The present findings outline a new signaling cascade in the induction of PAF-induced lung edema, in that stimulation of ASM causes recruitment of TRPC6 channels to caveolae, thus allowing for Ca(2+) influx and subsequent increases in endothelial permeability that are amplified in the absence of endothelial NO synthesis.
Platelet-activating factor (PAF) is a mediator of pulmonary oedema in acute lung injury that increases vascular permeability within minutes, partly through activation of acid sphingomyelinase (ASM). Since caveolae are rich in sphingomyelin and caveolin-1, which block endothelial nitric oxide (NO) synthase (eNOS) by direct binding, we examined the relationship between ASM, caveolin-1 and eNOS activity in the regulation of vascular permeability by PAF.In caveolar fractions from pulmonary vascular endothelial cells (isolated from perfused rat lungs) the abundance of caveolin-1 and eNOS increased rapidly after PAF perfusion. PAF treatment decreased endothelial NO (eNO) formation as assessed by in situ fluorescence microscopy. Restoration of eNO levels with PAPA-NONOate ((Z)-1-[N-(3-ammoniopropyl)-N-(npropyl)amino]diazen-1-ium-1,2-diolate) mitigated the PAF-induced oedema.PAF treatment increased the ASM activity in caveolar fractions and perfusion with ASM decreased eNO production. Pharmacological inhibition of the ASM pathway with imipramine, D609 or dexamethasone blocked the PAF-induced increase of caveolin-1 and eNOS in caveolae, and the decrease in eNO production and oedema formation.We conclude that PAF causes ASM-dependent enrichment of caveolin-1 in caveolae of endothelial cells, leading to decreased eNO production which contributes to pulmonary oedema formation. These findings suggest rapid reduction in eNO production as a novel mechanism in the regulation of vascular permeability.
Increased endothelial permeability and vascular barrier failure are hallmarks of inflammatory responses in both the pulmonary and the systemic circulation. Platelet-activating factor (PAF) has been implicated as an important lipid mediator in the formation of pulmonary and extrapulmonary edema. Ostensibly, the PAF-induced signaling pathways in endothelial cells utilize similar structures and molecules including acid sphingomyelinase, ceramide, caveolae, endothelial nitric oxide synthase, and nitric oxide, in pulmonary and systemic microvessels. Yet, the constituents of these signaling pathways act and respond in distinctly different and frequently opposing ways in the lung versus organs of the systemic circulation. By confronting seemingly discrepant findings from the literature, we reconstruct the differential signaling pathways by which PAF regulates edema formation in the systemic and the pulmonary vascular bed, and trace this dichotomy from the level of myosin light chain kinase via the regulation of endothelial nitric oxide synthase and sphingomyelinase signaling to the level of caveolar trafficking. Here, we propose that PAF regulates vascular barrier function in individual organs by opposing signaling pathways that culminate in increased respectively decreased nitric oxide synthesis in the systemic and the pulmonary endothelium. The present review may provide a physiological explanation for the overall disappointing results of previous pharmacological strategies in conditions of generalized barrier failure such as sepsis, and instead advertises the development of organ-specific interventions by targeting the individual composition or trafficking of endothelial caveolae.
Chronic kidney disease (CKD) is associated with excessive mortality from cardiovascular disease (CVD). Endothelial dysfunction, an early manifestation of CVD, is consistently observed in CKD patients and might be linked to structural defects of the microcirculation including microvascular rarefaction. However, patterns of microvascular rarefaction in CKD and their relation to functional deficits in perfusion and oxygen delivery are currently unknown. In this in-vivo microscopy study of the cremaster muscle microcirculation in BALB/c mice with moderate to severe uremia, we show in two experimental models (adenine feeding or subtotal nephrectomy), that serum urea levels associate incrementally with a distinct microangiopathy. Structural changes were characterized by a heterogeneous pattern of focal microvascular rarefaction with loss of coherent microvascular networks resulting in large avascular areas. Corresponding microvascular dysfunction was evident by significantly diminished blood flow velocity, vascular tone, and oxygen uptake. Microvascular rarefaction in the cremaster muscle paralleled rarefaction in the myocardium, which was accompanied by a decrease in transcription levels not only of the transcriptional regulator HIF-1α, but also of its target genes Angpt-2, TIE-1 and TIE-2, Flkt-1 and MMP-9, indicating an impaired hypoxia-driven angiogenesis. Thus, experimental uremia in mice associates with systemic microvascular disease with rarefaction, tissue hypoxia and dysfunctional angiogenesis.
BackgroundThe sympathetic nervous system is considered to modulate the endotoxin-induced activation of immune cells. Here we investigate whether thoracic epidural anesthesia with its regional symapathetic blocking effect alters endotoxin-induced leukocyte-endothelium activation and interaction with subsequent endothelial injury.MethodsSprague Dawley rats were anesthetized, cannulated and hemodynamically monitored. E. coli lipopolysaccharide (Serotype 0127:B8, 1.5 mg x kg-1 x h-1) or isotonic saline (controls) was infused for 300 minutes. An epidural catheter was inserted for continuous application of lidocaine or normal saline in endotoxemic animals and saline in controls. After 300 minutes we measured catecholamine and cytokine plasma concentrations, adhesion molecule expression, leukocyte adhesion, and intestinal tissue edema.ResultsIn endotoxemic animals with epidural saline, LPS significantly increased the interleukin-1β plasma concentration (48%), the expression of endothelial adhesion molecules E-selectin (34%) and ICAM-1 (42%), and the number of adherent leukocytes (40%) with an increase in intestinal myeloperoxidase activity (26%) and tissue edema (75%) when compared to healthy controls. In endotoxemic animals with epidural infusion of lidocaine the values were similar to those in control animals, while epinephrine plasma concentration was 32% lower compared to endotoxemic animals with epidural saline.ConclusionsThoracic epidural anesthesia attenuated the endotoxin-induced increase of IL-1β concentration, adhesion molecule expression and leukocyte-adhesion with subsequent endothelial injury. A potential mechanism is the reduction in the plasma concentration of epinephrine.
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