ADP and epinephrine-induced release of platelet fibrinogen

Human platelets gel-filtered into Tyrode's buffer containing 1 mM Mg++ and 0.35% bovine serum albumin were studied to determine whether they would undergo biphasic aggregation and release of alpha-granule proteins in response to adenosine diphosphate (ADP) or epinephrine without addition of exogenous fibrinogen. Fibrinogen concentration in the supernatant of unaggregated gel-filtered platelets was less than 1 pmole/ml. With addition of ADP or epinephrine, biphasic aggregation was seen, with release of platelet fibrinogen, beta-thromboglobulin, and platelet factor 4. Fibrinogen concentration in the supernatant after aggregation ranged from 15 to 70 pmole/ml. Release of the alpha-granule proteins by epinephrine was coincidental with release of the dense granule adenine nucleotides. Aggregation and alpha-granule protein release by both ADP and epinephrine were inhibited by added Ca++ at 1--2 mM. The ability of gel-filtered platelets to undergo ADP- and epinephrine-induced aggregation and release in the absence of exogenous fibrinogen suggests that released platelet fibrinogen may be able to fulfill the requirement for fibrinogen in ADP- and epinephrine-induced platelet aggregation and release.