Increased platelet aggregation due to chilling to 20 degrees C is not related to increased sensitivity to agonists

BACKGROUND: Previous studies suggested that chilled platelets have a greater sensitivity to agonists than do platelets aggregated at 37 degrees C. STUDY DESIGN AND METHODS: Washed platelets were aggregated at 20 or 37 degrees C with ADP (0-20 mu M), arachidonic acid (0-200 mu M), or the thromboxane mimetic U46619 (0-9 nM). RESULTS: Chilling caused a significant (p less than or equal to 0.05) increase in spontaneous platelet aggregation (>27% at 20 degrees C vs. <5% at 37 degrees C) and spontaneous dense granule release (>0.01 nM of ATP at 20 degrees C vs. 0 nM of ATP at 37 degrees C). ADP and U46619 caused a significantly greater aggregation response and dense granule release at 20 degrees C, although there was no change in agonist sensitivity of platelets (effective dose of agonist necessary to induce 50% aggregation [ED50]: 1.00 +/- 0.35 mu M ADP at 20 degrees C and 1.63 +/- 0.47 mu MADP at 37 degrees C; 8.26 +/- 3.65 pM U46619 at 20 degrees C and 18.97 +/- 4.82 pM U46619 at 37 degrees C). Platelets aggregated with arachidonic acid showed a significant decrease in aggregation and agonist sensitivity at 20 degrees C (ED50 118.7 +/- 44.4 mu M) from those at 37 degrees C (ED50 25.6 +/- 7.2 mu M), possibly as a result of the reduced enzymatic activity of cyclooxygenase and thromboxane synthase at the lower temperature. CONCLUSION: The data suggested that washed platelets chilled to 20 degrees C and aggregated are not more sensitive to agonists than are 37 degrees C controls, but rather that chilled platelets undergo greater spontaneous aggregation.