Fibrinogen receptor exposure and aggregation of human blood platelets produced by ADP and chilling

ADP-induced platelet aggregation is associated with the specific binding of fibrinogen to receptors. Since fibrinogen is also a cofactor for the spontaneous aggregation of chilled human platelets during rewarming, we examined the ability of these platelets to bind purified ¹²⁵I-labeled fibrinogen. Specific binding similar to ADP-induced binding at room temperature was observed after the platelets had been chilled for 20 min at 0-2°C. Aggregation, however, occurred only when the ambient temperature was increased and was followed within 15-30 sec by loss of most of the bound fibrinogen and disaggregation. ADP- and cold-induced fibrinogen binding and aggregation were inhibited by 0.3 μM PGE₁, 1 mM dibutyryl cyclic AMP, and 2.5 mM EDTA, unaffected by 208 μM colchicine, and absent from thrombasthenic platelets. Platelets change shape from discs to spiny spheres when they are exposed to ADP or cold. PGE₁ and dibutyryl cyclic AMP prevented the shape change induced by ADP but not by chilling. Shape change, fibrinogen binding, and aggregation induced by ADP, but not by chilling, were inhibited by incubating platelets with 7.3 μM antimycin A and 4.9 mM 2-deoxy-D-glucose. Fixed platelets bound fibrinogen only if they were exposed to ADP before fixation but they failed to aggregate. Studies with fixed platelets showed that EDTA and a low pH prevent the interaction of fibrinogen with exposed receptors, whereas platelets stimulated with ADP in the absence of divalent cations or at low pH prior to fixation bound fibrinogen at physiologic pH, and this binding was inhibited by EDTA. The present studies provide the following clues about the requirements for fibrinogen receptor exposure and platelet aggregation. (1) ADP and chilling activate platelets by exposing fibrinogen receptors via a common pathway that is blocked by elevating cyclic AMP. (2) Chilling can bypass the seemingly ATP-dependent step for ADP-induced fibrinogen receptor exposure. (3) Receptor exposure by both ADP and chilling is independent of platelet shape. (4) Increased cyclic AMP inhibits the shape change induced by ADP but not that caused by chilling. (5) EDTA and low pH prevent aggregation by interfering with the binding of fibrinogen to exposed receptors. (6) Receptor mobility may be as important as fibrinogen binding in supporting platelet aggregation.