Evidence for multiple mechanisms of interaction between wheat germ agglutinin and human platelets

Wheat germ agglutinin induced aggregation and secretion of serotonin from human platelets in plasma. This aggregation of platelets was blocked by ethylenediaminetetraacetate, azide or prostaglandin E1. The secretion of serotonin was not affected by ethylenediaminetetraacetate but was inhibited by prostaglandin E1. Thus, wheat germ agglutinin acts on platelets in plasma as a true aggregating agent. Washed platelets showed increased light transmission when treated with the lectin which was not blocked by ethylenediaminetetraacetate or prostaglandin E1. The capacity to inhibit platelet clumping by the above agents was restored if plasma was added back to the cell suspension. Washed platelets did not release serotonin under the conditions of cell clumping. Thus, in contrast to platelets in plasma, washed platelets are agglutinated by the lectin. Platelets fixed in formaldehyde were not agglutinated by the lectin in the aggregometer but agglutination was observed in the microtiter plate. This agglutination may be mediated by interplatelet bridging. The results show that the same agent may act on platelets by different mechanisms depending on the state of the cell and its environment.