Platelet alpha-granule proteins: studies on release and subcellular localization.

Four platelet proteins are secreted by platelets during the release reaction: platelet factor 4 (PF4).1 $-thromboglobulin ( 9TG). platelet-derived growth factor (PDGF). and fibrinogen. The characteristics of release of these proteins and 14Cserotonin (14C-5HT) were studied and compared with secretion of acid hydrolases as described by others.’ Distinct differences were found between the release of PF4, /9TG. and fibrinogen and the release of acid hydrolases. Among these were the following: (1) ADP and epinephrine induced release of PF4 and $TG but not acid hydrolases. (2) low concentrations of thrombin caused release of PF4. /9TG. and fibrinogen. but high concentrations were required for secretion of acid hydrolases. (3) Aspirin and indomethacin inhibited PF4. flTG, and fibrinogen release, but in vitro addition of aspirin caused minimal inhibition of acid hydrolase secretion. Secretion of protein by platelets also differed from release of 14C-5HT in that PF4. /9TG, and fibrinogen were secreted to a greater extent than was 14C-5HT by low concentrations of thrombin. collagen. and U4661 9 (a structural analogue of prostaglandin H2), and arachidonic acid caused more extensive release of PF4, /9TG. and fibrinogen than of ‘4C-5HT at all concentrations that induced release. When platelets were subjected to subcellular fractionation, specific radioimmunoassay techniques indicated that PF4, $TG, and fibrinogen were localized in the platelet a-granule fraction. The PDGF was localized to the a-granule fraction by the use of a bioassay. This localization was distinct from that of platelet acid hydrolases and dense granule components. Thus there were three types of platelet granule populations, each with unique constituents and with characteristic patterns of release. In vitro release of a-granule proteins by ADP, epinephrine, and arachidonic acid was prevented by inhibition of platelet cyclooxygenase, and this suggested that release was dependent on oxygenation of arachidonic acid by cyclooxygenase. In contrast, release by collagen and thrombin was only partially mediated by this pathway. since high concentrations of these agents bypassed the inactivated cyclooxygenase and induced a-granule protein release. Initial studies were performed to determine whether in vivo release could be inhibited by aspirin. Plasma levels of PF4 and /ITG in normal volunteers were decreased following aspirin ingestion. thus suggesting a contribution of cyclooxygenase-related platelet release to circulating levels of PF4 and /9TG in normal subjects.