Effect of heparin and a low-molecular weight heparinoid on PAF-induced airway responses in neonatally immunized rabbits.

1. We have investigated the effect of an unfractionated heparin preparation, a low-molecular weight heparinoid (Org 10172) and the polyanionic molecule polyglutamic acid against PAF-induced airway hyperresponsiveness and pulmonary cell infiltration in neonatally immunized rabbits in vivo. 2. Exposure of neonatally immunized rabbits to aerosolized platelet activating factor (PAF) (80 micrograms ml-1 for 60 min) elicited an increase in airway responsiveness to inhaled histamine 24 h and 72 h following challenge which was associated with an infiltration of inflammatory cells into the airways, as assessed by bronchoalveolar lavage (BAL). 3. A significant increase in the total numbers of cells recovered from BAL fluid was associated with significantly increased cell numbers of neutrophils, eosinophils and mononuclear cells 24 h following PAF exposure. The numbers of eosinophils and neutrophils in the airways remained elevated 72 h after challenge. 4. The intravenous administration of an unfractionated preparation of heparin (100 units kg-1) or Org 10172 (100 micrograms kg-1) 30 min prior to PAF exposure significantly inhibited the airway hyperresponsiveness induced by PAF, 24 h and 72 h following challenge. PAF-induced hyperresponsiveness was not significantly affected by prior intravenous administration of polyglutamic acid (100 micrograms kg-1). 5. The intravenous administration of unfractionated heparin (100 units kg-1), Org 10172 (100 micrograms kg-1) or polyglutamic acid (100 micrograms kg-1) 30 min prior to PAF exposure significantly inhibited the expected increase in total cell infiltration. 6. This study shows that unfractionated heparin and a low-molecular weight heparinoid, Org 10172, are capable of inhibiting both the airway hyperresponsiveness and pulmonary cell infiltration induced by PAF in the rabbit.