ATP-secretion occurs as an initial response in collagen induced platelet activation

The time course and degree of collagen induced platelet shape change, aggregation and ATP-secretion in citrated human platelet rich plasma was monitored with a lumi-aggregometer. Onset of ATP-secretion was measurable within seconds after collagen challenge and always distinctly before onset of shape change. In absence of platelet aggregation, the degree of collagen stimulation was correlated to onset of secretion, the secretion rate and onset of subsequent shape change. It is concluded that the secretion originated from the collagen-adherent platelets. Oral aspirin [1000 mg] did neither delay onset of secretion nor reduce the final amounts of ATP indicating that formation of TxA2 is not a necessary prerequisite for dense granule secretion during the primary activation process. EDTA and EGTA decreased the initial secretion rate, but may have interfered with the adhesion process. Micromolar concentrations of verapamil and yohimbine also reduced secretion, while the effect of theophylline was 100 times weaker. The results suggest that the primary activation process is vulnerable at the alpha-receptor-level and that the initial ATP-secretion requires Ca2+-entry rather than intra-platelet Ca2+-mobilization.