Relationship between flow rate and NO production in postnatal mesenteric arteries

Maturation of Intestinal Oxygenation: A Review of Mechanisms and Clinical Implications for Preterm Neonates

Nutrient requirements of preterm neonates may be substantial, to support growth and maturation processes in the presence of challenging post-natal circumstances. This may be accompanied by substantial intestinal oxygen requirements. Preterm neonates may not be able to meet these oxygen requirements, due to a developmental delay in intestinal oxygenation regulation mechanisms. This review summarizes the available literature on post-natal maturation of intestinal oxygenation mechanisms and translates these changes into clinical observations and potential implications for preterm neonates. The different mechanisms that may be involved in regulation of intestinal oxygenation, regardless of post-natal age, are first discussed. The contribution of these mechanisms to intestinal oxygenation regulation is then evaluated in newborn and mature intestine. Finally, the course of clinical observations is used to translate these findings to potential implications for preterm neonates.

Does red blood cell irradiation and/or anemia trigger intestinal injury in premature infants with birth weight ≤ 1250 g? An observational birth cohort study

BACKGROUND

Necrotizing enterocolitis (NEC) is a leading cause of neonatal morbidity and mortality in premature infants. To date, no effective biomarkers exist to predict which premature infants will develop NEC, limiting targeted prevention strategies. Multiple observational studies have reported an association between the exposure to red blood cell (RBC) transfusion and/or anemia and the subsequent development of NEC; however, the underlying physiologic mechanisms of how these factors are independently associated with NEC remain unknown.

METHODS

In this paper, we outline our prospective, multicenter observational cohort study of infants with a birth weight ≤ 1250 g to investigate the associations between RBC transfusion, anemia, intestinal oxygenation and injury that lead to NEC. Our overarching hypothesis is that irradiation of RBC units followed by longer storage perturbs donor RBC metabolism and function, and these derangements are associated with paradoxical microvascular vasoconstriction and intestinal tissue hypoxia increasing the risk for injury and/or NEC in transfused premature infants with already impaired intestinal oxygenation due to significant anemia. To evaluate these associations, we are examining the relationship between prolonged irradiation storage time (pIST), RBC metabolomic profiles, and anemia on intestinal oxygenation non-invasively measured by near-infrared spectroscopy (NIRS), and the development of NEC in transfused premature infants.

DISCUSSION

Our study will address a critical scientific gap as to whether transfused RBC characteristics, such as irradiation and metabolism, impair intestinal function and/or microvascular circulation. Given the multifactorial etiology of NEC, preventative efforts will be more successful if clinicians understand the underlying pathophysiologic mechanisms and modifiable risk factors influencing the disease.

TRIAL REGISTRATION

Our study is registered in ClinicalTrials.gov Identifier: NCT02741648 .

Impact of Continuous vs Bolus Feeding on Splanchnic Perfusion in Very Low Birth Weight Infants: A Randomized Trial

OBJECTIVE

To detect changes in splanchnic perfusion and oxygenation induced by 2 different feeding regimens in infants with intrauterine growth restriction (IUGR) and those without IUGR.

STUDY DESIGN

This was a randomized trial in 40 very low birth weight infants. When an enteral intake of 100 mL/kg/day was achieved, patients with IUGR and those without IUGR were randomized into 2 groups. Group A (n = 20) received a feed by bolus (in 10 minutes), then, after at least 3 hours, received the same amount of formula by continuous nutrition over 3 hours. Group B (n = 20) received a feed administered continuously over 3 hours, followed by a bolus administration (in 10 minutes) of the same amount of formula after at least 3 hours. On the day of randomization, intestinal and cerebral regional oximetry was measured via near-infrared spectroscopy and Doppler ultrasound (US) of the superior mesenteric artery was performed. Examinations were performed before the feed and at 30 minutes after the feed by bolus and before the feed, at 30 minutes after the start of the feed, and at 30 minutes after the end of the feed for the 3-hour continuous feed.

RESULTS

Superior mesenteric artery Doppler US showed significantly higher perfusion values after the bolus feeds than after the continuous feeds. Near-infrared spectroscopy values remained stable before and after feeds. Infants with IUGR and those without IUGR showed the same perfusion and oxygenation patterns.

CONCLUSION

According to our Doppler US results, bolus feeding is more effective than continuous feeding in increasing splanchnic perfusion.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT01341236.

Endothelial nitric oxide weakens arterial contractile responses and reduces blood pressure during early postnatal development in rats

OBJECTIVE

During maturation the vascular system undergoes structural and functional remodeling. At the systemic level it results in a gradual increase of arterial blood pressure during postnatal ontogenesis. The mechanisms of maintaining the blood pressure at a comparatively low level during the early postnatal development are not completely understood. Recently we showed that the hindlimb arteries of young (1-2 wk-old) rats exhibited an enhanced endothelial NO-pathway activity, which weakened their contractile responsiveness compared to the arteries of adult rats. Here we tested the hypothesis that an increased tonic endothelial NO production can take place in the whole vascular system leading to a decreased level of systemic blood pressure in young rats.

DESIGN AND METHODS

Segments of small mesenteric, saphenous, sural and intrarenal arteries were isolated from the young (2 wk-old), juvenile (4 wk-old) and adult (10-12 wk-old) male rats and tested in a wire isometric myograph. Anticontractile effect of NO was evaluated by the effects of NOS inhibitor L-NNA on both arterial spontaneous tone and constrictor responses to methoxamine (α1-adrenoceptor agonist). In addition, eNOS and arginase-2 mRNA expression in arterial preparations by qPCR and serum nitrite/nitrate levels by Griess reaction were estimated. Blood pressure with an intra-carotid artery catheter was measured in conscious rats.

RESULTS

In all arteries of 2 wk rats except the renal ones, L-NNA exposure resulted in a considerable tonic contraction and a remarkable enhancement of contractile responses to methoxamine. The effect of L-NNA gradually decreased with age and by 10-12 weeks became very small in the mesenteric arteries and disappeared in the sural and saphenous arteries. Although no difference in eNOS mRNA expression was found, the content of arginase-2 mRNA was significantly lower in young rats compared to adults. Serum levels of NO metabolites were two-fold higher in 2 wk-old rats than in adult rats. Along with that, arterial blood pressure was by half lower but rose more prominently after administration of l-NAME in young rats than in adults.

CONCLUSIONS

In young rats, tonic release of NO by the endothelium considerably weakens contractile responses of arteries supplying intestine, skin and skeletal muscles, which receive a high proportion of the cardiac output. Such anticontractile effect of NO can be an important mechanism responsible for the blood pressure reduction in immature circulatory system.

The role of the intestinal microcirculation in necrotizing enterocolitis

Necrotizing enterocolitis (NEC) continues to be a devastating inflammatory disease of the newborn intestine. Despite advances in management, morbidity and mortality remain high. While it is clear that intestinal ischemia plays a large role in disease pathogenesis, attempts to link NEC to intestinal macrovascular derangement have been largely unsuccessful. More recently, there has been a concerted effort to characterize the pathologic changes of the intestinal microcirculation in response to intestinal injury, including NEC. This microcirculatory regulation is controlled by a balance of vasoconstrictor and vasodilator forces. Vasoconstriction is mediated primarily by endothelin-1 (ET-1), while vasodilation is mediated primarily by nitric oxide (NO). These chemical mediators have been implicated in many aspects of intestinal ischemic injury and NEC, with the balance shifting toward increased vasoconstriction associated with intestinal injury. With a proper understanding of these antagonistic forces, potential therapeutic avenues may result from improving this pathologic microcirculatory dysregulation.

3D network model of NO transport in tissue

We developed a mathematical model to simulate shear stress-dependent nitric oxide (NO) production and transport in a 3D microcirculatory network based on published data. The model consists of a 100 μm × 500 μm × 75 μm rectangular volume of tissue containing two arteriole-branching trees, and nine capillaries surrounding the vessels. Computed distributions for NO in blood, vascular walls, and surrounding tissue were affected by hematocrit (Hct) and wall shear stress (WSS) in the network. The model demonstrates that variations in the red blood cell (RBC) distribution and WSS in a branching network can have differential effects on computed NO concentrations due to NO consumption by RBCs and WSS-dependent changes in NO production. The model predicts heterogeneous distributions of WSS in the network. Vessel branches with unequal blood flow rates gave rise to a range of WSS values and therefore NO production rates. Despite increased NO production in a branch with higher blood flow and WSS, vascular wall NO was predicted to be lower due to greater NO consumption in blood, since the microvascular Hct increased with redistribution of RBCs at the vessel bifurcation. Within other regions, low WSS was combined with decreased NO consumption to enhance the NO concentration.

Endothelium-dependent control of vascular tone during early postnatal and juvenile growth

Endothelial dysfunction can develop at an early age in children with risk factors for cardiovascular disease. A clear understanding of the nature of this dysfunction and how it can worsen over time requires detailed information on the normal growth-related changes in endothelial function on which the pathological changes are superimposed. This review summarizes our current understanding of these normal changes, as derived from studies in four different mammalian species. Although the endothelium plays an important role in controlling vascular tone from birth onward, the vasoactive molecules that mediate this control often change during postnatal or juvenile growth. The specifics of this transition to an adult endothelial cell phenotype can vary depending on the vascular bed. During growth, the contribution of nitric oxide to endothelium-dependent dilation generally increases in the lung, cerebral cortex, and skeletal muscle, but decreases in the intestine. Endothelial capacity for release of other vasoactive factors (e.g., cyclooxygenase products, hydrogen peroxide, carbon monoxide) can also increase or decrease during growth. Although these changes have been well documented, there is less information on their underlying cellular or molecular events. Further research is required to clarify these mechanisms, and to evaluate the functional significance of such shifts in endothelial phenotype.

Peculiaridades da circulação mesentérica em recém-nascidos e suas implicações em doenças gastrintestinais do período neonatal

OBJETIVO:Descrever peculiaridades da circulação mesentérica neonatal e caracterizar fatores de suscetibilidade ao desenvolvimento de doenças gastrintestinais e alterações do fluxo sanguíneo da artéria mesentérica superior por meio da dopplerfluxometria. FONTES DE DADOS: Livros-textos e publicações indexadas no Medline e SciELO nos últimos 20 anos, utilizando-se as palavras chaves: "mesenteric artery", "superior mesenteric artery", "newborn intestinal circulation", "necrotizing enterocolitis", "doppler flow velocimetry". SÍNTESE DOS DADOS: Alterações do fluxo sanguíneo mesentérico são um dos fatores predisponentes da enterocolite necrosante, doença neonatal de alta morbimortalidade que acomete principalmente prematuros. A circulação mesentérica é peculiar no período neonatal tanto em relação ao seu estado basal, quanto à sua resposta frente a estímulos fisiológicos. Variações da irrigação mesentérica podem ser inerentes à própria fase de desenvolvimento vascular intestinal do recém-nascido pré-termo, com possíveis agravos de fatores perinatais como: insuficiência placentária, asfixia, infecção, cateterismo umbilical, drogas (indometacina e cafeína), fototerapia, alimentação artificial e progressão rápida da dieta. A dopplerfluxometria permite o estudo da irrigação de órgãos-alvo e pode quantificar o fluxo sanguíneo, a resistência vascular e predizer situações de risco para doenças do trato gastrintestinal no período neonatal. CONCLUSÕES: O recém-nascido apresenta peculiaridades de irrigação sanguínea gastrintestinal. A dopplerfluxometria da artéria mesentérica superior é um método não invasivo que determina as condições circulatórias no território intestinal.

Neonatal colour Doppler ultrasound study: normal values of abdominal blood flow velocities in the neonate during the first month of life

BACKGROUND

Doppler US to measure abdominal blood flow velocities (ABFV) is increasingly used to investigate intestinal haemodynamics in several clinical conditions in neonates. Studies that provide reference values of ABFV during the entire neonatal period are currently lacking.

OBJECTIVE

To make available normal reference values of ABFV and Doppler indices in the coeliac trunk and superior mesenteric artery during the first month of life in term and healthy preterm infants.

MATERIALS AND METHODS

ABFV were obtained with colour Doppler US in 69 neonates (12 term, 57 preterm) divided into four gestational age groups (25-28 weeks, 29-32 weeks, 33-36 weeks, and 37-41 weeks).

RESULTS

ABFV increased with increasing gestational and postnatal age. We also provide normal reference values of ABFV and Doppler indices to compare with measurements of abdominal blood flow changes during the neonatal period for diagnostic, therapeutic and prognostic purposes.

CONCLUSION

These longitudinal reference values provide a useful tool for assessing possible alteration in ABFV secondary to neonatal pathologies.

IL-1beta alters hemodynamics in newborn intestine: role of endothelin

Studies were carried out to determine the effects of IL-1beta on newborn intestinal hemodynamics. IL-1beta increased the release of ET-1 by primary endothelial cells in a dose-dependent manner; as well, it reduced expression of the endothelin (ET) type B (ET(B)) receptor on endothelial cells and increased expression of the ET type A (ET(A)) receptor on vascular smooth muscle cells. IL-1beta increased endothelial cell endothelial nitric oxide (NO) synthase (eNOS) expression but did not enhance eNOS activity as evidenced by release of NO(x) into conditioned medium in response to acetylcholine or shear stress. The effects of IL-1beta on flow-induced dilation were evaluated in terminal mesenteric arteries in vitro. Pretreatment with IL-1beta (1 ng; 4 h) significantly attenuated vasodilation in response to flow rates of 100 and 200 microl/min. This effect was mediated, in part, by the endothelin ET(A) receptor; thus selective blockade of ET(A) receptors with BQ610 nearly restored flow-induced dilation. In contrast, exogenous ET-1 only shifted the diameter-flow curve downward without altering the percent vasodilation in response to flow. The effects of IL-1beta on ileal oxygenation were then studied using in vivo gut loops. Intramesenteric artery infusion of IL-1beta upstream of the gut loop caused ileal vasoconstriction and reduced the arterial-venous O(2) difference across the gut loop; consequently, it reduced ileal oxygenation by 60%. This effect was significantly attenuated by pretreatment with BQ610. These data support a linkage between the proinflammatory cytokine IL-1beta and vascular dysfunction within the intestinal circulation, mediated, at least in part, by the ET system.

Intestinal O2 consumption in necrotizing enterocolitis: role of nitric oxide

We tested the hypothesis that inducible isoform of nitric oxide synthase (iNOS)-derived nitric oxide (NO) inhibits oxygen consumption (VO2) in human intestine resected for necrotizing enterocolitis (NEC). Each NEC resection specimen was divided into two sections based on histologic appearance: healthy or diseased. Intestine removed from infants for reasons other than NEC was used as control. The tissue injury score (0-6, with 6 indicating complete necrosis) was 0.4 +/- 0.2 in control tissue, 1.2 +/- 0.4 in NEC-healthy tissue, and 4.6 +/- 0.5 in NEC-diseased tissue. Prominent iNOS staining was present in villus enterocytes in NEC-healthy tissue but not in the other tissue types. Intestinal VO2 (per direct oximetry, in nM O2/min/g) was significantly greater in control tissue than in NEC-healthy or NEC-diseased tissues. Accumulation of NO into buffer bathing intestinal slices (in nM NO/microL/g) was greater in NEC-healthy tissue than control or NEC-diseased tissues. The specific iNOS antagonist L-Nomega-(1-iminoethyl)-lysine (L-NIL) reduced buffer NO concentration 76% and increased VO2 by 90% in NEC-healthy tissue; however, L-NIL had no effect on NO or VO2 in control or NEC-diseased tissue. Addition of exogenous NO via S-nitroso-N-acetylpenicillamine depressed VO2 in NEC-healthy and control tissues but not NEC-diseased tissue. A significant correlation was present between buffer NO concentration and VO2 in NEC-healthy tissue. We conclude that iNOS-derived NO suppresses VO2 in intestine resected for NEC that demonstrates a relatively normal histology on light microscopy.

Platelet-activating factor increases endothelial [Ca2+]iand NO production in individually perfused intact microvessels

We have demonstrated that inhibition of NO synthase (NOS) in endothelial cells by either the NOS inhibitor Nω-monomethyl-l-arginine (l-NMMA) or the internalization of caveolin-1 scaffolding domain attenuated platelet-activating factor (PAF)-induced increases in microvessel permeability ( Am J Physiol Heart Circ Physiol 286: H195–H201, 2004) indicating the involvement of an NO-dependent signaling pathway. To investigate whether an increase in endothelial cytoplasmic Ca2+concentration ([Ca2+]i) is the initiating event and Ca2+-dependent NO production is crucial for permeability increases, PAF (10 nM)-induced changes in endothelial [Ca2+]iand NO production were measured in individually perfused rat mesenteric venular microvessels via fluorescence microscopy. When venular microvessels were exposed to PAF, endothelial [Ca2+]iincreased from 69 ± 8 nM to a peak value of 374 ± 26 nM within 3 min and then declined to a sustained level at 190 ± 12 nM after 15 min. Inhibition of NOS did not modify PAF-induced increases in endothelial [Ca2+]i. PAF-induced NO production was visualized and quantified at cellular levels in individually perfused microvessels using 4,5-diaminofluorescein diacetate and fluorescence imaging. Increased fluorescence intensity (FI), which is an indication of increased NO production, occurred in 75 ± 7% of endothelial cells in each vessel. The mean maximum FI increase was 140 ± 7% of baseline value. This increased FI was abolished by pretreatment of the vessel with l-NMMA and attenuated in the absence of extracellular Ca2+. These results provide direct evidence from intact microvessels that increased endothelial [Ca2+]iis the initial signal that activates endothelial NOS, and the subsequent increased NO production contributes to PAF-induced increases in microvessel permeability.

Developmental expression of eNOS in postnatal swine mesenteric artery

Developmental changes in the expression of endothelial nitric oxide synthase (eNOS) within the mesenteric artery of swine were studied in fetal (110 days postconception/117 days total gestation) and on postnatal days 1, 3, 10, and 30. Subjects in the 1-day-old group were subdivided into fed and nonfed. Transcription of eNOS was determined by real-time PCR, protein expression was evaluated by Western blotting, and hemodynamic and oxygenation parameters were measured within in situ gut loops before and after the administration of N(G)-monomethyl-L-arginine (L-NMMA). The abundance of eNOS mRNA remained steady throughout all ages. In contrast, expression of eNOS protein was twofold greater in the 1-day-old fed subjects compared with fetal or 1-day-old nonfed subjects. eNOS protein expression remained elevated on day 3, increased on day 10, and then declined to a level similar to the day 1 nonfed group by postnatal day 30. Intestinal vascular resistance was 31% lower in the day 1 fed group when compared to the day 1 nonfed group; resistance continued to decline through day 10 but then significantly increased on day 30. We conclude that the expression of eNOS changes within the mesenteric artery during early postnatal development at a posttranscriptional level.

Simultaneous in situ monitoring of intracellular Ca2+ and NO in endothelium of coronary arteries

We developed an in situ assay system to simultaneously monitor intracellular Ca(2+) concentration ([Ca(2+)](i), fura 2 as indicator) and nitric oxide (NO) levels [4,5-diaminofluorescein as probe] in the intact endothelium of small bovine coronary arteries by using a fluorescent microscopic imaging technique with high-speed wavelength switching. Bradykinin (BK; 1 microM) stimulated a rapid increase in [Ca(2+)](i) followed by an increase in NO production in the endothelial cells. The protein tyrosine phosphatase inhibitor phenylarsine oxide (PAO; 10 microM) induced a gradual, small increase in [Ca(2+)](i) and a slow increase in intracellular NO levels. Removal of extracellular Ca(2+) and depletion of Ca(2+) stores completely blocked BK-induced increase in NO production but had no effect on PAO-induced NO production. However, a further reduction of [Ca(2+)](i) by application of BAPTA-AM or EGTA with ionomycin abolished the PAO-induced NO increase. These results indicate that a simultaneous monitoring of [Ca(2+)](i) and intracellular NO production in the intact endothelium is a powerful tool to study Ca(2+)-dependent regulation of endothelial nitric oxide synthase, which provides the first direct evidence for a permissive role of Ca(2+) in tyrosine phosphorylation-induced NO production.