Platelet-activating factor and sepsis-induced small intestinal microvascular hypoperfusion

Is choline deficiency an unrecognized factor in necrotizing enterocolitis of preterm infants?

We undertook this review to determine if it is plausible that choline or phosphatidylcholine (PC) deficiency is a factor in necrotizing enterocolitis (NEC) after two clinical trials found a dramatic and unexpected reduction in NEC in an experimental group provided higher PC compared to a control group. Sources and amounts of choline/PC for preterm infants are compared to the choline status of preterm infants at birth and following conventional nutritional management. The roles of choline/PC in intestinal structure, mucus, mesenteric blood flow, and the cholinergic anti-inflammatory system are summarized. Low choline/PC status is linked to prematurity/immaturity, parenteral and enteral feeding, microbial dysbiosis and hypoxia/ischemia, factors long associated with the risk of developing NEC. We conclude that low choline status exists in preterm infants provided conventional parenteral and enteral nutritional management, and that it is plausible low choline/PC status adversely affects intestinal function to set up the vicious cycle of inflammation, loss of intestinal barrier function and worsening tissue hypoxia that occurs with NEC. In conclusion, this review supports the need for randomized clinical trials to test the hypothesis that additional choline or PC provided parenterally or enterally can reduce the incidence of NEC in preterm infants. IMPACT STATEMENT: Low choline status in preterm infants who are managed by conventional nutrition is plausibly linked to the risk of developing necrotizing enterocolitis.

Signalling mechanisms in PAF-induced intestinal failure

Capillary leakage syndrome, vasomotor disturbances and gut atony are common clinical problems in intensive care medicine. Various inflammatory mediators and signalling pathways are involved in these pathophysiological alterations among them platelet-activating factor (PAF). The related signalling mechanisms of the PAF-induced dysfunctions are only poorly understood. Here we used the model of the isolated perfused rat small intestine to analyse the role of calcium (using calcium deprivation, IP-receptor blockade (2-APB)), cAMP (PDE-inhibition plus AC activator), myosin light chain kinase (inhibitor ML-7) and Rho-kinase (inhibitor Y27632) in the following PAF-induced malfunctions: vasoconstriction, capillary and mucosal leakage, oedema formation, malabsorption and atony. Among these, the PAF-induced vasoconstriction and hyperpermeability appear to be governed by similar mechanisms that involve IP3 receptors, extracellular calcium and the Rho-kinase. Our findings further suggest that cAMP-elevating treatments - while effective against hypertension and oedema - bear the risk of dysmotility and reduced nutrient uptake. Agents such as 2-APB or Y27632, on the other hand, showed no negative side effects and improved most of the PAF-induced malfunctions suggesting that their therapeutic usefulness should be explored.

Quinidine, but not eicosanoid antagonists or dexamethasone, protect the gut from platelet activating factor-induced vasoconstriction, edema and paralysis

Intestinal circulatory disturbances, atony, edema and swelling are of great clinical relevance, but the related mechanisms and possible therapeutic options are poorly characterized, in part because of the difficulties to comprehensively analyze these conditions. To overcome these limitations we have developed a model of the isolated perfused rat small intestine where all of these symptoms can be studied simultaneously. Here we used this model to study the role of eicosanoids, steroids and quinidine in platelet-activating factor (PAF)-induced intestinal disorders. A vascular bolus of PAF (0.5 nmol) triggered release of thromboxane and peptidoleukotrienes into the vascular bed (peak concentration 35 nM and 0.8 nM) and reproduced all symptoms of intestinal failure: mesenteric vasoconstriction, translocation of fluid and macromolecules from the vasculature to the lumen and lymphatics, intestinal edema formation, loss of intestinal peristalsis and decreased galactose uptake. All effects of PAF were abolished by the PAF-receptor antagonist ABT491 (2.5 μM). The COX and LOX inhibitors ASA and AA861 (500 μM, 10 μM) did not exhibit barrier-protective effects and the eicosanoid antagonists SQ29548 and MK571 (10 μM, each) only moderately attenuated the loss of vascular fluid, the redistribution to the lumen and the transfer of FITC dextran to the lumen. The steroid dexamethasone (10 μM) showed no barrier-protective properties and failed to prevent edema formation. Quinidine (100 μM) inhibited the increase in arterial pressure, stabilized all the intestinal barriers, and reduced lymph production and the transfer of FITC dextran to the lymph. While quinidine by itself reduced peristalsis, it also obviated paralysis, preserved intestinal functions and prevented edema formation. We conclude that quinidine exerts multiple protective effects against vasoconstriction, edema formation and paralysis in the intestine. The therapeutic use of quinidine for intestinal ailments deserves further study.

Ingestion of (n-3) fatty acids augments basal and platelet activating factor-induced permeability to dextran in the rat mesenteric vascular bed

Loss of intestinal barrier function and subsequent edema formation remains a serious clinical problem leading to hypoperfusion, anastomotic leakage, bacterial translocation, and inflammatory mediator liberation. The inflammatory mediator platelet activating factor (PAF) promotes eicosanoid-mediated edema formation and vasoconstriction. Fish oil-derived (n-3) fatty acids (FA) favor the production of less injurious eicosanoids but may also increase intestinal paracellular permeability. We hypothesized that dietary (n-3) FA would ameliorate PAF-induced vasoconstriction and enhance vascular leakage of dextran tracers. Rats were fed either an (n-3) FA-rich diet (EPA-rich diet; 4.0 g/kg EPA, 2.8 g/kg DHA) or a control diet (CON diet; 0.0 g/kg EPA and DHA) for 3 wk. Subsequently, isolated and perfused small intestines were stimulated with PAF and arterial pressure and the translocation of fluid and macromolecules from the vasculature to lumen and lymphatics were analyzed. In intestines of rats fed the EPA-rich diet, intestinal phospholipids contained up to 470% more EPA and DHA at the expense of arachidonic acid (AA). The PAF-induced increase in arterial pressure was not affected by the EPA-rich diet. However, PAF-induced fluid loss from the vascular perfusate was higher in intestines of rats fed the EPA-rich diet. This was accompanied by a greater basal loss of dextran from the vascular perfusate and a higher PAF-induced transfer of dextran from the vasculature to the lumen (P = 0.058) and lymphatics. Our data suggest that augmented intestinal barrier permeability to fluid and macromolecules is a possible side effect of (n-3) FA-rich diet supplementation.

A model of the isolated perfused rat small intestine

Intestinal edema remains a serious clinical problem, and novel approaches to study its pathophysiology are needed. It was our aim to develop a long-term stable isolated perfused rat small bowel preparation permitting analysis of vascular, luminal, interstitial, and lymphatic compartments and to demonstrate the utility of this model by studying the effects of the proinflammatory mediator platelet-activating factor (PAF). A temperature-controlled chamber with an integrated balance was designed to perfuse isolated intestines through the mesenteric artery and the gut lumen. Steroids or oxygen carriers were not needed. Functional and morphological integrity of the tissue was preserved for several hours as confirmed by oxygen consumption, venous lactate-to-pyruvate ratio, arterial and venous pH, lactose digestion and galactose uptake, intravascular and luminal pressures, maintained fluid homeostasis, gut motility, and quantitative light microscopic analysis. Administration of PAF caused typical effects such as vasoconstriction, gut atony, and loss of galactose uptake. PAF also elicited a transient loss of 20% of the perfusate liquid from the mesenteric vascular bed, two-thirds of which were transferred to the lumen. All these responses were entirely reversible. This new model provides detailed insights into the physiology of the small intestine and will allow to study fundamental processes such as fluid homeostasis, barrier functions, transport mechanisms, and immune responses in this organ. Using this model, here we show a dramatic and yet reversible response of the rat small bowel to PAF, suggesting luminal water clearance as a novel safety factor in the intestine that may be of clinical relevance.

Involvement of platelet activating factor in the endotoxin-induced effects on gastrointestinal electrical activity and some haematological parameters in the conscious miniature pig

In conscious miniature pigs the influence of intravenous dose of lipopolysaccharide (LPS), 10 microg/kg over 10 min, with and without pretreatment with a platelet activating factor (PAF) antagonist, SAH 63-675 10 mg/kg, on gastrointestinal electrical activity, arterial pressure and clinical and haematological parameters was studied. Dose of LPS provoked mild clinical signs and hypotension, which were prevented by PAF antagonism. The LPS induced leukocytosis and increase in mature neutrophils, however, were PAF independent. Pretreatment with the PAF antagonist attenuated the LPS-provoked inhibition of electrical activity in the antrum, jejunum, ileum and caecum. These results suggest a beneficial effect of PAF antagonism in porcine endotoxaemia.

Regulation of intestinal blood flow

The gastrointestinal system anatomically is positioned to perform two distinct functions: to digest and absorb ingested nutrients and to sustain barrier function to prevent transepithelial migration of bacteria and antigens. Alterations in these basic functions contribute to a variety of clinical scenarios. These primary functions intrinsically require splanchnic blood flow at both the macrovascular and microvascular levels of perfusion. Therefore, a greater understanding of the mechanisms that regulate intestinal vascular perfusion in the normal state and during pathophysiological conditions would be beneficial. The purpose of this review is to summarize the current understanding regarding the regulatory mechanisms of intestinal blood flow in fasted and fed conditions and during pathological stress.

Beneficial effects of lexipafant, a PAF antagonist on gut barrier dysfunction caused by intestinal ischemia and reperfusion in rats

BACKGROUND

Platelet-activating factor (PAF) may play a pivotal role in the pathogenesis of intestinal ischemic injury.

METHODS

The potential role of PAF in intestinal ischemia and reperfusion (I/R) and the development of gut endothelial and epithelial barrier dysfunction and distant organ injury were investigated by pretreatment with a PAF antagonist, lexipafant. Bidirectional permeability of the intestinal barrier, enteric bacterial translocation, protease-antiprotease balance and mucosal histology, and also changes in pulmonary and liver endothelial barrier permeability were measured following intestinal ischemia for 40 min with 6 h of reperfusion in rats.

RESULTS

Intestinal mucosal endothelial and epithelial permeabilities significantly increased in animals with I/R. Lexipafant prevented the increase in albumin leakage from blood to the mucosal interstitium and the intestinal lumen during reperfusion, and the mucosal albumin leakage from the gut lumen to blood during I/R. Bacterial translocation was frequently noted in animals with I/R, while only a few positive cultures were obtained in animals with I/R administered lexipafant. Less leakage of fluorescein isothiocyanate dextran 70,000 into the interstitial space and gut lumen in I/R animals with lexipafant pretreatment was found under fluorescein microscopy. Lexipafant also partly prevented C1 inhibitor, prekallikrein, and factor X consumption in I/R animals and partly prevented changes in pulmonary and liver albumin leakage.

CONCLUSIONS

PAF seems to play an important role in I/R-associated intestinal dysfunction and the development of distant organ dysfunction, probably by triggering endothelial and epithelial barrier dysfunction. Furthermore, PAF seems to be partly involved in activation of the protease-antiprotease system. The use of PAF antagonists may provide a mode of treatment against I/R-associated organ dysfunction.

Platelet-activating factor and bacteremia-induced pulmonary hypertension

BACKGROUND

Acute lung injury is a common complication of gram-negative sepsis. Pulmonary hypertension and increased lung vascular permeability are central features of lung injury following experimental bacteremia. Platelet-activating factor is a prominent proinflammatory mediator during bacterial sepsis. Our previous studies have demonstrated that exogenous administration of platelet-activating factor (PAF) induces pulmonary edema without causing pulmonary hypertension. Interestingly, inhibition of PAF activity during Escherichia coli bacteremia prevents the development of both pulmonary hypertension and pulmonary edema. These data suggest that PAF contributes to pulmonary hypertension during sepsis, but that this is unlikely to be a direct vascular effect of PAF. The goal of the present study was to investigate the mechanism by which acute E. coli bacteremia induces pulmonary injury and to define the role that PAF plays in this injury. We hypothesized that the effects of PAF on pulmonary hypertension during bacteremia are due to the effects of PAF on other vascular mediators. Several studies suggest that PAF induces the expression of endothelin-1 (ET), a potent peptide vasoconstrictor. Further, our previous studies have implicated ET as a central mediator of systemic vasoconstriction during bacteremia. We therefore sought to assess whether ET is modulated by PAF. E. coli has also been demonstrated to increase endothelial production of nitric oxide (NO), which contributes to maintenance of basal vascular tone in the pulmonary circulation. We hypothesized that PAF might increase pulmonary vascular resistance during bacteremia by activating neutrophils, increasing expression of ET, and decreasing the tonic release of NO. Furthermore, we hypothesized that hypoxic vasoconstriction did not contribute to pulmonary vasoconstriction during the first 120 min of E. coli bacteremia.

METHODS

Pulmonary artery pressure (PAP), blood pressure (BP), heart rate (HR), and arterial blood gases (ABG) were measured in anesthetized spontaneously breathing adult male Sprague-Dawley rats. E. coli (10(9) CFU/100 g body wt) was injected at t = 0, and hemodynamic data were obtained at 10-min intervals and ABG data at 30-min intervals for a total of 120 min. Sham animals were treated equally but received normal saline in place of E. coli. In treatment groups, a 2.5 mg/kg dose of WEB 2086, a PAF receptor antagonist, was administered intravenously 15 min prior to the onset of sepsis or sham sepsis. The groups were (1) intravenous E. coli (n = 5); (2) intravenous WEB 2086 pretreatment + intravenous E. coli (n = 5); (3) intravenous WEB 2086 alone (n = 5); and (4) intravenous normal saline (n = 6). Nitric oxide metabolites (NOx) and ET concentrations were assayed from arterial serum samples obtained at the end of the protocol. Lung tissue was harvested for measurement of myeloperoxidase (MPO) activity and pulmonary histology.

RESULTS

E. coli bacteremia increased HR, PAP, and respiratory rate early during sepsis (within 20 min), while hypoxemia, hypotension, and hemoconcentration were not manifest until the second hour. Pretreatment with WEB 2086 completely abrogated all of these changes. E. coli bacteremia increased the activity of serum ET, lung MPO, and neutrophil sequestration in the lung parenchyma via a PAF-dependent mechanism. However, the mechanism of increased production of NO appears to be PAF independent.

CONCLUSIONS

These data support the hypothesis that E. coli bacteremia rapidly induces pulmonary hypertension stimulated by PAF and mediated at least in part by endothelin-1 and neutrophil activation and sequestration in the lung. Microvascular injury with leak is also mediated by PAF during E. coli bacteremia, but the time course of resultant hypoxemia and hemoconcentration is slower than that of pulmonary hypertension. The contribution of hypoxic vasoconstriction in exacerbating pulmonary hypertension in gram-negative sepsis is probably a late

Influence of platelet activating factor on gastrointestinal electrical activity and some haematological and clinical parameters in the conscious miniature pig

The effects of intravenous (i.v.) infusion of platelet-activating factor (PAF), 100 ng/kg/min for 10 min, with and without pretreatment with a selective PAF-antagonist on gastrointestinal electrical activity, arterial pressure and clinical and haematological parameters were studied. Conscious miniature pigs with electrodes implanted in the wall of the antrum pylori and small and large intestine were used. Platelet-activating factor induced restlessness or depression, shivering, tachypnoea and coughing, retching and vomiting, hypotension and a delayed and sustained increase in leucocyte count with an increase in percentage of segmented neutrophils. The PAF-antagonist, SAH 63-675, administered at 10 mg/kg intravenously, inhibited these effects. Platelet-activating factor resulted in a decrease in electrical activity in the antrum and large intestine, whereas small intestinal activity was not significantly influenced. Pretreatment with the antagonist suppressed these inhibitory effects.