Physiological role for nitrate-reducing oral bacteria in blood pressure control

Abstract

Circulating nitrate (NO(3)(-)), derived from dietary sources or endogenous nitric oxide production, is extracted from blood by the salivary glands, accumulates in saliva, and is then reduced to nitrite (NO(2)(-)) by the oral microflora. This process has historically been viewed as harmful, because nitrite can promote formation of potentially carcinogenic N-nitrosamines. More recent research, however, suggests that nitrite can also serve as a precursor for systemic generation of vasodilatory nitric oxide, and exogenous administration of nitrate reduces blood pressure in humans. However, whether oral nitrate-reducing bacteria participate in "setting" blood pressure is unknown. We investigated whether suppression of the oral microflora affects systemic nitrite levels and hence blood pressure in healthy individuals. We measured blood pressure (clinic, home, and 24-h ambulatory) in 19 healthy volunteers during an initial 7-day control period followed by a 7-day treatment period with a chlorhexidine-based antiseptic mouthwash. Oral nitrate-reducing capacity and nitrite levels were measured after each study period. Antiseptic mouthwash treatment reduced oral nitrite production by 90% (p < 0.001) and plasma nitrite levels by 25% (p = 0.001) compared to the control period. Systolic and diastolic blood pressure increased by 2-3 .5mmHg, increases correlated to a decrease in circulating nitrite concentrations (r(2) = 0.56, p = 0.002). The blood pressure effect appeared within 1 day of disruption of the oral microflora and was sustained during the 7-day mouthwash intervention. These results suggest that the recycling of endogenous nitrate by oral bacteria plays an important role in determination of plasma nitrite levels and thereby in the physiological control of blood pressure.

Fig. 1

Effects of a 7-day twice-daily use of antiseptic mouthwash on (A) salivary nitrite concentration, (B) plasma nitrite concentration, (C) urinary nitrite concentration, (D) clinic systolic blood pressure, and (E) the relationship between plasma nitrite concentration and clinic systolic blood pressure. Statistical significance was determined using paired Student t test for n = 19. Correlations were determined using Pearson’s correlation coefficient determination.

Fig. 2

Effects of a 7-day, twice-daily use of antiseptic mouthwash on (A) salivary nitrate concentration, (B) plasma nitrate concentration, and (C) urinary nitrate concentration in healthy volunteers. Statistical significance was determined using paired Student t test for n = 19.

Fig. 3

Effects of a 7-day, twice-daily use of antiseptic mouthwash on oral nitrate-reducing capacity. The amount of nitrite in the expelled contents of the oral cavity, after instillation of 10 ml of 0, 800 μmol/L, or 8 mmol/L KNO₃ solution delivering 0, 0.8, or 80 μmol of nitrate, respectively, which was held in the mouth for 5 min, of healthy volunteers was measured. Data are expressed as means ± SEM (n = 19) and statistical significance was determined using repeated-measures two-way ANOVA followed by Bonferroni posttests.

Fig. 4

Effects of a 7-day, twice-daily use of antiseptic mouthwash on systolic and diastolic blood pressure in healthy volunteers measured (A) in the clinic, (B) by 24-h ambulatory monitoring, and (C) at home and the daily profiles of home (D) systolic and (E) diastolic blood pressure during the 7-day study periods in healthy volunteers. Data are expressed as means ± SEM (n = 19) and statistical significance was determined using repeated-measures two-way ANOVA.

Fig. 5

Effects of a 7-day, twice-daily use of antiseptic mouthwash on 24-h ambulatory blood pressure, separated into daytime (A) systolic and (B) diastolic blood pressure and nighttime (C) systolic and (D) diastolic blood pressure. Data are expressed as means ± SEM (n = 19) and statistical significance was determined using paired Student t test.