Adhesion of Platelets to Collagen

Platelets adhere to collagen fibers, undergo the release reaction and aggregate, soluble collagen, however, does not cause release or aggregation. Platelets, however, recognize soluble collagen since adhesion (but not aggregation) occurs when a soluble collagen coated surface is exposed to washed rabbit platelets in Tyrode and in competitive experiments preincubation of platelets with soluble collagen reduces the number adhering to a collagen surface. The latter effect is concentration dependent; preincubation with 100 μg/ml of soluble collagen reduces platelet adhesion by 70%. Similar competitive experiments were performed to examine the features on collagen responsible for platelet adhesion. Preincubation of platelets with a variety of soluble collagens, denatured soluble collagen, pepsin- and periodate-treated soluble collagen and cyanogen bromide peptides all decrease the ability of platelets to bind to a collagen surface. Similar effects arise moreover with the synthetic copolypeptides (Gly-Pro-Ala-Gly-Pro-Pro)n,(Gly-Pro-Pro)n and (Pro2Gly)n. In fact, the homopolymers, polyproline and polyhydroxyproline (but not proline or hydroxyproline) as well give effects similar to those of the collagen materials. The effect is rather specific since preincubation with plasma proteins and synthetic polypeptides based on other major amino acids present in collagen, viz. polyalanine, polyglutamic acid, polyaspartic acid, polyarginine and polylysine do not inhibit the binding of platelets. It would therefore appear that proline and hydroxyproline sequences incorporated in a macromolecular chain are the important, if perhaps not the only determinants involved in platelet recognition of collagen. Soluble collagen coated surfaces were used in this study, however, similar effects of the test material on platelet adhesion to collagen fibers could be demonstrated.