Collagen-induced platelet shape change. The role of collagen quaternary structure

Multimerization of collagen molecules appears to be a prerequisite for collagen-induced platelet aggregation and release. Whether a similar requirement exists for collagen-initiated platelet disc-to-sphere transformation, one of the earliest events in the platelet:collagen interaction, is not known. We have studied platelet shape change by highly purified type I collagen in the presence of EDTA to prevent interference from platelet aggregation and release. The ability of soluble collagen to induce platelet shape change was dependent on prior fibril formation. Tropocollagen from which multimers were removed was not able to initiate the disc-to-sphere transformation, indicating that monomeric collagen is insufficient as a platelet activator. Increasing the extent of multimerization by preincubation at 37 degrees led to an increase in the response rate and a decrease in the lag time for initiation of shape change, but even after maximal multimerization, the lag time was not eliminated. The increase in the turbidity of the collagen solution preceded its ability to stimulate platelet shape change, suggesting that a minimum size of the multimer is needed for platelet activation. L-Arginine, an inhibitor of multimerization, prevented collagen-induced shape change in a dose-dependent relationship. Enhancement of the shape change response rate as a function of the concentration of collagen multimers was found.