Renal nitrate clearance in chronic kidney disease

BACKGROUND

Nitric oxide (NO) is rapidly oxidised in humans to nitrite and nitrate, with nitrate being present in much greater abundance. These oxidation products can be recycled back into nitric oxide via a complex entero-salivary pathway, thus preserving NO activity. Approximately 65% of circulating nitrate is excreted in the urine in 48 h, with the excretory pathway of the remainder unknown. The effect of declining renal function on nitrate clearance is unknown METHODS: Forty five subjects, 21 M, 24F, median age 69 (range 27-75 years) with renal function assessed by CKD-EPI eGFR between 9 and 89 ml/min/1.73 m² completed the study. Following a 24 h low nitrate diet a microplate spectrophotometric method was employed to measure plasma nitrate concentration and 24 h urinary nitrate excretion were measured to determine renal nitrate clearance.

RESULTS

There was a strong positive correlation between urinary nitrate clearance and eGFR, (Spearman R = 0.7665, p < 0.0001) with a moderate negative correlation between plasma nitrate concentration and CKD-EPI eGFR, (Spearman's R = -0.37, p = 0.012). There was a trend between fractional excretion of nitrate and CKD-EPI eGFR (ml/min/1.73 m²) Spearman's R 0.27, p = 0.07 though this did not reach statistical significance. Plasma nitrate concentration and serum creatinine concentration were positively correlated, Spearman's R = 0.39, p = 0.008.

CONCLUSIONS

We have observed a strong positive association between renal nitrate clearance and renal function such that plasma nitrate rises as renal function falls. Fractional excretion of nitrate appears to decline as renal function falls. As such, urinary nitrate excretion is unlikely to be a reliable marker of endogenous NO synthesis in settings where renal function is altered.

Influence of inflammation and nitric oxide upon platelet aggregation following deposition of diesel exhaust particles in the airways

Background and Purpose

Exposure to nanoparticulate pollution has been implicated in platelet‐driven thrombotic events such as myocardial infarction. Inflammation and impairment of NO bioavailability have been proposed as potential causative mechanisms. It is unclear, however, whether airways exposure to combustion‐derived nanoparticles such as diesel exhaust particles (DEP) or carbon black (CB) can augment platelet aggregation in vivo and the underlying mechanisms remain undefined. We aimed to investigate the effects of acute lung exposure to DEP and CB on platelet activation and the associated role of inflammation and endothelial‐derived NO.

Experimental Approach

DEP and CB were intratracheally instilled into wild‐type (WT) and eNOS^−/− mice and platelet aggregation was assessed in vivo using an established model of radio‐labelled platelet thromboembolism. The underlying mechanisms were investigated by measuring inflammatory markers, NO metabolites and light transmission aggregometry.

Key Results

Platelet aggregation in vivo was significantly enhanced in WT and eNOS^−/− mice following acute airways exposure to DEP but not CB. CB exposure, but not DEP, was associated with significant increases in pulmonary neutrophils and IL‐6 levels in the bronchoalveolar lavage fluid and plasma of WT mice. Neither DEP nor CB affected plasma nitrate/nitrite concentration and DEP‐induced human platelet aggregation was inhibited by an NO donor.

Conclusions and Implications

Pulmonary exposure to DEP and subsequent platelet activation may contribute to the reports of increased cardiovascular risk, associated with exposure to airborne pollution, independent of its effects on inflammation or NO bioavailability.

Role of inorganic nitrate and nitrite in driving nitric oxide-cGMP-mediated inhibition of platelet aggregation in vitro and in vivo

BACKGROUND

Nitric oxide (NO) is a critical negative regulator of platelets that is implicated in the pathology of thrombotic diseases. Platelets generate NO, but the presence and functional significance of NO synthase (NOS) in platelets is unclear. Inorganic nitrate/nitrite is increasingly being recognized as a source of bioactive NO, although its role in modulating platelets during health and vascular dysfunction is incompletely understood.

METHODS

We investigated the functional significance and upstream sources of NO-cGMP signaling events in platelets by using established methods for assessing in vitro and in vivo platelet aggregation, and assessed the bioconversion of inorganic nitrate to nitrite during deficiency of endothelial NOS (eNOS).

RESULTS

The phosphodiesterase 5 (PDE5) inhibitor sildenafil inhibited human platelet aggregation in vitro. This inhibitory effect was abolished by a guanylyl cyclase inhibitor and NO scavengers, but unaffected by NOS inhibition. Inorganic nitrite drove cGMP-mediated inhibition of human platelet aggregation in vitro and nitrate inhibited platelet function in eNOS(-/-) mice in vivo in a model of thromboembolic radiolabeled platelet aggregation associated with an enhanced plasma nitrite concentration as compared with wild-type mice.

CONCLUSIONS

Platelets generate transient, endogenous cGMP signals downstream of NO that are primarily independent of NOS and may be enhanced by inhibition of PDE5. Furthermore, nitrite can generate transient NO-cGMP signals in platelets. The absence of eNOS leads to enhanced plasma nitrite levels following nitrate administration in vivo, which negatively impacts on platelet function. Our data suggest that inorganic nitrate exerts an antiplatelet effect during eNOS deficiency, and, potentially, that dietary nitrate may reduce platelet hyperactivity during endothelial dysfunction.