Role of nitrite in regulation of fetal cephalic circulation in sheep

A physiologically relevant role for NO stored in vascular smooth muscle cells: A novel theory of vascular NO signaling

S-nitrosothiols (SNO), dinitrosyl iron complexes (DNIC), and nitroglycerine (NTG) dilate vessels via activation of soluble guanylyl cyclase (sGC) in vascular smooth muscle cells. Although these compounds are often considered to be nitric oxide (NO) donors, attempts to ascribe their vasodilatory activity to NO-donating properties have failed. Even more puzzling, many of these compounds have vasodilatory potency comparable to or even greater than that of NO itself, despite low membrane permeability. This raises the question: How do these NO adducts activate cytosolic sGC when their NO moiety is still outside the cell? In this review, we classify these compounds as ‘nitrodilators’, defined by their potent NO-mimetic vasoactivities despite not releasing requisite amounts of free NO. We propose that nitrodilators activate sGC via a preformed nitrodilator-activated NO store (NANOS) found within the vascular smooth muscle cell. We reinterpret vascular NO handling in the framework of this NANOS paradigm, and describe the knowledge gaps and perspectives of this novel model.

Evidence for placental-derived iron-nitrosyls in the circulation of the fetal lamb and against a role for nitrite in mediating the cardiovascular transition at birth

Circulating metabolites of nitric oxide, such as nitrite, iron nitrosyls (FeNO), and nitrosothiols, have vasodilatory bioactivity. In both human and sheep neonates, plasma concentrations of these NO metabolite (NOx) concentrations fall >50% within minutes after birth, raising the possibility that circulating NOx plays a role in maintaining low fetal vascular resistance and in the cardiovascular transition at birth. To test whether the fall in plasma NOx concentrations at birth is due to either ligation of the umbilical cord or oxygenation of the fetus to newborn levels, plasma NOx concentrations were measured during stepwise delivery of near-term fetal lambs. When fetal lambs were intubated and mechanically ventilated with 100% O₂ to oxygenate the arterial blood while still in utero with the umbilical circulation still intact, there was no change in plasma NOx levels. In contrast, when the umbilical cord was ligated while fetal lambs were mechanically ventilated with O₂ levels that maintained fetal arterial blood gases, plasma NOx levels decreased by nearly 50%. Characterization of the individual NOx species in plasma revealed that the overall fall in NOx at birth was attributable mainly to FeNO compounds. Finally, when the typical fall in NOx after birth was prevented by intravenous nitrite infusion, birth-related changes in blood pressure, heart rate, and carotid flow changes were little affected, suggesting the cardiovascular transition at birth is not dependent on a fall in plasma NOx. In conclusion, this study shows FeNO is released from the placenta and that its decline accounts for most of the measured fall in plasma NOx at birth.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.

Nitrite potentiates the vasodilatory signaling of S-nitrosothiols

Nitrite and S-nitrosothiols (SNOs) are both byproducts of nitric oxide (NO) metabolism and are proposed to cause vasodilation via activation of soluble guanylate cyclase (sGC). We have previously reported that while SNOs are potent vasodilators at physiological concentrations, nitrite itself only produces vasodilation at supraphysiological concentrations. Here, we tested the hypothesis that sub-vasoactive concentrations of nitrite potentiate the vasodilatory effects of SNOs. Multiple exposures of isolated sheep arteries to S-nitroso-glutathione (GSNO) resulted in a tachyphylactic decreased vasodilatory response to GSNO but not to NO, suggesting attenuation of signaling steps upstream from sGC. Exposure of arteries to 1 μM nitrite potentiated the vasodilatory effects of GSNO in naive arteries and abrogated the tachyphylactic response to GSNO in pre-exposed arteries, suggesting that nitrite facilitates GSNO-mediated activation of sGC. In intact anesthetized sheep and rats, inhibition of NO synthases to decrease plasma nitrite levels attenuated vasodilatory responses to exogenous infusions of GSNO, an effect that was reversed by exogenous infusion of nitrite at sub-vasodilating levels. This study suggests nitrite potentiates SNO-mediated vasodilation via a mechanism that lies upstream from activation of sGC.

Sex-specific associations between cerebrovascular blood pressure autoregulation and cardiopulmonary injury in neonatal encephalopathy and therapeutic hypothermia

BACKGROUND

Cardiopulmonary injury is common in neonatal encephalopathy, but the link with cerebrovascular dysfunction is unknown. We hypothesized that alterations of cerebral autoregulation are associated with cardiopulmonary injury in neonates treated with therapeutic hypothermia (TH) for neonatal encephalopathy.

METHODS

The cerebral hemoglobin volume index (HVx) from near-infrared spectroscopy was used to identify the mean arterial blood pressure (MAP) with optimal autoregulatory vasoreactivity (MAPOPT). We measured associations between MAP relative to MAPOPT and indicators of cardiopulmonary injury (duration of mechanical respiratory support and administration of inhaled nitric oxide (iNO), milrinone, or steroids).

RESULTS

We identified associations between cerebrovascular autoregulation and cardiopulmonary injury that were often sex-specific. Greater MAP deviation above MAPOPT was associated with shorter duration of intubation in boys but longer ventilatory support in girls. Greater MAP deviation below MAPOPT related to longer intensive care stay in boys. Milrinone was associated with greater MAP deviation below MAPOPT in girls.

CONCLUSION

MAP deviation from MAPOPT may relate to cardiopulmonary injury after neonatal encephalopathy, and sex may modulate this relationship. Whereas MAP above MAPOPT may protect the brain and lungs in boys, it may be related to cardiopulmonary injury in girls. Future studies are needed to characterize the role of sex in these associations.

Fetal-maternal nitrite exchange in sheep: Experimental data, a computational model and an estimate of placental nitrite permeability

INTRODUCTION

Nitrite conveys NO-bioactivity that may contribute to the high-flow, low-resistance character of the fetal circulation. Fetal blood nitrite concentrations depend partly on placental permeability which has not been determined experimentally. We aimed to extract the placental permeability-surface (PS) product for nitrite in sheep from a computational model.

METHODS

An eight-compartment computational model of the fetal-maternal unit was constructed (Matlab(®) (R2013b (8.2.0.701), MathWorks Inc., Natick, MA). Taking into account fetal and maternal body weights, four variables (PS, the rate of nitrite metabolism within red cells, and two nitrite distribution volumes, one with and one without nitrite metabolism), were varied to obtain optimal fits to the experimental plasma nitrite profiles observed following the infusion of nitrite into either the fetus (n = 7) or the ewe (n = 8).

RESULTS

The model was able to replicate the average and individual nitrite-time profiles (r(2) > 0.93) following both fetal and maternal nitrite infusions with reasonable variation of the four fitting parameters. Simulated transplacental nitrite fluxes were able to predict umbilical arterial-venous nitrite concentration differences that agreed with experimental values. The predicted PS values for a 3 kg sheep fetus were 0.024 ± 0.005 l∙min(-1) in the fetal-maternal direction and 0.025 ± 0.003 l∙min(-1) in the maternal-fetal direction (mean ± SEM). These values are many-fold higher than the reported PS product for chloride anions across the sheep placenta.

CONCLUSION

The result suggests a transfer of nitrite across the sheep placenta that is not exclusively by simple diffusion through water-filled channels.

Local and systemic vasodilatory effects of low molecular weight S-nitrosothiols

S-nitrosothiols (SNOs) such as S-nitroso-L-cysteine (L-cysNO) are endogenous compounds with potent vasodilatory activity. During circulation in the blood, the NO moiety can be exchanged among various thiol-containing compounds by S-transnitrosylation, resulting in SNOs with differing capacities to enter the cell (membrane permeability). To determine whether the vasodilating potency of SNOs is dependent upon membrane permeability, membrane-permeable L-cysNO and impermeable S-nitroso-D-cysteine (D-cysNO) and S-nitroso-glutathione (GSNO) were infused into one femoral artery of anesthetized adult sheep while measuring bilateral femoral and systemic vascular conductances. L-cysNO induced vasodilation in the infused hind limb, whereas D-cysNO and GSNO did not. L-cysNO also increased intracellular NO in isolated arterial smooth muscle cells, whereas GSNO did not. The infused SNOs remained predominantly in a low molecular weight form during first-passage through the hind limb vasculature, but were converted into high molecular weight SNOs upon systemic recirculation. At systemic concentrations of ~0.6 μmol/L, all three SNOs reduced mean arterial blood pressure by ~50%, with pronounced vasodilation in the mesenteric bed. Pharmacokinetics of L-cysNO and GSNO were measured in vitro and in vivo and correlated with their hemodynamic effects, membrane permeability, and S-transnitrosylation. These results suggest local vasodilation by SNOs in the hind limb requires membrane permeation, whereas systemic vasodilation does not. The systemic hemodynamic effects of SNOs occur after equilibration of the NO moiety amongst the plasma thiols via S-transnitrosylation.

A stereotaxic, population-averaged T1w ovine brain atlas including cerebral morphology and tissue volumes

Standard stereotaxic reference systems play a key role in human brain studies. Stereotaxic coordinate systems have also been developed for experimental animals including non-human primates, dogs, and rodents. However, they are lacking for other species being relevant in experimental neuroscience including sheep. Here, we present a spatial, unbiased ovine brain template with tissue probability maps (TPM) that offer a detailed stereotaxic reference frame for anatomical features and localization of brain areas, thereby enabling inter-individual and cross-study comparability. Three-dimensional data sets from healthy adult Merino sheep (Ovis orientalis aries, 12 ewes and 26 neutered rams) were acquired on a 1.5 T Philips MRI using a T1w sequence. Data were averaged by linear and non-linear registration algorithms. Moreover, animals were subjected to detailed brain volume analysis including examinations with respect to body weight (BW), age, and sex. The created T1w brain template provides an appropriate population-averaged ovine brain anatomy in a spatial standard coordinate system. Additionally, TPM for gray (GM) and white (WM) matter as well as cerebrospinal fluid (CSF) classification enabled automatic prior-based tissue segmentation using statistical parametric mapping (SPM). Overall, a positive correlation of GM volume and BW explained about 15% of the variance of GM while a positive correlation between WM and age was found. Absolute tissue volume differences were not detected, indeed ewes showed significantly more GM per bodyweight as compared to neutered rams. The created framework including spatial brain template and TPM represent a useful tool for unbiased automatic image preprocessing and morphological characterization in sheep. Therefore, the reported results may serve as a starting point for further experimental and/or translational research aiming at in vivo analysis in this species.

Role of blood and vascular smooth muscle in the vasoactivity of nitrite

Recent evidence from humans and rats indicates that nitrite is a vasodilator under hypoxic conditions by reacting with metal-containing proteins to produce nitric oxide (NO). We tested the hypothesis that near-physiological concentrations of nitrite would produce vasodilation in a hypoxia- and concentration-dependent manner in the hind limb of sheep. Anesthetized sheep were instrumented to measure arterial blood pressure and femoral blood flows continuously in both hind limbs. Nitrite was infused into one femoral artery to raise the nitrite concentration in the femoral vein by 10 to 15-fold while the sheep breathed 50%, 14% or 12% oxygen in inspired air. In contrast to reports in humans and rats, the nitrite infusion had no measurable effect on mean femoral blood flows or vascular conductances, regardless of inspired O2 levels. In vitro experiments showed no significant difference in the release of NO from nitrite in sheep and human red blood cells. Further experiments demonstrated nitrite is converted to NO in rat artery homogenates faster than sheep arteries, and that this source of NO production is attenuated in the presence of a heme oxidizer. Finally, western blots indicate that concentrations of the heme-containing protein cytoglobin, but not myoglobin, are markedly lower in sheep arteries compared with rats. Overall, the results demonstrate that nitrite is not a physiological vasodilator in sheep. This is likely due to a lack of conversion of nitrite to NO within the vascular smooth muscle, perhaps due to deficient amounts of the heme-containing protein cytoglobin.