Fetal pulmonary vasodilation after exogenous platelet-activating factor

Platelet Activating Factor (PAF): A Mediator of Inflammation

Platelet-activating factor (PAF) is a phospholipid-derived mediator with an established role in multiple inflammatory states. PAF is synthesized and secreted by multiple cell types and is then rapidly hydrolyzed and degraded to an inactive metabolite, lyso-PAF, by the enzyme PAF acetylhydrolase. In addition to its role in platelet aggregation and activation, PAF contributes to allergic and nonallergic inflammatory diseases such as anaphylaxis, sepsis, cardiovascular disease, neurological disease, and malignancy as demonstrated in multiple animal models and, increasingly, in human disease states. Recent research has demonstrated the importance of the PAF pathway in multiple conditions including the prediction of severe pediatric anaphylaxis, effects on blood-brain barrier permeability, effects on reproduction, ocular diseases, and further understanding of its role in cardiovascular risk. Investigation of PAF as both a biomarker and a therapeutic target continues because of the need for directed management of inflammation. Collectively, studies have shown that therapies focused on the PAF pathway have the potential to provide targeted and effective treatments for multiple inflammatory conditions.

PAF increases vascular permeability without increasing pulmonary arterial pressure in the rat

In vivo pulmonary arterial catheterization was used to determine the mechanism by which platelet-activating factor (PAF) produces pulmonary edema in rats. PAF induces pulmonary edema by increasing pulmonary microvascular permeability (PMP) without changing the pulmonary pressure gradient. Rats were cannulated for measurement of pulmonary arterial pressure (Ppa) and mean arterial pressure. PMP was determined by using either in vivo fluorescent videomicroscopy or the ex vivo Evans blue dye technique. WEB 2086 was administered intravenously (IV) to antagonize specific PAF effects. Three experiments were performed: 1) IV PAF, 2) topical PAF, and 3) Escherichia coli bacteremia. IV PAF induced systemic hypotension with a decrease in Ppa. PMP increased after IV PAF in a dose-related manner. Topical PAF increased PMP but decreased Ppa only at high doses. Both PMP (88 +/- 5%) and Ppa (50 +/- 3%) increased during E. coli bacteremia. PAF-receptor blockade prevents changes in Ppa and PMP after both topical PAF and E. coli bacteremia. PAF, which has been shown to mediate pulmonary edema in prior studies, appears to act in the lung by primarily increasing microvascular permeability. The presence of PAF might be prerequisite for pulmonary vascular constriction during gram-negative bacteremia.

Platelet-activating factor receptors in lamb lungs are downregulated immediately after birth

Platelet-activating factor (PAF) is a phospholipid with diverse biological functions mediated by a G protein-coupled receptor. We determined PAF receptor binding in lung membranes of four groups of perinatal lambs. Membrane protein (100 microg/ml) was incubated for 60 min at 30 degrees C with 0.5-24 nM of acetyl-[(3)H]PAF in 30 mM Tris buffer, pH 7.2, containing 0.25% BSA, 10 mM MgCl(2), and 125 mM choline chloride. PAF bound to membrane was isolated and quantified by scintillation spectrometry, followed with Scatchard analysis for receptor density (B(max)). The B(max) (means +/- SE, fmol/mg protein) were 445.8 +/- 12.3, 244.2 +/- 3.3, 250.6 +/- 3.6, and 419. 9 +/- 8.6 for the fetal, 90-min-old, <1-day-old, and 6- to 12-day-old lambs, respectively. The B(max) for the 90-min-old and <1-day-old lambs were not different but were significantly lower than those of either the term fetal or 6- to 12-day-old lambs. These data show a significant decrease in PAF binding to its receptor and in PAF B(max) in lung membranes of immediate newborn lambs. The dissociation constants (K(D), nM) were 7.7 +/- 0.52, 11.5 +/- 0.34, 6.9 +/- 0.48, and 5.0 +/- 0.53 for fetal, 90-min-old, <1-day-old, and 6- to 12-day-old newborn lamb lungs, respectively. The K(D) of the 90-min-old lamb was the highest of all. PAF receptor gene measured by RT-PCR showed a significant downregulation of PAF receptor gene mRNA in lungs of lambs <1 day old, suggesting a transcriptional regulation of PAF receptor gene expression in the immediate newborn period. We speculate that decreased PAF receptor binding immediately after birth will facilitate the fall in pulmonary vascular resistance in the immediate newborn period.

Platelet activating factor acetylhydrolase activity in lamb lungs is up-regulated in the immediate newborn period

We recently showed that platelet activating factor (PAF) is an important modulator of pulmonary vasomotor tone in the fetus, with a significant decrease in circulating PAF levels in the immediate newborn period. In this study, we have determined PAF catabolism by PAF acetylhydrolase (PAF-Ah) in lungs of near-term fetal and newborn 2- to 16-h (<1 day) and 6- to 12-day-old lambs. The rate of PAF catabolism by lung homogenate protein from the three groups of lamb lungs was studied at 37 degrees C in 30 mM Tris buffer, pH 7.5, containing 0.01% BSA. Each lung homogenate protein was incubated for 10 min with 50 microM [(3)H]acetyl-PAF at pO(2) <50 Torr (hypoxia) and approximately 100 Torr (normoxia). PAF-Ah activity was quantified as amount of lyso-PAF produced. PAF-Ah activity (means +/- SEM, nmol lyso-PAF/min/mg protein) in fetal lung homogenate was 1.19 +/- 0.14 and 2.46 +/- 0.05 during hypoxia and normoxia, respectively. The corresponding values for the newborns were newborn <1 day, 1.65 +/- 0.26 and 2.95 +/- 0.07 and newborn 6-12 days, 1.25 +/- 0.10 and 2.84 +/- 0.05. In all groups, PAF-Ah activity was higher in normoxia than in hypoxia. During normoxia, PAF-Ah activity in newborn <1 day was significantly higher than the activity in fetus, but similar to the activity in newborn 6- to 12-day-old lamb lungs. These data show a significant up-regulation of PAF-Ah activity in lungs in the immediate newborn period. PAF-Ah gene expression measured by RT-PCR showed a significant up-regulation of the PAF-Ah gene in lungs of lambs <1 day old, suggesting a transcriptional regulation of the PAF-Ah gene in the immediate newborn period. These results suggest that up-regulation of PAF-Ah activity after birth with oxygenation will result in a decrease in circulating PAF levels, thereby facilitating the fall in pulmonary vascular resistance in the immediate newborn period.

Platelet-activating factor modulates pulmonary vasomotor tone in the perinatal lamb

Eight near-term fetal lambs were studied acutely in utero to determine role of platelet-activating factor (PAF) in the regulation of vasomotor tone in systemic and pulmonary circulations in the immediate perinatal period. Four fetal lambs were studied predelivery and 2 h postdelivery to determine circulating PAF levels. Aortic and pulmonary arterial pressures and cardiac output were measured continuously, and systemic and pulmonary vascular resistances were calculated. Left pulmonary arterial blood flow was also measured in four fetal lambs. After delivery and oxygenation, circulating PAF levels fell significantly. When WEB-2170, a specific PAF-receptor antagonist, was infused to block effect of endogenous PAF in the eight near-term fetal lambs, systemic vascular resistance fell 30% but pulmonary vascular resistance fell dramatically by 68%. Specificity of WEB-2170 was tested in juvenile lambs and was found to be very specific in lowering vasomotor tone only when tone was elevated by action of PAF. Our data show that endogenous PAF levels in the fetus contribute to maintain a high basal systemic and pulmonary vasomotor tone and that a normal fall in circulating PAF levels after birth and oxygenation may facilitate fall in pulmonary vascular resistance at birth.

Role of platelet-activating factor on extravascular lung water after coronary reperfusion in dogs

Platelet-activating factor (PAF), one of the harmful substances released after coronary reperfusion, has been reported to increase pulmonary vascular permeability and induce pulmonary edema. In this study, we sought to examine the possible role of PAF in the genesis of pulmonary edema after coronary reperfusion. Extravascular lung water (EVLW) was measured by the thermal-dye double indicator dilution method during coronary ligation and after reperfusion in situ in dogs. The proximal left anterior descending coronary artery was occluded for 15 min and reperfused in 5 dogs (group 1), while five other dogs (group 2) were treated with PAF-antagonist (TCV-309, 1 mg/kg) before coronary artery occlusion. EVLW and hemodynamic indices were measured at baseline, 15 min of coronary occlusion, and 15 and 30 min after coronary reperfusion. EVLW increased at 15 min of coronary occlusion in both groups, but there was no significant difference between the two groups (6.4 to 10.3 ml/kg and 5.4 to 7.1 ml/kg in groups 1 and 2, respectively). After coronary reperfusion, EVLW increased further in group 1 (6.4 to 16.5 ml/kg, p < 0.01), but no further increase was observed in group 2 at 30 min after coronary reperfusion. There were no significant differences in hemodynamic indices between the two groups throughout the test. Thus, PAF-antagonist attenuated the increase in EVLW after coronary reperfusion independent of hemodynamic indices, and hence, PAF may play an important role in the genesis of pulmonary edema caused by coronary reperfusion.

The effects of cGMP on fetal sheep pulmonary blood flow and lung liquid production

This study was designed to determine the effects of a membrane permeant phosphodiesterase-resistant analog of cGMP on lung liquid production and pulmonary blood flow at the time of birth. Experiments were performed on seven fetal sheep prepared for chronic measurements of lung liquid production (Jv), pulmonary blood flow (Qp) and pressure, as well as systemic pressure. Injection of either 8-bromo-cGMP or saline were made via a catheter inserted in the left pulmonary artery. Experiments consisted of 1 h of control, 1 h of infusion, and 2 h of recovery. Data were analyzed by ANOVA and Newman-Keuls test. After infusion of 8-bromo-cGMP, Jv was decreased by 70 and 44% from control in h 3 and 4, respectively. Qp was elevated by 100 mL/min in h 2 and 3 and continued to be elevated by 50 mL/min in h 4. Saline infused animals showed no significant changes in Qp and Jv. This study demonstrates that 8-bromo-cGMP decreases lung liquid production and increases pulmonary blood flow in near term fetal sheep. Although blood flow increased in h 2, lung liquid production did not decrease at this time, suggesting a time dissociation between changes in pulmonary blood flow and lung liquid production. Thus, it is possible that a common transduction pathway involving cGMP may be responsible for lung liquid reduction and elevation of pulmonary blood flow at birth. However, Qp and Jv may not be causally related.

Effects of a paf-receptor antagonist on hemodynamics during and after cardiopulmonary bypass

OBJECTIVES

To assess after cardiopulmonary bypass (CPB) the role of paf-acether (paf), a phospholipid mediator whose injection in animal mimics the hemodynamics observed after CPB.

DESIGN

Prospective double-blind randomized study.

SETTING

Single institutional university hospital.

PARTICIPANTS

18 patients scheduled to undergo coronary artery bypass graft.

INTERVENTIONS

18 patients randomly received a placebo (n = 8) or 120 mg BN52021 (n = 10), a paf-receptor antagonist injected twice just before vascular cannulation and before cross-clamp release.

MEASUREMENTS AND MAIN RESULTS

Hemodynamic measurements were performed with a pulmonary artery and a radial artery catheter before and after the first injection of BN52021 or placebo, at the end of CPB, 1, 15, and 30 minutes after protamine infusion, then 6 hours and 24 hours postoperatively. BN52021 infusion, did not affect hemodynamic parameters. After CPB, the pulmonary artery pressures, the cardiac index, and the pulmonary artery occlusion pressures were statistically the same between groups. By contrast, the pulmonary vascular resistances (1.5 +/- 0.5 IU v 4.5 +/- 0.6 IU, p < 0.05), the right ventricular systolic work index (5.3 +/- 0.91 g m m-2 v 9.37 +/- 1.02 g m m-2, p < 0.05) and the transpulmonary gradient (4.7 +/- 1.1 mmHg v 12.0 +/- 1.2 mmHg, p < 0.05) were lower in the BN52021 group as compared with the placebo group. After protamine infusion, these differences between groups disappeared.

CONCLUSION

Because the inotropic and vasodilator therapy and the volume loading were the same between groups, this study suggests that pretreatment with a paf-receptor antagonist improves post-CPB pulmonary resistance. Nevertheless, this beneficial effect is transient without consequences on left ventricular function indices.