Differential effects of platelet depletion on the physiologic alterations of IgE anaphylaxis and acetyl glyceryl ether phosphorylcholine infusion in the rabbit

Forty Years Since the Structural Elucidation of Platelet-Activating Factor (PAF): Historical, Current, and Future Research Perspectives

In the late 1960s, Barbaro and Zvaifler described a substance that caused antigen induced histamine release from rabbit platelets producing antibodies in passive cutaneous anaphylaxis. Henson described a ‘soluble factor’ released from leukocytes that induced vasoactive amine release in platelets. Later observations by Siraganuan and Osler observed the existence of a diluted substance that had the capacity to cause platelet activation. In 1972, the term platelet-activating factor (PAF) was coined by Benveniste, Henson, and Cochrane. The structure of PAF was later elucidated by Demopoulos, Pinckard, and Hanahan in 1979. These studies introduced the research world to PAF, which is now recognised as a potent phospholipid mediator. Since its introduction to the literature, research on PAF has grown due to interest in its vital cell signalling functions and more sinisterly its role as a pro-inflammatory molecule in several chronic diseases including cardiovascular disease and cancer. As it is forty years since the structural elucidation of PAF, the aim of this review is to provide a historical account of the discovery of PAF and to provide a general overview of current and future perspectives on PAF research in physiology and pathophysiology.

Effect of PAF on rat lung vascular permeability: role of platelets and polymorphonuclear leucocytes

1. The objectives of the present experiments were to assess the contribution of polymorphonuclear leucocytes (PMNLs), platelets and their products such as thromboxane A2 (TxA2), histamine and 5-hydroxytryptamine to platelet activating factor (PAF)-mediated protein extravasation in rat lungs. 2. Intravenous injection of PAF (1.0 and 5.0 micrograms kg-1) increased dose-dependently (up to 7.5 fold) the vascular permeability of the trachea, upper and lower bronchi to Evans blue dye (EB), a marker of albumin extravasation. The permeability of the pulmonary parenchyma was not affected significantly by PAF. 3. Thrombocytopenia induced by administration of the IgG fraction of goat anti-rat platelet serum (APS; 15 mg 100 g-1, i.p., 16-18 h) reduced by 55, 58 and 40% the effects of the lower dose of PAF (1.0 microgram kg-1) and by 31, 23 and 15% the effects of the higher dose of PAF (5.0 micrograms kg-1) on the permeability of the trachea, upper and lower bronchi respectively to albumin. 4. PMNL depletion induced by administration of rabbit anti-rat polymorphonuclear serum (ANS; 2 mg kg-1, i.v., 24 h) did not reduce significantly the effects of the lower dose of PAF (1.0 microgram kg-1) on the airways, however the effects of the higher dose of PAF (5.0 micrograms kg-1) on the permeability of the trachea, upper and lower bronchi to albumin were reduced by 43, 25 and 23% respectively. 5. The injection of both the anti-platelet and the anti-PMNL sera reduced by 61, 62 and 96% the effects of the lower dose of PAF (1.0 microg kg-1) and by 44, 39 and 47% the effects of the higher dose of PAF (5.0 microg kg-1) on the permeability of the trachea, upper and lower bronchi respectively.6. The combined injection of the TxA2-mimetic (U-44069; 5.0 microg kg-1) and PAF (1.0 and 5.0 microg kg-1)in thrombocytopenic rats overcame the vascular permeability decrease induced by APS treatment.7. Pretreatment of the animals with a combination of antagonists to histamine (mepyramine;3.0 mg kg-1) and 5-hydroxytryptamine (methysergide; 2.5 mg kg-1) did not cause a significant inhibition of the effect of PAF (1.0 and 5.0 microg kg-1) on EB extravasation in the airways.8. These data show that the effect of intravenous PAF on rat vascular permeability is partly modulated by polymorphonuclear leucocyte and platelet activation. Our results suggest that following its release,TxA2 could increase postcapillary hydrostatic pressure by inducing a venoconstriction and potentiate the extravasation elicited by PAF. These results do not suggest a major role for histamine and/or 5-hydroxytryptamine on PAF-induced albumin extravasation.

Mechanism of action of platelet activating factor in the pulmonary circulation: an investigation using a novel isotopic system in rabbit isolated lung

1. Rabbit isolated lungs were perfused via the pulmonary artery with Tyrode solution containing 4.5% Ficoll and 0.1% bovine serum albumin at a constant rate of 20 ml min-1. Lung perfusate was drawn for alternating 5 min periods from two reservoirs, one containing 125I-albumin and the other unlabelled albumin to wash out the intravascular label. Microvascular 125I-albumin leakage was determined from the count remaining at the end of the washout phase with an external gamma scintillation probe. In addition, perfusion pressure was monitored continuously. Each experiment comprised 6 cycles over a total period of 60 min. 2. Infusion of platelet activating factor (PAF, 3 nmol min-1 for 10 min) resulted in microvascular 125I-albumin leakage, whereas lyso-PAF was without effect. During PAF infusions there was also an increase in perfusion pressure. Both the permeability and pressor effects of PAF were inhibited by the PAF antagonist L-652731. 3. Infusion of the thromboxane analogue U-46619 (0.6 nmol min-1 for 10 min) caused an increase in perfusion pressure but protein accumulation was not significantly different from that observed with control infusions. 4. Bolus injections of PAF (1 nmol) caused increases in perfusion pressure which were reduced by indomethacin, dazmegrel and BW 755C. Bolus injections of PAF, repeated at 30 min intervals caused reproducible pressor responses; however, repeated injections at 60 min intervals resulted in augmented responses. This augmentation did not occur in the presence of indomethacin. 5. Retrograde perfusion of PAF via the pulmonary vein induced increased perfusion pressure and microvascular 125I-albumin leakage. The observed increase in leakage when compared with forward perfusion suggests that PAF produces predominantly arteriolar constriction i.e. proximal to the site of leakage during forward perfusion. 6. These results indicate that PAF is a vasoconstrictor in the rabbit pulmonary circulation and augmented responses occur with repeated injections at 60 min intervals. Cyclo-oxygenase inhibition abolished this vascular hyperresponsiveness induced by PAF. PAF also caused protein accumulation in the lungs. Both these actions of PAF appear to be receptor-mediated because they were inhibited by PAF antagonists. Another pulmonary vasoconstrictor, U-46619 did not cause protein accumulation suggesting that the extravasation of protein with PAF is not merely secondary to changes in vascular tone.

Effect of a PAF antagonist, BN52063, on PAF-induced bronchoconstriction in normal subjects

1. The effects of an oral platelet activating factor (PAF) antagonist, BN52063, or matched placebo on inhaled PAF challenge were assessed in a double-blind study in eight normal subjects. 2. PAF (24 micrograms) induced an immediate bronchoconstriction, with a maximum fall in flow at 30% of vital capacity from a partial flow volume manoeuvre (Vp30) of 47.1 +/- 7% (mean +/- s.e. mean) 5 min after inhalation following placebo treatment. Repeated PAF challenges at 15 min intervals resulted in tachyphylaxis of the bronchoconstrictor response. 3. Two hours after the ingestion of BN52063 (120 mg) the maximum bronchoconstriction induced by inhaled PAF was attenuated (35.9 +/- 9% fall at 5 min) with a significant reduction (P less than 0.05) in response after the first and second inhalations. 4. Inhaled PAF induced an immediate neutropenia (73.2 +/- 9% fall 5 min after inhalation) followed by a rebound neutrophilia, which were unaffected by pretreatment with BN52063. 5. Oral ingestion of a dose of BN52063 which is effective in reducing skin responses to PAF gave partial protection against the bronchoconstrictor effect of inhaled PAF in normal subjects.

Bronchial and vascular effects of Paf in the rat isolated lung are completely blocked by WEB 2086, a novel specific Paf antagonist

1 The effect of the platelet-activating factor (Paf) antagonist, WEB 2086, on Paf-induced increase of pulmonary artery perfusion pressure (Pp), bronchial inflation pressure (Pi) and wet-to-dry lung weight ratios (W/D) was investigated in the rat isolated lung. 2 Lungs were perfused with Krebs-Ringer solution (KRS) as controls or with KRS containing WEB 2086 (0.1, 1.0, 10.0 or 100 micrograms ml-1) and then injected with a bolus of 20 micrograms Paf. 3 A dose-related inhibition of the Paf-induced increase of Pp, Pi and W/D was observed, being almost maximal for the 10.0 micrograms ml-1 and complete for the 100 micrograms ml-1 doses of WEB 2086 when compared to controls. 4 It is concluded that WEB 2086 is a highly effective and specific Paf antagonist in the pulmonary vasculature and bronchial tract.

Metabolism of platelet-activating factor in isolated perfused rat lung

The administration of platelet-activating factor (PAF) into the airway system of the lung is known to cause profound effects, yet little is known about the metabolism of this active lipid mediator. 3H-Labeled PAF administered into the airway of isolated rat lungs was rapidly and extensively metabolized. The tissue retained 96% of the administered radiolabel while the perfusate contained 4%. Characterization of the tissue retained lipid indicated metabolism into lyso-PAF (3.3%), phosphatidylcholine (82.3%), neutral lipid (1.7%) and intact PAF (10.2%). Analysis of tissue phosphatidylcholine by mass spectrometric techniques revealed metabolism of PAF to 1-0-hexadecyl-2-arachidonoyl-GPC, which represented 20-23% of the administered radiolabeled hexadecyl-PAF. These findings support the hypothesis that a relationship between PAF and arachidonate metabolism exists at the intact organ level. Autoradiographic analysis of the cellular distribution of the radiolabeled PAF metabolites in the lung tissue indicated labeling of two cell types, the alveolar type II cell and the nonciliated bronchiolar epithelial cell (Clara cell).

Acetyl glyceryl ether phosphorylcholine-stimulated human platelets cause pulmonary hypertension and edema in isolated rabbit lungs. Role of thromboxane A2

Macrophages, neutrophils, and platelets may play a role in acute edematous lung injury, such as that seen in the adult respiratory distress syndrome (ARDS), but their potential actions and interactions are unclear. Because stimulated human macrophages and neutrophils can release acetyl glyceryl ether phosphorylcholine (AGEPC), a potent platelet activator, we hypothesized that in ARDS, leukocyte release of AGEPC might stimulate platelets to release thromboxane A2 (TXA2), which then produces pulmonary hypertension and lung edema. In support of this premise, we found that pulmonary hypertension and edema occurred in isolated rabbit lungs perfused with human platelets and AGEPC, but not with platelets or AGEPC alone. Infusion of a vasodilator (nitroglycerin) to maintain base-line pulmonary artery pressures in lungs perfused with platelets and AGEPC prevented the development of lung edema suggesting that platelet and AGEPC-induced edema was hydrostatic in nature. Additional experiments suggested that the increased pressure was a result of TXA2 release from platelets stimulated by AGEPC. Specifically, preincubation of platelets with imidazole, a thromboxane synthetase blocker, prior to infusion with AGEPC significantly diminished pulmonary hypertension and prevented lung edema. Furthermore, pretreating lung preparations with 13-azaprostanoic acid, a TXA2 antagonist, before infusion of AGEPC and untreated platelets also reduced the pulmonary hypertension and blocked the lung edema. The role of TXA2 was further suggested when perfusates from lungs infused with platelets and AGEPC developed high levels of TXA2, whereas perfusates from controls did not. These results suggest that platelet aggregation induced by AGEPC may contribute to ARDS by releasing TXA2, which raises microvascular pressure and increases edema formation, especially when an underlying permeability defect is present.