EFFECTS OF PLATELET-ACTIVATING-FACTOR ON THE DISTRIBUTION OF RADIOLABELED LEUKOCYTES AND PLATELETS IN NORMAL HORSES AND ASYMPTOMATIC HORSESWITH CHRONIC OBSTRUCTIVE PULMONARY-DISEASE

Antigen challenge is known to cause the recruitment of neutrophils to the lungs of horses with chronic obstructive pulmonary disease (COPD). To evaluate a possible role of platelet activating factor (PAF) in th is process, the effects of PAF on the distribution of radiolabelled ne utrophils were compared in normal horses and asymptomatic horses with COPD. Changes in lung function, heart rate and the distribution of pla telets and eosinophils were also measured. PAF (5 ng kg/(-1) intraveno usly) caused immediate but transient increases in the number of radiol abelled neutrophils in the lungs and a concomitant decrease in the per ipheral neutrophil count. The total numbers of circulating leucocytes and neutrophils were also significantly decreased by PAF. Rapid and re versible increases in heart rate, respiratory rare and pleural pressur e were also observed. In separate experiments, the numbers of radiolab elled eosinophils and platelets in the lungs increased transiently aft er the administration of PAF. The responses to PAF were qualitatively and quantitatively similar in normal horses and asymptomatic COPD hors es. The PAF receptor antagonist WEB2086 (3 mg kg(-1) intravenously) in hibited the effects of PAF. These results suggest that PAF, if release d in the lungs of horses with COPD during an antigen challenge, might contribute to the recruitment of leucocytes and the respiratory change s.