Original Contribution
Active secretion and protective effect of salivary nitrate against stress in human volunteers and rats

https://doi.org/10.1016/j.freeradbiomed.2012.12.015Get rights and content

Abstract

Up to 25% of the circulating nitrate in blood is actively taken up, concentrated, and secreted into saliva by the salivary glands. Salivary nitrate can be reduced to nitrite by the commensal bacteria in the oral cavity or stomach and then further converted to nitric oxide (NO) in vivo, which may play a role in gastric protection. However, whether salivary nitrate is actively secreted in human beings has not yet been determined. This study was designed to determine whether salivary nitrate is actively secreted in human beings as an acute stress response and what role salivary nitrate plays in stress-induced gastric injury. To observe salivary nitrate function under stress conditions, alteration of salivary nitrate and nitrite was analyzed among 22 healthy volunteers before and after a strong stress activity, jumping down from a platform at the height of 68 m. A series of stress indexes was analyzed to monitor the stress situation. We found that both the concentration and the total amount of nitrate in mixed saliva were significantly increased in the human volunteers immediately after the jump, with an additional increase 1 h later (p<0.01). Saliva nitrite reached a maximum immediately after the jump and was maintained 1 h later. To study the biological functions of salivary nitrate and nitrite in stress protection, we further carried out a water-immersion-restraint stress (WIRS) assay in male adult rats with bilateral parotid and submandibular duct ligature (BPSDL). Intragastric nitrate, nitrite, and NO; gastric mucosal blood flow; and gastric ulcer index (UI) were monitored and nitrate was administrated in drinking water to compensate for nitrate secretion in BPSDL animals. Significantly decreased levels of intragastric nitrate, nitrite, and NO and gastric mucosal blood flow were measured in BPSDL rats during the WIRS assay compared to sham control rats (p<0.05). Recovery was observed in the BPSDL rats upon nitrate administration. The WIRS-induced UI was significantly higher in the BPSDL animals compared to controls, and nitrate administration rescued the WIRS-induced gastric injury in BPSDL rats. In conclusion, this study suggests that stress promotes salivary nitrate secretion and nitrite formation, which may play important roles in gastric protection against stress-induced injury via the nitrate-dependent NO pathway.

Highlights

► Active secretion of salivary nitrate is one of the human adaptive responses to stress. ► Salivary nitrate and its derivates can prevent stress-induced gastric ulcer. ► Salivary nitrate and its derivates restore blood flow in the stomach mucosa. ► The nitrate–nitrite–NO pathway may contribute to stress-injury protection.

Section snippets

Human bungee jump protocol

Twenty-two healthy volunteers naive to bungee jumping or skydiving (14 males and 8 females, age range 21–45 years, mean age 27 years) were involved in the human stress trial. All volunteers had been living in Beijing, China, for more than 1 year and shared similar dietary habits. The trial was reviewed and approved by the ethics committees of Capital Medical University of China. Written informed consent was obtained from all subjects.

All subjects participated in a bungee jump from a height of 68

Primary physical indexes significantly changed after acute stress

To confirm the stress status of each subject, primary physical indexes, including breath, pulse, systolic pressure, diastolic pressure, plasma adrenalin, and plasma noradrenalin were determined 3 h before and immediately after the jump. The average frequency of breaths increased from 15±4 to 23±6 per minute (p<0.001) and the average pulse rate increased from 67±8 to 114±20 per minute (p<0.001, Fig. 1A and 1B). Blood pressure increased in a similar fashion (p<0.001, Figs. 1C and 1D). The average

Discussion

In this study, we found that the levels of salivary nitrate and nitrite were immediately increased in human volunteers after a strong acute stress caused by high-platform jumping. To explore the biological function of salivary nitrate and nitrite, we carried out a rat WIRS assay. We found that blocking nitrate secretion from the salivary glands results in a significant decrease in gastric mucosal blood flow and increased gastric ulcers in rats treated by BPSDL, which leads to >60% decreases in

Acknowledgments

This study was supported by grants from the National Natural Science Foundation of China (30430690, 30125042, and 81170975) and Beijing Municipal Committee for Science and Technology (Z121100005212004). The authors acknowledge the volunteers who participated in this study.

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    These authors contributed equally to this work.

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