INTRACELLULAR CALCIUM MOBILIZATION IS TRIGGERED BY CLUSTERING OF MEMBRANE-GLYCOPROTEINS IN CONCANAVALIN A-STIMULATED PLATELETS

Stimulation of human platelets with concanavalin A resulted in a signi ficant increase in the concentration of cytoplasmic free Ca2+. This ef fect was due to two different processes: Ca2+ mobilization from intern al stores and Ca2+ influx from the extracellular medium. Kinetic analy sis revealed that the release of Ca2+ from internal storage sites occu rred sooner than the opening of plasma membrane Ca2+ channels. The abi lity of concanavalin A to induce a sustained increase in cytoplasmic C a2+ concentration was antagonized and reversed by methyl proportional to-D-mannopyranoside, demonstrating that it was promoted by the intera ction of the lectin with cell surface glycoproteins. Succinyl-concanav alin A, a dimeric derivative of the lectin, that does not promote patc hing/capping of the receptor, was able to bind to the platelet surface , and antagonized the effects of native concanavalin A. In addition, s uccinyl-concanavalin A, per se, was unable to induce Ca2+ mobilization in human platelets. Therefore, the action of the native concanavalin A was mediated by receptor clustering events. Concanavalin A mobilized Ca2+ from the same internal stores from which Ca2+ was mobilized in r esponse to strong platelet agonists, such as thrombin and arachidonic acid. However, while thrombin was ineffective in inducing Ca2+ release after stimulation of platelets with ConA, Con A was able to cause a f ull discharge of Ca2+ from internal stores even in platelets previousl y stimulated with thrombin. These results demonstrate for the first ti me that the clustering of specific membrane glycoproteins can trigger platelet activation. The physiological implications during platelet ag gregation are discussed.