Lipoxygenase metabolites mediate increased airways responsiveness to histamine after acute platelet activating factor exposure in the guinea-pig

Effect of a 5-lipoxygenase inhibitor and leukotriene antagonist (PF 5901) on PAF-induced airway responses in neonatally immunized rabbits

1. Aerosol administration of platelet activating factor (PAF) (80 micrograms ml-1 for 60 min) to neonatally immunized rabbits caused bronchoconstriction which was far in excess of that produced by a comparable aerosol of bovine serum albumin (BSA), the carrier molecule for PAF. Bronchoconstriction of a similar magnitude was elicited by PAF in immunized, sham-immunized and normal rabbits. 2. Aerosol administration of PAF to immunized rabbits induced enhanced airway responsiveness to inhaled histamine in all animals tested, 24 h and 72 h after exposure. In not all cases had airways responsiveness returned to basal levels at 1 week following PAF challenge. In contrast, following exposure of immunized rabbits to BSA, no significant changes in airway responsiveness to histamine were evident at any of the measured time points. 3. A significant increase in the total number of inflammatory cells recovered in bronchoalveolar lavage (BAL) fluid was determined 24 h and 72 h following PAF exposure in immunized rabbits. This was associated with a significant increase in the number of neutrophils and eosinophils. Similar changes were observed following exposure of PAF to normal and sham-immunized rabbits. No change in the total number of inflammatory cells was obtained in BAL after BSA challenge to immunized rabbits; however, neutrophil numbers were significantly increased. 4. PF 5901, a specific inhibitor of the 5-lipoxygenase pathway of arachidonic acid metabolism and a leukotriene D4 antagonist, at a dose of 10 mg (direct intratracheal administration) significantly inhibited the airway resistance (RL) component of the bronchoconstriction induced by PAF in neonatally-immunized rabbits. Doses of 10 mg, 3 mg and 1 mg PF 5901 (direct intratracheal administration) were sufficient to inhibit significantly the PAF-induced increase in airways responsiveness to inhaled histamine in immunized rabbits. PF 5901 however, failed to alter the pulmonary cell infiltration induced by PAF,as assessed by BAL.5. We suggest from the results of the present study that PAF induces consistent and long-lasting increases in airways responsiveness to histamine in immunized rabbits, which is mediated, at least in part, by products of the 5-lipoxygenase metabolic pathway. Furthermore, the inability of PF 5901 to inhibit the influx of inflammatory cells into the airway lumen following PAF challenge may suggest that bronchial hyperresponsiveness and cellular infiltration are not strictly associated events.

Animal models for testing anti-inflammatory drugs for treatment of bronchial hyperreactivity in asthma

In the first part of this review the important role played by the bronchial hyperreactivity caused by chronic bronchopulmonary inflammation in asthma is described. Deliberately, more emphasis is placed on the role of pro-inflammatory eosinophils, alveolar macrophages, lymphocytes and platelets rather than on mast cells and neutrophils or the numerous mediators. The reason for this is that, on account of the large number of mediators and their multitude of functions and interactions in asthma, antagonism of a specific mediator will probably not be clinically relevant for optimally effective curative treatment of asthma. Inhibition of the infiltration and activation of pro-inflammatory cells is likely to be a more successful approach. In the second part, various animal models of bronchial hyperreactivity, which could be suitable for testing anti-asthmatic drugs, are discussed. Most animal models pay too little attention to chronic bronchopulmonary inflammation as the cause of bronchial hyperreactivity in asthma. In various models the bronchial hyperreactivity is provoked by a single mediator and this leads to selection of specific antagonists which are unlikely to be of clinical benefit. Rats appear to have certain advantages over guinea-pigs as experimental animals for bronchial hyperreactivity.

Platelet activating factor and tracheobronchial respiratory glycoconjugate release in feline and human explants: involvement of the lipoxygenase pathway

It has been suggested that platelet activating factor (PAF) may participate in many aspects of bronchial asthma, including stimulation of mucus secretion. Feline tracheal and human bronchial explant production of respiratory glycoconjugates (RGC) in response to platelet activating factor (PAF) was investigated, in order to differentiate the actions of this putative mediator on mucus secretion. PAF caused a dose-dependent increase in RGC release in concentrations ranging from 100-0.5 microM during a 1-2 hours incubation with either feline or human explants, and the effect was inhibited by the PAF receptor antagonists Ro 19-3704. Several lines of evidence suggest that PAF enhances RGC release indirectly through stimulation of the production of lipoxygenase metabolites of arachidonic acid. 1) Incubation of 10 microM PAF together with arachidonic acid (100 micrograms/ml) enhances PAF's stimulatory effect on RGC release in cats. 2) The cyclooxygenase inhibitor ibuprofen (65 and 420 microM) either failed to effect or slightly enhanced PAF induced RGC release in both species. 3) The combined cyclooxygenase and lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA) as well as the putatively specific 5-lipoxygenase inhibitor L-651,392 (both at 50 microM) inhibited the response to PAF in both species. 4) The putative LTD4 receptor antagonists (L-660,711, 100 microM) slightly reduced the PAF secretory response in human bronchi. We conclude that PAF causes specific receptor mediated RGC release. This response is indirectly mediated through the generation of lipoxygenase metabolite formation including 5-lipoxygenase pathway metabolites.