Development of the myogenic response in postnatal intestine: role of PKC

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.

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.

Capitalizing on diversity: an integrative approach towards the multiplicity of cellular mechanisms underlying myogenic responsiveness

The intrinsic ability of resistance arteries to respond to transmural pressure is the single most important determinant of their function. Despite an ever-growing catalogue of signalling pathways that underlie the myogenic response, it remains an enigmatic mechanism. The myogenic response's mechanistic diversity has largely been attributed to 'hard-wired' differences across species and vascular beds; however, emerging evidence suggests that the mechanistic basis for the myogenic mechanism is, in fact, 'plastic'. This means that the myogenic response can change quantitatively (i.e. change in magnitude) and qualitatively (i.e. change in mechanistic basis) in response to environmental challenges (e.g. disease conditions). Consequently, understanding the dynamics of how the myogenic response capitalizes on its mechanistic diversity is key to unlocking clinically viable interventions. Using myogenic sphingosine-1-phosphate (S1P) signalling as an example, this review illustrates the remarkable plasticity of the myogenic response. We propose that currently unidentified 'organizational programmes' dictate the contribution of individual signalling pathways to the myogenic response and introduce the concept that certain signalling elements act as 'divergence points' (i.e. as the potential higher level regulatory sites). In the context of pressure-induced S1P signalling, the S1P-generating enzyme sphingosine kinase 1 serves as a divergence point, by orchestrating the calcium-dependent and -independent signalling pathways underlying microvascular myogenic responsiveness. By acting on divergence points, the proposed 'organizational programmes' could form the basis for the flexible recruitment and fine-tuning of separate signalling streams that underlie adaptive changes to the myogenic response and its distinctiveness across species and vascular beds.

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.

Heparin-binding EGF-like growth factor is a potent dilator of terminal mesenteric arterioles

OBJECTIVE

We have previously shown that heparin-binding EGF-like growth factor (HB-EGF) protects the intestines from multiple forms of injury via direct cytoprotective effects on the intestinal mucosa. In this study, we examined the effects of HB-EGF on the hemodynamics of intestinal arterioles, the major resistance vessels that regulate blood flow to the intestines, as an additional mechanism of HB-EGF-mediated intestinal protection.

METHODS

The hemodynamic effects of HB-EGF in rodent terminal mesenteric arterioles and human submucosal arterioles were examined ex vivo using a video dimension analyzer. Cultured human intestinal microvascular endothelial cells (HIMEC) were used to elucidate the mechanisms of HB-EGF-induced vasodilation.

RESULTS

HB-EGF significantly increased vessel diameter under conditions of increasing intraluminal pressure and increased flow rate. These HB-EGF-mediated vasodilatory effects were observed in terminal mesenteric arterioles from adult rats and 3 day old rat pups. These effects were confirmed in submucosal arterioles from human intestine. Furthermore, HB-EGF significantly reduced endothelin-1-induced mesenteric arteriolar vasoconstriction. The vasodilatory effects of HB-EGF were blocked by ET(B) receptor antagonism in adult rat arterioles, and also by nitric oxide synthase inhibition in rat pup and human infant arterioles. In HIMEC, HB-EGF significantly increased endothelin B (ET(B)) receptor protein expression and provoked intracellular calcium mobilization.

CONCLUSIONS

HB-EGF is a potent vasodilator of the intestinal microvasculature, further supporting its use in diseases manifested by decreased intestinal blood flow, including necrotizing enterocolitis.

Increased myogenic responsiveness of skeletal muscle arterioles with juvenile growth

Previous studies from this laboratory suggest that during juvenile growth, structural changes in the arteriolar network are accompanied by changes in some of the mechanisms responsible for regulation of tissue blood flow. To test the hypothesis that arteriolar myogenic behavior is altered with growth, we studied gracilis muscle arterioles isolated from Sprague-Dawley rats at two ages: 21-28 and 42-49 days. When studied at their respective in vivo pressures, the myogenic index (instantaneous slope of the active pressure-diameter curve) of arterioles from 42-49-day-old rats was more negative than that of arterioles from 21-28-day-old rats, indicating greater myogenic responsiveness. Endothelial denudation, or prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) receptor antagonism without denudation, significantly reduced the myogenic responsiveness of arterioles from the older rats over a wide range of pressures but had no consistent effects on the myogenic responsiveness of arterioles from the younger rats. The heme oxygenase inhibitor chromium (III) mesoporphyrin IX chloride had no effect on the myogenic activity of arterioles from either age group. These findings indicate that microvascular growth in young animals is accompanied by an increase in the myogenic behavior of arterioles, possibly because PGH(2) or TxA(2) assumes a role in reinforcing myogenic activity over this period. As a result, the relative contribution of myogenic activity to blood flow regulation in skeletal muscle may increase during rapid juvenile growth.

Necrotizing enterocolitis

In necrotizing enterocolitis (NEC) the small (most often distal) and/or large bowel becomes injured, develops intramural air, and may progress to frank necrosis with perforation. Even with early, aggressive treatment, the progression of necrosis, which is highly characteristic of NEC, can lead to sepsis and death. This article reviews the current scientific knowledge related to the etiology and pathogenesis of NEC and discusses some possible preventive measures.

Myogenic contractility is more dependent on myofilament calcium sensitization in term fetal than adult ovine cerebral arteries

Regulation of cytosolic calcium and myofilament calcium sensitivity varies considerably with postnatal age in cerebral arteries. Because these mechanisms also govern myogenic tone, the present study used graded stretch to examine the hypothesis that myogenic tone is less dependent on calcium influx and more dependent on myofilament calcium sensitization in term fetal compared with adult cerebral arteries. Term fetal and adult posterior communicating cerebral arteries exhibited similar myogenic responses, with peak tensions averaging 24 and 26% of maximum contractile force produced in any given tissue in response to an isotonic Krebs buffer containing 122 mM K+ (Kmax) at optimum stretch ratios (working diameter/unstressed diameter) of 2.19 and 2.23, respectively. Graded stretch increased cytosolic Ca2+ concentration at stretch ratios >2.0 in adult arteries, but increased Ca2+ concentration only at stretch ratios >2.3 in fetal arteries. In permeabilized arteries, myogenic tone peaked at a stretch ratio of 2.1 in both fetal and adult arteries. The fetal %Kmax values at peak myogenic tone were not significantly different at either pCa 7.0 (23%) or pCa 5.5 (25%) but were significantly less at pCa 8.0 (8.4 ± 2.3%). Conversely, adult %Kmax values at peak myogenic tone were significantly less at both pCa 8.0 (10.4 ± 1.8%) and pCa 7.0 (16%) than at pCa 5.5 (27%). The maximal extents of stretch-induced increases in myosin light chain phosphorylation in intact fetal (20%) and adult (17%) arteries were similar. The data demonstrate that the cerebrovascular myogenic response is highly conserved during postnatal maturation but is mediated differently in fetal and adult cerebral arteries.

Age-related differences in vasoconstrictor responses to isoprostanes in piglet pulmonary and mesenteric vascular smooth muscle

Isoprostanes are prostaglandin (PG)-like compounds produced nonenzymatically by free radical-catalyzed peroxidation of arachidonic acid. Isoprostanes evoke potent vascular effects but their actions in the neonatal vasculature are poorly known. We aimed to study the effects of 8-iso-PGE(1), 8-iso-PGE(2), 8-iso-PGF(1alpha), 8-iso-PGF(1beta), 8-iso-PGF(2alpha), and 8-iso-PGF(2beta) in pulmonary arteries (PA), pulmonary veins (PV), and mesenteric arteries (MA) from newborn and 2-wk-old piglets. Isoprostanes produced concentration-dependent contractions of PA, PV, and MA (magnitudes up to 1.5- to 2-fold greater than the responses to 62.5 mM KCl) but they were markedly less potent vasoconstrictors than the thromboxane A(2) (TXA(2)) mimetic U46619. Neonatal PA were more sensitive to 8-iso-PGF(1alpha), 8-iso-PGF(1beta), and 8-iso-PGF(2beta) than 2-wk-old PA. Neonatal PV were more sensitive to 8-iso-PGE(2) and 8-iso-PGF(1alpha), and neonatal MA were more sensitive to 8-iso-PGE(2), 8-iso-PGF(1alpha), 8-iso-PGF(1beta), 8-iso-PGF(2alpha), and 8-iso-PGF(2beta) than the corresponding 2-wk-old vessels. The sensitivity to U46619 decreased with postnatal age in MA but did not change in PA and PV. The contractile responses to all the isoprostanes and to U46619 were reverted by the TXA(2) receptor (TP) antagonist SQ 29,548. Moreover, isoprostane-evoked contractions in 2-wk-old PA were reduced by inhibitors of tyrosine kinase (genistein) and Rho kinase (Y 27632 and hydroxyfasudil) but not by inhibitors of protein kinase C (chelerythrine), mitogen-activated protein kinase kinase (PD 98059) or p38-kinase (SB 203580). In conclusion, isoprostanes produced compound-, tissue-, and age-dependent constriction of neonatal porcine pulmonary and mesenteric vascular smooth muscle. Isoprostane-evoked PA vasoconstriction involved TP receptors and activation of tyrosine kinases and Rho kinases.

Feeding strategies and necrotizing enterocolitis

PURPOSE OF REVIEW

To update clinical trials concerning feeding strategies or feeding interventions to prevent necrotizing enterocolitis (NEC).

RECENT FINDINGS

The overall incidence of NEC remains unchanged. Several studies have shown that it is safe to use small feeding volumes early in life even in selected circumstances, such as feeding during the use of indomethacin to treat symptomatic patent ductus arteriosus. Although descriptive case reports have linked some feeding interventions such as thickened feedings to NEC, there is no evidence to establish a causal relation. Interestingly, one report showed that neither the presence of-nor characteristics of-gastric residuals was helpful to identify those infants who subsequently develop NEC. Numerous studies in animals show that vascular and host defense responses are not adequate in the immature gut. A few small clinical trials showed that various feeding strategies may alter these responses in preterm infants. However, larger clinical trials around these issues await further characterization of the mechanisms that regulate these two aspects of gastrointestinal function with respect to feeding.

SUMMARY

Studies continue to demonstrate how fragile the immature gut is, but further work directed to understanding the mechanisms that regulate gastrointestinal responses to feeding is needed to design meaningful interventions for large prospective trials.

alphavbeta3- and alpha5beta1-integrin blockade inhibits myogenic constriction of skeletal muscle resistance arterioles

In isolated resistance arterioles with spontaneous tone, ligation of alpha4beta1- and alpha5beta1-integrins induces vasoconstriction whereas ligation of alphavbeta3-integrin induces vasodilation. However, whether integrins directly participate in myogenic constriction to pressure elevation is not known. To answer this question, isolated rat skeletal muscle arterioles were exposed to step increments in pressure in the absence or presence of peptides and function-blocking antibodies known to bind alpha4beta1-, alpha5beta1-, or alphavbeta3-integrins while vessel diameter was continually monitored. Myogenic constriction, as assessed by the ability of isolated arterioles to reduce their diameter in response to two consecutive increments in intraluminal pressure (90-110 and 110-130 cmH2O), was not affected by treatment with any of the control peptides (RAD, LEV), a control antibody (anti-rat major histocompatibility complex), an alpha4beta1-integrin-binding peptide (LDV), or an anti-alpha4-integrin antibody. In contrast, alpha5beta1-integrin blockade with either anti-alpha5- or anti-beta1-integrin antibody caused a significant inhibition of myogenic constriction. Also, both RGD peptide and anti-beta3-integrin antibody inhibited myogenic constriction. These results indicate that alpha5beta1- and alphavbeta3-integrins are necessary for myogenic constriction and further suggest that integrins are part of the mechanosensory apparatus responsible for the ability of vascular smooth muscle cells to detect and/or respond to changes in intraluminal pressure.

Alterations in PKC signaling underlie enhanced myogenic tone in exercise-trained porcine coronary resistance arteries

The intracellular mechanisms underlying enhanced myogenic contraction (MC) in coronary resistance arteries (CRAs) from exercise-trained (EX) pigs have not been established. The purpose of this study was to test the hypothesis that exercise-induced alterations in protein kinase C (PKC) signaling underlie enhanced MC. Furthermore, we sought to determine whether modulation of intracellular Ca2+signaling by PKC underlies enhanced MC in EX animals. Male Yucatan miniature swine were treadmill trained ( n = 7) at ∼75% of maximal O2uptake for 16 wk (6 miles/h, 60 min) or remained sedentary (SED, n = 6). Diameter measurements in response to intraluminal pressure (60, 75, and 90 cmH2O) or 60 mM KCl were determined in single, cannulated CRAs (∼100 μm ID) with and without the PKC inhibitor chelerythrine (CE, 1 μM). Confocal imaging of Ca2+signaling [myogenic Ca2+(Cam)] was also performed in CRAs of similar internal diameter after abluminal loading of the Ca2+indicator dye fluo 4 (1 μM, 37°C, 30 min). We observed significantly greater MC in CRAs isolated from EX than from SED animals at 90 cmH2O, as well as greater reductions in MC after CE at all pressures studied. At intraluminal pressures of 75 and 90 cmH2O, CE produced greater decreases in Camin CRAs from EX than from SED animals (64% vs. 25%, P < 0.05). Inhibition of KCl constriction and Camby CE was also greater in EX animals ( P < 0.05). Western blotting revealed significant increases in Ca2+-dependent PKC-α (∼50%) but not Ca2+-independent PKC-ϵ levels in CRAs isolated from EX animals ( P < 0.05). We also observed significant group differences in phosphorylated PKC-α levels. Finally, voltage-gated Ca2+current (VGCC) was effectively blocked by CE, bisindolylmaleimide, and staurosporine in isolated smooth muscle cells from CRAs, providing evidence for a mechanistic link between VGCCs and PKC in our experimental paradigm. These results suggest that enhanced MC in CRAs from EX animals involves PKC-dependent modulation of intracellular Ca2+, including regulation of VGCCs.