Nitric Oxide Mediates Platelet Activating Factor-Induced Microvascular Leakage in Rat Airways

Abstract

Platelet activating factor (PAF), a highly potent chemical mediator in inflammation and allergic reactions, induces microvascular leakage in several tissues. In rat airways, PAF-induced microvascular leakage is probably mediated by an endothelial cell receptor in the microvessels. Nitric oxide (NO), first identified as endothelium-derived relaxing factor, has been suggested to be a mediator of airway microvascular leakage. However, the role of NO in PAF-induced microvascular leakage in the airways has not yet been established. The aim of this study was to investigate the role of NO in PAF-induced microvascular leakage in rat nasal mucosa and trachea. We injected PAF (1 microg/kg) intravenously, and the amount of PAF-induced microvascular leakage was measured with extravasation of Evans blue dye (30 mg/kg, injected intravenously 5 minutes before the injection of PAF) by means of spectrophotometry and fluorescence microscopy. Five Sprague-Dawley rats were pretreated with NG-nitro-L-arginine methyl ester (L-NAME; 10 mg/kg, intravenously injected 1 hour before the injection of PAF) to inhibit NO synthase, and control rats (n = 4) were pretreated with normal saline solution. The average amount of extravasated Evans blue dye was significantly lower in the L-NAME-pretreated rats than in the control rats (t-test, p < .01). Tissue sections of the L-NAME-pretreated rats clearly showed a decreased extravasation of Evans blue dye on fluorescence microscopy. In conclusion, pretreatment with L-NAME clearly inhibited PAF-induced microvascular leakage in the nasal mucosa and trachea of rats. This finding implies that PAF may activate the constitutive endothelial NO synthase in the microvessels, and that activated endogenous NO may mediate PAF-induced microvascular leakage in rat airways.

Platelet-activating factor reduces endothelial nitric oxide production: role of acid sphingomyelinase

“…One possible explanation is that this differential regulation of NO represents another example of the frequently contrarian regulatory mechanisms of the systemic and the pulmonary circulation, for example high versus low arterial blood pressure or hypoxic dilatation versus hypoxic constriction. However, that would not explain why NO donors are protective against PAF-induced oedema in both the lungs [16] and the intestine [36]. Alternatively, these differences might be explained in terms of the bell-shaped dose-response curve of NO where either too little or too much NO increases vascular permeability.…”

“…Probable reasons to explain conflicting reports showing either attenuation [16] or aggravation [29,30] of pulmonary oedema by NO synthase inhibitors are: 1) the different timing of the measurements; 2) the numerous and diverse effects of NO on leukocytes and endothelial cells; and 3) the notion that both too much and too little NO can increase vascular permeability [11]. The model of the isolated blood-free perfused rat lung allows circumvention of most of these confounders.…”

“…In the lungs, exogenous NO has reduced vascular leak formation in animal models of acute lung injury [13,14] and even in patients [15], although under certain conditions a blockade of NO production may be beneficial as well [16]. These, and other seemingly contradictory, findings have not yet been completely reconciled [17], although it seems likely that vascular permeability is increased by both too little and too much NO.…”

Role of Nitric Oxide and Reactive Oxygen Species in Platelet-Activating Factor-Induced Microvascular Leakage

“…The mechanism by which PAF increases vascular permeability is not clearly understood although there is evidence that PAF-induced leakage is associated with activation of protein kinase C [12, 13]and tyrosine kinase pathways [14], and opening of L-type calcium channels [15]. Furthermore, studies have shown that PAF-induced leakage can be attenuated by inhibitors of nitric oxide synthase (NOS) [16, 17, 18]or by scavenging reactive oxygen species (ROS) [7, 9, 11, 19]suggesting that both NO and ROS are involved in PAF-induced leakage. The relationship between PAF and NO is particularly interesting in the hamster cheek pouch microcirculation because, while NOS inhibition attenuates PAF-induce leakage, it is not because NO mediates the leakage.…”

Candidate's Thesis: Platelet‐Activating Factor–Induced Hearing Loss: Mediated by Nitric Oxide?

“…Recent studies reported that PAF stimulates nitric oxide synthase (NOS) to release nitric oxide (NO) and that PAF actions are mediated by this endogenous NO in endotoxic shock, cardiopulmonary actions, microvascular permeability, and bowel injury in animal experiments. 24–28 NO is produced by a family of NOS, including neuronal NOS (nNOS, type 1), inducible NOS (iNOS, type 2), and endothelial NOS (eNOS, type 3). All three major isoforms of NOS have been demonstrated immunohistochemically in different cells of the organ of Corti and other parts of the auditory systems of guinea pigs.…”