Increased releasability of platelet products and reduced heparin-induced platelet factor 4 release from endothelial cells in bronchial asthma.

To determine whether or not platelet activation is involved in the mechanism of exacerbation of bronchial asthma, we evaluated adenosine triphosphate (ATP) release from thrombin-stimulated washed platelets, plasma levels of beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4), and plasma beta-TG/PF4 ratios during symptomatic and asymptomatic periods in 15 patients with bronchial asthma compared with 16 normal control subjects. We also measured these parameters during allergen provocation tests and acetylcholine inhalation tests in 6 allergic asthmatics. ATP release, plasma levels of beta-TG and PF4 were significantly increased during symptomatic periods and after the allergen provocations but not after acetylcholine inhalations. However, these findings were not accompanied by the elevation of plasma beta-TG/PF4 ratios. The heparin-induced PF4 release, which is reported to reflect release of PF4 attached on endothelial cells, was significantly reduced in 12 asymptomatic asthmatic patients compared with 11 normal subjects, and it was much more reduced in 7 symptomatic asthmatic patients, suggesting the possibility of the reduced PF4 binding on endothelial cell surface. This finding may represent the prolonged half life of PF4 in asthmatics. We concluded that 1) increased releasability of platelet products and in vivo release of the platelet granular contents are involved in the mechanism of the exacerbation of bronchial asthma, 2) some functional alteration in platelet-endothelial cell interaction may be involved in bronchial asthma, and 3) plasma beta-TG/PF4 ratios are not elevated possibly because of both increased platelet releasability and prolonged half-life of PF4 in the blood in asthmatic patients.