CHARACTERIZATION OF ADHESION OF NON-EXOGENOUSLY STIMULATED AND RESTING PLATELETS IN NORMAL PLASMA TO FIBRINOGEN AND ITS FRAGMENTS

Platelet adhesion to various forms of fibrinogen was studied using pla telets in plasma and washed platelets. The study was designed to deter mine if platelets prepared with minimal handling in plasma at physiolo gical pH containing normal levels of Ca2+ have different requirements for adhesion to immobilized fibrinogen than do washed platelets tested in the absence of plasma. Exposure of platelets to citrate and low pH did not seem to affect the requirements of washed platelets for adhes ion to fibrinogen. Nonetheless, behavioural differences between these two types of platelets were seen. Surprisingly, in the absence of exog enous activation normal platelets in plasma behaved qualitatively as s timulated washed platelets. That is, both types of platelets adhered t o all forms of fibrinogen which possessed at least one gamma-chain car boxyl terminal platelet binding site. Platelets in plasma treated with prostaglandin E(1) (resting platelets) adhered only to forms of fibri nogen which contained two gamma-chain platelet binding sites. These ob servations also demonstrate that the fibrinogen alpha-chain arginine-g lycine-aspartic acid-phenylalanine and arginine-glycine-aspartic acid- serine sequences are not necessary or sufficient to mediate the adhesi on of resting or stimulated platelets in plasma to fibrinogen. The pre sence of endogenous adenosine diphosphate appears to account, at least in part, for the ability of normal platelets in plasma to adhere to f orms of fibrinogen which have only one gamma-chain platelet binding si te.