive oxygen species (Zalewska et al., 2014). In middle-aged men, there is an inverse association between salivary secretory levels and testosterone and 8-hydroxy-2-deoxyguanosine, an indicator of oxidative DNA damage, which is helpful as a predictor of severe erectile dysfunction and hypogonadism (Yasuda et al., 2008). In turn, in normotensive Wistar rats, an increase in salivary lipid and protein oxidative damage markers was found two months after orchiectomy (Cypriano et al., 2021).

In this research study, we tested the hypothesis that the reduction in plasma testosterone concentration induced by orchiectomy impairs the salivary flow rate, biochemical composition, and redox status in SHR. Therefore, this research analyze the salivary flow rate, pH, buffer capacity, amylase, total protein, electrolytes, and saliva redox status from bilaterally orchiectomized SHR.