NADPH Oxidase in the Renal Microvasculature Is a Primary Target for Blood Pressure–Lowering Effects by Inorganic Nitrate and Nitrite

Abstract

Abstract-Renal oxidative stress and nitric oxide (NO) deficiency are key events in hypertension. Stimulation of a nitrate-nitrite-NO pathway with dietary nitrate reduces blood pressure, but the mechanisms or target organ are not clear. We investigated the hypothesis that inorganic nitrate and nitrite attenuate reactivity of renal microcirculation and blood pressure responses to angiotensin II (ANG II) by modulating nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and NO bioavailability. Nitrite in the physiological range (10 −7-10 −5 mol/L) dilated isolated perfused renal afferent arterioles, which were associated with increased NO. Contractions to ANG II (34%) and simultaneous NO synthase inhibition (56%) were attenuated by nitrite (18% and 26%). In a model of oxidative stress (superoxide dismutase-1 knockouts), abnormal ANG II-mediated arteriolar contractions (90%) were normalized by nitrite (44%). Mechanistically, effects of nitrite were abolished by NO scavenger and xanthine oxidase inhibitor, but only partially attenuated by inhibiting soluble guanylyl cyclase. Inhibition of NADPH oxidase with apocynin attenuated ANG II-induced contractility (35%) similar to that of nitrite. In the presence of nitrite, no further effect of apocynin was observed, suggesting NADPH oxidase as a possible target. In preglomerular vascular smooth muscle cells and kidney cortex, nitrite reduced both basal and ANG II-induced NADPH oxidase activity. These effects of nitrite were also abolished by xanthine oxidase inhibition. Moreover, supplementation with dietary nitrate (10 −2 mol/L) reduced renal NADPH oxidase activity and attenuated ANG II-mediated arteriolar contractions and hypertension (99±2-146±2 mm Hg) compared with placebo (100±3-168±3 mm Hg). In conclusion, these novel findings position NADPH oxidase in the renal microvasculature as a prime target for blood pressure-lowering effects of inorganic nitrate and nitrite. Research during the past decade has revealed that the endogenous anions nitrate and nitrite are used to generate NO and that stimulation of this nitrate-nitrite-NO pathway can compensate for disturbances in NO generation from NO synthases (NOS). [14][15][16][17] This has interesting nutritional implications because nitrate is also a normal constituent of our daily diet, with vegetables being the main source. Oral commensal bacteria seem obligatory for the reduction of nitrate to nitrite, whereas several mammalian enzymes are capable of subsequent reduction of nitrite to NO and other bioactive nitrogen oxides. [15][16][17][18] Recent clinical and experimental studies have independently shown that supplementation with inorganic nitrate, by dietary means or with nitrate salt, reduces blood pressure in healthy normotensive individuals [19][20][21] as well as in hypertensives. 22 In addition, these anions can partly compensate for metabolic disturbances in aged endothelial NOS knockout mice, 23 reduce hypertension and markers of oxidative stress, and ameliorate organ injuries in models of renal a...

Blood Pressure–Lowering Effect of Orally Ingested Nitrite Is Abolished by a Proton Pump Inhibitor

“…The absence of somatic motor reflexes in response to tail pitching or blinking in response to a lowpressure corneal stimulation indicated deep anesthesia, and at least 15 minutes of stabilization was allowed before drug infusions. Rats were 42 Arterial rings (2 mm) were mounted on 25-µm tungsten wires in a small-vessel myograph (Model 620M; Danish Myo Technology, Denmark) for recording isometric force by transducers (PowerLab 4/30; AD Instruments). The chambers were filled with Krebs solution (37 °C, pH 7.4) aerated with carbogen (95% O 2 ; 5% CO 2 ).…”

“…Resting tension of the arteries was set as described previously. 42,43 After stabilization and washout protocols, 1 µmol/L norepinephrine was used to preconstrict mesenteric resistance vessels to obtain a basal tone of ≈50% of its maximal diameter. When the constriction reached a steady-state plateau, sodium nitrite (10 …”

Oxidative Stress, Inflammation, and Vascular Aging in Hypertension

“…They showed that inorganic nitrate and nitrite attenuate reactivity of the renal microcirculation and BP responses to angiotensin II by modulating Nox activity and NO bioavailability. 37 In a model of oxidative stress (SOD1 −/− ), abnormal angiotensin II-mediated arteriolar contractions were normalized by nitrite with Nox oxidases as a possible target.…”

Nitrate, Nitrite, and Nitric Oxide Find a Home in the Kidney by Offsetting Angiotensin II–Mediated Hypertension

“…9 The authors test the hypothesis that stimulation of a nitrate-nitrite-NO pathway, with inorganic nitrate or nitrite, limits afferent arteriolar responsiveness to Ang II and markedly attenuates Ang II-induced hypertension by inhibiting superoxide production via nitric oxide synthase-independent NO generation ( Figure). The authors show convincingly that Ang II-induced constriction of isolated preglomerular afferent arteriolar ex vivo preparations can be blunted by inorganic nitrite at physiological concentrations as well as through dietary nitrate supplementation.…”

“…9 Future studies will undoubtedly be aimed at translating these findings to human patients. Interestingly, one cannot but wonder whether xanthine oxidase inhibitors (eg, allopurinol, febuxostat) currently prescribed to treat a variety of uric acid-based disorders (eg, gout) may inadvertently limit nitritederived NO and thereby elevate blood pressure in such individuals.…”