Summary
Spontaneous platelet aggregation (SPA) was studied in human whole blood at 3, 5, 10,
20, 30, 40 and 60 minutes after venepuncture. Using a whole blood platelet counter,
SPA was quantified by measuring the fall in single platelet count upon rollermixing
aliquots of citrated blood at 37° C. The extent of SPA increased with the time after
venepuncture, with a correlation coefficient of 0.819. The inhibitory effect of dipyridamole
(Dipy) on SPA was studied: (a) 10 μM at each time interval; (b) 0.5-100 μM at 3 and
30 minutes and (c) 15 μM in combination with 100 μM adenosine, 8 μM 2-chloroadenosine
(2ClAd, an ADP receptor blocker) and 50 μM aspirin. There was a rapid decrease in
the inhibitory effect of Dipy with the time after venepuncture; the correlation coefficient
was -0.533. At all the concentrations studied, Dipy was more effective at 3 minutes
than at 30 minutes after venepuncture. A combination of Dipy with adenosine, 2ClAd
or aspirin was a more effective inhibitor of SPA than either drug alone. However,
when 15 μM Dipy and 10 μM Ad were added together, the inhibitory effect of Dipy was
not increased significantly, suggesting that Dipy inhibits platelet aggregation independent
of Ad. The increase in SPA with the time after venepuncture was abolished when blood
was taken directly into the anticoagulant containing 5 μM 2ClAd. It is suggested that
ADP released from the red blood cells is responsible for the increased platelet aggregability
with the time after venepuncture and makes a serious contribution to the artifacts
of in vitro platelet function studies.
Keywords
Time after venepuncture - Spontaneous platelet aggregation - Dipyridamole - 2-chloroadenosine
- ADP
