Interaction of subcomponent C1q and collagen with isolated platelet membranes in platelet aggregation

Human platelet membranes were isolated by the glycerol-lysis technique. When tested by a turbidimetric method in the presence of fibrinogen and calcium ions, the membrane vesicles exhibited patterns of reactivity to collagen. As with human platelet-rich plasmas, the aggregating effects of collagen were inhibited by preincubating the membrane suspensions with hemolytically active human C1q. Upon addition to platelet-rich plasma, the isolated membranes reduced collagen-provoked platelet aggregation. This inhibitory effect was partially suspended by prior incubation of the membranes with native C1q. The results show that the receptors for collagen and C1q are preserved during the membrane isolation procedure and support the competitive nature of inhibition by C1q of collagen-induced platelet aggregation.