Nitrite mediated vasorelaxation in human chorionic plate vessels is enhanced by hypoxia and dependent on the NO-sGC-cGMP pathway.

Adequate perfusion of the placental vasculature is essential to meet the metabolic demands of fetal growth and development. Lacking neural control, local tissue metabolites, circulating and physical factors contribute significantly to blood flow regulation. Nitric oxide (NO) is a key regulator of fetoplacental vascular tone. Nitrite, previously considered an inert end-product of NO oxidation, has been shown to provide an important source of NO. Reduction of nitrite to NO may be particularly relevant in tissue when the oxygen-dependent NO synthase (NOS) activity is compromised, e.g. in hypoxia. The contribution of this pathway in the placenta is currently unknown. We hypothesised that nitrite vasodilates human placental blood vessels, with enhanced efficacy under hypoxia. Placentas were collected from uncomplicated pregnancies and the vasorelaxant effect of nitrite (10-6 -5x10-3  M) was assessed using wire myography on isolated pre-constricted chorionic plate arteries (CPAs) and veins (CPVs) under normoxic (pO2 ∼5%) and hypoxic (pO2 ∼1%) conditions. The dependency on the NO-sGC-cGMP pathway and known nitrite reductase (NiR) activities was also investigated. Nitrite caused concentration-dependent vasorelaxation in both arteries and veins, and this effect was enhanced by hypoxia, significantly in CPVs (P < 0.01) and with a trend in CPAs (P = 0.054). Pre-incubation with NO scavengers (cPTIO and oxyhemoglobin) attenuated (P < 0.01 and P < 0.0001, respectively), and the sGC inhibitor ODQ completely abolished nitrite-mediated vasorelaxation, confirming the involvement of NO and sGC. Inhibition of potential NiR enzymes xanthine oxidoreductase, mitochondrial aldehyde dehydrogenase and mitochondrial bc1 complex did not attenuate vasorelaxation. This data suggests that nitrite may provide an important reservoir of NO bioactivity within the placenta to enhance blood flow when fetoplacental oxygenation is impaired, as occurring in pregnancy diseases such as pre-eclampsia and fetal growth restriction.