A role for H2S in the microcirculation of newborns: the major metabolite of H2S (thiosulphate) is increased in preterm infants

Hydrogen Sulfide-Clues from Evolution and Implication for Neonatal Respiratory Diseases

Reactive oxygen species (ROS) have been the focus of redox research in the realm of oxidative neonatal respiratory diseases such as bronchopulmonary dysplasia (BPD). Over the years, nitric oxide (NO) and carbon monoxide (CO) have been identified as important gaseous signaling molecules involved in modulating the redox homeostasis in the developing lung. While animal data targeting aspects of these redox pathways have been promising in treating and/or preventing experimental models of neonatal lung disease, none are particularly effective in human neonatal clinical trials. In recent years, hydrogen sulfide (H2S) has emerged as a novel gasotransmitter involved in a magnitude of cellular signaling pathways and functions. The importance of H2S signaling may lie in the fact that early life-forms evolved in a nearly anoxic, sulfur-rich environment and were dependent on H2S for energy. Recent studies have demonstrated an important role of H2S and its synthesizing enzymes in lung development, which normally takes place in a relatively hypoxic intrauterine environment. In this review, we look at clues from evolution and explore the important role that the H2S signaling pathway may play in oxidative neonatal respiratory diseases and discuss future opportunities to explore this phenomenon in the context of neonatal chronic lung disease.

Remote ischemic conditioning counteracts the intestinal damage of necrotizing enterocolitis by improving intestinal microcirculation

Necrotizing enterocolitis (NEC) is a devastating disease of premature infants with high mortality rate, indicating the need for precision treatment. NEC is characterized by intestinal inflammation and ischemia, as well derangements in intestinal microcirculation. Remote ischemic conditioning (RIC) has emerged as a promising tool in protecting distant organs against ischemia-induced damage. However, the effectiveness of RIC against NEC is unknown. To address this gap, we aimed to determine the efficacy and mechanism of action of RIC in experimental NEC. NEC was induced in mouse pups between postnatal day (P) 5 and 9. RIC was applied through intermittent occlusion of hind limb blood flow. RIC, when administered in the early stages of disease progression, decreases intestinal injury and prolongs survival. The mechanism of action of RIC involves increasing intestinal perfusion through vasodilation mediated by nitric oxide and hydrogen sulfide. RIC is a viable and non-invasive treatment strategy for NEC.

Necrotizing enterocolitis (NEC) is one of the most lethal gastrointestinal emergencies in neonates needing precision treatment. Here the authors show that remote ischemic conditioning is a non-invasive therapeutic method that enhances blood flow in the intestine, reduces damage, and improves NEC outcome.

Relevance of Microvascular Flow Assessments in Critically Ill Neonates and Children: A Systematic Review

OBJECTIVES

Resolution of impaired microvascular flow may lag the normalization of macrocirculatory variables. The significance of microcirculatory dysfunction in critically ill children and neonates is unknown, but microcirculatory variables can be measured using Doppler or videomicroscopy imaging techniques. We outline the current understanding of the role of the microcirculation in critical illness, review methods for its assessment, and perform a systematic review of how it has been monitored in critically ill neonates and children.

DESIGN

Systematic review (PROSPERO CRD42019117993).

SETTING

Not applicable.

SUBJECTS

Not applicable.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

We systematically searched MEDLINE, EMBASE, PubMed, and Web of Science. We included studies of critically ill patients 0 to 18 years old investigating microcirculatory blood flow. Two reviewers analyzed abstracts and articles. Results were qualitatively analyzed due to study heterogeneity. A total of 2,559 abstracts met search criteria, of which 94 underwent full-text review. Of those, 36 met inclusion criteria. Seven studies investigated microcirculatory changes in critically ill children. Twenty studies investigated the microcirculatory changes in neonates with variable diagnoses compared with a diverse set of clinical endpoints. Nine studies assessed the effects of age, sex, and birth weight on microvascular flow in neonates. Across all studies, microcirculatory dysfunction was associated with poor outcomes and may not correlate with observed macrovascular function.

CONCLUSIONS

Assessment of microvascular flow in critically ill children and neonates is possible, although significant challenges remain. In many such patients, microvascular blood flow is disrupted despite medical management targeting normalized macrovascular variables. Future studies are needed to define normal pediatric microvascular flow variables and to assess the impact of patient and treatment factors on its function.

Neonatal comorbidities and gasotransmitters

Hydrogen sulfide, nitric oxide, and carbon monoxide are endogenously produced gases that regulate various signaling pathways. The role of these transmitters is complex as constitutive production of these molecules may have anti-inflammatory, anti-microbial, and/or vasodilatory effects whereas induced production or formation of secondary metabolites may lead to cellular death. Given this fine line between friend and foe, therapeutic attenuation of these molecules' production has involved both inhibition of endogenous formation and therapeutic supplementation. All three gases have been implicated as regulators of critical aspects of neonatal physiology, and in turn, comorbidities including necrotizing enterocolitis, hypoxic ischemic encephalopathy, and pulmonary hypertension. In this review, we present current perspectives on these associations, highlight areas where insights remain sparse, and identify areas for potential for future investigations.

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.

Microvascular circulatory dysregulation driven in part by cystathionine gamma-lyase: A new paradigm for cardiovascular compromise in the preterm newborn

OBJECTIVE

H₂ S may explain the dysregulation of microvascular tone associated with poor outcome following preterm birth. In adult vasculature, H₂ S is predominantly produced by CSE. We hypothesized that vascular CSE activity contributes to microvascular tone regulation during circulatory transition.

METHODS

Preterm (GA62) and full-term (GA69) guinea pig fetuses and neonates were studied. Microvascular blood flow was assessed by laser Doppler flowmetry. Thiosulfate, primary urinary metabolite of H₂ S, was determined by high-performance liquid chromatography. Real-time H₂ S production was assessed using a microrespiration system in fetal and postnatal (10, 24 hours) skin and heart samples. CSE contribution was investigated by inhibition via propargylglycine.

RESULTS

In preterm animals, postnatal H₂ S production capacity in peripheral vasculature increased significantly and was significantly reduced by the inhibition of CSE. Urinary thiosulfate correlated with both microvascular blood flow and capacity of the vasculature to produce H₂ S. H₂ S produced via CSE did not correlate directly with microvascular blood flow.

CONCLUSIONS

In preterm neonates, H₂ S production increases during fetal-to-neonatal transition and CSE contribution to total H₂ S increases postnatally. CSE-dependent mechanisms may therefore underpin the increase in H₂ S production over the first 72 hours of life in preterm human neonates, associated with both central and peripheral cardiovascular instability.

Supporting preterm cardiovascular function

Preterm infants are at higher risk of adverse neurodevelopmental outcomes. Inadequate cerebral oxygen delivery resulting from poor cardiovascular function is likely to be a significant contributor to preterm brain injury. In this context, improved support of cardiovascular function is integral to improving preterm outcomes. Many of the treatments used to support preterm cardiovascular function are based on adult physiology and may not be appropriate for the unique physiology of the preterm infant. The preterm heart is structurally immature with reduced contractility and low cardiac output. However, there is limited evidence that inotropic support with dopamine and/or dobutamine is effective in preterm babies. Hypovolemia may also contribute to poor preterm cardiovascular function; there is evidence that capillary leakage results in considerable loss of plasma from the circulation of newborn preterm babies. In addition, the vasoconstrictor response to acute stimuli does not develop until quite late in gestation and is limited in the preterm infant. This may lead to inappropriate vasodilatation adding to functional hypovolemia. The first line treatment for hypotension in preterm infants is volume expansion with crystalloid solutions, but this has limited efficacy in the preterm infant. More effective methods of volume expansion are required. Effective support of preterm cardiovascular function requires better understanding of preterm cardiovascular physiology so that treatments can target mechanisms that are sufficiently mature to respond.

Videomicroscopy as a tool for investigation of the microcirculation in the newborn

The perinatal period remains a time of significant risk of death or disability. Increasing evidence suggests that this depends on microcirculatory behavior. Sidestream dark‐field orthogonal polarized light videomicroscopy (OPS) has emerged as a useful assessment of adult microcirculation but the values derived are not delineated for the newborn. We aimed to define these parameters in well term newborn infants. Demographic details were collected prospectively on 42 healthy term neonates (n = 20 females, n = 22 males). OPS videomicroscopy (Microscan) was used to view ear conch skin microcirculation at 6, 24, and 72 h of age. Stored video was analyzed by a masked observer using proprietary software. There were no significant differences between the sexes for any structural parameters at any time point. There was a significant increase over time in small vessel perfusion in female infants only (P = 0.009). A number of 6‐ and 72‐h measurements were significantly correlated, but differed from the 24‐h values. These observations confirm the utility of the ear conch for neonatal microvascular videomicroscopy. They provide a baseline for studies into the use of OPS videomicroscopy in infants. The changes observed are comparable with previous studies of term infants using these and other microvascular techniques. It is recommended that studies for examining the mature neonatal microvascular structure be delayed until 72 h of life, but studies of the physiology of cardiovascular transition should include the 24‐h time point after delivery.

Influence of sympathetic activity in the control of peripheral microvascular tone in preterm infants

BACKGROUND

Microvascular dysregulation following preterm birth is associated with increased illness severity and hypotension, particularly in males. Sympathetic nervous vascular regulation is evident in females. We hypothesized that sympathetic dysfunction in male preterm infants may contribute to a failure of peripheral microvascular vasoconstriction.

METHODS

Microvascular blood flow of infants 24-43 wk gestational age was assessed at 6, 24, and 72 h of age by laser Doppler. Blood flow Fourier transformed frequency distribution spectra (low frequency/high frequency ratio) were used to assess the influence of sympathetic tone on microvascular regulation. Total sympathetic output was assessed as urinary normetanephrine.

RESULTS

Microvascular sympathetic activity at 24 h postnatal age decreased in early preterm males, but not females. Peripheral sympathetic activity increased with advancing postnatal age in females, but decreased in males. In early preterm infants, total normetanephrine outputs increase significantly with postnatal age, in both sexes.

CONCLUSION

Sympathetic activation following preterm birth is sexually dimorphic, with preterm males having reduced sympathetic tone and reduced upregulation of sympathetic tone following birth. There is evidence of a disconnect between central sympathetic activity and local peripheral microcirculatory sympathetic drive. This may relate to autonomic nervous immaturity and highlights the need to understand how preterm birth may affect autonomic function.

A versatile method for analysis of saliva, plasma and urine for total thiols using HPLC with UV detection

A simple and rapid HPLC method using 2-chloro-1-methyllepidinium tetrafluoroborate (CMLT) as a derivatization reagent was developed for simultaneous determination of homocysteine (Hcy), glutathione (GSH), γ-glutamylcysteine (γ-GluCys), cysteinylglycine (CysGly), N-acetylcysteine (NACys) and cysteine (Cys) in human saliva, plasma and urine. Separation of the analytes was achieved in just 7min using an HPLC, followed by UV detection at 355nm. Chromatographic separation was accomplished on Aeris PEPTIDE XB-C18 (150mm×4.6mm, 3.6µm) column from Phenomenex with a gradient elution: 0-4.0min, 7-30% B; 4.0-5.5min, 30-7% B; 5.5-7.5min, 7% B; (A: B, v/v); (A) 0.5% CH3COOH and (B) EtOH. Mobile phase was delivered at a flow rate 1.0mLmin(-1). Linearity in detector response for total thiols was observed over the range of 0.1-20μmolL(-1) for Hcy, GSH and γ-GluCys, 0.25-50μmolL(-1) for NACys and CysGly and 5-300 for Cys. The LOQ values for Hcy, GSH, γ-GluCys, NACys, CysGly and Cys were 0.05, 0.05, 0.10, 0.06, 0.12 and 0.08μmolL(-1), respectively. The method was successfully implemented to analysis of the samples donated by 15 apparently healthy volunteers and 10 patients.

H2S- and NO-Signaling Pathways in Alzheimer's Amyloid Vasculopathy: Synergism or Antagonism?

Alzheimer's type of neurodegeneration dramatically affects H2S and NO synthesis and interactions in the brain, which results in dysregulated vasomotor function, brain tissue hypoperfusion and hypoxia, development of perivascular inflammation, promotion of Aβ deposition, and impairment of neurogenesis/angiogenesis. H2S- and NO-signaling pathways have been described to offer protection against Alzheimer's amyloid vasculopathy and neurodegeneration. This review describes recent developments of the increasing relevance of H2S and NO in Alzheimer's disease (AD). More studies are however needed to fully determine their potential use as therapeutic targets in Alzheimer's and other forms of vascular dementia.

Interactions of the gasotransmitters contribute to microvascular tone (dys)regulation in the preterm neonate

BACKGROUND & AIMS

Hydrogen sulphide (H2S), nitric oxide (NO), and carbon monoxide (CO) are involved in transitional microvascular tone dysregulation in the preterm infant; however there is conflicting evidence on the interaction of these gasotransmitters, and their overall contribution to the microcirculation in newborns is not known. The aim of this study was to measure the levels of all 3 gasotransmitters, characterise their interrelationships and elucidate their combined effects on microvascular blood flow.

METHODS

90 preterm neonates were studied at 24h postnatal age. Microvascular studies were performed by laser Doppler. Arterial COHb levels (a measure of CO) were determined through co-oximetry. NO was measured as nitrate and nitrite in urine. H2S was measured as thiosulphate by liquid chromatography. Relationships between levels of the gasotransmitters and microvascular blood flow were assessed through partial correlation controlling for the influence of gestational age. Structural equation modelling was used to examine the combination of these effects on microvascular blood flow and derive a theoretical model of their interactions.

RESULTS

No relationship was observed between NO and CO (p = 0.18, r = 0.18). A positive relationship between NO and H2S (p = 0.008, r = 0.28) and an inverse relationship between CO and H2S (p = 0.01, r = -0.33) exists. Structural equation modelling was used to examine the combination of these effects on microvascular blood flow. The model with the best fit is presented.

CONCLUSIONS

The relationships between NO and H2S, and CO and H2S may be of importance in the preterm newborn, particularly as NO levels in males are associated with higher H2S levels and higher microvascular blood flow and CO in females appears to convey protection against vascular dysregulation. Here we present a theoretical model of these interactions and their overall effects on microvascular flow in the preterm newborn, upon which future mechanistic studies may be based.