Platelets in asthma

Failure of inhibitory effects by platelets on super-oxide anion generation from stimulated neutrophils in a severe bronchial asthmatic

Blood was drawn from healthy human volunteers and a 30-year-old woman with severe bronchial asthma (atopic and perennial type), whose peripheral blood platelets counts were continuously higher than 60 x 10(4)/mm3. Neutrophils were purified on a Conray-Ficoll gradient and platelets were isolated by a gel filtration method with Sepharose 2B. Superoxide anion (O2-) generation from the cells was measured by 2-methyl-6-[p-methoxyphenyl]-3,7-dihydroimidazo [1,2-a]purazin-3-one (MCLA)-dependent luminescence. Addition of 0.5 microM MCLA and 10(-8) M N-formyl-methionyl-leucyl-phenylalanine (fMLP) or 10 ng/ml phorbol myristate acetate (PMA) to a suspension of platelets obtained from the healthy controls and the patient caused no significant MCLA-dependent luminescence, 02- generation by neutrophils from healthy human volunteers, activated by 10(-8) M fMLP or 10 ng/ml PMA, was inhibited by their platelets in a concentration-dependent manner. Platelets from the patient failed to inhibit O2- generation from neutrophils activated by 10(-8)M fMLP. Cu-Zn superoxide dismutase concentrations in platelets from healthy human volunteers were not higher than the patient's. These data suggest that the absence of inhibition of superoxide generation from activated neutrophils by platelets might be involved in the pathogenesis of bronchial asthma.

Platelet activation in nocturnal asthma

Platelet activation may be a factor in the bronchial hyperresponsiveness that characterises asthma. As hyperresponsiveness is increased at night, changes in platelet activation over 24 hours were related to the diurnal changes in peak expiratory flow and plasma catecholamine concentrations in five subjects with asthma and five normal subjects. The effect of muscarinic receptor blockade with intravenous atropine at 0400 hours on these measurements was also studied. Platelet activation, assessed as the ration of beta thromboglobulin to platelet factor 4, was highest when the peak expiratory flow rate was at its lowest in the asthmatic subjects. There was no correlation between platelet activation and plasma catecholamine concentrations. Intravenous atropine did not alter the ratio of beta thromboglobulin to platelet factor 4, suggesting that parasympathetic activity is not the cause of the increased platelet activation at night.

Venous blood platelets decrease during allergen-induced asthmatic reactions

To determine whether circulating platelets alter during asthmatic reactions induced by allergens, we studied nine subjects previously shown to develop an early or dual asthmatic reaction after inhalation challenge with extracts of house dust mite or grass pollen. In each subject, FEV1, circulating platelets and leucocytes were measured before, 15, 30 and 60 min, and 2, 4, 6 and 8 hr after inhalation of allergen and diluent control administered in a single-blind, randomized fashion. The same procedure was repeated in six of the nine subjects after bronchoconstriction induced by methacholine. Each subject developed an early asthmatic reaction after allergen inhalation challenge, which was followed by a late asthmatic reaction in six subjects and by an equivocal late asthmatic reaction in two of them (fall in FEV1 of 15 and 17% respectively). Compared with the control day, circulating platelets significantly decreased during the allergen-induced early asthmatic reaction (P less than 0.025, at 30 min). Platelet counts returned to baseline values within 4 hr and remained steady thereafter both in subjects who did and did not develop a late asthmatic reaction. No changes in platelet counts occurred after bronchoconstriction induced by methacholine. Diurnal increase of leucocyte numbers occurred after challenge with both allergen and diluent control. These results suggest that platelets may be involved in the pathogenesis of allergen-induced asthmatic reactions.

The effect of histamine challenge on platelet count and platelet activation

There is conflicting evidence of platelet involvement in asthmatic bronchoconstriction. We sought direct evidence of platelet activation and indirect evidence of platelet sequestration following histamine challenge (HC) in 14 asthmatics and seven non asthmatic control subjects. Peripheral platelet counts and levels of beta thromboglobulin (BTG) were measured immediately before and following HC in the 21 subjects. There were no significant differences in the platelet counts or BTG levels, before or after HC, within or between the two groups. These results do not support platelet activation or accumulation in the lungs occurring immediately following histamine induced bronchoconstriction in asthmatics.