Aggregation and Release Reaction Induced in Human Blood Platelets by Zymosan

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Abstract
When citrated platelet-rich plasma was shaken with zymosan particles, platelet aggregation and 14C-serotonin release occurred within 15 min. The platelet response occurred much more rapidly if the zymosan was first incubated with heparinized or citrated plasma for 20 to 30 min. After such incubation, the activity was on the zymosan; the plasma had no effect on platelets and could not generate further activity when incubated with fresh zymosan. Plasma was not necessary for the reaction between washed active zymosan and platelets. However, albumin enhanced the response. The usual ratio of zymosan to platelets was 1:20, and mixed aggregates formed after a delay of a minute or more. ADP, ATP, and heparin neutralizing activity were released from the platelets but beta glucuronidase and lactic dehydrogenase were not. Aggregation and release were inhibited by aspirin or by enzymes which remove ADP, and did not occur with thrombasthenic platelets. The reaction between zymosan and plasma required magnesium ions, and failed to occur in plasma incubated with cobra venom factor or hydrazine or heated to 50°C for 30 min. A mixture of hydrazine-treated and heated plasma generated activity on zymosan as did plasma from a patient with congenital C6 deficiency. Serum was much less effective than plasma in generating activity on zymosan unless fibrinogen was added or more than 15 µg/ml of residual fibrinogenfibrin-related antigen was present. This antigen was detected on zymosan prepared from citrated plasma but not on that prepared from serum or EDTA plasma. Thus the release-promoting activity of zymosan particles depends on their exposure to both the properdin (complement-bypass) mechanism and to fibrinogen.

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